Metastructure surface device for laser beam splitting and overlapping and magneto-optical trap chip prepared by same

Abstract

The invention provides a metasurface device for laser beam splitting and overlapping and a magneto-optical trap chip prepared by the metasurface device, belongs to the technical field of optics, and designs the metasurface device with a specific phase gradient by designing an arrangement mode of unit structures on a metasurface. According to the device, a rear-end laser beam emitted from the rear end of the device can form three deflected light beams in three areas, the three deflected light beams are deflected and overlapped towards the center of the device, a front-end laser beam opposite to the rear-end laser beam is arranged at the front end of the device, and the rear-end laser beam and the front-end laser beam form a light field center in the overlapped area; a magnetic field zero point of the planar coil coincides with a light field center to form a magneto-optical trap; the transmission-type magneto-optical trap chip is designed by adopting the combination of double light beams and a planar coil, compared with a reflection-type chip, the magneto-optical trap chip provided by the invention can add other laser subsequently, and the contradiction between the integrated construction requirements of single-beam cooling light and a dipole trap array is solved.

Description

Technical Field

[0001] The present invention relates to the field of optical technologies, and particularly to a metasurface device for laser beam splitting and overlapping and a magneto-optical trap chip for preparing the same. Background Art

[0002] A magneto-optical trap (MOT) is one of the most fundamental and core components in a single-atom system quantum computer. The miniaturized design thereof is of extremely important significance for the integration and chipization of the quantum computer of this system. And how to develop an integrated atomic chip of a dipole trap array and a magneto-optical trap is a key problem faced by the research on the integration and chipization of a single-atom system quantum computer. At present, the research on the magneto-optical trap (MOT) still faces challenges in miniaturization and integration. Especially when constructing the integration of a magneto-optical trap and a dipole trap array, the traditional single-beam reflection-type cooling light scheme is difficult to meet the requirement of dipole light transmission, which restricts the further integrated development of the system.

[0003] A planar coil is a miniaturized electromagnetic component, which is made by printing copper wires on a ceramic substrate using printed circuit board (PCB) technology and is designed to generate a magnetic field with a specific configuration to realize a magneto-optical trap. This kind of chip usually includes two coaxial but different-radius toroidal coils, which have different numbers of turns and are passed with currents in opposite directions. The purpose of the planar coil chip is to generate a quadrupole magnetic field several millimeters above the chip surface, and this magnetic field matches the working point of the grating chip, so as to realize the capture and manipulation of cold atoms. The design of this chip allows the working point of the magnetic field, that is, the position where the magnetic field strength is zero, and the strength of the magnetic field gradient to be precisely controlled by adjusting the ratio of the currents in the coils. This highly integrated and adjustable coil design is of great significance for the development of portable and low-power cold atom sensors and quantum devices.

[0004] In 2013, Nshii et al. proposed a scheme of a GMOT (grating magneto-optical trap) (A surface-patterned chip as a strong source of ultracold atoms for quantum technologies), and this scheme constructed a chip-level magneto-optical trap by using a single beam of light passing through a specifically etched diffraction grating. However, in this scheme, the device for generating a quadrupole magnetic field still uses a traditional anti-Helmholtz coil. In order to improve the integration degree, in 2022, Chen L. et al. developed a planar coil chip (Planar Integrated MagnetoOptical Trap), which can generate a quadrupole magnetic field and constructs a magneto-optical trap with a higher integration degree. In the above magneto-optical trap preparation schemes, the magneto-optical trap chips are all single-beam reflection types and cannot be compatible with other lasers, which is contradictory to the requirement of dipole light transmission when constructing the integration with a dipole trap array. SUMMARY OF THE INVENTION

[0005] In traditional magneto-optical trap preparation schemes, magneto-optical trap chips are all single-beam reflection types, which cannot be compatible with other lasers and conflict with the need for dipole light transmission when integrated with a dipole trap array. To overcome the above limitations in the prior art, the present invention proposes a metasurface device for laser beam splitting and overlapping and a magneto-optical trap chip prepared therefrom.

[0006] The present invention is implemented as follows: A metasurface device for laser beam splitting and overlapping, the metasurface device includes a SiO2 substrate as the metasurface substrate and TiO2 nanocolumns arrayed on the SiO2 substrate. Draw three dividing lines outward from the center of the metasurface substrate as the origin. The dividing lines divide the metasurface substrate into three parts. Adjacent two dividing lines form a part. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. Adjacent two dividing lines form an included angle, which are α, β, and γ respectively. The phases of the unit structures in each part decrease in the direction pointed to the origin along the angle bisector of the included angle; the metasurface device can make a laser beam incident from its bottom pass through the metasurface device and then form three deflected beams in the three part regions respectively. The deflection angles of the three deflected beams at the metasurface device interface are the same, and the deflection directions are different. The three deflected beams are deflected towards the center direction of the metasurface device and overlap. The light rays in each deflected beam are parallel to each other.

