Method for forming a nanostructure on a side surface of an optical nanofiber

Abstract

Method for forming a nanostructure on a side surface of an optical nanofiber Method for forming a nanostructure on a side surface of an optical nanofiber (30), the optical nanofiber being formed as a subwavelength waist of a suspended single-mode tapered optical fiber (20), the method comprising: applying an additional elongation to the optical fiber (20) to increase fiber tension, subjecting the optical nanofiber (30) under increased fiber tension to electron beam induced deposition to fabricate at least one nanostructure (NP) on the side surface of the optical nanofiber (30), wherein an electron beam (BB) is deliberately blurred to decrease pressure exerted by the electron beam (BB) on the optical nanofiber (30) during the fabrication, and subjecting the optical nanofiber (30) with the at least one nanostructure (NP) to a plasma oxygen treatment.

Description

[0001] Method for forming a nanostructure on a side surface of an optical nanofiber

[0002] The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 828972 (nanoBRIGHT) and under the Marie Sklodowska-Curie grant agreement No. 101106602 (SPEEDBUMPS).

[0003] The present invention relates to a method for forming a nanostructure on a side surface of an optical nanofiber.

[0004] Optical nanofibers (ONFs) are single-mode optical fibers whose diameter is reduced at subwavelength dimensions in a controlled way to promote strong confinement of the evanescent electromagnetic field onto its surface while maintaining high transmission levels of the electromagnetic field from one end to the other. ONFs have proven to be a promising and widely used platform, in particular in two fields:

[0005] (i) in the field of quantum optics and photonics, where ONFs’ characteristics are leveraged to couple with solid-state single photon emitters (SPE);

[0006] (ii) sensing and biosensing, where the properties of the evanescent field are strongly dependent on the external environment (i.e., refractive index sensing).

[0007] For field (i), quantum optics and photonics, to further increase the spontaneous emission rate of SPE integrated on ONFs and to overcome the severe constraints on the system’s overall efficiency imposed by the isotropic nature of light emitted by SPEs, which limits the efficiency to 10%-20% of the total emitted photons, it has been proposed to couple the SPE to a dielectric, high-quality factor (Q) optical cavity directly fabricated on the ONF. The characteristics of these systems are unsuitable for broadband SPEs at room temperature (RT), thus the integration of SPE with plasmonic resonances in metallic low Q cavities has been proposed, demonstrating a strong enhancement of the radiative rate of the emitter at RT. However, no strategy has been proposed for the deterministic fabrication of plasmonic structures on ONFs. In addition, the integration of plasmonic nanoantennas on ONFs would greatly improve the performance of sensors and biosensors concerning the application in the field (ii), enhancing the interaction between guided modes and the environment surrounding the ONF.

[0008] In these frameworks, precise control of the physical properties of both the guided light and the portion of light affected by the interaction with the optical cavity, in terms of spectral characteristics and polarization, is desirable, yet not demonstrated because of the absence of reliable devices and methods able to perform the aforementioned control.

[0009] M. Sugawara et al, “Plasmon-enhanced single photon source directly coupled to an optical fiber”, Phys. Rev. Res. 4, 043146 (2022), highlights the potential of the enhancement provided by plasmonic resonances on the single photon emission properties from a quantum dot, and their integration of a tapered optical nanofiber. The coupling on the nanotaper is achieved by subsequent drop-casting of a gold nanorod (GNR) able to sustain a Localized Surface Plasmon Resonance (LSPR) and of a Quantum Dot (QD). The theoretical model described in the work focuses on several factors, like the distance between the GNR and the QD or, as highlighted in other works from the same group, the angle between the GNR long axis and the QD, which ultimately influences the effect of the LSPR on the QD emission properties. Although these factors are theoretically analyzed, the utilized drop-casting technique, which represents the only example of achieving LSPR-coupled QD on a nanotaper in the literature, puts several limitations:

[0010] - positioning of a plasmonic particle on the nanotaper. The drop used for the dropcasting could not be smaller than several tens of micrometers, which is various orders of magnitude bigger than the deposited GNR;

[0011] - relative positioning of GNR and QD. As the QD is deposited in the same way, it introduces the same uncertainty in the positioning, which sums to the one introduced in the previous point;

[0012] - orientation of the GNR long axis with respect to the fiber optical axis. This has a strong effect on the possibility of enhancing single photon emission properties through LSPR, but again the used technique makes it impossible to decide the orientation of the GNR long axis;

[0013] - relative positioning of multiple GNRs. Drop-casting makes it impossible to decide how many GNRs are deposited during the casting, not to mention their relative separation or orientation. These latter are crucial factors to achieve collective effects, like redirecting the emission radiative pattern;

[0014] - control of the geometry and composition of the plasmonic structures. As this example is based on the deposition of previously synthesized GNR, the resonator geometry and composition are established during their synthesis, and no active control by the user during their deposition can be carried out. It does not give the possibility to design asymmetric structures.

