Rotationally symmetric dielectric structure for optical beam shaping, method for its manufacture, single-mode optical waveguide with such a structure and beam shaping system

Abstract

Rotationally symmetric dielectric structure (2) for optical beam shaping and for capturing and manipulating individual particles and living biological cells in an aqueous medium, which is mounted concentrically on the facet of a single-mode optical fiber (1), wherein the structure (2) comprises at least three total reflection surfaces (S1, S1', S2) configured to divide a light field emerging from the single-mode optical fiber (1) into at least two separate light paths, and wherein the at least three total reflection surfaces (S1, S1', S2) are further configured to combine the separate light paths as a ring beam at a common focal point.

Description

[0001] The present disclosure relates to a rotationally symmetric dielectric structure, in particular for optical beam shaping and for capturing and manipulating individual particles and living biological cells in an aqueous medium. Furthermore, the disclosure includes optical tweezers based on total internal reflection of micro-optics and mounted at the tip of a single-mode optical fiber.

[0002] Optical trapping is one of the most important functions in the study of single cells in biophysics, biochemistry, and health sciences. However, in its conventional form (high numerical aperture (NA) microscope objectives), it has not yet been fully utilized for in vivo applications. This is due to the following reasons: light field distortions are caused by optical distortions that occur in highly turbid media such as thick biological tissue. In addition, access to confined spaces is limited by the bulky optical setup, and the working distance is restricted due to the short focal point of the trap lenses, which limits the possibility of combining optical trapping with other experiments. In the search for an alternative, optical fibers have attracted much attention due to their ability to guide light to the desired point while protecting it from aberrations and distortions.However, for a highly focused beam generated by a single-mode optical fiber, additional optical elements must be attached to the tip of the fiber. Given the difference between the refractive index of the optics and the immersion medium, generating such a highly focused beam for in vivo applications requires highly curved surfaces, for which the tolerances of current manufacturing techniques may not be sufficient.

[0003] At the same time, the working distances of highly focused beams are very small, which limits the freedom to integrate optical trapping with other functions.

[0004] Creating optical traps with a standard single-mode fiber is not easy. There are reports of microstructured optical fibers with lens-shaped tips [1-3], sharp tips [4], specific structures for generating structured light [5], diffractive object elements [6], and even silicon-based metal lenses [7] capable of trapping particles, but none of them can create a 3D optical trap. In all cases, the axial force is too high (or even too low), so only a 2D trap in the transverse plane is possible. Axial trapping is achieved either by electrostatic forces between the particle and the tip of the optical element or the fiber, or by sticking the particle to the glass slide or the chamber wall.

[0005] Liberale et al. [8] demonstrated a 3D optical trap using a device on the scale of optical fibers to create a counter-propagating trap with tilted beams. The result is a focal spot that efficiently captures a wide range of particle sizes and types at a working distance of 46 µm. Another approach used a hollow-core photonic crystal fiber with a polished tip to generate a focused ring beam. This fiber successfully demonstrated the capture of oil droplets at a long working distance of about 30 µm from the lens tip [9] and was later used for droplet lasing

[10] and particle shooting

[11] . In another work, the Liberale group designed a hybrid photonic structure that can be used for optical trapping of single particles.However, it remains to be demonstrated that the above-mentioned structure works at the tip of an optical fiber

[12] .

[0006] AU 2020 101 129 A4 describes a self-assembled laser system with microspheres based on a coaxial three-waveguide optical fiber. AU 2020 101 132 A4 discloses a cellular multicore laser system with stretching function. CN 110 927 879 A describes an optical nanometer beam scanning probe based on optical fiber tweezers.

[0007] None of the current solutions are single-mode fibers capable of creating full 3D optical traps, and the disadvantages of the current solutions are summarized below: - Only 2D trapping (trapping in the transverse plane). - In the case of 3D trapping, current solutions require either at least two single-mode fibers for trapping. Or they require a photonic crystal fiber with a toroidal core, and additionally, the fiber tip must be modified, which is a less flexible and perhaps even costly approach.

[0008] The present disclosure utilizes beam shaping as a solution to overcome the need for highly focused Gaussian beams. By designing a micro-optical component at the tip of a single-mode optical fiber, a focused annular beam is created that distributes the optical forces and creates an optical trap for a variety of particles, both in size and type, at a large distance from the tip of the fiber (long working distance).