[0007] Further, the design method of the metasurface device includes the following steps: S1: Change the unit structure size. Through simulation, establish a database of unit structures, which includes the size data of the unit structures and the actual transmittance and actual transmission phase φ corresponding to the unit structure size trans ; S2: Set the deflection angle θ t and the working wavelength λ0 of the magneto-optical trap. Using the formula, calculate the required phase gradient dφ / dX for each region. The calculation formula is: (9); S3: Establish a metasurface substrate, divide the substrate into multiple unit structure arrays, draw three dividing lines outward with the center of the metasurface substrate as the origin. The dividing lines divide the metasurface substrate into three parts. The unit structures of adjacent two dividing lines form a part. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. An included angle is formed between adjacent two dividing lines, which are α, β, and γ respectively. The unit structures with decreasing phase are arranged along the direction pointing to the origin along the angle bisector of the included angle in each of the three parts; establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the central position coordinates of each unit structure in the unit structure array ; Calculate the theoretical phase φ at each position coordinate idea , and select the unit structure with the actual transmission phase φ idea having the smallest difference from the ideal phase φ trans from the database; Arrange the selected unit structures according to the designed phase gradient to form the final phase gradient metasurface device; The arrangement of the unit structures in step S3 specifically includes the following steps: S31: After dividing the substrate into multiple unit arrays, draw three dividing lines outward with the center of the metasurface substrate as the origin. The dividing lines divide the metasurface substrate into three parts. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. An included angle is formed between adjacent two dividing lines, which form a part. Denote the upper left region as region A, the corresponding included angle as α, the upper right region as region B, the corresponding included angle as β, and the lower region as region C, the corresponding included angle as γ; The phase decreases along the direction pointing to the origin along the angle bisector of the α angle in region A, the phase decreases along the direction pointing to the origin along the angle bisector of the β angle in region B, and the phase decreases along the direction pointing to the origin along the angle bisector of the γ angle in region C; S32: Establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the central position coordinates of each unit structure in the unit structure array ; According to the phase gradient dφ / dX calculated in step S2, calculate the theoretical phase φ at each position coordinate idea , and the calculation method is: Integrate formula (9) to obtain the calculation formula for the ideal phase φ idea of the unit structure: (10) where X is the distance between the unit structure in the three regions and the origin along the direction of decreasing phase; In region A, the distance between the unit structure and the origin is denoted as X along the angle bisector of angle α or on a ray parallel to the angle bisector of angle α; in region B, the distance between the unit structure and the origin is denoted as X along the angle bisector of angle β or on a ray parallel to the angle bisector of angle β; in region C, the distance between the unit structure and the origin is denoted as X along the angle bisector of angle γ or on a ray parallel to the angle bisector of angle γ; S34: According to the ideal phase φ calculated in step S33 idea , perform a search in the database established in step S1, and select the actual transmission phase φ idea with the smallest difference from the ideal phase φ trans of the unit structure, place the corresponding unit structure at the corresponding position coordinates , and repeat step S3 to form a metasurface device with a phase gradient.

[0008] A magneto-optical trap chip includes the above-mentioned metasurface device for splitting and overlapping laser beams. A rear-end laser beam is provided behind the metasurface device. After passing through the metasurface device, the rear-end laser beam forms three deflected light beams, and all three light beams are deflected towards the center direction of the metasurface device and overlap; a front-end laser beam opposite to the rear-end laser beam is provided at the front end of the metasurface device, and the rear-end laser beam and the front-end laser beam form a light field center in the overlapping area; a planar coil is arranged around the metasurface device so that the magnetic field zero point of the planar coil coincides with the light field center.

