Optical sensing device with asymmetric plasmonic antenna

Abstract

The invention is notably directed to an optical sensing device (1, 1a) having a layer structure comprising: a substrate (10) structured to laterally delimit a cavity (40); a dielectric layer (11), which extends on top of the substrate (10) and forms a membrane (11) spanning the cavity (40), the membrane (11) including n apertures (30) to the cavity (40), where n ≥ 1, preferably n ≥ 100, and more preferably n ≥ 400; and plasmonic antennas comprising n pairs of opposite antenna elements (17, 17a) patterned on top of the dielectric layer (11) on opposite lateral sides of respective ones of the n apertures (30), so as to define n respective gaps (g) on top of the n apertures, wherein the gaps extend between opposite antenna elements (17, 17a) of the n pairs along respective directions parallel to an average plane (x, y) of the dielectric layer (11) to define n molecular passages, each extending from the cavity (40) through a respective one of the n apertures (30) and a respective one of the n gaps, and the two antenna elements of each of the n pairs have triangular shapes in a bowtie configuration, but one antenna element only of said two antenna elements has a slit extending from one lateral side thereof, such that said bowtie configuration is asymmetric. The invention is further directed to related apparatuses, fabrication methods, and methods of optically sensing analytes.

Description

[0001] OPTICAL SENSING DEVICE WITH ASYMMETRIC PLASMONIC ANTENNA

[0002] TECHNICAL FIELD

[0003] The invention is directed to an optical sensing device, as well as related apparatuses, fabrication methods, and method for optically sensing analytes (such as DNA and RNA molecules). In particular, it is directed to a nanofabricated sequencing device, involving asymmetric bowtie antennas arranged about respective apertures, which form molecular passages. The apertures are bordered by pairs of bowtie antenna elements, where one of the antenna elements includes a lateral slit, making the corresponding plasmonic antenna asymmetric. Analyte molecules can be controllably trapped in the molecular passages for sensing applications, e.g., based on surface-enhanced Raman spectroscopy techniques.

[0004] BACKGROUND

[0005] WO2023117078A1 (to LSPR AG) discloses plasmonic nanopore single-molecule DNA sequencing devices and methods of fabrication thereof. The devices have a layer structure, which involves a substrate, a dielectric layer, and opposite antenna elements. The substrate is structured to laterally delimit a cavity. The dielectric layer extends on top of the substrate and forms a membrane spanning the cavity. The membrane includes n apertures (i.e., nanopores) to the cavity, where n can typically be larger than or equal to 100 or 400. There are n pairs of opposite antenna elements, which are patterned on top of the dielectric layer, on opposite lateral sides of respective ones of the n apertures. The n pairs of opposite antenna elements define n respective gaps extending between opposite antenna elements of the n pairs along respective directions parallel to an average plane of the substrate. The n pairs of opposite antenna elements define, together with the respective apertures, n molecular passages. Each passage extends from the cavity through a respective one of the n apertures (i.e., through the membrane) and a respective one of the n gaps, along a direction transverse to the average plane of the substrate. The average length of the n gaps along said respective directions is between 4 nm and 20 nm. The n gaps define respective electromagnetic field enhancement regions, in which electromagnetic radiation can be concentrated upon irradiating the antenna elements, for optically sensing molecules. The average diameter of the n apertures is larger than or equal to the average length of the gaps along their respective directions. Thus, the minimal cross- sectional dimension of each of n the passages is limited by a respective one of the n gaps along said respective directions.

[0006] According to the above design, the antenna elements can be substantially flush with, or even hang over the edges of the aperture underneath. However, the antenna elements will not be laterally recessed with respect to the edges of this aperture, subject to fabrication tolerances (on the order of the nm). This means that the minimal cross-sectional dimension of each passage (as defined by a corresponding aperture in the dielectric layer) is actually limited by the gap between opposite antenna elements bordering this aperture. And this allows the electromagnetic field enhancement volume defined between the opposite antenna elements to be minimized, for a given cross-sectional dimension of the corresponding aperture. This, in turn, allows the electromagnetic field enhancement to be increased.

[0007] The above device configuration is extremely challenging to process with high accuracy and low tolerance (on the order of the nm), because the gaps between antenna elements are defined above respective apertures defined in a thin membrane, as opposed to contexts where antenna elements lay flat on a thick substrate. Now, WO2023117078A1 further discloses fabrication methods that allow nanostructures as described above to be accurately defined, i.e., with an accuracy on the order of the nm. In addition, such fabrication methods allow nanostructures to be obtained with deterministic and reproducible shapes, dimensions, and positions, unlike prior fabrication methods. Previous nanofabrication methods required the patterned antenna element to be laterally recessed with respect to the edges of the corresponding aperture.

[0008] While the above antenna configuration already makes it possible to increase the electromagnetic field enhancement for two given antenna elements on either side of the opening, the inventors set themselves the challenge of improving the electromagnetic field enhancement further, under the constraints that nanostructures can be obtained with deterministic and reproducible shapes, dimensions, and positions.

[0009] SUMMARY

[0010] According to a first aspect, the invention is embodied as an optical sensing device. The device has a layer structure, which involves a substrate, a dielectric layer, and plasmonic antennas. The substrate is structured to laterally delimit a cavity. The dielectric layer extends on top of the substrate and forms a membrane spanning the cavity. The membrane includes n apertures to the cavity, where n > 1. Two or more apertures are preferably provided (n > 2). More preferably, the number of apertures is larger than or equal to 100 or, even, 400. The plasmonic antennas comprise n pairs of opposite antenna elements patterned on top of the dielectric layer on opposite lateral sides of respective ones of the n apertures. As a result, the n pairs of opposite antenna elements define n respective gaps on top of the n apertures. The gaps extend between opposite antenna elements of the n pairs along respective directions parallel to an average plane of the dielectric layer to define n molecular passages. Accordingly, each gap defines a respective electromagnetic field enhancement region, in which electromagnetic radiation can be concentrated upon irradiating the respective antenna elements for optically sensing molecules passing through the gaps, in operation. Each of the n molecular passages extends from the cavity through a respective one of the n apertures and a respective one of the n gaps. Finally, the two antenna elements of each of the n pairs have triangular shapes in a bowtie configuration. However, one antenna element only of these two antenna elements has a slit extending from one lateral side thereof, such that the bowtie configuration is asymmetric.

[0011] The above design relies on plasmonic antennas, where the symmetry of the bowtie configuration is broken by introducing a slit (i.e., a slot or notch, or a strip-like cut-out) laterally in one element (call it the slit antenna element) of the two antenna elements and, this, for each of the pairs of antenna elements. This effectively results in an asymmetric bowtie configuration. I.e., compared with the usual, ideal bowtie configuration, the bowtie structure proposed above is no longer symmetrical with respect to the gap or aperture, i.e., with respect to the median plane extending perpendicular to the average plane of the antenna elements. This makes it possible to obtain Fano resonances and further increase the electric field enhancement at the level of the gap. Analyte molecules can be controllably trapped in the molecular passages for sensing application, e.g., using surface-enhanced Raman spectroscopy techniques.

[0012] Various strategies can be contemplated to optimize the asymmetrical geometry of the bowtie structure and, in turn, improve the field enhancement. For instance, in embodiments, the slit extends over an entire thickness of the slit antenna element, to accentuate the asymmetry of the bowtie configuration and thus the desired effects in terms of Fano resonances and electric field enhancement.

[0013] Beyond the asymmetry resulting from the slit, whereby one antenna element differs from the other, the slit antenna element may further be asymmetric with respect to a transverse plane containing the line segment that passes through the apex and is perpendicular to the base (i.e., the side opposite the apex) of the slit element. That is, apices of the two antenna elements typically point towards each other (subject to fabrication tolerances, typically on the order of the nm), as a result of the bowtie configuration. Now, the slit antenna element may advantageously be asymmetric with respect to the longitudinal plane that contains the line segment passing through the apices and is perpendicular to the average plane of the dielectric layer.

[0014] To that aim, the lateral side (or edge) from which the slit extends is preferably a lateral side joining the apex (meaning one of the two apices as defined above) of the slit antenna element. In particular, this slit may extend from this lateral side to at least the longitudinal plane but preferably not up to the opposite lateral side, i.e., opposite with respect to the longitudinal plane.

[0015] Furthermore, one may seek to optimize the apparent surface of the slit. For example, in embodiments, the ratio of the apparent area of the sole slit (call it the first area) to the apparent area (the second area) of the slit antenna element is between 0.10 and 0.20. Each of the first and second area is measured parallel to the average plane of the dielectric layer. The second area corresponds to the actual prism and not the ideal prism (without the notch).

[0016] Moreover, the slit may be curved or bent along its direction of extension, or have a non-constant profile along its direction of extension. Note, this direction of extension is not necessarily straight; it is, however, parallel to the average plane of the dielectric layer.

[0017] For completeness, the slit may be filled with a different material. That is, while the antenna elements are made of a first material (e.g., a metal such as gold), the slit is preferably filled by a second material that is distinct from the first material. The second material may for instance be a dielectric material having a distinct permittivity (typically higher than the permittivity of the first material). Optionally, the second material fills the slit only and thus stops at the lateral side (or edge) of the slit antenna element.

[0018] In embodiments, an average length of the n gaps along said respective directions is between 4 nm and 20 nm. The average diameter of the n apertures is equal to the average length of the gaps, subject to ± 2 nm, whereby inner ends of the antenna elements of each of the n pairs are substantially flush with inner walls of the respective n apertures in the membrane. That is, the vertices of the antenna elements are brought as close as possible to the edge of the respective aperture. This allows the electromagnetic field enhancement to be further increased, all things being otherwise equal. In variants, the apices of the antenna elements even hang over the edges of the aperture underneath, which further reduces the electromagnetic field enhancement region. For completeness, the diameters of the apertures and the lengths of the gaps will preferably be essentially constant, subject to a dispersion (i.e., a standard deviation) of less than (or equal to) 3 nm. More preferably, an average in-plane separation distance between two closest apertures is between 1 and 10 microns, preferably between 2 and 7 microns.

[0019] In embodiments, the dielectric layer is a first dielectric layer, the device further includes a second dielectric layer, the substrate is on top of the second dielectric layer, and the substrate and the second dielectric layer are jointly structured to form a recess delimiting said cavity. Preferably, the substrate comprises silicon, each of the two dielectric layers comprises SislS , and each of the antenna elements essentially comprises gold (Au). In variants, the antenna elements may comprise any other metal that exhibits plasmonic behaviour in the desired wavelength region, e.g., in the range between 200 and 3000 nm, preferably between 700 and 1000 nm. The thickness of each of the first and second dielectric layers is preferably between 10 and 60 nm, and more preferably between 15 and 35 nm. Despite their thicknesses, such dielectric layers are found to be surprisingly stable, mechanically and chemically speaking. In addition, the optical sensing device may further comprise one or more pairs of electrodes, wherein the electrodes of each of the pairs are on opposite sides of the first dielectric layer. In variants, such electrodes do not form part of the device but are nevertheless brough in contact with a liquid in which the device is immersed.