[0009] The current disclosure enables the miniaturization of optical tweezers to the size of a single optical fiber, which was previously impossible. This represents a major advance for the field of optical trapping and manipulation in health sciences, biophysics, and chemistry, opening up new applications for in vivo trapping, manipulation, and investigation.

[0010] The disclosure is defined in the independent claims. The dependent claims describe preferred embodiments.

[0011] The present disclosure relates to a rotationally symmetric dielectric structure for optical beam shaping and for trapping and manipulating individual particles and living biological cells in an aqueous medium, which structure is mounted concentrically on the facet of a single-mode optical fiber, wherein the structure comprises at least three total reflection surfaces configured to split a light field emerging from the single-mode optical fiber into at least two separate light paths, and wherein the at least three total reflection surfaces are further configured to combine the separate light paths as a ring beam at a common focal point.

[0012] Various embodiments may preferably have the following features.

[0013] Preferably, the structure and the optical waveguide share a common axis of symmetry. Preferably, a first total reflection surface is configured such that a first light path of the emerging light field, whose radial distance from the axis of symmetry is less than a threshold value, is reflected away from the axis of symmetry toward a second total reflection surface, and the second total reflection surface is configured such that it directs the light path to the focal point.

[0014] Preferably, a third total reflection surface is designed such that a second light path of the emerging light field, whose radial distance from the axis of symmetry is greater than the threshold value, is directed towards the focal point.

[0015] Preferably, the structure is designed such that the second and third light paths lie within a numerical aperture of the optical fiber.

[0016] Preferably, the total reflection surfaces are designed such that the difference in the optical path length of the facet to the focal point between the first light path and the second light path is an integer multiple of the light wavelength in an ambient medium.

[0017] Preferably, the total reflection surfaces are designed to achieve total internal reflection for an ambient refractive index between 1 and 1.4.

[0018] Preferably, a surface at which the light field exits the structure is concentric to the focal point.

[0019] Preferably, the structure is configured such that a ratio between a length of a propagation path of an expansion of the beam and a remaining length of the structure is in a range of 0.5 to 5.

[0020] Preferably, the numerical aperture for focusing the light in an ambient medium is between 0.3 and 1.5.

[0021] Preferably, the structure is designed to capture particles and direct the light scattered by the captured particles into the optical fiber.

[0022] Preferably, the focal point of the structure is configured to act as optical tweezers for capturing particles and / or cells.

[0023] The present disclosure also includes a single-mode optical fiber having a structure as described above.

[0024] Furthermore, the present disclosure includes a beamforming system comprising a single-mode optical fiber as described above having a structure as described above, wherein the optical fiber is configured to be movable in an axial and / or transverse direction with respect to a common axis of symmetry, and wherein the structure is configured to maintain the position of a trapped particle.

[0025] The present disclosure also relates to a method for fabricating a rotationally symmetric dielectric structure for beam shaping as described above, wherein the structure is fabricated directly on a fiber tip by 3D printing or fabricated on a substrate and then attached to the fiber.

[0026] The disclosure is further described with reference to the accompanying figures, which show Fig. 1a) to c) are ray diagrams of the structure according to an exemplary embodiment of the disclosure and Fig. 2 a three-dimensional representation of the front part of the structure according to Fig. 1 with cut-open area.

[0027] In the present disclosure, a focused annular beam is used, but generated from a standard single-mode optical fiber. The present disclosure shows a standard single-mode optical fiber with a 3D-printed microstructured probe at the tip, which generates a focused annular beam with a numerical aperture NA ≈ 1 at the focal point 50 µm from the fiber (see FIG. Fig. 1 and Fig. 2).

[0028] The probe's optical design was conceived with a number of specific requirements in mind: The probe should enable focusing with a high numerical aperture while ensuring annular illumination to avoid axial optical forces. Furthermore, as much of the light exiting the single-mode fiber (SMF) as possible should be directed to the focal point to avoid wasting optical power for capture. Finally, the device should operate in water without requiring any post-processing or additional steps.