[0009] Furthermore, the design method of the magneto-optical trap chip includes the following steps: S1: Change the unit structure size, and through simulation, establish a database of unit structures, which includes the size data of the unit structures and the actual transmittance and actual transmission phase φ corresponding to the unit structure sizes trans ; S2: Set the deflection angle θ t and the working wavelength λ0 of the magneto-optical trap, and use the formula to calculate the required phase gradient dφ / dX for each region. The calculation formula is: (9); S3: Establish a metasurface substrate, divide the substrate into multiple unit structure arrays, draw three dividing lines outward with the center of the metasurface substrate as the origin. The dividing lines divide the metasurface substrate into three parts. The unit structures of adjacent two dividing lines form a part. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. An angle is formed between adjacent two dividing lines, which are α, β, and γ respectively. The unit structures with decreasing phase are arranged along the direction pointing to the origin along the angle bisector of the angle for each of the three parts; establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the central position coordinates of each unit structure in the unit structure array ; Calculate the theoretical phase φ at each position coordinate idea , and select the unit structure with the actual transmission phase φ idea having the smallest difference from the ideal phase φ trans from the database; Arrange the selected unit structures according to the designed phase gradient to form the final phase gradient metasurface device; The arrangement of the unit structures in step S3 specifically includes the following steps: S31: After dividing the substrate into multiple unit arrays, draw three dividing lines outward with the center of the metasurface substrate as the origin. The dividing lines divide the metasurface substrate into three parts. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. An angle is formed between adjacent two dividing lines, and a part is formed by adjacent two dividing lines. Denote the upper left region as region A, with the corresponding angle as α, the upper right region as region B, with the corresponding angle as β, and the lower region as region C, with the corresponding angle as γ; Region A sets the phase to decrease along the direction pointing to the origin along the angle bisector of angle α, region B sets the phase to decrease along the direction pointing to the origin along the angle bisector of angle β, and region C sets the phase to decrease along the direction pointing to the origin along the angle bisector of angle γ; S32: Establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the central position coordinates of each unit structure in the unit structure array ; S33: According to the phase gradient dφ / dx calculated in step S2, calculate the theoretical phase φ at each position coordinate idea , and the calculation method is: Integrate formula (9) to obtain the calculation formula for the ideal phase φ idea of the unit structure: (10) where X is the distance between the unit structure in the three regions and the origin along the direction of decreasing phase; S34: Retrieve in the database established in step S1 according to the ideal phase φ calculated in step S33, and select the actual transmission phase φ with the smallest difference from the ideal phase φ idea , and place the corresponding unit structure at the corresponding position coordinates idea of the unit structure, and repeat step S3 to form a metasurface device with a phase gradient. trans and repeat step S3 to form a metasurface device with a phase gradient.

[0010] S4: Simulate the metasurface device with a phase gradient obtained in step S3 in CST to obtain the electric field distribution data after the laser beam propagates through the metasurface device. The part with the highest electric field intensity is the center of the optical field, and obtain the position coordinates (x c , y c , z c ) of the center of the optical field; S5: Input the radius, current, and number of turns of the inner coil and outer coil in the planar coil into Maxwell software, perform electromagnetic simulation on the coil using Maxwell software, and mark the point where the magnetic field component is zero as the working point (x0, y0, z0) of the planar coil; adjust the parameters of the inner coil and outer coil by observing the position of the working point to make the working point of the planar coil coincide with the center of the optical field in step S4, and determine the radius, current, and number of turns of the inner coil and outer coil in the planar coil.

[0011] Compared with the prior art, the present invention designs the arrangement of unit structures on the metasurface to design a metasurface device with a specific phase gradient, which can deflect a beam of rear-end laser beam incident from its bottom through the metasurface device into three deflected light beams in three regions respectively, and the three deflected light beams deflect towards the center of the metasurface device and overlap; The magneto-optical trap chip designed by the present invention introduces two counter-propagating laser beams. Among them, the rear-end laser beam arranged behind the metasurface device deflects and overlaps after passing through the precisely designed metasurface chip, and coincides with the front-end laser beam on the other side of the metasurface device, thereby forming an enhanced cooling effect in a specific region. This system not only significantly reduces the device volume of the cold atom experiment, but also improves the efficiency and stability of cold atom capture and trapping by optimizing the spatial matching degree of the optical field and the magnetic field; by adopting the combination of a double-beam and a planar coil, a transmissive magneto-optical trap chip is designed. Compared with the reflective chip, the magneto-optical trap chip provided by the present invention can add other lasers in the follow-up, solving the contradiction between the single-beam cooling light and the integrated construction requirement of the dipole trap array. Description of the Drawings

[0012] Figure 1 is a traditional anti-Helmholtz coil with a three-dimensional cylindrical geometry.​

[0013] Figure 2 is a planar coil: parameters R , N , I are the radius, number of turns, and current of each toroidal coil respectively. The magnetic field lines are represented by black curves.

[0014] Figure 3 is the design diagram of the magneto - optical trap chip.

[0015] Figure 4 is the schematic diagram of the generalized Snell's law.

[0016] Figure 5 is the schematic diagram of the metasurface device structure.

[0017] Figure 6 is the schematic diagram of the optical path of one of the deflected light beams after a laser beam source is transmitted through the metasurface device.

[0018] Figure 7 is the schematic diagram of the optical paths of two deflected light beams after a laser beam source is transmitted through the metasurface device.

[0019] Figure 8 is the schematic diagram of the optical paths of three deflected light beams after a laser beam source is transmitted through the metasurface device.

[0020] Figure 9 is the transmittance distribution of the metasurface device.

[0021] Figure 10 is the phase distribution of the metasurface device.

[0022] Figure 11 is the Y - plane electric field diagram of CST simulation.