[0020] According to another aspect, the invention is embodied as an optical sensing apparatus. The apparatus notably comprises an optical sensing device as described above. It further includes a distributed electromagnetic source (e.g., comprising a Laser), which is configured to irradiate the antenna elements of at least some of the n pairs of opposite antenna elements. This way, electromagnetic radiation can be concentrated in respective electromagnetic field enhancement regions as delimited by the n gaps, with a view to optically sensing molecules in the respective gaps. The apparatus further includes a detector, which is configured to optically detect optical signals as modulated and / or generated by the molecules in said regions, in operation. The detector is preferably a Raman spectrometer. It may for instance be a spatial heterodyne Raman spectrometer, a snapshot hyperspectral spectrometer or a scanning hyperspectral spectrometer.

[0021] In embodiments, the apparatus further comprises an electrical circuit comprising one or more pairs of electrodes, wherein the electrodes of each of the pairs are on opposite sides of the (first) dielectric layer. The electrical circuit is notably configured to apply a voltage bias between electrodes of each of the pairs to urge molecules through the passages. The optical sensing device of the apparatus may include two dielectric layers as described above, where the substrate extends on top of the second dielectric layer, so as to form a recess delimiting said cavity.

[0022] According to another aspect, the invention is embodied as a method of fabrication of an optical sensing device as described above. The method comprises providing a substrate, depositing a dielectric layer on top of the substrate, and patterning fiducial marks on both the dielectric layer and the substrate. In addition, n pairs of opposite antenna elements are patterned on top of the dielectric layer (e.g., using electron beam lithography) to form plasm onic antennas, based on an alignment protocol exploiting the fiducial marks previously patterned. According to this patterning step, the two antenna elements of each of the n pairs patterned have triangular shapes in a bowtie configuration, but one antenna element only of said two antenna elements has a slit extending from one lateral side thereof, whereby said bowtie configuration is asymmetric. This is further done so as to define n respective gaps extending between opposite antenna elements (in fact between apices of antenna elements) of respective ones of the n pairs (n > 1) along respective directions parallel to an average plane of the substrate. The average length of the n gaps along their respective directions is preferably between 4 nm and 20 nm. Optionally, a protective layer is then deposited, so as to coat inner ends of the opposite antenna elements of each of the n pairs. The dielectric layer is subsequently dry etched (preferably using reactiveion etching) at locations defined according to the fiducial marks, to open n apertures through the dielectric layer, between opposite antenna elements of respective ones of the n pairs. As a result, the opposite antenna elements of the n pairs are on opposite lateral sides of respective ones of the n apertures. Moreover, the antenna elements and the dielectric layer are coated with a protective polymer, for it to plug the gaps and the apertures. Next, the substrate is structured to form a recess extending up to the dielectric layer, so as for the latter to extend on top of residual, peripheral portions of the substrate and form a membrane spanning a cavity delimited by the recess. Eventually, the protective polymer is removed, to free up n molecular passages, each extending from the cavity through a respective one of the n apertures and a respective one of the n gaps along a direction transverse to the average plane of the substrate. The method is performed to obtain an optical sensing device, where the n gaps effectively define respective electromagnetic field enhancement regions, in which electromagnetic radiation can be concentrated upon irradiating the antenna elements, for optically sensing molecules, in operation of the resulting device. Moreover, the average diameter of the n apertures is preferably larger than or equal to the average length of the gaps along their respective directions, whereby a minimal cross-sectional dimension of each of n the passages is limited by a respective one of the n gaps along said respective directions.

[0023] The above fabrication method makes it possible to achieve clean and precise asymmetric nanostructures, where a pair of antenna elements are arranged on opposite lateral sides of nanoapertures formed through a membrane spanning a cavity, while ensuring deterministic and reproducible positions, shapes, and dimensions of the nanostructures. The above fabrication approach revolves around patterning the asymmetric antenna elements (e.g., using electron beam lithography, or EBL for short), based on an alignment protocol exploiting previously patterned fiducial marks, prior to etching the apertures. More precisely, the apertures are etched only after having patterned the asymmetric antenna element and after having protected the inner ends (i.e., the apices) of the antenna elements. The recess is then formed on the back side, but only after having coated the upper structures to protect the gaps and apertures.

[0024] In preferred embodiments, the dielectric layer is a first dielectric layer, and the method further comprises depositing a second dielectric layer below the substrate, preferably while depositing the first dielectric layer. The second dielectric layer is patterned (after patterning the fiducial marks but prior to patterning the n pairs of opposite antenna elements) so as for residual, peripheral portions of the second dielectric layer to delimit the recess to be formed next.

[0025] Preferably, the fiducial marks are patterned using EBL and a dry etching procedure. The fiducial marks may for example be patterned as slits extending through the dielectric layer and partly in the substrate. Such slits should not be confused with the slit cut out from the slit antenna element.

[0026] Preferably, the n pairs of opposite antenna elements are patterned as follows. First, a photoresist is deposited on top of the dielectric layer, for the photoresist to plug the slits serving as fiducial marks. As a result, the photoresist protects the fiducial marks from being filled with metal (e.g., gold) during the subsequent deposition of the metallic layer. Second, the photoresist is structured by photolithography, for it to form residual plugs at a level of the slits. A metallic layer is then deposited on top of the dielectric layer and the residual plugs, and the residual plugs are then removed to define openings at the level of the slits (i.e., the fiducial marks). Next, an electron beam resist is deposited on top of the metallic layer. The electron beam resist is subsequently structured by electron beam lithography in accordance with shapes of the antenna elements. Finally, the metallic layer is etched through the structured electron beam resist using ion beam etching, to obtain the desired antenna elements. In preferred embodiments, the protective layer is an alumina layer, which is obtained by coating an electron beam resist, opening cavities at the level of the gaps, and depositing the protective layer by atomic layer deposition at a temperature that is less than 90 C (to keep it compatible with materials in other layers). As a result, the protective layer notably coats the inner ends (i.e., apices) of the opposite antenna elements of each of the n pairs.

[0027] Preferably, the substrate essentially comprises silicon, each of the two dielectric layers essentially comprises SislS , and each of the antenna elements essentially comprises Au.

[0028] According to another aspect, the invention is embodied as a method of optically sensing an analyte. This method relies on an optical sensing device as described above. The method comprises irradiating pairs of antenna elements of this device, to concentrate electromagnetic radiation in the electromagnetic field enhancement regions delimited by the gaps, and sensing molecules in the gaps by optically detecting optical signals that are modulated and / or generated by the molecules in the gaps, thanks to an optical detector, which preferably includes a Raman spectrometer, more preferably a spatial heterodyne Raman spectrometer or another hyperspectral Raman spectrometer. In particular, the present approach can be used to identify a nucleic acid sequence and can be applied to characterize both single and double stranded DNA sections.

[0029] Preferably, the sensing method further comprises applying an electric field across the membrane to urge molecules to the passages and trap the molecules at the gaps, by virtue of a combined effect of the electric field applied and the electromagnetic radiation concentrated in the respective electromagnetic field enhancement regions.

[0030] In preferred embodiments, the optical signals are detected with a Raman spectrometer, according to a surface-enhanced Raman spectroscopy technique, such as, surface enhanced Coherent anti-Stokes Raman spectroscopy (SECARS), surface-enhanced resonance Raman scattering (SERRS), surface enhanced hyper Raman scattering (SEHRS), or surface-enhanced Raman scattering (SERS). The molecules may notably comprise DNA or RNA molecules. In that case, the optical signals detected may be exploited to identify a nucleic acid sequence of the molecules, e.g., through the respective Raman fingerprint spectrum.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. The illustrations are for clarity in facilitating one skilled in the art in understanding the invention in conjunction with the detailed description. In the drawings:

[0033] FIG. 1 is a 2D cross-sectional view of an optical sensing device (partial view) according to embodiments. The device contains a single membrane with a single cavity in this example, for the sake of simplicity of the depiction. Moreover, the cross-sectional view shows a single aperture, for simplicity. The inner ends of the antenna elements are aligned (i.e., flush) with edges of the aperture (or nanopore) in this example. One of the two antenna element includes a slit, resulting in an asymmetric bowtie configuration of the plasmonic antenna, according to embodiments;

[0034] FIG. 2 is a 3D view of an upper portion of the device of FIG. 1, showing a single pair of antenna elements in an asymmetric bowtie configuration, according to embodiments;

[0035] FIG. 3 is a 3D view of a device showing multiple plasmonic antennas, where the membrane includes a 2D arrangement of apertures and corresponding antenna elements, according to embodiments;

[0036] FIG. 4A is a 3D view of a sensing apparatus including a device such as shown in FIG. 1 or 2, which further illustrates the operation of the apparatus, as in embodiments;

[0037] FIG. 4B is a 2D cross-sectional view of the apparatus of FIG. 4A, further illustrating how a liquid containing DNA molecules can be spilled over the device, while the cavity underneath is filled with liquid, to trap and sense DNA molecules, as in embodiments;

[0038] FIG. 5 is a 2D cross-sectional view of a variant to the optical sensing device of FIG. 1, where the antenna elements partly hang over the aperture, as in embodiments; and

[0039] FIG. 6 is a corresponding 3D view;

[0040] FIG. 7 shows a 3D model (top view) of a plasmonic antenna with two antenna elements in a bowtie configuration, where the lower antenna element includes a slit extending from a lateral edge, according to embodiments;

[0041] FIGS. 8A- 80 show top views of potential, ideal patterns of plasmonic antennas as written in negative tone electron beam resist, as involved in embodiments;

[0042] FIGS. 9A - 90 show top views of simulated patterns, as obtained after transferring the ideal patterns of FIGS. 8A- 80 to a metal layer, as done in embodiments; FIG. 10A shows a segmented version of a scanning electron microscope (SEM) image of a plasmonic antenna pattern as written in a negative-tone electron beam resist, as in embodiments (top view, black-and-white segmented image);

[0043] FIG. 10B shows a segmented version of a SEM micrograph of the plasmonic antenna pattern of FIG. 10A, once transferred from the negative-tone e-beam resist to a gold layer, as in embodiments (top view, contour line); and

[0044] FIG. 11 is a Surface-Enhanced Raman Scattering (SERS) spectrum obtained for thymine, showing a number of high-quality Raman peaks (after background subtraction), as obtained with a plasmonic antenna such as shown in 10B.

