Optical clock metasurface

Abstract

The invention provides an optical clock metasurface which is characterized in that the metasurface is composed of a plurality of units which are integrated on a substrate; when incident ray polarized light with an initial polarization direction is input along a main optical axis, a first group of non-circular symmetrical focusing spot clusters are generated on a focal plane through metasurface modulation; after the polarization direction of the incident ray polarized light is rotated by a certain angle theta1, incident ray polarized light with a new polarization direction is formed, after the incident ray polarized light is modulated by the metasurface, a second group of non-circularly symmetrical focusing light spot clusters are generated on the focal plane, and the relative angle theta2 between the two groups of focusing light spot clusters corresponds to the rotation angle theta1 of the polarization direction. By changing the polarization direction of incident ray polarized light, synchronous rotation of output light spots is achieved, coordinated movement similar to clock pointers is achieved, the rotation angle of a light field is accurately controlled, and the device can be widely applied to the fields of light field regulation and control, polarization detection, optical manipulation and the like.

Description

(I) Technical field

[0001] The present invention provides an optical clock metasurface, which can be used in the fields of light field regulation, polarization detection and optical manipulation. (II) Background technology

[0002] Optical metasurfaces achieve precise control of light fields, including multiple parameters such as amplitude, phase, and polarization, by designing nanostructures on a subwavelength scale, thereby effectively adjusting the transmission characteristics of light and showing great application potential. In recent years, optical metasurfaces have shown significant advantages in the fields of miniaturization, integration, and multifunctionalization of optical devices, and are widely used in the development of new optical devices such as super lenses, holographic displays, and sensors, promoting the further development of integrated optical systems.

[0003] With the advancement of micro-nano manufacturing technology, research on optical metasurfaces has gradually focused on improving the efficiency and accuracy of multifunctional integration and light field control. Significant results have been achieved in phase control and wavelength multiplexing. For example, Xu et al. proposed a metasurface that realizes multi-focal optical tweezers through polarization multiplexing for the capture and manipulation of micro-nano particles (Journal of Physics D: Applied Physics, 2024, 57(42): 425104.). This work demonstrates its advantages in the integration of multiple optical manipulation tasks, but it still has limitations in multi-focal simultaneous manipulation and precise position control. Wang's team designed a multifunctional superlens based on genetic algorithm optimization to achieve efficient control of wavelength multiplexing (Optical Materials Express, 2021, 11(11): 3908-3916.). The design is compact and easy to integrate, but it still faces challenges in spectral response. Deng et al. proposed a broadband metasurface based on a dual-cell structure, which improves the optical performance in a wide frequency range by bypassing phase control (Nano Letters, 2018, 18(5): 2885-2892.), which performs well in wide-spectrum imaging, but still needs to be optimized in terms of bandwidth coverage and system complexity; Wang et al. proposed an information separation encryption method based on a multi-layer cascaded metasurface, which realizes information encryption and physical space separation through multiple independent channels (Chinese Optics Letters, 2023, 21(2): 020003.), although this design has significant advantages in information capacity and security, the complex phase control increases the difficulty of implementation; Allayarov et al. proposed a dynamic control method based on a non-local medium metasurface, which achieves the controllability of lattice resonance by adjusting the refractive index contrast between the substrate and the upper medium (Advanced Photonics Research, 2024, 5(1): 2300268.), although this method performs well in spectral selectivity and dynamic control, its implementation process involves complex optical coupling control. In summary, although the design and application of metasurfaces have made significant progress, existing technologies still face challenges in multi-function, high integration and precise control, especially in the dynamic regulation of complex light fields, efficient integration of multiple functions, and precise control of light beam direction and focal position.

[0004] The present invention provides an optical clock metasurface, which realizes the optical clock effect by precisely focusing and controlling the polarization direction of left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP) on the focal plane. The present invention has the following significant advantages: first, the present invention flexibly controls the rotation of the light spot by changing the polarization direction of the incident linear polarized light, realizes the optical movement similar to the clock, and improves the efficiency and applicability of the light field control; second, the present invention adopts signal function modulation to ensure the continuous and periodic phase profile, avoids the phase discontinuity problem in the traditional metasurface multi-region design, thereby reducing the diffraction loss and improving the efficiency of the light field control; finally, multi-focus control can be realized by adjusting the parameters of the signal function, without the need for complex multi-region design, simplifying the design process, and having high flexibility, and can accurately control the number, position and spacing of the focus to meet different application requirements; in summary, the present invention overcomes the shortcomings of the prior art in terms of multi-functional integration, high-precision control and structural complexity, and provides an innovative solution for high-precision optical control. (III) Summary of the invention

