Method and system for realizing orbital angular momentum multiplier multiplexing based on metasurface

Abstract

According to the method and system for achieving orbital angular momentum multiplier multiplexing based on the metasurfaces, two cascaded metasurfaces are arranged, the first metasurface executes different orbital angular momentum multiplication on circularly polarized light which is incident in a left-and-right rotation mode at an input plane to convert phases, and the second metasurface executes different orbital angular momentum multiplication on circularly polarized light which is incident in a left-and-right rotation mode; the left-hand circularly polarized light and the right-hand circularly polarized light respectively execute multiplication operation of different orders; and the light beam evolves after being propagated for a certain distance, and is output after being subjected to phase collimation by the second metasurface at the output plane, so that multiplexing of different order multiplication functions of vortex light orbital angular momentum on the metasurface is realized. According to the method, the multiplication order of the orbital angular momentum is remarkably increased while the complexity of the system is not increased, the method can work in a broadband, the robustness is good, and the method is suitable for highly integrated and miniaturized development of an optical system.

Description

Technical Field

[0001] The present invention relates to the fields of vortex optical field regulation and optical computing, and specifically designs a technical solution for realizing the multiplexing of orbital angular momentum multipliers of vortex beams based on metasurfaces. Background Art

[0002] Orbital angular momentum (OAM) is a fundamental degree of freedom of light, which has theoretically unbounded states and plays an important role in fields such as optical communication, quantum entanglement, and key distribution. Traditional OAM regulation devices are large in volume and single in function, and it is difficult to meet the integration requirements. A metasurface is an artificial subwavelength structure that can uniquely regulate the amplitude, phase, polarization, and frequency of light, and has a wide range of applications in fields such as optical holography, optical computing, and optical communication. Utilizing the characteristics of metasurfaces can make the regulation of the optical field more compact and flexible, and is expected to greatly reduce the complexity of optical systems. In vortex optical communication, different orbital angular momentum vortex modes, as different optical carriers, need to realize the functions of channel switching and routing. The discovery of orbital angular momentum multipliers provides a mode conversion scheme, which is expected to play an important role in vortex optical communication. However, the existing technology has not fully utilized its multiplexing to achieve multifunctional OAM operations.

[0003] Therefore, there is an urgent need for a compact and efficient method to realize the multiplexing function of OAM multipliers. Summary of the Invention

[0004] Aiming at the deficiencies of the existing technology, the present invention provides a phase modulation scheme that combines the transmission phase and geometric phase of a metasurface to realize independent phase modulation of left-handed circularly polarized light and right-handed circularly polarized light, thereby enabling the multiplexing of phase holograms.

[0005] The technical solution of the present invention provides a method for realizing the multiplexing of orbital angular momentum multipliers based on a metasurface. Two cascaded metasurfaces are set. The first metasurface respectively performs different orbital angular momentum multiplication conversion phases on the left-handed and right-handed circularly polarized lights incident on the input plane. The left-handed circularly polarized light and the right-handed circularly polarized light respectively perform multiplication operations of different orders. After the light beam propagates for a certain distance and evolves, it passes through the second metasurface for phase collimation at the output plane and then outputs, realizing the multiplexing of different-order multiplication functions of the orbital angular momentum of the vortex light on the metasurface.

[0006] Moreover, the left-handed circularly polarized light performs a second-order multiplication operation, and the right-handed circularly polarized light performs a third-order multiplication operation; or the right-handed circularly polarized light performs a second-order multiplication operation, and the left-handed circularly polarized light performs a third-order multiplication operation.

[0007] Moreover, the light field incident with left / right circular polarization is converted into a beam with right / left polarization state after loading the conversion phase at the first metasurface, and is re-converted into collimated left / right circularly polarized light and emitted after loading the correction phase at the second metasurface after propagating a certain distance.

[0008] Moreover, the conversion phase is loaded at the first metasurface to conformally map the circular wavefront of the incident vortex beam into a complementary fan-shaped wavefront; the correction phase is loaded at the second metasurface to eliminate the phase distortion generated during the propagation of the light field and recombine it into a circular wavefront for output.

