All-fiber spinning controllable sub-wavelength Bessel beam and focused vortex beam generating device

Abstract

The invention provides an all-fiber spinning controllable sub-wavelength Bessel light beam and focusing vortex light beam generating device. The spin multiplexing metasurface optical fiber is characterized by comprising a single-mode optical fiber (1), a coreless optical fiber (2) and a spin multiplexing metasurface structure (3). The coreless optical fiber (2) is welded on the single-mode optical fiber (1) and plays a role in expanding a beam of a fiber core optical field of the single-mode optical fiber. The spin multiplexing metasurface structure (3) is arranged on the end face of the coreless optical fiber (2) and is composed of rectangular nanometer columns with the same size and different orientation angles, and the orientation angles of the rectangular nanometer columns at different positions are designed according to phase modulation needed by an emergent light field of the end face of the coreless optical fiber (2). When right-handed or left-handed circularly polarized light enters, the right-handed or left-handed circularly polarized light is expanded by the coreless optical fiber (2) and then irradiates the spin multiplexing metasurface structure (3), and after phase modulation, a sub-wavelength Bessel beam or a sub-wavelength focused vortex beam is generated at the optical fiber end. The invention belongs to the technical field of optical fiber microstructure devices.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber microstructure device technology, specifically involving the generation of spin-controllable subwavelength Bessel beams and subwavelength focused vortex beams by designing and fabricating spin-multiplexed metasurface structures on the end face of optical fibers and coupling different circularly polarized light. Background Technology

[0002] With the rapid development of nanophotonics, the generation and manipulation of subwavelength-scale structured beams has become an important research direction in the field of optics. This is because subwavelength structured beams can significantly improve the performance of application systems in many fields. For example, in the field of super-resolution imaging, subwavelength focused light fields can help break through the diffraction limit, significantly improve imaging resolution, and provide new tools for biomedical detection and nanomaterial characterization; in optical micromanipulation, subwavelength structured light fields can achieve precise capture and manipulation of tiny objects such as nanoparticles and biomolecules, promoting interdisciplinary research between nanotechnology and biomedicine; and in the fields of high-density optical storage and nanolithography, the use of subwavelength beams can greatly improve information storage density and processing accuracy, meeting the development needs of future highly integrated devices. Therefore, how to compress complex structured beams (such as vortex beams, Bessel beams, etc.) to the subwavelength scale is a hot topic of current interest in the field of optical field manipulation.

[0003] Meanwhile, optical fiber, as an optical field transmission platform possessing strong light field confinement capabilities, low transmission loss, and high flexibility and stability, offers another highly promising approach for the generation and manipulation of structured light fields. Compared to free-space optical systems or on-chip integrated devices, fiber optic devices are not limited by the "hard" connections between devices in a structured beam excitation platform, allowing the light field to be confined within the fiber core for transmission. They possess inherent miniaturization and ease of integration characteristics. Integrating metasurface technology into the fiber endface to construct an all-fiber optical field manipulation device can achieve precise wavefront shaping at the subwavelength scale and significantly simplify the system structure, potentially greatly improving the system's practicality and flexibility.

[0004] Building upon the aforementioned background technology, this invention successfully generates spin-controllable subwavelength Bessel beams and subwavelength focused vortex beams by designing a metasurface structure at the fiber end, providing a promising fiber integration solution for generating subwavelength structured beams at the fiber end. The structure designed in this invention not only possesses excellent spin multiplexing capabilities but also features high integration density, small size, and high flexibility, making it suitable for plug-and-play applications. It has significant application value in fields such as micro-nano optics, light-matter interactions, and integrated photonics. Summary of the Invention

[0005] This invention provides an all-fiber spin-controllable subwavelength Bessel beam and focused vortex beam generating device.

[0006] The all-fiber spin-controlled subwavelength Bessel beam and focused vortex beam generating device provided by this invention is implemented as follows:

[0007] The all-fiber spin-controlled subwavelength Bessel beam and focused vortex beam generating device, such as Figure 1 As shown, it consists of a single-mode fiber 1, a coreless fiber 2, and a spin-multiplexing metasurface structure 3. The coreless fiber 2 is fused to the single-mode fiber 1, serving to expand the transmitted optical field in the core of the single-mode fiber 1. The spin-multiplexing metasurface structure 3 is located at the end face of the coreless fiber 2. When right-handed or left-handed circularly polarized light is incident, it is expanded by the coreless fiber 2 and then illuminates the spin-multiplexing metasurface structure 3. After phase modulation by the spin-multiplexing metasurface structure 3, a subwavelength Bessel beam or a subwavelength focused vortex beam is generated at the fiber end.

[0008] The specific design principle of the spin-multiplexed metasurface structure 3 is as follows: In the device structure, the output optical field of the single-mode fiber 1 will be expanded in the coreless fiber 2 in the form of a Gaussian beam. Considering the refractive index of the coreless fiber 2... The Gaussian mode in single-mode fiber 1 is ,in It is a constant. Let be the mode field radius of the Gaussian mode in single-mode fiber 1. Then, the light field reaching the output end face of the coreless fiber 2 after beam expansion, i.e., illuminating the spin-multiplexed metasurface structure 3, is:

[0009] (1)

[0010] in , , H represents the length of the fusion spliced ​​coreless fiber. For simplicity, the transmission phase and Gouy phase are omitted here because these two phases have a uniform spatial distribution and do not affect the overall transverse phase distribution of the optical field.