[0023] Figure 12 is the Z - plane electric field diagram of CST simulation.

[0024] Figure 13 is the MATLAB simulation of the beam superposition in three regions.

[0025] Figure 14 is the electric field diagram of the deflected light in the Z - plane in three regions. Specific implementation manners

[0026] The present invention will be further described below in conjunction with specific embodiments, and the advantages and features of the present invention will become clearer as the description proceeds.

[0027] In the traditional preparation scheme of the magneto-optical trap, there is a contradiction between the requirements of the single-beam reflection cooling light of the magneto-optical trap and the need for the transmission of the dipole light when constructing the integration of the dipole trap array. To overcome these limitations, the present invention proposes a novel dual-beam-based magneto-optical trap design, which uses a planar coil chip to generate a quadrupole magnetic field and combines a metasurface device for design, and realizes the cooling and trapping of atoms through dual beams, providing a feasible solution for the realization of an integrated atom chip of the magneto-optical trap and the dipole trap array.

[0028] A metasurface is an ultrathin and inhomogeneous planar medium, usually composed of sub-wavelength planar nanostructures, which can precisely control the amplitude, phase, polarization, etc. of light. Compared with traditional concave and convex lenses, metasurface devices have the following characteristics: very flat, extremely thin in thickness, and convenient for integration.

[0029] The present invention provides a metasurface device for laser beam overlap, such as Figure 3 shown, the metasurface device includes a SiO2 substrate as the metasurface substrate and TiO2 nanocolumns arrayed on the SiO2 substrate. Taking the center of the metasurface substrate as the origin, draw three dividing lines outward. The dividing lines divide the metasurface substrate into three parts. Adjacent two dividing lines form a part. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. Adjacent two dividing lines form an included angle, which are α, β, and γ respectively. The phases of the unit structures in each part decrease in the direction pointed to the origin along the angle bisector of the included angle; the metasurface device can make a laser beam incident from its bottom pass through the metasurface device and form three deflected beams in three regions respectively. The deflection angles of the three deflected beams at the interface of the metasurface device are the same, and the deflection directions are different. The three deflected beams are deflected towards the center direction of the metasurface device and overlap, as Figures 6 - 8 shown. After the laser beam source is transmitted through the metasurface device, the plane light wave is deflected in three regions of the metasurface device to become three deflected beams. The three deflected beams are deflected towards the center of the metasurface device and overlap; the light rays in each deflected beam are parallel to each other. The function of the metasurface device is to deflect a plane wave laser beam by 20° in three directions in three regions respectively.

[0030] The design method of the metasurface device includes the following steps: S1: When designing the metasurface device, change the unit structure size, and through simulation, establish a database of unit structures, which includes the size data of the unit structures and the actual transmittance and actual transmission phase φ corresponding to the unit structure size trans ; S2: Set the deflection angle θ tUsing the working wavelength λ0 of the magneto-optical trap and the formula, calculate the phase gradient dφ / dX required for each region. The calculation formula is: (9); S3: Establish a metasurface substrate (such as a SiO2 thin film), divide the substrate into a plurality of unit structure arrays, draw three dividing lines outward from the center of the metasurface substrate as the origin, and the dividing lines divide the metasurface substrate into three parts. Adjacent two dividing lines form a part. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. Adjacent two dividing lines form an included angle, which are α, β, and γ respectively. The unit structures with decreasing phase are arranged along the angular bisector of the included angle in the direction pointing to the origin in the three parts. Establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the center position coordinates of each unit structure in the unit structure array. ; Calculate the theoretical phase φ at each position coordinate in turn. At, and select the unit structure with the smallest difference between the actual transmission phase φ idea And the ideal phase φ idea From the database. trans ; Arrange the selected unit structures according to the designed phase gradient to form the final phase gradient metasurface device. The arrangement of the unit structures in step S3 specifically includes the following steps: S31: After dividing the substrate into a plurality of unit arrays, draw three dividing lines outward from the center of the metasurface substrate as the origin, as Figure 5 Shown, the dividing lines are red lines. The dividing lines divide the metasurface substrate into three parts. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. Adjacent two dividing lines form a part. Adjacent two dividing lines form an included angle. Denote the upper left region as region A, the corresponding included angle as α, the upper right region as region B, the corresponding included angle as β, and the lower region as region C, the corresponding included angle as γ; Region A sets the phase to decrease along the angular bisector of the α angle in the direction pointing to the origin, region B sets the phase to decrease along the angular bisector of the β angle in the direction pointing to the origin, and region C sets the phase to decrease along the angular bisector of the γ angle in the direction pointing to the origin; that is, the structural units of each region are arranged with the actual transmission phase decreasing along the direction of the white arrow. S32: Establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the center position coordinates of each unit structure in the unit structure array. ; S33: According to the phase gradient dφ / dX calculated in step S2, calculate the theoretical phase φ at each position coordinate in turn. Atidea , the calculation method is as follows: Integrate formula (9) to obtain the ideal phase φ of the unit structure idea Calculation formula of (10) Where X is the distance between the unit structure in the three regions and the origin along the direction of decreasing phase; In region A, along the angle bisector of angle α or on the ray parallel to the angle bisector of angle α, the distance between the unit structure and the origin is denoted as X; in region B, along the angle bisector of angle β or on the ray parallel to the angle bisector of angle β, the distance between the unit structure and the origin is denoted as X; in region C, along the angle bisector of angle γ or on the ray parallel to the angle bisector of angle γ, the distance between the unit structure and the origin is denoted as X; S34: According to the ideal phase φ calculated in step S33 idea , retrieve in the database established in step S1, and select the actual transmission phase φ idea with the smallest difference from the ideal phase φ trans of the unit structure, place the corresponding unit structure at the corresponding position coordinates and repeat step S3 to form a metasurface device with a phase gradient.