[0045] The accompanying drawings show simplified representations of devices or parts thereof, as involved in embodiments. Technical features depicted in the drawings are not necessarily to scale. Similar or functionally similar elements in the figures have been allocated the same numeral references, unless otherwise indicated.

[0046] Devices, apparatuses, and methods embodying the present invention will now be described, by way of non-limiting examples.

[0047] DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0048] The following description is structured as follows. General embodiments and high-level variants are described in section 1. Section 2 addresses particularly preferred embodiments and technical implementation details.

[0049] 1. General embodiments and high-level variants

[0050] A first aspect of the invention, which concerns an optical sensing device 1, la, is now described in detail, in reference to FIGS. 1 - 3 and 5 - 9. Such a device 1 can be used in an apparatus 100 such as shown in FIGS. 4A or 4B, in accordance with another aspect of the invention, which is described later in detail.

[0051] Two variants 1, la of this device are shown in the accompanying drawings (compare FIGS. 1, 2 and 5, 6). In each case, the device 1, la has a layer structure, which notably comprises a substrate 10, a dielectric layer 11, and antenna elements 17, 17a, 17b, 17c. The substrate 10 is structured to laterally delimit a cavity 40. As seen in FIGS. 1, 3 — 5, the cavity 40 is typically formed by a recess defined in the substrate 10, whereby this recess laterally delimits the cavity 40. Initially, the substrate 10 may for example be provided as a double-side polished silicon (Si) wafer, the thickness of which is typically equal to 275, 375, or 525 pm. This wafer then undergoes several processing steps, to define the cavity 40.

[0052] The dielectric layer 11 extends on top of the substrate 10. The layer 11 forms a membrane 11, which spans the cavity 40. The thickness of layer 11 is typically between 10 and 60 nm, preferably between 15 and 35 nm, e.g., between 18 and 22 nm. It is preferably made of Sis Notwithstanding their low thicknesses, the resulting membranes can be surprisingly stable, mechanically and chemically speaking. The device 1, 1a may for example be embodied as a chip, e.g., a sequencing chip. In the accompanying drawings, the depicted devices 1, la include a single membrane, for simplicity. In variants, however, the chip may include several membranes, each spanning a respective cavity, to multiply the number of available nanopores.

[0053] The membrane 11 includes n apertures 30 to the cavity 40. These apertures are defined as through holes through the membrane 11. There is at least one aperture (n > 1). A single aperture already makes it possible to sense molecules, one at a time. In typical applications, however, the device 1, la includes several apertures (n > 2). For applications such as sequencing applications, one will typically seek to fabricate a device with hundreds to thousands to millions of apertures (n > 100). Such apertures are also referred to as nanopores herein, because of their preferred dimensions, as discussed later.

[0054] The apertures can for instance be arranged according to a 2D lattice, e.g., a hexagonal, square or rectangular lattice, as assumed in FIG. 3. The average, in-plane separation distance between the apertures will typically be between 1 and 100 microns, as measured centre-to-centre. In preferred embodiments, the average lattice step is between 1 and 10 microns, preferably between 2 and 7 microns. More preferably, it is between 3 and 6 microns. For example, the average distance between apertures can be of 5 microns. Correspondingly, the areal density of the apertures 30 is between 0.0001 and 1 pm'2, though preferably between 0.01 and 1 pm'2. This means that thousands to millions of apertures can be achieved with a chip of, e.g., 5 x 5 mm2. For instance, a low areal density, single membrane structure may already include at least 400 nanopores. Now, a chip may for example include an array of / n / n membranes (where m > 2, e.g., m = 3), each including 400 nanopores or more. The device 1, 1a further includes n pairs of opposite antenna elements 17, 17a, 17b, 17c (i.e., plasmonic structures), where each pair of antenna elements form a plasmonic antenna. I.e., a plasmonic antenna includes two antenna elements. These antenna elements are patterned on top of the dielectric layer 11. They generally have a bowtie configuration, as shown in the accompanying drawings. The antenna elements 17, 17a, 17b, 17c preferably have a form factor. That is, their largest dimension preferably extends parallel to the plane (x, y). They are ideally made as sharp as possible toward the centre to enhance the electromagnetic hot spot, although they will likely exhibit rounded edges due to the reality of the fabrication steps.

[0055] The antenna elements 17, 17a, 17b, 17c of each pair are on arranged on opposite lateral sides of the respective apertures 30. In each pair, the antenna elements are separated by a respective gap g. There are n gaps g in total. Such gaps extend between opposite antenna elements 17, 17a (or 17b, 17c) of respective ones of the n pairs. The gaps extend along respective directions, which are all parallel to an average plane (x, y) of the substrate 10. The pairs of antenna elements 17, 17a will typically be all aligned along parallel directions, as assumed in FIG. 3, where all pairs extend along an axis parallel to direction x. In general, the relative orientation of the bowties will be consistent with the orientation of the polarization of the incident light. The bowties are typically designed to be excited by light with a linear polarization along the long axis of the bowties. Orienting one or many bowties away from this preferred orientation reduces the field enhancement, and thus the efficiency of the sensing apparatus. When illuminating the pairs of antenna elements with a laser source, maximal efficiency is obtained if all the bowties are aligned in the same direction and parallel to the polarization direction of incident light.

[0056] The n gaps g define, together with the respective apertures, n molecular passages. Each passage extends from the cavity 40, i.e., across the membrane 11, along a direction z that is transverse to the main plane or the average plane (x, y) of the substrate 10. That is, each molecular passage extends through a respective aperture 30 and a respective gap g.

[0057] The average length of the n gaps (as measured along their respective directions) is preferably between 4 nm and 20 nm. More preferably, this dimension is between 4 nm and 15 nm, and even more preferably between 5 nm and 12 nm. The gaps separating each antenna element pair are ideally constant, subject to fabrication tolerances, which can be as low as ± 1 to ± 2 nm, owing to preferred fabrication methods as discussed herein. In fact, both the diameters of the apertures and the gap lengths may essentially be constant, subject to a dispersion of less than 2 nm. The structures and their dimensions were checked using scanning electron microscopy (SEM), as well as transmission electron microscopy (TEM). The high resolution permitted by TEM allows the variability (dispersion) of the dimensions of the nanostructures to be accurately estimated. Note, the variability (or dispersion) is measured as a standard deviation, actually an uncorrected sample standard deviation.

[0058] The n gaps define (i.e., laterally delimit) respective electromagnetic field enhancement regions, also referred to as hot spot regions. That is, electromagnetic radiation can be concentrated in such regions upon irradiating the antenna elements 17, 17a, 17b, 17c, with a view to optically sensing analytes (molecules), in operation. Thus, the antenna elements enable an electromagnetic field enhancement mechanism, which can generate a field-enhanced hot spot, wherein electromagnetic radiation can be concentrated. This, in turn, can be exploited to sense an analyte (e.g., a DNA molecule, by surface-enhanced Raman spectrometry), by irradiating the antenna elements. Owing to the dimensions and arrangement of the opposite antenna elements (e.g., elements 17 and 17a), the resulting electromagnetic field enhancement region is essentially confined to the gap in-between (as illustrated in FIG. 4B), above the apertures 30.

[0059] As noted earlier, the antenna elements generally have a bowtie configuration. They can be regarded as flat prisms, pointing at each other. I.e., seen from above, they essentially have triangular shapes pointing at each other. Now, the symmetry of antenna elements can be altered to further improve the electromagnetic field enhancement. According to the present invention, one antenna element only of the two antenna elements of each plasmonic antenna is pattern so as to have a slit 18 (i.e., a slot or notch, or a strip-like cut-out) extending from one lateral side thereof. The antenna element 17a, 17c with the slit is referred to as a slit antenna element in the following. As a result of the slit 18, the bowtie configuration is asymmetric. That is, the symmetry of the antenna elements is broken by introducing a lateral slit 18 in one of the two antenna elements, which effectively results in an asymmetric bowtie configuration. I.e., compared with the ideal bowtie configuration, the antenna structure proposed herein is no longer symmetrical with respect to the gap or aperture, i.e., with respect to the median plane IE. parallel to the plane (y, z), i.e., the plane extending perpendicular to the average plane of the antenna elements, the average plane of the substrate 10, or the average plane of the dielectric layer 11. This makes it possible to obtain Fano resonances and increase the electric field enhancement.

[0060] An example of the modification of the geometrical layout of the plasmonic bowtie antenna is shown in FIG. 7. In this example, the proposed geometry breaks the symmetry of the bowtie configuration by introducing a strip-like cut-out 18 from the lower antenna element. The dimensions and the position of the cut-out determine the wavelength position and strength of the desired resonance. The maximum field enhancement will normally occur at the position of the resonance.

[0061] Various strategies can be contemplated to optimize the asymmetry of the bowtie structure and, in turn, improve the field enhancement, as discussed in detail in the following.

[0062] For instance, the slit 18 may extends over an entire thickness of the slit antenna element, as assumed in FIGS. 2, 3, 5, 6, and 7. This accentuates the asymmetry of the bowtie configuration and thus the desired effects in terms of Fano resonances and electric field enhancement. In variants, the slit extends only partly, albeit substantially, though the thickness of the antenna element, which already results in an asymmetry. It is worth noting, in that respect, that preferred fabrication methods as discussed herein rely on grayscale lithography, such that the slit does not necessarily need to extend through the entire depth of the slit antenna element 17a, 17c. However, given the aspect ratio typically intended for the antenna elements, the slit 18 preferably extends over the entire thickness of the slit antenna element 17a, 17c.

[0063] Beyond the asymmetry resulting from the slit 18, whereby the two antenna elements 17, 17a (or 17b, 17c) differ from each other, the slit antenna element 17a, 17c may further be asymmetric with respect to the longitudinal transverse plane H._-, parallel to (x, z), as assumed in the accompanying drawings. This plane contains the line segment passing through the apices 19, 19a of the antenna elements and is perpendicular to the average plane of the dielectric layer 11. As noted earlier, a bowtie configuration implies that the apices 19, 19a are located inwardly and point towards each other, subject to slight deformations that may result from fabrication tolerances, typically on the order of the nm. Note, in that respect, that although the antenna elements generally evoke perfect geometrical shapes (e.g., triangles or prisms having apices and edges), one keeps in mind that apices 19, 19a of such antenna elements will actually show a certain radius of curvature (or osculating radius) resulting from actual fabrication steps, as illustrated in FIG. 7. In practice, this radius will preferably be less than 30 nm, preferably less than 20 nm, and more preferably on the order of 5 nm, thanks to fabrication methods as discussed herein. This radius is assumed to be approximately equal to 8 nm in FIG. 7.