[0029] According to the present disclosure, a purely reflective structure or probe based on total internal reflection (TIR) is provided with a focusing NA of approximately or equal to 1.0 and a back focal length of 50 µm. To obtain the TIR on all surfaces, all reflection angles should be kept above the critical angle. Considering the small difference between the refractive index of water (as the capture medium) and the probe (build) material (Nanoscribe IP-DIP photopolymer in the present example), the critical angle at the design wavelength of 808 nm is 59.6°. As a result, the Gaussian beam emerging from the facet or ground surface of the SMF is expanded over a length of 500 µm with an NA of 0.13 (characteristic of 780HP fibers in air) and then split into two different paths, as shown in the Fig. 1 a) to c). The structure and the optical fiber preferably have a common axis of symmetry. The first path is shown hatched here and has a waveform and includes light with a radial distance smaller than a first threshold, while the second path is shown as a dotted area and thus includes light with a radial distance greater than the first threshold and smaller than a second threshold. The end surface at which the light leaves the probe is curved and corresponds to the shape of the wavefront to avoid any refraction. This also means that the design works regardless of the wavelength and the immersion medium, as long as the refractive index of the environment is that of water or lower.

[0030] With reference to the Fig. 1 and Fig. 2 describes a rotationally symmetric dielectric structure for optical beam shaping and for capturing and manipulating individual particles and living biological cells in an aqueous medium, which structure is mounted concentrically on the facet of a single-mode optical fiber. The structure has at least three total reflection surfaces S1, S1', S2 configured to split a light field emerging from the single-mode optical fiber into at least two separate light paths. The at least three total reflection surfaces S1, S1', S2 are further configured to combine the separate light paths into a ring beam at a common focal point.

[0031] As mentioned above, the structure and the optical fiber share a common axis of symmetry. A first total reflection surface S1 is designed such that a first light path of the emerging light field, whose radial distance from the axis of symmetry is less than a threshold value, is reflected away from the axis of symmetry to a second total reflection surface S1'. The threshold value is equal to the maximum distance of the total reflection surface S1 from the axis of symmetry. The second total reflection surface S1' is designed to direct the light path to the focal point. This configuration is shown in Fig. 1a). The area marked with wavy hatching indicates the first light path. Reference numeral 1 denotes the optical (single-mode) glass fiber, and reference numeral 2 denotes an expansion cylinder of the rotationally symmetric dielectric beam-forming structure.

[0032] A third total reflection surface S2 is designed such that a second light path of the emerging light field, whose radial distance from the symmetry axis is greater than the threshold value, is directed toward the focal point. The second light path thus includes light whose radial distance from the common symmetry axis is greater than the maximum distance of the total reflection surface S1 from the symmetry axis and less than the minimum distance of the total reflection surface S1' from the symmetry axis. This is Fig. 1b), where the dashed area indicates the second light path.

[0033] Fig. 1c) shows a superposition of the first and second light paths according to Fig. 1a) and Fig. 1b).

[0034] Fig. Figure 2 shows a three-dimensional representation of the front part of the structure according to Fig. 1 with cut-open area.

[0035] The second and third light paths are located within a numerical aperture of the optical fiber.

[0036] The total internal reflection surfaces S1, S1', S2 are designed such that the difference in the optical path length of the facet to the focal point between the first light path and the second light path is an integer multiple of the light wavelength in an ambient medium. Furthermore, total internal reflection is preferably achieved at an ambient refractive index between 1 and 1.4.

[0037] A surface where the light field leaves the structure is concentric to the focal point.

[0038] The ratio between the length of the propagation path for expanding the beam and the remaining length of the structure is preferably in the range of 0.5 to 5.

[0039] A numerical aperture for focusing light in an ambient medium is preferably between 0.3 and 1.5.

[0040] The structure is configured to capture particles and direct the light scattered by the captured particles into the optical fiber. The focal point of the structure is configured to act as optical tweezers for capturing particles and / or cells.

[0041] The disclosure further includes a single-mode optical fiber having a structure as described above.

[0042] Furthermore, the disclosure includes a beamforming system comprising a single-mode optical fiber having a structure as described above, wherein the optical fiber is configured to be movable in an axial and / or transverse direction with respect to a common axis of symmetry, and wherein the structure is configured to maintain the position of a trapped particle.