[0031] The arrangement of the unit structure in the above step S3 can also adopt the following method: According to the size data of the unit structure in step S1 and the phase gradient dφ / dX calculated in step S2, determine the phase difference dφ that needs to be achieved between adjacent unit structures along the direction pointing to the origin in the three regions, select appropriate unit structures from the database, and arrange the selected unit structures in a spatial order according to the actual transmission phase φ trans of the unit structure and the phase difference dφ between adjacent unit structures in the direction of decreasing phase to form the final phase gradient metasurface device; The specific steps are as follows: 1): After dividing the substrate into multiple unit arrays, draw three dividing lines outward from the center of the metasurface substrate as the origin, such as Figure 5As shown, the demarcation line is a red line. The demarcation line divides the metasurface substrate into three parts. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different phase gradient change directions. Two adjacent demarcation lines form a part, and two adjacent demarcation lines form an included angle. Denote the upper left region as region A, with the corresponding included angle as α, the upper right region as region B, with the corresponding included angle as β, and the lower region as region C, with the corresponding included angle as γ. In region A, the phase is set to decrease along the direction pointing to the origin along the angle bisector of angle α. In region B, the phase is set to decrease along the direction pointing to the origin along the angle bisector of angle β. In region C, the phase is set to decrease along the direction pointing to the origin along the angle bisector of angle γ. That is, the structural units in each region are arranged with the actual transmitted phase decreasing along the direction of the white arrow.

[0032] 2): According to the size data of the unit structure in step S1, obtain the distance dX between the centers of adjacent unit structures in the phase decreasing direction in regions A, B, and C. According to the phase gradient dφ / dX calculated in step S2, determine the phase difference dφ required between adjacent unit structures in regions A, B, and C along the phase decreasing direction. Among them, adjacent unit structures refer to two adjacent unit structures in the phase decreasing direction. Since the intervals between adjacent structural units along the phase decreasing direction in the three regions are different, among them, the intervals between adjacent structural units along the phase decreasing direction in regions A and B are the same, and the adjacent structural units are two adjacent structural units along the diagonal. In region C, the adjacent unit cells are adjacent structural units along the direction perpendicular to the center of the substrate. Therefore, the phase differences between adjacent structural units in regions A and B are the same, while the phase difference from that in region C is different.

[0033] 3): Select multiple suitable unit structures from the database according to the phase difference dφ. In the direction of phase decrease in each region, according to the actual transmitted phase φ of the unit structure trans and the phase difference dφ between adjacent unit structures, arrange the selected unit structures in space to form a final metasurface device with a specific phase gradient.

[0034] A magneto-optical trap chip includes the above-mentioned metasurface device for splitting and overlapping laser beams. A rear-end laser beam is provided behind the metasurface device. After passing through the metasurface device, the rear-end laser beam forms three deflected light beams, and all three light beams are deflected towards the center direction of the metasurface device and overlap. A front-end laser beam that shoots opposite to the rear-end laser beam is provided in front of the metasurface device. The rear-end laser beam and the front-end laser beam form a light field center in the overlapping region. A planar coil is arranged around the metasurface device so that the magnetic field zero point (i.e., the working point) of the planar coil coincides with the center of the light field.

[0035] The rear-end laser beam and the front-end laser beam are cooling light beams.