[0005] The purpose of the present invention is to provide an optical clock metasurface. The implementation of the present invention is as follows: the device is mainly composed of a plurality of dielectric block units, which are integrated on a substrate. When the incident linear polarized light with an initial polarization direction is input along the main optical axis, after being modulated by the metasurface, a first group of focused light spot clusters are generated on the focal plane; after the polarization direction is rotated by a certain angle θ1, the incident linear polarized light is modulated by the metasurface again to form a second group of focused light spot clusters, wherein the relative angle θ2 between the two groups of light spots corresponds to the rotation angle θ1 of the polarization direction. By changing the polarization direction of the incident linear polarized light, the synchronous rotation of the output light spot can be achieved, simulating the coordinated movement of the clock hands, thereby accurately controlling the rotation angle of the light field. In addition, the present invention innovatively introduces a signal modulation method to regulate the phase profile of the metasurface to achieve precise spatial coincidence of multiple foci and stable control of phase interference.

[0006] The following is a detailed description of the working principle of an optical clock metasurface. The phase shift along the y-axis satisfy When the LCP and RCP are independently encoded, two phase profiles can be generated simultaneously. and The phase profiles of the two can be expressed as:

[0007] In order to realize the optical clock effect, on the one hand, LCP and RCP must be precisely focused on the same position on the focal plane. The spatial overlap of the two is the key to ensure the generation of coherent interference. On the other hand, the circular symmetry of the phase profile is a necessary condition to ensure that the shape of the light spot remains stable during the rotation process. It ensures that the interference light spot does not deform during rotation, thus ensuring the accuracy of the effect. When the above conditions are met, the interference pattern will produce a light spot rotation as the incident polarization direction changes, thereby forming an optical clock effect. Therefore, in order to realize the rotation of the light spot on the focal plane, there must be a difference in phase between the two components of LCP and RCP. This difference is introduced by the geometric phase, and the phase profile needs to be adjusted. Additional geometric phase modulation term

[0008] Among them, the geometric phase modulation term Introduced by the rotation angle θ of the metasurface unit, the key to ensuring that the geometric phase difference between the LCP and RCP components is π is to adjust the rotation angle θ to achieve phase matching. The expression for the difference between the geometric phase modulation terms of LCP and RCP is:

[0009] The rotation angle θ is obtained by calculation, and then the phase profile of the metasurface is obtained, such as Figure 2 As shown in (a), the output light field produces a significant interference effect on the focal plane, causing the light spot on the focal plane to rotate along with the polarization direction of the incident linear polarization light, as shown in Figure 2 (b) as shown.

[0010] In order to achieve simultaneous focusing of multiple focal points, the present invention introduces a signal function to modulate the phase profile. Taking the sawtooth wave signal function as an example, by introducing a sawtooth wave signal function with periodic phase jump, its phase profile is as follows: Figure 2 As shown in (c), the incident linear polarized light is decomposed into multiple spatial frequency components. These components produce multiple diffraction orders at different positions on the focal plane, forming multiple focal points that rotate with the polarization direction of the incident linear polarized light, such as Figure 2 (d) The phase profile after the sawtooth signal function modulation is:

[0011] Among them, m(x) and m(y) are sawtooth signal functions along the x and y directions, respectively, defined as:

[0012] Where A is the amplitude, which determines the spacing between the foci, and L x and L y is the period of the function, controlling the number and position of the foci, φ xand φ y is the phase shift, which is used to adjust the starting position of the focus. The introduction of the signal function makes the phase jump periodically in space, resulting in the spatial frequency decomposition of the incident linear polarization light and the formation of multiple focal points on the focal plane.

[0013] Through the method of signal function modulation, the phase profile of the metasurface is made periodic and continuous in space, without the need to divide it into multiple areas. This overcomes the diffraction loss caused by phase discontinuity in traditional metasurface design, improves the efficiency and flexibility of light field regulation, and thus realizes the multi-focus optical clock effect. Secondly, by adjusting the parameters of the signal function, the number, position and spacing of the foci can be flexibly controlled, thereby simplifying the design process and adapting to different application requirements. (IV) Description of the drawings

[0014] Figure 1 This is a schematic diagram of the working principle of an optical clock metasurface.

[0015] Figure 2 It is a phase contour diagram of an optical clock metasurface and a light field distribution diagram in the focal plane under the excitation of incident linear polarized light with different polarization directions: (a) and (b) are the phase contour diagram and light field distribution diagram of the metasurface that produces a single focused light spot (the number in the upper right corner is the polarization direction of the incident linear polarized light); (c) and (d) are the phase contour diagram and light field distribution diagram of the metasurface that produces two focused light spots.

[0016] Figure 3 This is a schematic diagram of the preparation of an optical clock metasurface.

[0017] Figure 4 The present invention is a system working principle diagram of an optical clock metasurface: (a) light field distribution at the fiber end of a large mode field optical fiber; (b) and (c) are light field distribution diagrams in the focal plane of a device that generates two focused light spots under the excitation of incident linear polarized light in different polarization directions, where (b) corresponds to polarization direction (301) and (c) corresponds to polarization direction (401).