[0015] An aim of the invention is to provide a method for forming a nanostructure on a side surface of an optical nanofiber, which method is capable of at least partially overcoming the drawbacks of the known method.

[0016] In accordance with this aim, the invention proposes a method for forming a nanostructure on a side surface of an optical nanofiber, said optical nanofiber being formed as a subwavelength waist of a suspended single-mode tapered optical fiber, said method comprising: applying an additional elongation to the optical fiber to increase fiber tension, subjecting the optical nanofiber under increased fiber tension to electron beam induced deposition to fabricate at least one nanostructure on the side surface of the optical nanofiber, wherein an electron beam is deliberately blurred to decrease pressure exerted by the electron beam during the fabrication, and subjecting the optical nanofiber with the at least one nanostructure to a plasma oxygen treatment.

[0017] The proposed method lets the user define the position of the fabricated structures with a resolution of the order of 10 nm, also concerning SPEs that are deposited on the nanofiber before the fabrication. As the presented method is based on Electron Beam Induced Deposition (EBID), it exploits the same technique optimizations that allow for defining, along with several accessory geometrical characteristics, also the orientation of the deposited structures. Furthermore, multiple structures can be deposited at once with the same high resolution of the order of 10 nm. The composition of the structures can be dynamically modified from structure to structure by modifying the beam parameters, and the geometry is easily adaptable by changing the shape parameters. Therefore, the invention offers distinct technical advantages with respect to the state-of-the- art:

[0018] - it offers a reliable solution to precisely positioning the nanostructures along the nanometric section of the fiber with a precision of the order of 10 nm;

[0019] - it allows for the serial fabrication of nanoparticles with different material properties and geometry in a single process;

[0020] - the controlled beam blurring reduces the mechanical solicitations on the nanofiber, hence reducing the chance of breaking the fibers during the process;

[0021] - the process is compatible with the techniques traditionally used to deposit single photon emitters on the tapered nano fibers;

[0022] - the fabricated nanostructures show a broadband spectral resonance that can be tuned by acting on the fabrication parameters and the number of structures, thus representing a promising technique to offer plasmonic enhanced single photon emission from solid state emitters at room temperature integrated on the collection platform represented by the fiber.

[0023] The broadband resonance can be exploited in the biosensing framework, offering hotspots for broadband SERS detection. As the spectral position of the resonance is influenced also by the environment refractive index (RI), by spectrally monitoring the structures’ resonance it is also possible to measure RI variations, opening to other sensing capabilities, for example, gas sensors or liquid phase RI sensors.

[0024] Further characteristics and advantages of the proposed device will be presented in the following detailed description, which refers to the attached drawings, provided purely by way of non-limiting example, in which:

[0025] - Figure 1 shows the scheme of a tapered nanofiber glued to a U-shaped microscope holder;

[0026] - Figure 2 shows the sketch of the application of an additional tension through additional elongation after the fiber waist production process;

[0027] - Figure 3 shows: (a) SEM micrograph of a tapered nanofiber when no additional tension has been added after the fiber waist production process, (b) SEM micrograph of a tapered nanofiber at which an elongation of AE = 100 pm has been applied; - Figure 4 shows an example fabrication of two identical nanopillars with a height of 300 nm and a base diameter of 100 nm. In the first case, the beam is tightly focused onto the nanofiber surface, while in the second case, the beam is blurred with a blurring factor equal to the base diameter. The scale bar is 500 nm;

[0028] - Figure 5 shows a sketch of different blurring methods, top-view. The first case (a) represents the typical EBID configuration without blurring, while the last two represent the blurring methods (b) and (c), respectively;

[0029] - Figure 6 shows: (a) backscattered electron SEM micrographs of a dummy sample fabricated to evaluate the effect of the beam parameters on the deposited material characteristics, (b) EDX measurements of the relative platinum atomic fraction with respect to the sum of platinum plus carbon, carried out on the structures shown in figure 6(a);