[0009] Moreover, the transmission phases of the nanobricks on the metasurface are selected with several different sizes through discretization processing, and the geometric phase is regulated by the rotation angle of the nanobricks.

[0010] Moreover, the phase mask of the metasurface system is realized by the superposition of the transmission phase component and the geometric phase component, where the transmission phase component is controlled by the size of the nanobricks and the geometric phase component is controlled by the rotation angle of the nanobricks.

[0011] Moreover, discretization processing is performed on the transmission phase component, and the nanobricks are selected based on the error from the ideal phase.

[0012] Moreover, it is applicable to the visible light band and realizes the orbital angular momentum multiplexing function in optical communication or optical computing through cascaded metasurfaces.

[0013] On the other hand, the present invention also provides a device for realizing orbital angular momentum multiplication multiplexing based on a metasurface, which sets two cascaded metasurfaces for realizing the method for realizing orbital angular momentum multiplier multiplexing based on a metasurface as described above.

[0014] Moreover, the metasurface is composed of a silica substrate and titanium dioxide nanobricks.

[0015] A scheme for realizing orbital angular momentum multiplier multiplexing based on a metasurface designed by the present invention has the following advantages and positive effects: 1. It can significantly increase the multiplication order of the orbital angular momentum while not increasing the system complexity.

[0016] 2. The modulation method using geometric phase plus transmission phase has a broadband phase response, which enables the multiplier functional device to operate in a wide wavelength band.

[0017] 3. The design of the geometric phase allows for errors in the processing accuracy, making the entire system have good robustness.

[0018] 4. The structural unit of the metasurface is of sub-wavelength scale. The design based on the metasurface can make the functional device structure more compact, which is suitable for the highly integrated and miniaturized development of optical systems and can be used in optical communication and optical computing systems. Description of the Drawings

[0019] Figure 1 is a schematic diagram of realizing orbital angular momentum multiplier multiplexing in the cascaded metasurface system according to an embodiment of the present invention; Figure 2 is a schematic diagram of a single metasurface unit according to an embodiment of the present invention; Figure 3 is a transmittance scan diagram of a single structure of the metasurface according to an embodiment of the present invention; Figure 4 is a phase scan diagram of a single structure of the metasurface according to an embodiment of the present invention; Figure 5 is a conversion phase diagram of the metasurface according to an embodiment of the present invention; Figure 6 is a corrected phase diagram of the metasurface according to an embodiment of the present invention.

[0020] Figure 7 is an interference diagram of a vortex beam with a topological charge of and a reference beam in a two - times multiplier and a three - times multiplier according to an embodiment of the present invention. Detailed Embodiments

[0021] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0022] The present invention proposes to achieve independent phase modulation of left-handed circularly polarized light and right-handed circularly polarized light based on the combination of transmission phase and geometric phase of the metasurface, so as to realize the multiplexing of phase holograms. In the orbital angular momentum multiplier, light realizes the reshaping of the wavefront through two-phase modulations in space, and realizes the multiplication effect of orbital angular momentum by phase compression in the angular direction of the vortex phase. Specifically, the wavefront of a vortex beam in a circular ring shape is conformally mapped to the fan-shaped wavefront on the output plane, and multiple fans are recombined into a circular ring on the output plane, thereby realizing the regulation of the orbital angular momentum of the vortex beam. Among them, the conversion phase and correction phase of different orbital angular momentum multipliers on the input plane and the output plane can be realized through the metasurface multiplexing mask, so that left-handed light and right-handed light can perform calculations of different-order multipliers. It should be noted that in the design of the present invention, the left-handed and right-handed incident circularly polarized lights respectively realize the functions of two-fold and three-fold orbital angular momentum multipliers. The function of multiplexing the orbital angular momentum multiplier is realized by cascading two metasurface systems. The invention can multiplex multiple optical computing functions on a single metasurface system (multiplexing of different-order multiplication functions of the orbital angular momentum of vortex light on the metasurface), and can significantly reduce the complexity of device design while retaining the device functions.