[0011] Design a spin-multiplexing metasurface structure 3 to achieve the following phase modulation: , Representing left- and right-hand circularly polarized light, the initial light field distribution output from spin-recombined metasurface structure 3 can be expressed as:

[0012] (2)

[0013] in Reflecting the properties of spin-reused metasurface structure 3, when right-handed circularly polarized light is incident... When left-handed circularly polarized light is incident To generate subwavelength Bessel beams and subwavelength focused vortex beams respectively when different circularly polarized light is incident, it is necessary to set:

[0014] (3)

[0015] (4)

[0016] in Numerical aperture, Focal length The topological charge number is given. To achieve the phase distribution described above, we use a rectangular nanopillar array to construct the spin-reusing metasurface structure 3. A schematic diagram of the spin-reusing metasurface structure 3 is shown below. Figure 2 As shown. Figure 3 The diagram shows a schematic of the rectangular nanopillar structure that makes up the spin-recombining metasurface structure 3. Each rectangular nanopillar is equivalent to an anisotropic scatterer because the complex scattering coefficients of the incident light polarized along the long and short axes of the rectangular nanopillar are different. This allows the rectangular nanopillars to convert the incident circularly polarized light (CP) into two parts: one part is co-polarized circularly polarized light with the same polarization as the incident light and no phase shift; the other part is cross-polarized circularly polarized light with opposite polarization to the incident light and a sudden phase change. .in The deflection angle of the nanopillar, where "±" depends on the circular polarization state of the input beam. When left-handed circularly polarized light is incident, the phase abruptly changes to "+", and when right-handed circularly polarized light is incident, the phase abruptly changes to "-". Therefore, we can use the nanopillar to construct two phase profiles to achieve polarization multiplexing. For convenience, as shown in Figure 3, we set the deflection angle of the nanopillar... To generate the negative phase profile required for a Bessel beam in a rectangular nanopillar array A (i.e., the deflection angle of the nanopillars in the rectangular nanopillar array A). This allows the rectangular nanopillar array B to generate the positive phase profile required to produce a vortex beam (i.e., the deflection angle of the nanopillars in the rectangular nanopillar array B). The nanorods are nested together to form a spin-reused metasurface structure 3, where the distance between adjacent nanorods is P. When right-handed circularly polarized light is incident on the spin-reused metasurface structure 3, the nanorod array A will produce a positive phase profile. Meanwhile, the nanopillar array B produces The initial optical field distribution output from the spin-recombined metasurface structure 3 can be expressed as:

[0017] (5)

[0018] in To generate the desired subwavelength Bessel beam, and Phase carrying divergence As the transmission continues to diverge, a weak background field is generated. When left-handed circularly polarized light is incident on the self-selected multiplexed metasurface structure 3, the rectangular nanopillar array A will produce a positive phase profile. Meanwhile, the rectangular nanopillar array B produces The initial light field distribution output from the self-selected reusable metasurface structure 3 can be expressed as:

[0019] (6)

[0020] in To generate the desired focused vortex beam, and Phase with outward deflection With transmission, only a weak background field will be generated. Therefore, we can achieve the generation of subwavelength Bessel beams and subwavelength focused vortex beams through nested structures. The phase distribution carried by the nested and merged self-selected multiplexed metasurface structure 3 can be expressed as:

[0021] (7)

[0022] in, and is an integer value. P is the period of the unit nanopillar.

[0023] In this invention, silicon nitride material is selected to construct rectangular nanopillars with a height of T, T=637nm. The polarization conversion efficiency of the silicon nitride rectangular nanopillars is scanned using FDTD simulation software when the length L and width W range from 100nm to 400nm. Ultimately, a length L=350nm and a width W=120nm are chosen for the silicon nitride rectangular nanopillars. Furthermore, a self-selected multiplexed metasurface structure 3 is constructed using the silicon nitride rectangular nanopillars, thereby realizing the generation of subwavelength Bessel beams and focused vortex beams.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. The all-fiber spin-controllable subwavelength Bessel beam and focused vortex beam generating device proposed in this invention has all the advantages of fiber photonic devices.

[0026] 2. The all-fiber spin-controllable subwavelength Bessel beam and focused vortex beam generating device proposed in this invention uses ordinary commercial single-mode fiber and coreless fiber, without the need to separately draw special fiber, thus effectively controlling costs.

[0027] 3. The all-fiber spin-controlled subwavelength Bessel beam and focused vortex beam generator proposed in this invention can generate the required subwavelength Bessel beam and subwavelength focused vortex beam by designing a spin-multiplexed metasurface. It features plug-and-play integration, high density, small size, and high flexibility, which is beneficial for meeting diverse application needs. It also provides a promising fiber optic integration solution for generating subwavelength structured beams at fiber ends. Attached Figure Description

[0028] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the structure of an all-fiber spin-controllable subwavelength Bessel beam and focused vortex beam generating device provided in an embodiment of the present invention.