[0030] - Figure 7 shows: (a) SEM micrograph of a single nanopillar with a height of h = 500 nm and a width of w = 100 nm, and two identical nanopillars arranged in a nanoantenna configuration, separated by a g = 10 nm nanometric gap. The insets show the top view of the fabricated structures with higher magnification, (b) SEM micrograph of two pillars fabricated tangent to a previously deposited 56 nm Au nanoparticle;

[0031] - Figure 8 shows a sCMOS image of an ONF equipped with a single pillar (a) before and (b) after the plasma oxygen treatment;

[0032] - Figure 9 shows: (a) effect of a plasma oxygen treatment on the base diameter size of a series of nanopillars with different heights, evaluated by SEM inspection, (b) Effect of the plasma oxygen treatment on the height of a series of nanopillars with different heights before the treatment, evaluated by SEM inspection;

[0033] - Figure 10 shows: (a) Experimental broadband spectral response of light scattered by two example structures, a single 500 nm nanopillar (NP) and two nanopillars separated by 10 nm in a nanoantenna configuration (NA), (b) Numerical simulation of the broadband spectral response of the same structures characterized experimentally in figure 10a;

[0034] - Figure 11 shows: (a) Experimental (dots) and numerical (dashed line) scattered light intensity dependence on the linear polarization angle of the beam at the input of the nanofiber, (b) Experimental (dots) and numerical (dashed line) characterization of the scattered light when the input polarization is fixed at the angle that gave the highest scattering intensity in figure I la. A method is discussed for forming a nanostructure on a side surface of an optical nanofiber, said optical nanofiber being formed as a subwavelength waist of a suspended single-mode tapered optical fiber.

[0035] This method allows for example to fabricate a device based on an ONF integrated with an array of plasmonic structures with controlled size and material properties that offer a polarization and / or energy filtering effect on the light propagating inside the fiber.

[0036] According to an embodiment, the method comprises essentially:

[0037] (i) Pre-processing of the nanotaper to add additional tension, thus allowing for Scanning Electron Microscope (SEM) imaging and processing.

[0038] (ii) Blurred Electron Beam Induced Deposition of nanometric structures on the nanotaper and definition of material properties.

[0039] (iii) Post-processing Plasma Oxygen treatment to clean the fiber and increase the metal concentration in the nanostructures.

[0040] The result of the fabrication procedure is a device exhibiting plasmonic polarization filtering and energy filtering on the input light beam, whose parameters can be tailored on the base of the grown structure(s). This is shown in the following by experimental data and numerical simulations.

[0041] (i) Pre-processing to add additional tension

[0042] A nanofiber 30 (i.e. subwavelength waist of a single-mode tapered optical fiber) is fabricated using the oxyhydrogen flame-brush heat-and-pull method and glued to a U-shaped microscope holder 10 (Figure 1). In this configuration, a fiber 20 is suspended between two fixed points Pl, P2 and acts as a string. Exposure to the SEM beam exerts pressure pulses, which puts the fiber 20 in oscillation, thus making impractical any imaging and, a fortiori, any fabrication on the substrate. To remove the oscillations and reach an ideal processing condition, after the pulling process the oxyhydrogen flame is turned off, the fiber is cooled down to room temperature by continuously dry blowing of filtered forced air, and two piezoelectric stages pull the cooled down fiber in opposite directions for a total additional elongation of AL, where AL = 100 gm for example (as sketched in Figure 2). After this procedure, the fiber 20 is attached to the U-shaped holder 10 using a UV-curable resin. The beneficial effects of improving SEM imaging through the reduction of the oscillations are visible in the comparison between the SEM micrograph in Figure 3(a) and the one in Figure 3(b) in which two nanofibers are imaged, respectively, without the additional tension after the heat-and-pull procedure and with an additional tension of AL = 100 gm.

[0043] In view of a complete device, which must offer a long lifetime and good resilience, the additional tension must lie in a range that can be empirically defined (the inventors estimated it in an elongation range between 75 pm and 150 gm). Exceeding a certain threshold over an elongation of 150 gm will make the nanofiber too fragile, while if the additional tension is too low (elongation from 0 to 50 gm), it will not prevent the fiber from vibrating upon electron beam exposure.