[0036] The design method of the magneto-optical trap chip includes the design method of the metasurface device and the planar coil design method. The design method of the metasurface device is as described in the above steps S1 to S3; Among them, the planar coil design method includes the following steps: S4: Simulate the phase gradient metasurface device obtained in step S3 in CST to obtain the electric field distribution data after the laser beam propagates through the metasurface device. The part with the highest electric field intensity is the center of the optical field, and obtain the position coordinates (x c , y c , z c ) of the optical field center; S5: Input the radius, current, and number of turns parameters of the inner coil and the outer coil in the planar coil into Maxwell software, use Maxwell software to perform electromagnetic simulation on the coil, and mark the point where the magnetic field component is zero as the working point (x0, y0, z0) of the planar coil; adjust the parameters of the inner coil and the outer coil by observing the position where the working point is located, so that the working point of the planar coil coincides with the optical field center in step S4, and determine the radius, current, and number of turns of the inner coil and the outer coil in the planar coil.

[0037] The present invention provides a magneto-optical trap chip based on a metasurface. The overall structure of the chip is as Figure 3 shown. A metasurface structure is designed at the center on the glass substrate in the vacuum chamber, and two layers of coils are designed on the periphery. A beam of incident light is incident from the glass substrate, and a beam of incident light is added on the other side. A MOT is generated in the vacuum chamber to capture cold atom clusters. The chip design realizes the miniaturization and integration of the metasurface and the planar coil, and only requires a two-beam to generate a MOT.

[0038] Traditional magneto-optical traps use a pair of anti-Helmholtz coils to generate such a magnetic field. It consists of a pair of identical coils (identical radius R , current I and number of turns N ), with opposite current directions, as Figure 1 shown. The working point of this coil configuration is exactly its geometric center. This cylindrical geometry poses limitations on compact cold atom systems. For example, the vacuum chamber needs to be inserted between the pair of coils, thus limiting the minimum size of the coils. To achieve the required field gradient of about 10 G / cm, the current in the coils increases with the cube of the coil size, which can lead to heat dissipation problems. These limitations can be overcome by planar coil design to achieve a compact system.

[0039] During the design process of the planar coil chip, a coplanar toroidal coil chip such as Figure 2 is used, which consists of two different radii R 1 andR 2 ( R 1 < R 2), different number of turns N 1 and N 2, and opposite currents I 1 and -I 2 ( I 1,2 > 0) of coaxial coils. Define the center of the coil as the origin of the cylindrical coordinate system, and the z-axis is perpendicular to the coil plane. At the point (ρ = 0, , z = z0) on the z-axis, the coil generates a magnetic field strength , where: (1) (2) and the corresponding magnetic field gradient: (3) (4) Due to cylindrical symmetry, on the z-axis , for simplicity, first consider the case of balanced currents I 1 = I 2 = I , which allows the two coils to be connected in series to a current supply. For appropriate R 1 and R 2, N 1 and N 2 can be adjusted such that at the desired height h, i.e., the target MOT operating point (0, 0, h), the field strength is zero. Therefore, this coplanar coil configuration can provide the quadrupole magnetic field required to achieve MOT, and its operating point is located above the coil plane.

[0040] Use Maxwell software to perform electromagnetic simulation on the coil. After the simulation is completed, post-processing is performed in Maxwell, and the field calculator is set to extract the z-axis magnetic field component and the x-axis magnetic field component. The point where the magnetic field component is 0 is denoted as the operating point, and the sizes of the inner and outer coils are adjusted by observing the position of the operating point of the z-axis magnetic field component. As the radii of the inner and outer coils decrease, the operating point also decreases. Set the operating point height of the coil according to the ability to match the overlapping part of the operating point of the coil chip with the light beam of the above-mentioned metasurface.

[0041] As Figure 4 shown, with the incident light unchanged, changing the phase gradient dΦ / dX of the interface, the refraction angle θ t changes accordingly, which is the generalized Snell's law, and the formula is as follows: (5) Since the other side of the designed device is air, n t is 1, the light beam is incident perpendicularly from the metasurface substrate, and the incident angle θ i is 0, so the formula is simplified to: (6) Equation (6) shows that if an appropriate phase gradient is introduced at the interface, refracted light beams in any direction can be obtained.

[0042] The discontinuity of the interface phase is ingeniously achieved through a carefully designed optical metasurface structure. As an innovative artificial structure, the permittivity and permeability of this metasurface can be customized according to specific requirements, showing extremely high flexibility. The present invention focuses on the application of a dielectric metasurface. In this design, microstructural units of high refractive index dielectric materials are ingeniously embedded on the substrate of low refractive index dielectric. Each such unit structure acts as a low-quality factor resonator. When light waves pass through these carefully arranged microstructures, the resulting phase accumulation effect can be approximately expressed as a specific function, thereby achieving precise control of the phase of the light wavefront. The phase accumulation generated can be approximately expressed as: (7) In the formula: n eff represents the effective refractive index of the microstructure, h represents the height of the microstructure, and λ0 represents the working wavelength.

[0043] Method 1: Setting the number of nanocolumns In Equation (6), dφ / dX is the phase gradient, dφ = 2π / n, where n is the number of structural units required to cover at least 2π phase, and dX is the lattice period P.