[0030] Figure 2 This is a schematic diagram of the spin-reusable metasurface structure provided in an embodiment of the present invention.

[0031] Figure 3 This is a schematic diagram of a rectangular nanopillar structure that forms a spin-reusable metasurface structure, as provided in an embodiment of the present invention.

[0032] Figure 4 The following is a normalized intensity distribution diagram of the outgoing light field above the end face of the optical fiber when right-handed circularly polarized light is incident, provided in an embodiment of the present invention; (a) is the normalized intensity distribution in the xz plane; (b1)-(b4) are the normalized intensity distributions in the xy plane when the transmission distances are z=9, 18, 27, 36μm respectively; (c1)-(c4) are the normalized intensity distributions on the x-axis extracted from the results of (b1)-(b4) respectively.

[0033] Figure 5 The following is a normalized intensity distribution diagram of the outgoing light field above the end face of the optical fiber when left-handed circularly polarized light is incident, provided by an embodiment of the present invention; (a) is the normalized intensity distribution in the xz plane; (b) is the normalized intensity distribution in the xy plane when the transmission distance is z=42μm; (cd) are the normalized intensity distributions on the x-axis and y-axis extracted from the results in (b), respectively. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other instances obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] A schematic diagram of the all-fiber spin-controlled subwavelength Bessel beam and focused vortex beam generating device proposed in this invention is shown below. Figure 1 It consists of a single-mode fiber 1, a coreless fiber 2, and a spin-multiplexing metasurface structure 3. In this embodiment, commercially available visible single-mode fiber and commercially available coreless fiber are selected, along with the spin-multiplexing metasurface structure 3 constructed from silicon nitride rectangular nanopillars of the same size but different orientation angles.

[0036] In this embodiment, left-handed or right-handed circularly polarized light with an incident wavelength of λ=532nm is coupled into the all-fiber device. The mode field radius of the transmitted optical field in the visible single-mode fiber is... The length of coreless optical fiber refractive index ,focal length Numerical aperture and topological load A spin-recombining metasurface structure was constructed using silicon nitride rectangular nanopillars with a height T = 637 nm, a length L = 350 nm, and a width W = 120 nm. The diameter of the spin-recombining metasurface structure was 60 μm. The output light field distribution of the spin-recombining metasurface structure was simulated using FDTD simulation. The normalized intensity distribution of the output light field when right-handed circularly polarized light is incident is shown in the figure below. Figure 4 As shown, the output light field is a Bessel beam, as... Figure 4 (c1-c4) represent the normalized intensity distributions on the x-axis at distances of 9, 18, 27, and 36 μm from the fiber end face (i.e., the z=0 plane), where the black arrows indicate the FWHM of the central main intensity lobe. The resulting Bessel beam has a subwavelength dimension (smaller than the incident light wavelength of 532 nm). The normalized intensity distribution of the output light field when left-handed circularly polarized light is incident is shown in the figure. Figure 5 As shown, a focused vortex beam is generated at z=42μm. Figure 5 (c) shows the normalized intensity distribution of the focused vortex beam on the x and y axes at the focal plane position of z=42μm, where the black arrows represent the ring intensity FWHM of the focused vortex beam, which also has a subwavelength dimension.

[0037] The above description discloses only one preferred embodiment of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the invention.

Claims

1. A device for generating all-fiber spin-controlled subwavelength Bessel beams and focused vortex beams, characterized in that: It consists of a single-mode fiber (1), a coreless fiber (2), and a spin-multiplexed metasurface structure (3); the coreless fiber (2) is fused to the single-mode fiber (1) and serves to expand the transmitted optical field in the core of the single-mode fiber (1); the spin-multiplexed metasurface structure (3) is located on the end face of the coreless fiber (2); the spin-multiplexed metasurface structure is composed of rectangular nanopillars of the same size but different orientation angles, and the orientation angles of the rectangular nanopillars at different positions are designed according to the phase modulation required for the emitted optical field from the end face of the coreless fiber (2); when right-handed or left-handed circularly polarized light is incident, it is expanded by the coreless fiber (2) and then irradiates the spin-multiplexed metasurface structure (3), and after phase modulation by the spin-multiplexed metasurface structure (3), a subwavelength Bessel beam or a subwavelength focused vortex beam is generated at the fiber end.

2. The all-fiber spin-controlled subwavelength Bessel beam and focused vortex beam generating device according to claim 1, characterized in that: The spin-reused metasurface structure (3) determines the orientation angle of rectangular nanopillars at different positions through the geometric phase design principle. It realizes the spin-reused function by nesting and superimposing two different sets of geometric phases to generate the required subwavelength Bessel beam and subwavelength focused vortex beam.

3. The all-fiber spin-controlled subwavelength Bessel beam and focused vortex beam generating device according to claim 1, characterized in that: The spin-reusable metasurface structure (3) can be composed of a rectangular nanopillar array of all-dielectric material, which can be silicon nitride, gallium nitride or titanium dioxide.

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