[0044] (ii) Blurred Electron Beam Induced Deposition

[0045] Even considering the additional elongation, during fabrication through Electron Beam Induced Deposition (EBID) the electron beam is tightly focused on the nanofiber’s surface, producing pressure pulses that exceed the ones that the fiber must sustain during the imaging phase. The effect of this is that during the mechanism of EBID, which depends strongly on the interaction between the substrate and the beam, the nanofiber is locally displaced and the resulting deposited structure heavily differs from the desired result. As an example, in Figure 4 the EBID fabrication of a nominally 300 nm height and 100 nm base diameter pillar is shown. To decrease the pressure exerted by the beam during the fabrication, deliberate blurring of the beam is employed (the pillar in the box of Figure 4 is fabricated with a focused electron beam, while the pillar in the lower side of Figure 4 is fabricated with a blurred electron beam). Blurring of the beam has a marginal effect on the composition of the deposited structure, but it is an effective way to allow fabrication on the ONF. Due to the employment of the blurring, this method will be hereinafter designated as Blurred EBID (BEBID). Although the blurring factor b can be freely defined, two techniques are proposed, whose sketch is reported in Figure 5, that have empirically proven to give the best results in terms of fidelity between the desired geometry and the obtained one: method (b) is a “static” blurring, in which the blurring factor is set at the same size of the structure to be fabricated (i.e., a cross-section of the electron beam impinging on the side surface of the optical nanofiber has a diameter substantially equal to a cross-sectional dimension of the nanostructure to be fabricated), and method (c) in which the blurring factor is set at half of the size of the structure to be fabricated (more in general, the cross-section of the electron beam impinging on the side surface of the optical nanofiber should have a diameter smaller than, but of the same order of magnitude as a cross-sectional dimension of the nanostructure to be fabricated). In Figure 5, the area DS enclosed by a dashed line represents the desired shape to be fabricated, FB designates a tightly focused beam, BB designates a blurred beam, and the dashed line BT represents the beam trajectory. Method (b) is the configuration that minimizes the pressure exerted by the beam on the nanofiber, while method (c) corresponds to a 2x2 “binning” of the projection of the structure on the nanotaper surface. Once introduced the concept of BEBID, one can demonstrate that by finely tuning the deposition parameters, in particular the electron beam current I, and the electron beam accelerating voltage V, the composition of the structures can be tuned to show a metallic or a dielectric behavior. This can be seen in a dummy nanostructures sample fabricated using CgHiePt Platinum organometallic precursor reported in Figure 6(a), where a matrix of different nanostructures is fabricated for different combinations of I and V. Their relative Pt atomic concentration with respect to Pt+C (carbon is a subproduct of the precursor molecules dissociation) is evaluated by Energy Dispersive X-Ray Spectroscopy (EDX) and the results are reported in Figure 6(b). Deposition parameters that combine high I and low V showed a Pt concentration that reaches 50%, thus showing metallic behavior, while for low I and high V, Pt concentration stays below the 5%-10%, showing a dielectric behavior.

[0046] Combining the BEBID method with the findings obtained from the material characterization, nanopillars with custom height and width are fabricated directly on the nanotaper. As an example, two different structures geometries are here reported: a single nanopillar with a height of h = 500 nm and a width of w = 100 nm, and two identical nanopillars NP arranged in a nanoantenna configuration, separated by a g = 10 nm nanometric gap. The two examples of fabrication are reported in Figure 7(a). One of the most innovative aspects of the proposed method relies upon the fact that, unlike the other methods suggested in the literature, it allows for a fully deterministic positioning of the structures, which is crucial to achieving closely packed structures with precisely defined nanogaps. This can in turn be exploited to deterministically position the structures at precise separation distances from a previously deposited nanoparticle or SPE. As an example, in Figure 7(b) two pillars are fabricated tangent to a previously deposited ~50 nm gold nanoparticle GNP. In this framework, the nanoparticle is identified through SEM imaging and then the fabrication follows the method described above. This fabrication protocol can be in principle applied to any SPE previously deposited on the fiber, with the only factor that drives the applicability of the technique being the possible effect of the electron beam exposure on the SPE emission properties, which must be carefully evaluated.

[0047] (iii) Post-processing Plasma Oxygen treatment

[0048] After the fabrication of the structures, the obtained devices, formed by the integration of nanostructures on ONFs, are subjected to a post-processing Plasma Oxygen treatment (POT) for two main reasons:

[0049] (a) During the processing, dust and other contaminants can accumulate on the ONF surface, mainly trapped through electrostatic interactions, thus hindering the correct evanescent field propagation along the nanotaper, hence this treatment strongly helps in removing those contaminants, restoring the evanescent field that interacts only with the structures (an example of the same ONF equipped with a nanopillar, before and after the Plasma Oxygen is reported in Figure 8(a)-(b)).