[0043] Furthermore, a method for fabricating a rotationally symmetric dielectric structure for beam shaping as described above is disclosed, wherein the structure is fabricated directly on a fiber optic tip using 3D printing or fabricated on a substrate and then attached to the fiber.

[0044] Because the probe is a rotationally symmetric design, the final probe is constructed iteratively in 2D (along the beam axis) by manually tracing rays from the fiber core to the focus and numerically solving sets of equations to satisfy all external boundary conditions while observing the law of reflection (as well as total internal reflection) to ensure the correct reflection angles are achieved. The resulting shapes are then fitted with piecewise polynomials (splines), which are then exported and converted into a 3D model, for example, in the CAD software SolidWorks.

[0045] Microstructured optics, which utilize total internal reflection to shape or modify a Gaussian beam (eigenmode of the single-mode fiber), can produce a highly focused ring beam without losing the high-intensity part of the beam.

[0046] The large distance between the trapped particle and the fiber tip (long working distance) is an advantage that provides freedom for further manipulation of the trapped particle. At the same time, the current design also collects the backscattered signal from the trapped particle or cell. This advantage can be used to capture Raman or fluorescence signals from particles and cells for further studies of interest.

[0047] Although the disclosure is illustrated and described in detail with reference to the figures and the accompanying description, this illustration and detailed description are intended to be illustrative and exemplary, and not limiting of the disclosure. It is understood that those skilled in the art may make modifications and variations without departing from the scope of the following claims. In particular, the disclosure also encompasses embodiments having any combination of features mentioned or shown above with respect to various aspects and / or embodiments.

[0048] The disclosure also includes individual features shown in the figures, even if they are shown there in connection with other features and / or are not mentioned above.

[0049] Furthermore, the term "including" and its derivatives do not exclude other elements or steps. Likewise, the indefinite article "a" or "an" and its derivatives do not exclude a plurality. The functions of several features listed in the claims may be performed by a single unit. The terms "substantially," "about," "approximately," and the like, when used in conjunction with a feature or value, specifically define the feature or value. Any reference signs in the claims are not to be construed as limiting the scope of the claims.