[0064] In embodiments, the lateral side SI from which the slit 18 extends is a lateral side that joins the apex 19a of the slit antenna element 17a, 17c. In addition, the slit 18 advantageously extends from this lateral side SI to at least the longitudinal plane H._-. However, it does preferably not extend up to the opposite lateral side S2, i.e., the side opposite to SI with respect to H. as illustrated in FIG. 7. In other words, the slit extends up to the longitudinal plane nx z, see, e.g., FIGS. 9B, 9E, 91, 9J, 9K, 90, or even beyond this plane, i.e., partly into the opposite half of the slit antenna element, as exemplified in FIGS. 7, 9F, and 9L. This again accentuates the asymmetry of the bowtie configuration. Again, any gap g extends between respective antenna elements along a respective direction parallel to the plane (x, y). This direction passes through the apices pointing towards each other. Thus, the slit 18 may extend from one lateral side SI joining the apex 19a of the slit antenna element up to the plane Hx,z, or beyond said plane nv.z, but preferably not up to the opposite side S2. In variants, the slit may extend from the base S3, but then it should typically be sufficiently wide, or preferably bent, to create a substantial asymmetry. Best is for the slit 18 to extend laterally from SI (or equivalently S2).

[0065] In general, the slit 18 should be sufficiently pronounced to enable a sufficient asymmetry and, in turn, a meaningful improvement to the field enhancement. However, the slit 18 cannot be too large either as this would amount to making the slit element 17a, 17c disappear, hence a necessary trade-off. In the present context, the apparent area of the slit 18 can normally be characterized using a suitable characterization method, such as SEM or TEM. The relative importance of the slit can be appreciated thanks to the ratio of the apparent area (call it the slit area) of the slit 18 to the area of the triangle 17a, 17c (call it the reference area). Note, the reference area corresponds to the actual prism (with the slit) and not the ideal prism (without the slit). This ratio should normally be less than 0.5; it is typically between 0.05 and 0.25. In embodiments, however, this ratio is between 0.10 and 0.20, which gives rise to optimal field enhancements according to estimations by the inventors. Each of the slit area and the reference area is measured in a plane parallel to the plane (x, y), or in projection in the plane (x, y), which can for instance be taken as corresponding to the top surface of the dielectric layer 11.

[0066] As further seen in FIGS. 9A, 9C, 91, 9J, 9L, and 9N, the slit can possibly be curved (or bent) along its direction of extension. Moreover, the slit may have a non-constant profile along its direction of extension, parallel to the plane (x, y), see FIGS. 9H, 91, 9 J, 9K, 9L, and 90. The profile of the slit may even be dotted or dashed, see FIGS. 9D or 9M, i.e., formed of several segments. In principle, the slit may even extend (in a curved or bent manner) from one side SI back to the same side SI, see FIG. 9N. However, the slit is preferably “blind,” i.e., it forms a blind slit, similar to a blind hole, to accentuate the asymmetry.

[0067] The slit 18 may possibly be filled with a distinct material. That is, while the antenna elements are normally made of a same material (call it first material), preferably a metal such as gold, the slit 18 may advantageously be filled by a second material, i.e., a solid material (not air) that is distinct from the first material, therefore having a distinct permittivity. The second material is preferably a dielectric material, e.g., with a relatively high permittivity. Note, this second material optionally fills the slit 18 only, i.e., the second material stops at the edge (along SI in FIG. 7). In variants, the second material makes up another layer, on top of the dielectric layer 11, which notably fills the slit 18. This layer may even extend above the antenna layer. As one understands, various configurations can be contemplated, as long as the aperture 30 remains free, to preserve the molecular passages. For example, a solid, albeit transparent, layer (e.g., a thin film) may cover the antennae, provided the aperture 30 remains free.

[0068] As a result of the second material filling the slit 18, the intensity of the electromagnetic field tends to become lower in the slit 18, whereby more energy is available, which results in a larger field enhancement. Any material can be contemplated to fill the slit, as long as this material has a permittivity different from that of the material of the antenna elements. This second material may for instance be chosen so as to tune the resonance.

[0069] As said, the average length of the n gaps, as measured parallel to (x, y), is preferably between 4 nm and 20 nm, so as to define fairly small electromagnetic field enhancement regions, in which electromagnetic radiation can be concentrated upon irradiating the antenna elements 17, 17a. Moreover, the average diameter d of the n apertures 30 is preferably larger than or equal to the average length of the gaps along their respective directions. That is, the average diameter d is larger than or equal to a quantity that is between 4 nm and 20 nm. As a result, the minimal cross-sectional dimension of each of n the passages is limited by a respective one of the n gaps along said respective directions.

[0070] This constraint has important implications, keeping in mind that usual fabrication methods typically require the patterned antenna element to be laterally recessed with respect to the respective aperture edges. On the contrary, in embodiments as described just above, the minimal cross-sectional dimension of each molecular passage is limited by a respective gap (as measured along its respective direction), because the antenna elements can be substantially flush with the aperture edge (see FIGS. 1 and 2) or, even, hang over the aperture (see FIGS. 5 and 6). In other words, the dimensions of the electromagnetic field enhancement region are not primarily limited by the aperture dimensions.

[0071] To summarize, in the above embodiments, the antenna elements are flush with, or hang over the edges of, the aperture underneath, but are not laterally recessed with respect to the edges of this aperture. Doing so allows the electromagnetic field enhancement volume to be minimized, for given cross-sectional dimensions of the aperture. This, in turn, allows the electromagnetic field enhancement to be maximized, all things being otherwise equal. In particular, large SERS enhancement factors can be achieved, e.g., exceeding 1011.

[0072] In the example of FIGS. 1 - 3, the nano-antenna elements 17, 17a are flush with the apertures 30. More precisely, the optical sensing device 1 is designed in such a way that the average diameter d of the n apertures 30 is substantially equal to the average length of the gaps. I.e., inner ends of the antenna elements 17, 17a of each of the n pairs are substantially flush with inner walls of the respective n apertures 30 in the membrane 11. The equality between the average aperture diameter (in-plane) and the average gap is subject to fabrication tolerances, which, in the present context, can typically be of ± 2 nm or less (possibly down to ± 1 nm or less, e.g., ± 0.8 nm), according to images obtained by SEM and TEM. Achieving flush antenna elements reduces the hot-spot volume and thus increases the electromagnetic radiation concentrations in the gaps, with respect to configurations where the antenna elements are recessed with respect to the apertures. The electromagnetic field enhancement can be further improved by having antenna elements 17b, 17c that slightly protrude inwardly, so as to hang over the apertures in a cantilever configuration, as in the device la depicted in FIGS. 5 - 6.

[0073] In both cases, the depicted configurations can be practically obtained, thanks to fabrication methods described below. Such configurations allow the electromagnetic radiation concentrations to be optimized, because they make it possible to bring the inner apices of the antenna elements closer together, notwithstanding the apertures and the cavity underneath.

[0074] As further seen in the accompanying drawings, the present optical sensing devices 1, la typically include two dielectric layers 11, 12. That is, the dielectric layer 11 is a first dielectric layer 11 and the device 1, la further includes a second dielectric layer 12, which is preferably made of the same material as layer 11 and can have the same thickness. As best seen in FIGS. 1 and 5, the substrate 10 extends between the two dielectric layers 11, 12, on top of the second dielectric layer 12 according to the orientation chosen in the drawings, where the z axis points upwards. As further seen in FIGS. 1 and 5, the substrate 10 and the second dielectric layer 12 can be jointly structured to form a recess, which delimits the cavity 40. In embodiments, the substrate 10 is directly coated by each of the two dielectric layers 11, 12. The first dielectric layer 11 extends on a first side (top side) of the substrate 10, while the second dielectric layer 12 extends on a second side of the substrate, opposite to its first side. As said, the substrate 10 typically comprises silicon. In variants, however, other substrate materials can be contemplated, such as quartz or glass. Each of the two dielectric layers 11, 12 preferably comprises SisN4, although other compounds such as SiCL can be contemplated. Preferred embodiments rely on Si substrates 10 and SisN4 dielectric layers 11, 12. I.e., the substrate 10 is essentially made of Si, while the dielectric layers are essentially made of Sis The thickness of each of the first and second dielectric layers 11, 12 is preferably between 10 and 60 nm, and more preferably between 15 and 35 nm.

[0075] The antenna elements are preferably made of gold (Au). Other plasmonic materials (typically metals) can be contemplated. However, Au is preferred as it does not corrode, contrary to, e.g., silver, which may oxidize. Aluminium (Al) may potentially be used to fabricate the plasmonic structures acting at shorter wavelengths (i.e., in the UV region of the electromagnetic spectrum). Overall, various materials can be contemplated for the antenna elements, allowing an amplified field-enhancement over a wide energy range, i.e., from ultraviolet (UV) to near IR and full IR.

[0076] The antennas (e.g., consisting of gold) are preferably patterned on top of a bonding layer (e.g., including Cr or Ti, though Cr is preferred). In addition, a bonding layer may be used on top of the metallic layer 17, 17a, 17b, 17c (e.g., Au), in order to improve the adhesion of the electron beam lithography (EBL) resist. The extent of the adhesion of the EBL resist directly on top of the metallic layer may depend on the metal used.

[0077] Besides, the present layer stacks 10 - 12 may involve additional layers. In particular, the layer structure may include intermediate layers. For example, if a dry etching method is used to open the backside recess to the membrane 11, an intermediate Si O layer can be deposited on top of the Si layer to act as an as an etch stop. As another example, reflector layer structures (dielectric or metallic) may possibly be provided above or below the antennas, and / or laterally around the antenna elements to allow more efficient readouts from the far field or improve coupling of incident, scattered, or emitted light, or increase the sensitivity, through increased photonantenna interactions. Moreover, any of the layers 10, 11, 12 may possibly be deposited as a stack of several, superimposed layers. Preferred, however, is to rely on single layers of silicon and SisN4, where the SisN4 layers 11, 12 may directly coat the substrate 10 (possibly subject to a thin oxide layer surrounding the substrate), for simplicity.

[0078] The optical sensing device 1, 1a may further include one or more pairs of electrodes, wherein electrodes of each pair are arranged (e.g., patterned) on opposite sides of the first dielectric layer 11. The aim is to be able to apply a voltage to a liquid in which the device is immersed, in operation, as discussed later. In variants, such electrodes can be external, i.e., they are not integral with the device 1, lb and can thus be supplied separately.