[0018] Figure 5 The graph of the rotation angle of the focused light spot of an optical clock metasurface under the excitation of incident linear polarized light in different polarization directions: (a) a single focused light spot; (b) two focused light spots.

[0019] Figure 6 Light field distribution diagram of an optical clock metasurface in the focal plane under the excitation of incident circularly polarized light: (a) LCP excitation; (b) RCP excitation. (V) Specific implementation methods

[0020] The present invention will be further described below in conjunction with specific embodiments.

[0021] Combination Figure 1 The present invention mainly provides an optical clock metasurface, characterized in that: the metasurface (1) is composed of a plurality of units (101) and integrated on an optical substrate (2); when an incident linear polarized light (3) with an initial polarization direction (301) is input along a principal optical axis, it is modulated by the metasurface (1) to generate a first group of non-circularly symmetric focused light spot clusters (302) on a focal plane; when the polarization direction (301) is rotated by a certain angle θ1, an incident linear polarized light (4) with a new polarization direction (401) is modulated by the metasurface (1) to generate a second group of non-circularly symmetric focused light spot clusters (402) on a focal plane, wherein an angle θ2 between the two groups of focused light spot clusters (302) and (402) corresponds to an angle θ1 between the polarization directions of the two incident linear polarized lights (3) and (4); by adjusting the polarization direction of the incident linear polarized light, synchronous rotation of the output light spot is achieved, thereby achieving an optical clock effect similar to the coordinated movement of a clock hand.

[0022] When a large mode field optical fiber is used as the substrate (2), taking the example of integrating a metasurface device with an optical clock effect (generating two focused light spots) on the fiber end of the large mode field optical fiber, combined with Figure 3 , the preparation process of the device can be divided into the following eight steps:

[0023] Step 1: Evenly apply photoresist (6) on the silicon dioxide substrate (5), such as Figure 3 (a) shown.

[0024] Step 2: selectively expose the photoresist (6) by electron beam etching, scan the pattern onto the photoresist (6), and form an ordered sub-wavelength structure exposure area (6.1), which corresponds to the regional position of different sub-wavelength structure units (1) constituting the optical clock metasurface (2), such as Figure 3 (b) as shown.

[0025] Step 3: After the exposure is completed, the excess photoresist (6) is removed, and after cleaning and drying, the desired geometric pattern is completely retained on the photoresist (6), such as Figure 3 (c) as shown.

[0026] Step 4: Deposit a layer of full dielectric film (7) on the photoresist (6) by thermal evaporation, such as Figure 3 (d) as shown.

[0027] Step 5: After stripping the photoresist (6) in the unexposed area, a metasurface (1) is obtained, such as Figure 3 (e) as shown.

[0028] Step 6: Apply a heat-curing epoxy resin (9) on a glass slide (8), and then contact the fiber end of the large mode area optical fiber (2) with the epoxy resin (9) by dispensing, so as to form an epoxy resin (9) bonding layer on the fiber end. Figure 3 (f) as shown.

[0029] Step 7: Use an optical microscope to align the center of the metasurface (1) with the core of the large mode field optical fiber (2), and then solidify the metasurface (1) at room temperature, as shown in FIG. Figure 3 (g) as shown.

[0030] Step 8: Separate the silica substrate (5) from the fiber end of the large mode field optical fiber (2). Since the bonding effect between the optical clock metasurface (1) and the epoxy resin (9) is better than the bonding effect between the metasurface (1) and the silica substrate (5), the metasurface (1) is retained on the end face of the large mode field optical fiber (1). Figure 3 (h) shown.

[0031] According to the above steps, an optical clock metasurface with a large mode field optical fiber as a substrate (2) is prepared.

[0032] Combination Figure 4 The optical signal input of an optical metasurface is realized in the following manner: the optical signal output by a laser light source (10.1) is coupled into the core of a large mode field optical fiber (2) through an optical system and is transmitted in a guided mode (201) with a flat dispersion, wherein the optical field distribution at the fiber end of the large mode field optical fiber (2) is as follows: Figure 4 (a) shown.