[0023] Embodiment 1 The schematic diagram of the orbital angular momentum multiplier of the vortex beam based on the metasurface proposed by the present invention is as Figure 1 shown. The cascaded metasurface system has two-phase modulations on the incident light field, so that the wavefront of the vortex beam is reshaped in space, and the compression of the angular phase gradient doubles the topological charge number of the orbital angular momentum. The optical layout of this system consists of the cascade of two optical elements: the former performs a conformal optical transformation, while the latter corrects the phase distortion caused by the different paths traveled by different points of the beam and restores the required phase distribution. On the contrary, due to the invariance of the optical path to time reversal, the second optical element performs an inverse optical transformation.

[0024] The orbital angular momentum multiplier realized by the present invention involves two-phase masks in the light propagation direction. Different-order multipliers correspond to different conversion phases and correction phases, and the combination of the transmission phase and geometric phase of the metasurface can realize independent phase holograms for left-handed and right-handed circularly polarized lights. Therefore, the phase masks required for different-order multipliers can be realized on the same metasurface, and different multiplication factors can be multiplexed for different left-handed and right-handed circularly polarized lights.

[0025] The light field incident with left / right-handed circularly polarized light is converted into a beam with right / left-handed polarization state after loading the conversion phase at the first metasurface, and is re-converted into a collimated left / right-handed circularly polarized light and emitted after loading the correction phase at the second metasurface after propagating a certain distance.

[0026] For the metasurface phase mask that converts and corrects the phase, it is divided into the combination of the transmission phase component and the geometric phase component. The transmission phase component needs to be discretized. Select the bricks on the metasurface with relatively small phase error compared to the ideal phase among several nanobricks. The geometric phase component is achieved through the rotation angle of the bricks.

[0027] By using two metasurfaces with different phase masks for left - and right - handed circular polarizations, the phase modulation of light propagating in space twice is realized. At the same time, it is ensured that the polarization state returns to the incident polarization state after two conversions, achieving the multiplication effect based on the metasurface for different multiplication factors.

[0028] To achieve the orbital multiplier, it is required that the light field reshapes the optical wavefront after two - stage phase modulation. The circular beam splitting is mapped into complementary sectors and collimated and output in the output plane. The compression of the angular phase enables the doubling of the orbital angular momentum. That is, the working process of the orbital angular momentum multiplier is as follows: Use two cascaded metasurfaces to perform two - stage phase modulation on the incident light field respectively; Through the combination of the transmission phase and the geometric phase of the metasurface, independent phase holographic modulation is achieved for left - handed circularly polarized light and right - handed circularly polarized light; Load the conversion phase on the first metasurface at the input plane, and conformally map the circular wavefront of the incident vortex beam into a complementary sector wavefront; Load the correction phase on the second metasurface at the output plane to eliminate the phase distortion generated during the propagation of the light field and recombine it into a circular wavefront for output; By adjusting the conversion phase and the correction phase, different - order orbital angular momentum multiplication operations are performed on left - handed circularly polarized light and right - handed circularly polarized light respectively.

[0029] Therefore, the key to orbital angular momentum multiplication is to make the circular ring conformally map into a sector through optical transformation, and the azimuthal phase gradient is conformally mapped onto a circular sector with an amplitude of Here, corresponds to the multiplication coefficient of the orbital angular momentum multiplier . The required conversion phase is described by the following formula 2. The light beam is split into copies, and the complementary sectors after conversion reform a circular ring light field in the output plane and are collimated and output after conversion phase compensation to complete the multiplication operation. The design method proposed in the present invention is to utilize the unique characteristics of the metasurface for functional multiplexing of second - order and third - order multipliers based on conversion.

[0030] To conformally map the circular light field distribution of the vortex beam into a sector light field distribution, it is necessary to load the required conversion phase at the input plane position. The th conversion phase is expressed as:

[0031] Among them, is the wave vector in free space, is the polar coordinate system of the input plane, and act together to control the size of the light beam. is the propagation distance from the input plane to the output plane, is the multiplication order of the corresponding multiplier, is the deflection angle of the fan in the plane. By changing the deflection angle of the fan, the method of phase superposition is used to splice the head and tail of the fan into a ring in the angular direction. The conversion phase loaded on the input plane can be expressed in the following form:

[0032] Among them, is the natural base, is the correction phase of the corresponding region, and the order is marked as m = 1, 2,... n .