[0044] Therefore, Equation (6) can be simplified to (8) The gradient metasurface structure consists of n unit structures with a cell period of P, and their phase delays are arranged in a gradient. Taking the deflection of a left-handed circularly polarized light with a wavelength of 780 nm by 20° by this structure as an example, 9 structural units are selected to achieve beam deflection, and the phase interval is 360° / 9 = 40°.

[0045] Method 2: Setting the phase gradient Equation (6) is transformed into (9) Set the refraction angle θ of the metasurface tis 20 degrees, the working wavelength λ0 of the magneto-optical trap is 780 nm. According to different phase gradient arrangement methods, the phase difference between adjacent unit structures can be obtained. Integrating formula (9), the ideal phase φ required at any coordinate position on the metasurface device can be calculated. idea , the ideal phase φ idea The calculation formula is: (10) where X represents the distance between the unit structure and the origin along the direction of phase gradient arrangement.

[0046] In the above method 1, when setting the same refraction angle, the intensity of the overlapping part of the light beam is less than that of method 2, and its superiority is lower than that of method 2 when designing the MOT metasurface device.

[0047] When designing the metasurface device, the transmittance and actual transmission phase φ of each unit structure can be obtained by simulating unit structures of various different sizes trans , and a unit structure database containing this data is created. This database records the size information of the unit structure and the corresponding transmittance and phase data. Using this information, the metasurface device can be constructed. First, a substrate (such as a SiO2 thin sheet) is established, and then the substrate is divided into multiple units. According to the application of the device, the corresponding parameters, deflection angle θ t , working wavelength λ0 are designed. Using these parameters and the position coordinates of the unit, the ideal phase φ required at this position can be calculated idea . According to this ideal phase value, the unit structure with the actual transmission phase φ trans with the smallest difference from it is selected in the database, and the selected nanorods are placed at the corresponding coordinate positions, thus completing the design of the metasurface device.

[0048] The metasurface device includes an array of unit structures. The unit structure includes a dielectric substrate and dielectric nanorods, and the dielectric nanorods are placed on the dielectric substrate.

[0049] In one example, the material of the dielectric substrate is SiO2, the relative dielectric constant of SiO2 is 2.0736, the dielectric nanorods are cylindrical, the height of the cylinder is H, where the range of H is 600 nm < H < 650 nm, the radius of the cylinder is R, where the range of R is 50 nm < R < 200 nm, the height H and radius R of the cylinder are uncertain values. By changing the values of the height H and radius R, the nanorods can achieve different phase adjustments. The material of the dielectric nanorods is TiO2, the relative dielectric constant of TiO2 is 6.4516. Both of these two materials, silica and titanium dioxide, have the advantages of low loss and high refractive index.

[0050] The simulation software used for metasurface simulation is CST (Computer Simulation Technology). In the simulation process, the finite element method and the finite integration method are mainly used for electromagnetic simulation.

[0051] To verify the method described in this embodiment, a metasurface is designed. The metasurface consists of 20*20 structural units. As Figure 5 shown, the metasurface is divided into three regions. The red lines are the boundaries of the regions. Phase gradients are set in the three regions respectively. The phase gradient decreases along the direction of the white arrow, that is, the phase decreases along the angular bisector of the angle pointing to the origin. The metasurface device designed by the present invention has successfully achieved forming a beam of incident light into three beams deflected towards the center of the metasurface. The transmission rate distribution and phase distribution of the designed metasurface are as Figure 9 、 Figure 10 shown.

[0052] In CST, the boundary conditions in the x, y, and z directions are all set as open boundaries. The electric field diagrams of the metasurface in the Y and Z planes in the CST simulation are as Figure 11 、 Figure 12 shown. In the electric field diagram of the Y plane, it can be observed that the metasurface has achieved forming a beam of plane waves into three beams and generating overlap after passing through the metasurface. The electric field intensity in the overlapping part of the beams is significantly higher than that of the surrounding area.

[0053] To further verify the beam overlapping function of the metasurface device designed by the present invention, three left-handed circularly polarized light beams are set to deflect 20 degrees towards the center in MATLAB. It can be observed that the electric field diagram of the Z plane in the MATLAB simulation ( Figure 13 ) is consistent with the electric field diagram of the Z plane in the CST simulation ( Figure 12 ). Therefore, the beam superposition function of this metasurface is further verified.

[0054] The metasurface device is simulated in CST to obtain the electric field distribution data after the laser beam propagates through the metasurface device. The part with the highest electric field intensity is the center of the optical field, and the position coordinates (x c , y c , z c ) of the optical field center are obtained.