[0050] (b) As reported in the literature, Plasma Oxygen can remove part of the carbon from the structures fabricated using CgHiePt, hence increasing the Pt concentration.

[0051] Although the effect described in (b) is obtained only for some organometallic precursor, of which the precursor in use is an example, (a) remains valid independently from the material deposited, hence the described procedure must be run in any case.

[0052] With regards to (b), removing carbon has a quantifiable effect on the structures’ size, which is characterized by SEM inspection in Figure 9(a) and 9(b). The height is reduced by ~25 nm and the diameter by ~15 nm after a 30-minute, 30 W treatment. This reduction must be considered when fabricating the starting structures following the procedure described in (ii). The described procedure represents a non-invasive cleaning procedure on the nanotaper that can be performed before successive encapsulation strategies to prevent further contamination. Moreover, to avoid any effect on the composition of the structures as well as the material properties, using a lower power or shorter processing times can achieve the cleaning effect without affecting the material properties.

[0053] Optical characterization of the device

[0054] Numerical and experimental data

[0055] The presented method can be used to precisely define the geometry of the structures, their separation (in the case of multiple structures), and their azimuthal position on the transverse section of the nanotaper / nanofiber. For a fixed material composition, the first two points have a strong influence on the resonant response upon interaction with a broadband electromagnetic field, making the structures act as field enhancement hotspots for a selected broadband range. The separation between the structures regulates the intensity of the field enhancement factor and the spectral position and width of the plasmonic resonance. Additionally, the azimuthal position determines the dependence of the response of the structures upon different polarizations.

[0056] As an example, the inventors evaluated experimentally and numerically the broadband response of a single pillar with a height of 500 nm and a base diameter of 100 nm and two pillars identical to the single one separated by a g = 10 nm gap, in a nanoantenna configuration. The experimental data (Figure 10(a)) and the numerical simulations (Figure 10(b)) are in good agreement, showing that the spectral position of the resonance can be designed a priori. It is highlighted how adding a second identical element separated by a nanometric gap translates in a 50 nm red-shift of the nanoantenna resonance concerning the nanopillar one, again emphasizing the energy filtering capabilities of the device under consideration.

[0057] The polarization selection effect of the device can be observed in Figure 11(a), where both the nanoantenna and the nanopillar strongly scatter light linearly polarized at a specific angle while allowing for high transmission levels of the orthogonal linear polarization angle. In the same way, in Figure 11(b) it can be observed that the nanostructures scatter light mostly in a specific polarization angle, showing a stronger effect in the case of the nanoantenna.

Claims

CLAIMS1. Method for forming a nanostructure on a side surface of an optical nanofiber (30), said optical nanofiber being formed as a subwavelength waist of a suspended single-mode tapered optical fiber (20), said method comprising: applying an additional elongation to the optical fiber (20) to increase fiber tension, subjecting the optical nanofiber (30) under increased fiber tension to electron beam induced deposition to fabricate at least one nanostructure (NP) on the side surface of the optical nanofiber (30), wherein an electron beam is deliberately blurred to decrease pressure exerted by the electron beam on the optical nanofiber (30) during the fabrication, and subjecting the optical nanofiber (30) with the at least one nanostructure (NP) to a plasma oxygen treatment.

2. Method according to claim 1, wherein the optical nanofiber (30) is formed through a heat-and-pull process, and wherein the additional elongation is applied after cooling down the optical fiber (20) after the heat-and-pull process.

3. Method according to claim 2, wherein the additional elongation AL is such that 75 pm < AL < 150 pm.

4. Method according to claim 1 to 3, wherein a cross-section of the electron beam impinging on the side surface of the optical nanofiber has a diameter substantially equal to a cross-sectional dimension of the at least one nanostructure (NP) to be fabricated.

5. Method according to claim 1 to 3, wherein a cross-section of the electron beam impinging on the side surface of the optical nanofiber has a diameter smaller than, but of the same order of magnitude as a cross-sectional dimension of the at least one nanostructure (NP) to be fabricated.

6. Method according to any of the preceding claims, wherein subjecting the optical nanofiber (30) to electron beam induced deposition comprises adjusting at least one deposition parameter to tune composition of the at least onestructure (NP) to selectively show a metallic or a dielectric behavior.

7. Method according to claim 6, wherein said at least one deposition parameter comprises at least one of electron beam current and electron beam accelerating voltage.

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