[0050] The revelation is further described by the following aspects. 1. Rotationally symmetric dielectric structure for optical beam shaping and for capturing and manipulating single particles and living biological cells in an aqueous medium, which structure is mounted concentrically on the facet of a single-mode optical fiber, wherein the emerging light field is first expanded by free propagation and then split into at least two separate light paths by means of at least three total reflection surfaces, which then emerge from the structure combined to form a ring beam and converge at a common focal point having the necessary properties for capturing particles. 2. Rotationally symmetric dielectric structure for beam shaping according to aspect 1, wherein light components propagating near the common axis of symmetry of glass fiber and structure (inner light components) are reflected away from the axis of symmetry by a first total reflection surface (S1) and directed into the focal point by a second total reflection surface (S1') (light path L1). 3. Rotationally symmetric dielectric structure for beam shaping according to one or more of the preceding aspects, wherein light components that do not propagate near the common axis of symmetry of the optical fiber and the structure (external light components) are guided into the focal point (light path L2) by a single total reflection surface (S2). 4. Rotationally symmetric dielectric structure for beam shaping according to one or more of the preceding aspects, wherein all light components within the numerical aperture of the fiber do not take any light paths other than those described in aspects 2 and 3. 5. Rotationally symmetric dielectric structure for beam shaping according to one or more of the preceding aspects, wherein the difference in the optical path length from the fiber facet to the focal point between the two light paths of aspects 2 and 3 is an integer multiple of the wavelength of the light in the surrounding medium. 6. Rotationally symmetric dielectric structure for beam shaping according to one or more of the preceding aspects, wherein the condition for total internal reflection at the total reflection surfaces is met over a range of the ambient refractive index of 1 - 1.4. 7. Rotationally symmetric dielectric beam shaping structure according to one or more of the preceding aspects, wherein the surface at which the light exits the structure is concentric with the focal point so that no refraction occurs and the focusing is independent of the wavelength of the light (achromatic). 8. Rotationally symmetric dielectric beam-forming structure according to one or more of the preceding aspects, wherein the ratio of the length of the propagation path for expanding the beam to the remaining length of the structure is in the range of 0.5 to 5. 9. Rotationally symmetric dielectric structure for beam shaping according to one or more of the preceding aspects, wherein the numerical aperture at which the light is focused in the ambient medium is in a range between 0.3 and 1.5. 10. Rotationally symmetric dielectric structure for beam shaping according to one or more of the preceding aspects, wherein the structure is produced by 3D printing directly on a fiber optic tip or, as a second variant, is first produced on another substrate and then attached to the fiber. 11. A rotationally symmetric dielectric beam-shaping structure according to one or more of the preceding aspects, wherein light from the structure is used to capture particles (optical tweezers), the particles are excited to emit light at the same or a different wavelength, and the emitted light is guided back into the fiber through the structure. 12. A rotationally symmetric dielectric beam-shaping structure according to one or more of the preceding aspects, wherein light from the structure is used to capture particles (optical tweezers), the particles are illuminated with the same or a different wavelength, and the reflected light is guided back into the fiber by the structure. 13. A rotationally symmetric dielectric beamforming structure according to one or more of the preceding aspects, wherein the optical focus created at the front side of the dielectric structure creates a state of equilibrium of forces that traps individual particles and cells and acts like optical tweezers. 14. A rotationally symmetric dielectric beamforming structure according to one or more of the preceding aspects, wherein the trapped particle or cell can be moved (optically manipulated) by moving the single-mode fiber in axial and transverse directions while the trapped particle maintains its position relative to the fiber. Reference list [1] Hu, Z. [ua]: Manipulation and arrangement of biological and dielectric particles by a lensed fiber probe. In: Optics Express, Vol. 12, 2004, No. 17, pp. 4123-4128. ISSN 1094-4087 (E). DOI: 10.1364 / OPEX.12.004123. [2] Taguchi, K. [ua]: Rotational manipulation of a yeast cell using optical fibers. In: Electronics Letters, Vol. 33, 1997, No. 14, pp. 1249-1250. ISSN 1350-911X (E). [3] RODRIGUES RIBEIRO, Rita S. [et al.]: Optical fiber tweezers fabricated by guided wave photo-polymerization. In: Photonics, Vol. 2, 2015, No. 2, pp. 634-645. ISSN 2304-6732 (E). [4] LIU, Zhihai [u.a.]: Tapered fiber optical tweezers for microscopic particle trapping: fabrication and application. In: Optics Express, Vol. 14, 2006, No. 25, S. 12510-12516. ISSN 1094-4087 (E). [5] TAYLOR, R.S.; HNATOVSKY, C.: Particle trapping in 3-D using a single fiber probe with an annular light distribution. In: Optics Express, Vol. 11, 2003, No. 21, S. 2775-2782. ISSN 1094-4087 (E). [6] RODRIGUES RIBEIRO, Rita S. [u.a.]: Fabrication of fresnel plates on optical fibres by FIB milling for optical trapping, manipulation and detection of single cells. In: Scientific Reports, Vol. 7, 2017, Artikelnummer: 4485. ISSN 2045-2322 (E). [7] CHANTAKIT, Teanchai [u.a.]: All-dielectric silicon metalens for two-dimensional particle manipulation in optical tweezers. In: Photonics Research, Vol. 8, 2020, No. 9, S. 1435-1440. ISSN 2327-9125 (E). DOI: 10.1364 / PRJ.389200. [8] LIBERALE, C. [u.a.]: Integrated microfluidic device for single-cell trapping and spectroscopy. In: Scientific Reports, Vol. 3, 2013, Artikelnummer: 1258. ISSN 2045-2322 (E). [9] LIU, Zhihai [u.a.]: Single fiber optical trapping of a liquid droplet and its application in microresonator. In: Optics Communications, Vol. 381, 2016, S. 371-376. ISSN 1873-0310 (E); 0030-4018 (P). DOI: 10.1016 / j.optcom.2016.07.046.

[10] LIU, Zhihai [u.a.]: Single-fiber tweezers applied for dye lasing in a fluid droplet. In: Optics Letters, Vol. 41, 2016, No. 13, S. 2966-2969. ISSN 1539-4794 (E); 0146-9592 (P).