[0079] Referring to FIGS. 4A and 4B, another aspect of the invention is now described, which concerns an optical sensing apparatus 100. This apparatus includes an optical sensing device 1 la such as described above. In the following, this apparatus is assumed to include a device 1 such as shown in FIGS. 1 - 3 (with flush antenna elements), for the sake of exemplification. However, the apparatus may similarly be based on a device la such as shown in FIGS. 5 and 6 (with inwardly protruding antenna elements).

[0080] The apparatus 100 further includes a distributed electromagnetic source 70. The source 70, preferably comprises a Laser. For example, the apparatus 100 may be equipped with a microlens array, designed to form beamlets from a large area laser beam, in order to illuminate each antenna pair individually. In all cases, the source 70 is configured to irradiate the antenna elements 17, 17a of at least some of the n pairs of opposite antenna elements. That is, the source 70 may be configured to irradiate the antenna elements of each of the n pairs of opposite antenna elements or a subset thereof, e.g., one or more pairs of the n pairs. The aim is to be able to concentrate electromagnetic radiation in the respective electromagnetic field enhancement regions, for optically sensing molecules in the respective gaps g.

[0081] Moreover, the apparatus 100 includes a detector 60, which is configured to optically detect optical signals as modulated and / or generated by the molecules in these regions, in operation. The light source and the detector are typically arranged in reflection geometry. The light source 70 is used to optically excite the pairs of antenna elements, while the detector 60 is used to optically detect signals modulated or generated by analytes in the gaps. This detector 60 may for instance include one or more Raman spectrometers. In preferred embodiments, the detector includes a spatial heterodyne Raman spectrometer (SHRS), an integral field spectrograph (IFS) or another type of snapshot hyperspectral spectrometer. An SHRS, IFS or other snapshot hyperspectral spectrometer allows high resolution, broad spectral range, and high throughput, to be simultaneously achieved. Such spectrometers do not require scanning. However, an SHRS requires special care with data collection and processing as the measurement is an interferogram, which must be converted to a conventional spectrum. Although this document focuses on Raman spectroscopy, especially for applications such as DNA sequencing, it will be apparent to the skilled person that the present devices and apparatuses can also be used with other surface enhanced spectroscopies (e.g., infrared absorption or fluorescence as well as intensity and phase changes based on optical resonance shifts).

[0082] The apparatus 100 may further comprises an electrical circuit 50 such as shown in FIGS. 4A and 4B. This circuit 50 notably includes one or more pairs of electrodes on opposite sides of the first dielectric layer 11. The electrical circuit 50 is configured to apply a voltage bias between electrodes of each of the pairs to urge molecules through the passages defined at the apertures and possibly control the progression of the molecules through the passages, as described later in reference to another aspect of the invention. The circuit 50 should at least allow a constant electric field to be applied, in order to produce a transmembrane bias of a desired voltage difference. The examples of FIGS. 4A and 4B involve very simple electrical circuits, i.e., involving a voltage source, a voltmeter, and an ammeter connected to two electrodes. In practice, however, the circuit may possibly be more sophisticated. For example, the circuit may include multiple electrode pairs, e.g., including one pair for a respective (group of) aperture(s). Such a circuit may notably be used to cause DNA strands to tunnel from the front (top) side of the device (i.e., where the antenna elements are) to the backside (i.e., to the cavity). As noted earlier, the electrodes may possibly be formed integral with the device. In variants, such electrodes are mere end portions of electrical conductors brought in the vicinity of the membrane 11.

[0083] A further aspect of the invention is now described in detail (still referring to FIGS. 4A and 4B), which aspect concerns methods for optically sensing an analyte. The main aspects of such methods have already been described, be it implicitly, in reference to the previous aspects of the invention. Such methods rely on an optical sensing device 1, 1a such as described above. Essentially, they consist in irradiating pairs of antenna elements of this device 1, la (e.g., thanks to an electromagnetic source 70), to concentrate electromagnetic radiation in the electromagnetic field enhancement regions. The aim is to sense molecules in the gaps by optically detecting optical signals that are modulated and / or generated by the molecules in the gaps, thanks to an optical detector 60, e.g., a Raman spectrometer. In embodiments, the sensing methods further comprises applying 50 an electric field across the membrane 11, to urge molecules to the passages and trap the molecules at the respective gaps, by virtue of a combined effect of the electric field applied and the electromagnetic radiation concentrated in the respective electromagnetic field enhancement regions. That is, the electric field applied across the membrane urges a molecule along the z axis, while the electromagnetic radiation concentrated in the field enhancement region causes to trap this molecule in a respective gap (also known as optical trapping). As a result, it is possible to jointly control the irradiation of the antenna elements and the transmembrane electric field to control the progression (i.e., tunnelling) of molecules through the nanopores. It is possible to reverse the tunnelling direction, if necessary, and immobilize a molecule (at the level of a given portion thereof) between opposite antenna element apices, for characterizing specific molecule portions or units, such as specific DNA or RNA units, i.e., nucleotides.

[0084] In other words, the joint action of the hot spots and electric field can be used to trap the molecules in respective gaps and move the molecules in both directions. E.g., if the measurements obtained are not satisfactory, one may possibly pull back a DNA strand for a certain number of bases and measure the sequence once again.

[0085] In general, the device 1, la is meant to be immersed in a liquid. In practice, liquid can be pipetted above the membrane 11, as illustrated in FIG. 4B. E.g., a liquid drop 45 of a KC1 solution containing DNA molecules can be deposited to top of the dielectric surface 11. As a result, one or more top electrodes of the circuit contact the residual liquid above the layer 11. Meanwhile, the cavity 40 can be filled with a KC1 solution, too, so as to allow DNA molecules to reach the cavity via the nanopores. The bottom electrode(s) contact(s) the KC1 liquid in the cavity. A voltage bias (transmembrane bias) is applied through the electrodes, so as to urge DNA molecules towards the passages and trap them at the gaps, thanks to the combined action of the voltage bias applied and the antenna element irradiation. Meanwhile, the field enhancement enabled by the antenna irradiation allows the trapped or translocating molecules to be sensed. In variants to KC1 solutions, other applications may rely on water or oil, for example.

[0086] In detail, the electromagnetic field of light incident on a conductor drives the mobile / free charge carriers of the conductor into coherent oscillations, i.e., surface plasmon polaritons (SPP). These oscillations lead to a strong confinement of electromechanical energy near the surface of the conductor, enabling concentration and guiding of light below the diffraction limit. Coupling of SPPs between multiple structures gives rise to extremely high local field enhancements (with local field strengths exceeding 100 times the incident field) in the small gap in between them. The resulting field enhancements (“hot-spot”) are particularly beneficial for effects whose strength increases non-linearly with the field amplitude, such as surface- enhanced fluorescence, infrared absorption, and Raman scattering. The antenna assists both in coupling light into the sensing volume, as well as in transmitting the signal towards the detector 60.

[0087] That said, preferred applications exploit Raman detection to characterize DNA or RNA molecules. In such case, the device 1, la can be designed as a sequencing chip. The optical signals detected may notably be processed to identify particular nucleic acid sequences, as noted earlier. Note, the present sensing methods can be applied to characterize both single- and double-stranded DNA sections, as well as RNA strands. The same spectroscopic detection methods can further be applied to methylated nucleotides.

[0088] A final aspect of the invention is now described, which concerns methods of fabrication of optical sensing devices 1, la such as described earlier. Such methods are adapted from the methods disclosed in WO2023117078A1, in order to achieve asymmetric antenna configurations as described above, with clean nanostructures, where pairs of antenna elements are arranged on opposite lateral sides of nano-apertures formed through a membrane spanning a cavity, while ensuring deterministic and reproducible positions, shapes, and dimensions of the nanostructures.

[0089] Details and illustrative drawings of such methods are found in WO2023117078A1. Basically, the proposed fabrication revolves around patterning the antenna elements (e.g., using EBL), based on an alignment protocol exploiting previously patterned fiducial marks, prior to etching the apertures. However, the apertures are etched only after having protected inner ends (i.e., apices) of the antenna elements. The recess is then formed on the back side, but only after having coated the upper structures to protect the gaps and apertures. This way, the incompatibility noted earlier can be solved.

[0090] In detail, a substrate 10 is provided and a dielectric layer 11 is deposited on top of the substrate 10. Fiducial marks are then patterned on both the dielectric layer 11 and the substrate 10. Such fiducial marks are preferably patterned using both EBL and a dry etching procedure. The fiducial marks may for instance be patterned as slits extending through the dielectric layer 11 and partly in the substrate 10 (such slits are unrelated to the slit 18). Next, n pairs of laterally opposite antenna elements are patterned on top of the dielectric layer 11, preferably using EBL, to form asymmetric plasmonic antennas 17 - 17c as described earlier. I.e., one antenna element has a slit extending from a lateral side thereof. The processing steps are based on an alignment protocol exploiting the fiducial marks previously patterned. The pairs of opposite antenna elements are preferably patterned by depositing a photoresist on top of the dielectric layer 11, for the photoresist to plug the slits forming the fiducial marks. The aim is to protect the fiducial marks and prevent them from being filled with metal during the subsequent deposition of the metallic layer. Next, the photoresist can be structured by photolithography, for it to form residual plugs at a level of the slits. A metallic layer is then deposited on top of the dielectric layer 11 and the residual plugs. The residual plugs are then removed to define openings at the level of the slits. An electron beam resist can then be deposited on top of the metallic layer. The electron beam resist is then structured by EBL in accordance with the desired shapes for the antenna elements. Finally, the metallic layer is etched through the structured electron beam resist, e.g., using ion beam etching, to obtain the antenna elements 17 - 17c. This makes it possible to define n respective gaps g extending between the pairs of opposite antenna elements.

[0091] Optionally, a protective layer is subsequently deposited, for it to coat inner ends of the opposite antenna elements of each pair. The protective layer may for instance be an alumina layer, which is preferably obtained as follows. First, an electron beam resist is coated. Then, cavities are opened at the level of the gaps. Finally, the protective layer is deposited by atomic layer deposition (ALD). The deposited layer coats the inner ends of the opposite antenna elements.

[0092] Next, the dielectric layer 11 is etched (using a dry etching technique, preferably using reactiveion etching, or RIE), at locations defined according to the fiducial marks. This is done so as to open n apertures 30 through the dielectric layer 11, between opposite antenna elements of respective pairs. As a result, opposite antenna elements are on opposite lateral sides of respective apertures 30.

[0093] The antenna elements and the dielectric layer 11 are then coated with a protective polymer, for it to plug the gaps and the apertures 30. The substrate 10 can subsequently be structured to form a recess 40 extending up to the dielectric layer 11. As a result, the layer 11 extends on top of residual, peripheral portions of the substrate 10. The layer 11 accordingly forms a membrane 11 spanning the cavity delimited by the recess 40. After that, the protective polymer can finally be removed, to free up n molecular passages. Each passage accordingly extends from the cavity 40, through a respective aperture 30 and between a respective gap g. Each passage extends along a direction z transverse to the average plane (x, y) of the substrate 10, see FIG. 1.