[0033] After the light passes through the first phase mask on the input plane and propagates a certain distance, it evolves into a new annular light field at the output plane. In order to eliminate the distortion generated during the mapping process of the light, phase compensation is required at the output plane position, and the specific correction phase of the corresponding region is expressed as the following expression:

[0034] The method designed by this invention mainly uses two optical elements to effectively map the azimuthal phase gradient of the input vortex beam to the optical transformation of a circular sector. By combining multiple circular sector transformations into one optical element, the value of the input orbital angular momentum state can be multiplied by dividing the phase and mapping it to complementary circular sectors.

[0035] The phase of the optical element can be realized by a metasurface as a phase mask: different multiplications correspond to different conversion phases and correction phases. The metasurface can achieve independent phase responses for left- and right-handed circularly polarized light, so different phase modulations can be realized on a single metasurface.

[0036] It can be known from the above theoretical derivation that the implementation of second-order and third-order orbital angular momentum multipliers corresponds to different conversion phases and correction phases , that is, the second-order multiplication corresponds to the conversion phase and the correction phase , the third order is and . By using the geometric phase and transmission phase of the metasurface, independent phase responses for left- and right-handed circular polarizations can be achieved, thus realizing multiplexed phase loading. The specific design principle is that when left- and right-handed circularly polarized lights pass through a metasurface, the modulated phase is the transmission phase and the geometric phase combined:

[0037] Among them, when left-handed light is incident, the phase modulation is performed , and when right-handed light is incident, it is . Thus, after introducing an additional transmission phase through the geometric phase control of the metasurface, different phase delays can be introduced for left-handed and right-handed lights. While ensuring the metasupercell, the phase modulation is divided into two independent channels. Therefore, the conversion phase mask of the first metasurface can be designed as:

[0038] The correction phase mask of the second metasurface can be designed as:

[0039] Based on this, the present invention designs a scheme for multiplexing orbital angular momentum multipliers based on metasurfaces. The present invention uses left- and right-handed polarization multiplexing to respectively achieve multiplication operations of doubling and tripling the orbital angular momentum. Therefore, the present invention envisions that this compact orbital angular momentum multiplier can be applied in the miniaturization and integration applications in the fields of optical communication, optical computing, and large-scale optical processing systems.

[0040] Example 2 Based on the scheme of Example 1, the present invention further proposes a preferred implementation scheme for a single metasupercell: The designed metasurface uses silica as the substrate and titanium dioxide as the material of the bricks, ensuring a high transmittance of the orbital angular momentum multiplication device in the visible light region. The size and rotation angle of the nanobricks are optimized through electromagnetic simulation, and high transmittance and uniform phase distribution can be achieved.

[0041] Figure 2 is a schematic diagram of the metasurface nanobrick structure designed in the embodiment of the present invention. The substrate structure is silica, and the nanobricks are made of titanium dioxide material. By combining the transmission phase and geometric phase of the nanorods for independent phase modulation of left- and right-handed circularly polarized lights, the masks for the conversion phase and correction phase required for different multiplication orders are realized.

[0042] The present invention further proposes that the design of the metasurface requires determining different nanobricks with transmission phases. The nanobricks have a uniform phase distribution while satisfying as high a transmittance as possible. It is preferably recommended to select 8 nanobrick structures as alternatives for the transmission phase. In specific implementation, more types of nanobrick structures can also be selected according to needs.

[0043] Through electromagnetic simulation software, the transmittance and transmission phase of the nanobricks at different lengths and widths can be scanned, as Figure 3 and 4 shown. In the embodiment, for the size of the nanocolumns in the unit structure, it is determined through electromagnetic software simulation. The set optical wavelength is 633 , the height of the nanocolumns is 650 , and the unit interval of the nanocolumns is 400 . The simulation finds that the nanostructure has a high transmittance, indicating that the selected nanobrick structure of the present invention can realize a high-efficiency orbital angular momentum multiplier. Combining the requirements of high transmittance and uniform phase distribution, 8 nanobricks with different sizes are determined as options for phase modulation of the present invention, which are marked with asterisks in the figure.