[11] DENG, Hongchang [u.a.]: Fiber-based optical gun for particle shooting. In: ACS Photonics, Vol. 4, 2017, No. 3, S. 642-648. ISSN 2330-4022 (E).

[12] REDDY, Innem V.A.K. [u.a.]: 3D micro-printed hybrid photonic structure for single-fiber optical tweezers. In: 2021 Conference on Lasers and Electro-Optics / Europe - European Quantum Electronics Conference, 20-24 June 2021, Virtual Event. 2021, S. cl_5_3.

Claims

[1] Rotationally symmetric dielectric structure (2) for optical beam shaping and for capturing and manipulating single particles and living biological cells in an aqueous medium, which is mounted concentrically on the facet of a single-mode optical fiber (1), wherein the structure (2) comprises at least three total reflection surfaces (S1, S1', S2) configured to divide a light field emerging from the single-mode optical fiber (1) into at least two separate light paths, and wherein the at least three total reflection surfaces (S1, S1', S2) are further configured to combine the separate light paths as a ring beam at a common focal point. [2] Rotationally symmetric dielectric structure (2) for optical beam shaping according to claim 1, wherein the structure (2) and the single-mode optical fiber (1) have a common axis of symmetry, and a first total reflection surface (S1) is designed such that a first light path of the emerging light field, whose radial distance from the symmetry axis is smaller than a threshold value, is reflected away from the symmetry axis in the direction of a second total reflection surface (S1'), and wherein the second total reflection surface (S1') is designed to direct the light path into the focal point. [3] Rotationally symmetric dielectric structure (2) for optical beam shaping according to claim 2, wherein a third total reflection surface (S2) is designed such that a second light path of the emerging light field, whose radial distance from the axis of symmetry is greater than the threshold value, is directed towards the focal point. [4] Rotationally symmetric dielectric structure (2) for optical beam shaping according to claim 3, wherein the structure (2) is designed such that the second and third light paths lie within a numerical aperture of the single-mode optical fiber (1). [5] Rotationally symmetric dielectric structure (2) for optical beam shaping according to claim 3 or 4, wherein the total reflection surfaces (S1, S1', S2) are designed such that a difference in an optical path length of the facet to the focal point between the first light path and the second light path is an integer multiple of a light wavelength in an ambient medium. [6] Rotationally symmetric dielectric structure (2) for optical beam shaping according to one of the preceding claims, wherein the total reflection surfaces (S1, S1', S2) are designed to achieve total reflection for an ambient refractive index between 1 and 1.

4. [7] Rotationally symmetric dielectric structure (2) for optical beam shaping according to one or more of the preceding claims, wherein a surface at which the light field exits the structure (2) is concentric with the focal point. [8] Rotationally symmetric dielectric structure (2) for optical beam shaping according to one or more of the preceding claims, wherein the structure (2) is configured such that the ratio between the length of the propagation path of an expansion of the beam and the remaining length of the structure (2) is in the range of 0.5 to 5. [9] Rotationally symmetric dielectric structure (2) for optical beam shaping according to one or more of the preceding claims, wherein the numerical aperture for focusing light in an ambient medium is between 0.3 and 1.

5. [10] Rotationally symmetric dielectric structure (2) for optical beam shaping according to one or more of the preceding claims, wherein the structure is configured to trap particles and guide the light emitted by the trapped particles into the single-mode optical fiber (1). [11] Rotationally symmetric dielectric structure (2) for optical beam shaping according to one or more of the preceding claims, wherein the focal point of the structure is configured to serve as optical tweezers for capturing particles and / or cells. [12] Single-mode optical fiber (1) having a structure (2) according to one of the preceding claims. [13] Beam forming system with a single-mode optical fiber (1) according to claim 12 with a structure (2) according to one of claims 1 to 11, wherein the single-mode optical fiber (1) is configured to be movable in an axial and / or transverse direction with respect to a common axis of symmetry, wherein the structure is configured to hold a position of a confined particle. [14] A method for producing a rotationally symmetric dielectric structure (2) for optical beam shaping according to any one of claims 1 to 11, wherein the structure (2) is produced directly on a fiber tip by means of 3D printing or is produced on a substrate and then attached to the single-mode optical fiber (1).

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