[0094] Overall, a fabrication method as defined above makes it possible to obtain an optical sensing device 1, la in which the n gaps define respective electromagnetic field enhancement regions. The control and precision offered by this approach allow clean nanostructures to be obtained, in which the average diameter d of the n apertures 30 may be larger than or equal to the average length of the gaps along their respective directions.

[0095] There is one backside cavity 40 per membrane and all apertures in that membrane lead to that same cavity. However, the device 1, la (e.g., a chip) may also include several membranes extending over respective cavities. That is, every membrane 11 on the chip has its own backside cavity. The backside opening of the recess can notably be achieved via two different processes. A first possibility is to use a KOH etch. This, however, requires that the dimensions of the aperture on the backside are larger than the dimensions of the membrane on the front side, due to anisotropic KOH etching properties. A second possibility is to use deep reactive ion etching (DRIE) through the whole substrate 10 (e.g., a Si wafer). In that case, the dimension of the cavity (recess) on the backside is approximately the same as the membrane. This, in turn, allows the number of membranes on the front side to be increased, since space requirements caused by the KOH etch are reduced significantly. Using DRIE requires an etch stop layer on the front side, between the silicon and the silicon-nitride layer. Additional aspects of such fabrication methods can be found in WO2023117078A1.

[0096] The above embodiments have been succinctly described in reference to the accompanying drawings and may accommodate a number of variants. Several combinations of the above features may be contemplated. Examples are given in the next section.

[0097] 2. Particularly preferred embodiments and technical implementation details

[0098] This section describes preferred embodiments directed to DNA sequencing technology, which aim at improving Raman spectra acquired from single molecules. They rely on surface enhanced Raman spectroscopy (SERS), which requires nanometre sized, plasmonic nanostructures. Specifically, the SERS signal amplitude S scales with the fourth power of the electric field enhancement, i.e., S oc IEI4. Here, \E\ denotes the amplitude of the electric field. For simplicity, it is assumed that the amplitude of the source electric field is equal to 1.0. Thus, \E\ simultaneously denotes the amplitude of the electric field and the enhancement factor of the electric field.

[0099] The following addresses a modified geometrical layout of a bowtie plasmonic nanoantenna that leads to significantly higher electric field enhancement \E\ in the plasmonic hotspot, as discussed in section 1. The geometrical modification results from a cut-out of a metallic strip from one of the two antenna elements composing a plasmonic bowtie antenna, in order to break the symmetry of the usual bowtie layout. This results in Fano resonances, which, in turn, result in higher field enhancements and thus increase the Raman signal S.

[0100] This section further discusses experimental results demonstrating the excellent performance of the modified plasmonic bowtie antennas in SERS, as well as nanofabrication fabrication methods, hence demonstrating the practical usefulness and the real-world applicability of the proposed devices.

[0101] 2.1. Preliminary remarks

[0102] 2.1.1. Rationale

[0103] The following embodiments address improvements to plasmonic nanostructures. The underlying idea is to increase the signal level of the surface enhanced Raman (SERS) signal that originates from nucleotides (adenine, guanine, thymine, cytosine) arranged along the DNA helical structure.

[0104] Raman spectroscopy is widely known spectroscopic technique that allows to identify molecules through a spectrum that is unique for every molecule. Surface enhanced Raman spectroscopy (SERS) is a variant that allows the detection and identification of single molecules in solution. The SERS signal level scales as S oc IE)4, such that even a slight increase of the electric field enhancement significantly improves SERS efficiency.

[0105] As discussed in section 1, it is proposed to modify the geometrical layout of the plasmonic nanostructures to increase the electric field enhancement in the gap between the two prisms and thus increase the SERS signal level. The traditional bowtie layout is modified by laterally removing a strip from one of the two antenna elements, to break the symmetry of the original layout. This leads to a Fano-type resonance and increases the electric field enhancement. Beyond DNA sequencing, the techniques discussed herein can be applied to the sequencing of proteins, again using Raman spectroscopy. 2.1.2. Prior work

[0106] WO2023117078A1 (to LSPR AG) discloses plasmonic nanopore single-molecule DNA sequencing devices and methods of fabrication thereof. The reader is referred to WO2023117078A1 for a discussion of the background art. One practical problem remains when using SERS for identifying DNA nucleotides is to optimize the level of the SERS peaks measured from a few or only one single nucleotide.

[0107] 2.1.3. Present invention

[0108] As mentioned earlier, the electric field enhancement allowed by a plasmonic hotspot is key. The amplification of the SERS signal S scales as

[0109] S o |E|4(1)

[0110] Therefore, increasing the electric field enhancement \E\ in the plasmonic hotspot is key to an efficient implementation of plasmonic nanopore single molecule DNA sequencing. As described in WO2023117078 Al, a practical implementation of plasmonic, solid-state nanopore sequencing requires fabricating the necessary nanostructures in the sequencing chip, arranging the chip in a measurement setup to be able to read nucleotides and DNA sequence information from the chip, and implementing software to extract nucleotide information from measurements. This approach increases the throughput of DNA sequencing to unprecedented levels.

[0111] The present invention makes it possible to improve the signal level measured from SERS signal, something that benefits the data quality, starting with the signal-to-noise ratio and counts in Raman peaks. In particular, it is possible to increase the SERS signal level when measuring Raman spectra from single DNA molecules, i.e., nucleotides along the DNA strand, which eventually improves the performance of the sequencing chip. Such improvements result from the modification of the geometrical layout of the plasmonic bowtie nanoantennas and a specific modification of the nanofabrication process to fabricate the sequencing chip.

[0112] 2.2. Results

[0113] The following addresses theoretical and experimental results that demonstrate the validity, practical feasibility, and usefulness, of embodiments of the invention. 2.2.1. Experiments

[0114] The following outlines fabrication steps of nanoantennas with Fano slits. This requires high- resolution fabrication techniques. EBL is used, with settings optimized to write at the highest possible resolution. The e-beam resist plays a significant role in the fabrication process as it sets a limit for the attainable resolution of the patterned structures. Preferred is to use a hydrogen silsesquioxane (HSQ) resist, a negative tone resist, and the highest resolution e-beam resist available. Coating of resist requires proper adhesion between the resist and the underlying substrate. To enhance the adhesion of the HSQ on gold, a thin layer of chromium is deposited on top of the gold by evaporation prior to the coating. Bowties with a Fano slit 18 patterned using the nanofabrication process described herein are shown in FIG. 10 A. The nanostructures were imaged with a Carl Zeiss Gemini 550 scanning electron microscope.

[0115] Note, while FIG. 10A shows a segmented SEM image of the patterned nano-antenna as written in a negative tone e-beam resist, FIG. 10B is a segmented SEM micrograph of a Fano-resonant plasmonic nanostrucure written in gold. The slit in the lower structure can still be seen. Still, the resolution limit of the SEM is close to being reached in this case, making it difficult to acquire clear and detailed images of the slit shape and topography.

[0116] 2.2.2. Spectrographic results

[0117] FIG. 11 shows a Raman spectrum acquired from a pure thymine single-stranded DNA oligomer, using a Fano-resonant plasmonic bowtie as shown in FIG. 10B. The background signal is subtracted. The spectrum shows high-quality Raman peaks.

[0118] 2.2.3 Results from computational electromagnetics

[0119] The following addresses numerical results from computational electromagnetics that provide the theoretical basis for the practical implementation of plasmonic bowties using Fano resonances. These results were obtained with an advanced, large-scale, computational electrodynamics (Maxwell) solver, namely HADES3D / DGFD (i.e., the so-called “Discontinuous Galerkin Frequency Domain Electromagnetic Solver”), cf. section 2.3. 1.4. Two configurations were analysed, i.e., (1) a first configuration where the plasmonic bowtie antenna has no Fano cut-out and resides in air, and (2) a second configuration where the plasmonic bowtie antenna has a Fano cut-out (again in air). The maximum electric field enhancement value obtained is \E\ = 63 in the former case, against \E\ = 140 in the second case. Note, the geometric model used assumes a radius of curvature rCurv = 20 nm, which is a conservative estimate. Based on the SEM images, the radius of curvature is rather smaller, typically less than 17 nm. Nevertheless, the electric field enhancement is significant.

[0120] 2.3. Methods

[0121] 2.3.1. Theoretical design method

[0122] 2.3.1.1. SERS mechanism

[0123] The following briefly recapitulates the mechanism of SERS detection of single molecules. It focuses on the sole Stokes Raman bands; The anti-Stokes Raman bands are neglected, mainly because they are typically one to two orders of magnitude smaller than the former at room temperature.

[0124] The Raman signal power PRaman obtained from a sample is given by

[0125] P Raman KNkI, (2) where K is the fraction of photons emitted from the molecules, which are both collected and converted into electrons by the detector. The fraction K depends on a number of detector parameters, starting with the quantum efficiency (QE), which is a function of the wavelength. The apparatus may operate in the near-infrared (NIR) region of the electromagnetic spectrum and, thus, QE in the band 800 - 900 nm is most relevant. N is the number of molecule(s) involved and excited in the Raman spectrometry readout. In principle, in the case of plasmonic nanopore single molecule DNA sequencing, N = 1. This, however, is not necessarily the case in practice. Rather, one may typically come to capture at least three nucleotides, nk-i, nk, nk+i, along the DNA strand in a Raman spectrum. Here, nk-i, nk, and nk+i respectively correspond to the nucleotide that was previously, is currently, and will next be in the centre of the hotspot. It may well be needed to consider more nucleotides before and after entering the hotspot, thus contributing to the Raman spectrum measured for nucleotide nk. Such behaviour is expected, considered normal, and a consequence of the realities of experimental life. Deconvolution is the means to extract the ‘true’ Raman signal for nucleotide nk from the weighted superposition of spectra of nucleotides nk-i, nk, and nk+i. Then, <Jk (in cm2 / molecule) is the Raman scattering cross section for mode k, integrated over its relevant bandwidth and over the complete spatial emission angle. I denotes the intensity of laser incident onto the molecule(s).