[0044] Figure 5 and Figure 6 Specifically show the phase distributions of the conversion phase and correction phase required for different multipliers. The holographic phase described by formulas (5) and (6) can be decomposed into the modulation of the transmission phase component and the modulation of the geometric phase component. It should be noted that the transmission phase component needs to be quantized into 8 phase steps corresponding to the selection of 8 nanos, and the geometric phase component can be realized by precisely adjusting the rotation angle of the nanobricks.

[0045] In summary, by designing a transmission phase and geometric phase multiplexing metasurface, different holographic phase modulations can be realized, and different multiplication order functions of the multiplier can be multiplexed.

[0046] As Figure 7 shown, in the embodiment of the present invention, the interference patterns of the vortex beams with different topological charges of the double multiplier and triple multiplier with the reference beam. This figure shows that the left- and right-handed incident beam multipliers respectively perform the functions of the orbital angular momentum double multiplier and triple multiplier. The multiplication effect can be verified through the interference spiral arms, verifying the feasibility of the method of the present invention.

[0047] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should all be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claimed rights.

Claims

1. A method for realizing orbital angular momentum multiplier reuse based on a metasurface, characterized in that: Two cascaded metasurfaces are set up. The first metasurface performs different orbital angular momentum multiplication conversion phases on left-handed and right-handed circularly polarized light at the input plane, and performs different orders of multiplication operations on left-handed circularly polarized light and right-handed circularly polarized light respectively. The light beam evolves after propagating for a certain distance, and is output after being phase-collimated by the second metasurface at the output plane, thereby realizing the multiplexing of different orders of multiplication functions of the orbital angular momentum of the vortex light on the metasurface.

2. The method for realizing orbital angular momentum multiplier reuse based on a metasurface according to claim 1, characterized in that: A second-order multiplication operation is performed on left-handed circularly polarized light, and a third-order multiplication operation is performed on right-handed circularly polarized light; or, a second-order multiplication operation is performed on right-handed circularly polarized light, and a third-order multiplication operation is performed on left-handed circularly polarized light.

3. The method for realizing orbital angular momentum multiplier reuse based on a metasurface according to claim 1, characterized in that: The incident left / right circularly polarized light field is converted into a right / left polarized light beam after the conversion phase is loaded on the first metasurface. After propagating for a certain distance, the correction phase is loaded on the second metasurface and converted back into a collimated left / right circularly polarized light output.

4. The method for realizing orbital angular momentum multiplier reuse based on a metasurface according to claim 1, characterized in that: The conversion phase is loaded on the first metasurface to conformally map the circular wavefront of the incident vortex beam into a complementary fan-shaped wavefront; the correction phase is loaded on the second metasurface to eliminate the phase distortion generated during the propagation of the light field and recombine it into a circular wavefront output.

5. The method for realizing orbital angular momentum multiplier reuse based on a metasurface according to claim 1, characterized in that: The transmission phase of the nanobricks on the metasurface is obtained by selecting a number of structures of different sizes through discretization processing, and the geometric phase is regulated by the rotation angle of the nanobricks.

6. The method for realizing orbital angular momentum multiplier reuse based on a metasurface according to claim 5, characterized in that: The phase mask of the metasurface system is realized by the superposition of the transmission phase component and the geometric phase component, where the transmission phase component is controlled by the size of the nanobricks and the geometric phase component is controlled by the rotation angle of the nanobricks.

7. The method for realizing orbital angular momentum multiplier reuse based on a metasurface according to claim 6, characterized in that: The transmission phase component is discretized and nanobricks are selected based on the error from the ideal phase.

8. The method for realizing orbital angular momentum multiplier reuse based on a metasurface according to claim 1, characterized in that: It is applicable to the visible light band and can realize orbital angular momentum multiplexing function in optical communication or optical computing through cascading metasurfaces.

9. A device for realizing orbital angular momentum multiplication multiplexing based on a metasurface, characterized in that: Two cascaded metasurfaces are provided to implement the method for realizing orbital angular momentum multiplier multiplexing based on a metasurface as described in any one of claims 1 to 8.

10. The device for realizing orbital angular momentum multiplier reuse based on a metasurface according to claim 9, characterized in that: The super surface is composed of a silicon dioxide substrate and titanium dioxide nanobricks.

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