[0033] The output light field of the device is observed using Figure 4 The optical system shown is implemented as follows: the system uses two laser light sources (10.1) and (10.2). The optical signal output by the laser light source (10.1) passes through the polarization and power control module (11) and is then transmitted by the reflector (12) to the polarization beam splitter (13) and then collimated and condensed by the lens (14). The objective lens (15) couples the incident linear polarized light (3) with the initial polarization direction (301) into the core of the large mode field optical fiber (2), excites the formation of a dispersion-flat guided mode (201), and then transmits it to the metasurface (1) at the fiber end. The metasurface (1) modulates the guided mode (201) and outputs a focused light beam with a focused spot rotating with the polarization direction of the incident linear polarized light. The sample (17) is moved to the focal plane of the CCD digital camera (18) by controlling the electrically controlled translation stage (16) to observe the output light field of the sample (17), as shown in FIG. Figure 4As shown in (b), the output light field has a focused light spot cluster (302) with two light spots; when the polarization direction of the optical signal of the polarization and power control module (11) is changed, the polarization direction is rotated by a certain angle θ1 to obtain an incident linear polarized light (4) with a polarization direction (401), and the incident linear polarized light (4) is transmitted to the fiber end through the large mode field optical fiber (2) and modulated by the metasurface (1) to output a focused light beam. The corresponding output light field is observed, as shown in FIG. Figure 4 As shown in (c), the output focused light spot cluster (402) is rotated by θ2.

[0034] The focal plane light field distribution of the device generating a single focused light spot and two focused light spots under the excitation of incident linear polarized light in different polarization directions is observed respectively, and the relationship curve between the corresponding focused light spot rotation angle and the polarization direction of the incident linear polarized light is calculated, such as Figure 5 shown.

[0035] When the system uses two laser light sources (10.1) and (10.2), the polarization beam splitter (16) superimposes the optical signals output by the laser light sources (10.1) and (10.2) into incident circularly polarized light, which is then transmitted through the subsequent system and observed. The focal plane light field distribution obtained is as follows: Figure 6 As shown, it is a circular focused light spot, which cannot achieve the optical clock effect.

Claims

1. The present invention provides an optical clock metasurface, characterized in that: The metasurface (1) is composed of a plurality of units (101) and is integrated on an optical substrate (2); when incident linear polarized light (3) with an initial polarization direction (301) is input along a principal optical axis, it is modulated by the metasurface (1) to generate a first group of non-circularly symmetric focused light spot clusters (302) on a focal plane; when the polarization direction (301) is rotated by a certain angle θ1, incident linear polarized light (4) with a new polarization direction (401) is modulated by the metasurface (1) to generate a first group of non-circularly symmetric focused light spot clusters (302) on a focal plane. A second group of non-circularly symmetric focused light spot clusters (402) is generated, wherein the angle θ2 between the two groups of focused light spot clusters (302) and (402) corresponds to the angle θ1 between the polarization directions of the two incident linear polarized lights (3) and (4). By adjusting the polarization directions of the incident linear polarized lights, synchronous rotation of the output light spots is achieved, thereby achieving an optical clock effect similar to the coordinated movement of clock hands. To achieve the optical clock effect, the optical clock metasurface must meet the following conditions:

1. The phase shift of the unit (101) along the x-axis is The phase shift along the y-axis The absolute value of the difference satisfies That is, the unit (101) is equivalent to a half-wave plate. This characteristic ensures that when the incident linear polarized light passes through the unit (101), the phase information can be encoded into the left-handed circularly polarized light (LCP) and the right-handed circularly polarized light (RCP) components of the incident linear polarized light, respectively, so that the LCP and RCP can simultaneously obtain independent phase profiles. and When LCP or RCP is used as incident light of a single polarization state, the metasurface (1) encodes the corresponding phase profile only on the unit (101), thereby realizing independent optical manipulation of the two circular polarization states; secondly, the optical clock metasurface provides a unified phase profile through all units (101), so that LCP and RCP have a consistent basic phase distribution, thereby ensuring that the phases of LCP and RCP coincide precisely in space and focus on the same position on the focal plane; thirdly, by introducing a geometric phase modulation term controlled by the rotation angle of the unit (101), Phase Contour and The phase difference between them is equal to π, so as to achieve coherent interference between LCP and RCP and produce the expected interference effect; 4. Phase profile of the optical clock metasurface and It has approximate circular symmetry, which can effectively ensure that the shape of the non-circularly symmetric focused light spot generated by interference remains relatively stable during rotation, thereby ensuring the accuracy of the optical clock effect. When the above conditions are met, the focused light spot cluster can rotate with the change of the polarization direction of the incident linear polarized light, thereby forming an optical clock effect.

2. The optical clock metasurface according to claim 1, characterized in that: The optical substrate 2 is a transparent or semi-transparent material.

3. The optical clock metasurface according to claim 1, characterized in that: By introducing a signal function into the phase profile for modulation, multiple phase profiles with periodic phase jumps are formed, so as to realize multiple focuses at different positions on the focal plane. The modulation parameters of the phase profile include modulation amplitude, period and phase offset, which are respectively used to control the number, position and spacing of the focuses. The phase profile can be expressed as: Among them, m(x) and m(y) are signal functions along the x and y directions respectively. The amplitude of the signal function determines the spacing between the foci, the period of the signal function controls the number and position of the foci, and the phase offset of the signal function is used to adjust the starting position of the foci.

Images