[0126] Raman scattering is intrinsically a very weak phenomenon. While it is straightforward to measure Raman spectra from powders, acquiring a high-quality Raman spectrum from a few molecules, or even a single molecule, in solution is extremely difficult, hence the advantage of SERS. However, SERS requires the presence of appropriately structured nanoparticles or nanostructures, usually made from noble metals, e.g., gold (Au), silver (Ag), platinum (Pt), or even aluminium (Al), to generate the necessary localized electromagnetic field. In such a situation, the Raman power PSERS measured from a few molecules or a single molecule is amplified through the presence of the nanostructured surface. PSERScanbe written as where GSERSis the total SERS enhancement factor formed from the product of the electromagnetic enhancement factor Gs™RSand the chemical enhancement factor GER™. The chemical enhancement factor G(^”loriginates from a change in the polarizability of the molecule, and thus from a change in its Raman scattering cross section oe through the quantummechanical interactions between the Raman photon emitting molecule and its immediate neighbourhood. This mechanism is short ranged, i.e., at most a few A or even less. The chemical enhancement is a function of the molecule and its neighbourhood. Quantitatively, it is typically between 102and IO4The more important electromagnetic enhancement factor ^SERS originates from the extreme localization of light in a hotspot, which is characterized by the electric field enhancement factor \E\. As noted earlier, \E\ denotes the amplitude of the electric field under the condition that the amplitude of the source electric field is exactly 1.0, such that \E\ denotes both the amplitude of the electric field and the enhancement factor of the electric field.

[0127] In plasmonics, the spatial extent of the electromagnetic hotspot is significantly smaller than the wavelength of the incident light. Typically, the extent of the hotspot depends on the sharpness of corners and edges and is on the order of a few nanometres only. In the present context, the extent of the hotspot is estimated to be about 2 to 4 nanometres. The electromagnetic enhancement is a function of the nanostructured surface and independent on the molecule itself. While it is by far the strongest contribution in the amplification of the Raman signal, its quantitative value is in a range that goes up to IO10. Since electromagnetic enhancement \E\ is a long-range effect, it is effective at distances up to 10 nm away from the surface.

[0128] The electromagnetic enhancement \E\ has two main contributions, namely local (i.e., near-field) and reradiation enhancement. The near-field enhancement of the electric field amplitude \E\ is a consequence of surface plasmons that are excited when a metallic nanostructured surface is illuminated by laser light. The following focuses on metallic surfaces, which are most often used, although non-metallic surfaces may also work. Localized surface plasmons associated with nanometre sized and structured metallic particles create tiny electromagnetic hotspots where the electric field amplitude is significantly higher, compared with situations without the presence of nanoparticles.

[0129] The molecule that emits Raman photon radiates like a Hertzian dipole. The presence of metallic structures in the immediate vicinity of a Hertzian dipole modifies the way in which electromagnetic power is radiated by this Hertzian dipole. Therefore, reradiation enhancement is a consequence of the presence of metallic structures in the immediate vicinity of the Raman photon emitting molecule.

[0130] The electromagnetic SERS enhancement can, in the |E|4approximation, be formulated as: where M0C(mL) and M0C(mR) are the squared ratios of the enhanced electric field ELOC and the non-enhanced electric field E, along the z axis, at the laser and the Raman peak wavelengths, respectively. Under the assumption that the laser wavelength COL and the Raman wavelength COR are not too far from each other, Eq. (4) simplifies as where Eq. (5) is known as the zero Stokes shift limit of the |E|4approximation, which is widely used, although it is, strictly speaking, only valid at Raman shifts that are close to the exciting laser wavelength. Note, the functional behaviour of M0C(mL) in the immediate plasm onic vicinity of the Raman photon emitting molecule may deviate rapidly, particularly if the Raman peak wavenumber C R moves farther and farther away from the exciting laser wavelength COL. Furthermore, the functional behaviour of the zero Stokes shift limit of the |E|4approximation scales as IE]4, such that even a relatively small increment in \E\ will significantly increase the SERS signal. Such increments may well mark the difference between detection of SERS signals and no detection, according to evidence originating from pre- and post-measurement SEM investigations of plasmonic structures. As a consequence, the first and foremost priority when optimizing the SERS signal level is to increase the electric field enhancement in the nanostructured plasmonic hotspot. 2.3.1.2. Fano resonances to increase the electric field enhancement

[0131] Fano type resonances can lead to higher field enhancement in a plasmonic hotspot, which can, in turn, increase the SERS signal significantly. Several methods can be contemplated to excite Fano resonances in plasmonic nanostructures. Interestingly, both symmetric and non- symmetric geometrical antenna arrangements can exhibit Fano resonances, depending on the material, shape, and size, of the antenna elements. The decision as to which type of structure to use, symmetrical or not, may depend on the intended application.

[0132] 2.3.1.3. Breaking the geometrical symmetry to obtain Fano resonances

[0133] In the present context, use is made of an asymmetric plasmonic bowtie layout, because this asymmetric slit design, by its increased number of degrees of freedom and ease of tuning, provides a high control of the strength and frequency of the Fano resonance whereby the bowtie symmetry is broken in order to obtain Fano resonances and increase the electric field enhancement \E\. FIG. 7 illustrates a possible modification of the geometrical layout of the original plasmonic bowtie antenna, which breaks the symmetry of the bowtie antenna by introducing a strip-like cut-out from the metallic layer of one of the two prisms that form the bowtie antenna. The dimensions and the position of the cut-out determine the wavelength position and strength of the desired resonance. Specifically, the maximum field enhancement will occur at the position of the resonance. In this example the cut-out 18 is in the lower prism 17a, 17c. The size of the cut-out 18 is given by its length, its width, and its distance, from the baseline S3 of the (lower) prism 17a, 17c. Obviously, the cut-out may also be obtained in the upper prism 17, 17b as this does not alter the physics. The cut-out may also be curved or bent, thus further modifying the way the electromagnetic modes of the bowtie structure are changed, see FIGS. 8C, 8G, 81, 8J, 8L, 8N, and, correspondingly, FIGS. 9C, 9G, 91, 9J, 9L, 9N. Note, the corners of the bowtie antenna layout have a finite radius of curvature. While it would be desirable to have sharp corners and edges, such ideal geometric shapes cannot be fabricated in reality. Rather, corners and edges always have a finite radius of curvature; it is the objective and task of the process engineer to minimize this radius of curvature. The smaller the radius of curvature, the higher the field enhancement in the gap g of the plasmonic bowtie antenna.

[0134] 2.3.1.4. Computational electrodynamics analysis technique

[0135] In order to study, design, and optimize, the specific geometrical arrangement and associated dimensions of the modified bowtie antenna, use is made of the HADES3D / DGFD solver to solve the electric field vector wave equation on the basis of the interior-penalty discontinuous Galerkin frequency domain method. 2.3.2 Fabrication process

[0136] The nanofabrication process is carried out in a cleanroom. This process description represents one way of preparing the Fano plasm onic nanoantenna. Multiple variants can be contemplated, as exemplified in the following.

[0137] As explained in section 1, a preferred fabrication method revolves around patterning the antenna elements (e.g., using EBL) using an alignment protocol exploiting previously patterned fiducial marks, prior to etching the apertures. More precisely, the apertures are etched only after having patterned the asymmetric antenna element and optionally after having protected the inner ends (i.e., the apices) of the antenna elements thanks to a protective layer. The following focuses especially on process steps to pattern asymmetric antenna elements. The reader is otherwise referred to WO2023117078A1 for more details.

[0138] A substrate is selected, preferably a monocrystalline wafer of silicon, e.g., a 380 pm-thick, double-side-polished (DSP) silicon wafer. Other substrates could be used, which should ideally be flat, chemically, and structurally compatible with the fabrication systems used in the following steps, e.g., not so soft as to bend significantly under its own weight, but thin enough and small enough to fit in the machines.

[0139] The substrate is then cleaned to improve the structural quality of the following metal layers, and preferably coated with a material having different dielectric properties. The plasmon resonances occurring in the metallic plasmonic nanoantenna are influenced by the interface between the metal and the substrate and are especially impacted by the complex index of refraction of the substrate. So, preferred is to clean and coat the wafer, whereby the wafer undergoes a standard set of cleaning steps (“RCA Standard Clean”) before being coated on both sides with two layers. First, with a 400 nm silicon dioxide (SiCL) layer, then, with a 20 nm low-stress silicon nitride (SisN^ layer grown by Low Pressure Chemical Vapor Deposition (LPCVD). In the present context, the metal lies on a silicon nitride layer. The HADES3D / DGFD solver described in 2.3. 1.4 takes into account the refractive index of this substrate during the plasmonic bowtie design and optimization steps.

[0140] The prepared substrate is coated with a metallic layer that will later make up the plasmonic nanostructures. As discussed in section 2.3. 1.1, the choice of metals suitable for this layer includes (but is not limited to) gold, silver, and platinum. Preferred is to use gold as the main medium for the plasmonic nanostructure. In addition to gold, thin layers of chromium are deposited, on the one hand, between the substrate and the gold layer and, on the other hand, on top of the gold layer. The chromium layers serve as adhesion-promoting layers, one for the gold layer to the wafer, the other for future resist layers on top of the metals. The layer stack (e.g., 2 nm of chromium, 34 nm of gold, and 2 nm of chromium) is obtained thanks to an evaporator system. The lower layer of chromium is covered with gold without breaking vacuum and is not exposed to oxygen. The second chromium layer starts oxidizing immediately upon removing the wafer from the vacuum system and likely becomes fully oxidized shortly thereafter.

[0141] A layer of e-beam lithographic resist is deposited on the topmost metal layer. The process spincoats a 40 nm layer of 2% hydrogen silsesquioxane (HSQ) in methyl isobutyl ketone (MIBK) and toluene (CAS# 108-88-3) negative-tone e-beam resist. This specific resist is selected because it exhibits a small resolution, allowing fine structures to be written. Other resists may, however, be contemplated. In particular, positive-tone e-beam resists may be used. In that case, instead of writing the shape of the nanostructure into the resist with the electron beam, a negative pattern is written. The result after e-beam lithography and development must be that the nanostructures of resist remain on top of the metal layer.

[0142] The Fano plasmonic nanostructures are written to the resist- and metal-coated substrate with an e-beam lithography system, e.g., a Raith EBPG5000+ electron beam lithography system. After writing the design into the resist, the nanostructures are revealed by developing the sample. The patterns written in the resist are transferred to the underlying metal layers by ionbeam milling. Argon ions hit the top of the sample and eject tiny amounts of material away, thinning the sample from the top. The resist nanostructures shield the metal below them, thinning instead of the gold. Any uncovered gold is also thinned. To determine when to stop the process, a secondary ion mass spectrometry (SIMS) system is used as an end-point detection system. During this processing step, any remaining polymerized HSQ is mostly or completely etched away. Remaining process steps are described in WO2023117078A1.

[0143] 2.4. Discussion

[0144] The present approach makes it possible to tune the position and amplitude of individual resonances, thus providing a way to place each resonance in the spectral region of highest relevance. The underlying principle is based on Fano resonances, an effect originating from the coupling between a background mode and local excitation mode. Numerical simulations performed by the present Inventors have confirmed the resonant properties of Fano bowties as described in section 1. By optimizing the resonance amplitude, the electric field enhancement observed can be more than twice the enhancement obtained with the corresponding, symmetrical antenna layout. Such tunable resonance properties can be useful for bio-sensing applications.

[0145] While the present invention has been described with reference to a limited number of embodiments, variants, and the accompanying drawings, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted without departing from the scope of the present invention. In particular, a feature (device-like or method-like) recited in a given embodiment, variant or shown in a drawing may be combined with or replace another feature in another embodiment, variants, or drawing, without departing from the scope of the present invention. Various combinations of the features described in respect of any of the above embodiments or variants may accordingly be contemplated, that remain within the scope of the appended claims. In addition, many minor modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention is limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. In addition, many other variants than explicitly touched above can be contemplated. For example, other materials and dimensions than those explicitly indicated may be contemplated. Several variants are evoked in WO2023117078 Al, which may usefully apply in the present context, too.

[0146] REFERENCE LIST

[0147] 1, la Optical Sensing Device

[0148] 10 Structured Substrate (e.g., Si, forming a recess)

[0149] 11 First dielectric layer (membrane, e.g., SisN^

[0150] 12 Second dielectric layer (e.g., SisN^

[0151] 17, 17a, Bowtie Antenna Elements in Flush Configuration

[0152] 17b, 17c Bowtie Antenna Elements in Cantilever Configuration

[0153] 19, 19a Apices of Bowtie Antenna Elements

[0154] 30 Apertures

[0155] 40 Cavity

[0156] 45 Liquid droplet (e.g., KC1 solution containing DNA molecules)

[0157] 50 Electrical Circuit

[0158] 60 Detector (e.g., Raman spectrometer)

[0159] 70 Distributed Electromagnetic Source

[0160] 100 Optical Sensing Apparatus d Aperture diameter g Gaps between opposite antenna elements r Curvature Radius

[0161] SI, S2, S3 Lateral Sides of Cut-out Antenna Element

[0162] IL,z Longitudinal Tranverse Plane

[0163] IL - Median Tranverse Plane

Claims

CLAIMS1. An optical sensing device (1, la) having a layer structure comprising: a substrate (10) structured to laterally delimit a cavity (40); a dielectric layer (11), which extends on top of the substrate (10) and forms a membrane (11) spanning the cavity (40), the membrane (11) including n apertures (30) to the cavity (40), where n > 1, preferably n > 100, and more preferably n > 400; and plasmonic antennas comprising n pairs of opposite antenna elements (17, 17a) patterned on top of the dielectric layer (11) on opposite lateral sides of respective ones of the n apertures (30), so as to define n respective gaps (g) on top of the n apertures, wherein the gaps extend between opposite antenna elements (17, 17a) of the n pairs along respective directions parallel to an average plane (x, ) of the dielectric layer (11) to define n molecular passages, each extending from the cavity (40) through a respective one of the n apertures (30) and a respective one of the n gaps, and the two antenna elements of each of the n pairs have triangular shapes in a bowtie configuration, but one antenna element only of said two antenna elements has a slit extending from one lateral side thereof, such that said bowtie configuration is asymmetric.

2. The optical sensing device (1, la) according to claim 1, wherein said slit extends over an entire thickness of said one antenna element.

3. The optical sensing device (1, la) according to claim 1 or 2, wherein apices of the two antenna elements point towards each other, as a result of said bowtie configuration, and said one antenna element is asymmetric with a respect to a longitudinal plane (x, z) that contains a line segment passing through the apices and is perpendicular to said average plane of the dielectric layer (11).

4. The optical sensing device (1, la) according to claim 3, wherein said one lateral side is a lateral side of said one antenna element that joins one of the two apices.

5. The optical sensing device (1, la) according to claim 4, wherein said slit extends from said one lateral side to at least the longitudinal plane (x, z) but preferably not up to an opposite lateral side that is opposite to said one lateral side with respect to said longitudinal plane.

6. The optical sensing device (1, la) according to any one of claims 1 to 5, wherein a ratio of a first apparent area of the slit to a second apparent area of said one antenna element is between 0.10 and 0.20, each of the first apparent area and the second apparent area measured parallel to the average plane (x, ).

7. The optical sensing device (1, la) according to any one of claims 1 to 6, wherein the slit is curved or bent along its direction of extension.

8. The optical sensing device (1, la) according to any one of claims 1 to 7, wherein the slit has a non-constant profile along its direction of extension.

9. The optical sensing device (1, la) according to any one of claims 1 to 8, wherein the antenna elements are made of a first material, preferably a metal, more preferably gold, and the slit is filled by a second material that is distinct from the first material, preferably a dielectric material, having a distinct permittivity, the second material optionally filling the slit only.

10. The optical sensing device (1) according to any one of claims 1 to 9, wherein an average length of the n gaps along said respective directions is between 4 nm and 20 nm, the average diameter (d) of the n apertures (30) is equal to the average length of the gaps, subject to ± 2 nm, whereby inner ends of the antenna elements (17, 17a) of each of the n pairs are substantially flush with inner walls of the respective n apertures (30) in the membrane (11), preferably, the diameters of the apertures and the lengths of the gaps are essentially constant, subject to a dispersion of less than 2 nm, and, more preferably, an average in-plane separation distance between two closest apertures is between 1 and 10 microns, preferably between 2 and 7 microns.

11. The optical sensing device (1, la) according to any one of claims 1 to 10, wherein said dielectric layer is a first dielectric layer (11), the device (1, la) includes a second dielectric layer (12), the substrate (10) is on top of the second dielectric layer (12), and the substrate (10) and the second dielectric layer (12) are jointly structured to form a recess delimiting said cavity (40), preferably, the substrate (10) comprises silicon, each of the two dielectric layers (11, 12) comprises SislS , and each of the antenna elements (17, 17a) essentially comprises Au, and, more preferably, the optical sensing device (1, la) further comprises one or more pairs of electrodes, wherein the electrodes of each of the pairs are on opposite sides of the first dielectric layer (11).

12. An optical sensing apparatus (100), wherein the apparatus (100) comprises an optical sensing device (1, la) according to any one of claims 1 to 11, a distributed electromagnetic source (70), preferably comprising a Laser, configured to irradiate the antenna elements (17, 17a) of each of the n pairs of opposite antenna elements, so as to concentrate electromagnetic radiation in electromagnetic field enhancement regions delimited by the respective gaps (g) for optically sensing molecules in such gaps (g), and a detector (60) configured to optically detect optical signals as modulated and / or generated by the molecules in said electromagnetic field enhancement regions, in operation, the detector being preferably a Raman spectrometer, more preferably a spatial heterodyne Raman spectrometer or an integral field spectrograph.

13. The optical sensing apparatus (100) according to claim 12, wherein the apparatus (100) further comprises an electrical circuit (50) comprising one or more pairs of electrodes, wherein the electrodes of each of the pairs are on opposite sides of the dielectric layer (11), the electrical circuit configured to apply a voltage bias between electrodes of each of the pairs to urge molecules through the passages.

14. A method of fabrication of an optical sensing device (1) according to any one of claims 1 to 11, wherein the method comprises: providing a substrate (10); depositing a dielectric layer (11) on top of the substrate (10);patterning fiducial marks on both the dielectric layer (11) and the substrate (10); patterning n pairs of opposite antenna elements (17, 17a) on top of the dielectric layer(11) to form plasmonic antennas, preferably using electron beam lithography, based on an alignment protocol exploiting the fiducial marks patterned, wherein the two antenna elements of each of the n pairs have triangular shapes in a bowtie configuration, but wherein one antenna element only of said two antenna elements has a slit extending from one lateral side thereof, whereby said bowtie configuration is asymmetric, and define n respective gaps (g) extending between opposite antenna elements of respective ones of the n pairs along respective directions (x) parallel to an average plane (x, y) of the substrate (10), wherein n > 1, preferably n > 100, and more preferably n > 400, and an average length of the n gaps along said respective directions is preferably between 4 nm and 20 nm; optionally depositing a protective layer, for it to coat inner ends of the opposite antenna elements (17, 17a) of each of the n pairs; dry etching the dielectric layer (11), preferably using reactive-ion etching, at locations defined according to the fiducial marks, to open n apertures (30) through the dielectric layer (11), between opposite antenna elements (17, 17a) of respective ones of the n pairs, for the opposite antenna elements of the n pairs to be on opposite lateral sides of respective ones of the n apertures (30); coating the antenna elements (17, 17a) and the dielectric layer (11) with a protective polymer, for it to plug the gaps and the apertures (30); structuring the substrate (10) to form a recess (40) extending up to the dielectric layer (11), so as for the latter to extend on top of residual, peripheral portions of the substrate (10) and form a membrane (11) spanning a cavity delimited by the recess (40); and removing the protective polymer, to free up n molecular passages, each extending from the cavity (40) through a respective one of the n apertures (30) and a respective one of the n gaps along a direction (y) transverse to the average plane (x, y) of the substrate (10), to obtain an optical sensing device (1), in which an average diameter (d) of the n apertures (30) is preferably larger than or equal to said average length of the gaps along said respective directions, whereby a minimal cross-sectional dimension of each of n the passages is limited by a respective one of the n gaps along said respective directions.

15. A method of optically sensing an analyte, wherein the method comprises: providing an optical sensing device (1, la) according to any one of claims 1 to 11, irradiating (70) the pairs of antenna elements of this device (1, la), to concentrate electromagnetic radiation in electromagnetic field enhancement regions delimited by the n gaps, and sensing (60) molecules in the gaps by optically detecting optical signals that are modulated and / or generated by the molecules in the gaps, thanks to an optical detector, which is preferably a Raman spectrometer, more preferably a spatial heterodyne Raman spectrometer or an integral field spectrograph, wherein, preferably, the method further comprises applying (50) an electric field across the membrane (11) to urge molecules to the passages and trap the molecules at the respective gaps, by virtue of a combined effect of the electric field applied and the electromagnetic radiation concentrated in the respective electromagnetic field enhancement regions, and jointly controlling the applied electric field and an intensity at which the pairs of antenna elements are irradiated (70) to control a progression of the molecules through the n passages, while optically detecting the optical signals, the molecules preferably including DNA or RNA molecules and wherein, more preferably, the optical signals are detected with a Raman spectrometer, according to a surface- enhanced Raman spectroscopy technique, and the molecules comprise DNA or RNA and the optical signals are detected to identify a nucleic acid sequence.

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