[0055] Input the radius, current, and number of turns of the inner and outer coils in the planar coil into the Maxwell software. Use the Maxwell software to perform electromagnetic simulation on the planar coil. After the simulation is completed, perform post-processing in Maxwell. Set the field calculator to extract the z-axis magnetic field component and the x-axis magnetic field component. Mark the point where the magnetic field component is zero as the working point (x0, y0, z0); adjust the parameters of the inner and outer coils (such as the radius of the coil) by observing the position of the working point of the z-axis magnetic field component, so that the working point (x0, y0, z0) of the coil chip coincides with the beam overlapping position (x c , y c , z c ) of the metasurface device, to set the working point height h of the coil (h is equal to z0, z c ), and determine the parameters of the inner and outer coils. For example, Figure 14 is the electric field diagram of the three-region deflected light in the Z plane, which is used to calculate the working point. According to Figure 14 , it can be directly seen that the position of the working point is the point with the highest light intensity in the red area. When z is 3um, then h is 3um, and the radii of the inner and outer coils of the corresponding coplanar coil are 5.5um and 11um respectively.

[0056] The magneto-optical trap chip design proposed by the present invention realizes multiple technological breakthroughs and advantages by combining the double-beam scheme with the planar coil. The present invention carefully designs a metasurface structure, which has the ability to accurately convert the incident plane wave into three beams of deflected light towards the center, and these beams overlap at a position 3um above the metasurface. At the same time, the present invention abandons the traditional anti-Helmholtz coil configuration and adopts an innovative coplanar ring coil design. The coil is precisely tuned to the working point z = 3um, which perfectly coincides with the light field focus generated by the metasurface, thus constructing a compact and efficient magneto-optical trap environment. Through this integrated design, the metasurface chip and the coil chip are successfully combined to form a highly integrated magneto-optical trap system.

[0057] The magneto-optical trap chip provided by the present invention is a transmissive chip. Compared with the reflective chip, the magneto-optical trap chip provided by the present invention is more flexible during experimental debugging and can be compatible with other lasers added later, such as even light, Rydberg excitation light, Raman light and other operation lasers. This design cleverly solves the contradiction between the single-beam cooling light and the integrated construction requirement of the dipole trap array. By finely adjusting the position of the nanocolumns on the metasurface, precise control of the theoretical phase of the unit structure is achieved, thereby reducing the difference between the theoretical phase and the actual phase, reducing the aberration and improving the quality of the magneto-optical trap array. A metasurface device that can simultaneously replace the functions of the SLM and the strong focusing system is designed. At the same time, by utilizing the interaction between the two beams inside the chip, the overall performance and application potential of the system are significantly improved.

[0058] By introducing two cooling light beams, one of the cooling light beams is deflected after passing through a precisely designed metasurface chip and coincides with the other cooling light beam on the other side of the chip, thereby forming an enhanced cooling effect in a specific area. This system not only significantly reduces the device volume of cold atom experiments, but also improves the efficiency and stability of cold atom trapping and confinement by optimizing the spatial matching degree of the optical field and the magnetic field. In addition, this design adopts a transmissive structure, allowing other types of lasers to be added subsequently. Through the innovative dual-beam design and metasurface technology, the integration of the magneto-optical trap and the dipole trap array is realized, solving the contradiction between the integration of traditional single-beam cooling light and the dipole trap array, and opening up a new way for the exploration of miniaturization and integration in the fields of quantum information science, precision measurement, and basic physics research.

Claims

1. A metasurface device for laser beam splitting and overlapping, characterized in that, The metasurface device includes a SiO2 substrate serving as the metasurface substrate and TiO2 nanocolumns arrayed on the SiO2 substrate. Taking the center of the metasurface substrate as the origin, three dividing lines are drawn outward. The dividing lines divide the metasurface substrate into three parts. Adjacent two dividing lines form a part. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. Adjacent two dividing lines form an included angle, which are α, β, and γ respectively. The phases of the unit structures in each part decrease in the direction pointed by the angular bisector of the included angle towards the origin. The metasurface device can make a laser beam incident from its bottom pass through the metasurface device and then form three deflected light beams in the three part regions respectively. The light rays in each deflected light beam are parallel to each other. The deflection angles of the three deflected light beams at the metasurface device interface are the same, but the deflection directions are different. The three deflected light beams are all deflected towards the center direction of the metasurface device and overlap with each other.

2. The metasurface device for laser beam splitting and overlapping according to claim 1, characterized in that, The design method of the metasurface device includes the following steps: S1: Change the unit structure size. Through simulation, establish a database of unit structures, where the database includes the size data of the unit structures and the actual transmittance and actual transmission phase φ corresponding to the unit structure sizes trans ; S2: Set the deflection angle θ t and the working wavelength λ0 of the magneto-optical trap, and use the formula to calculate the required phase gradient dφ / dX for each region. The calculation formula is: （9）； S3: Establish a metasurface substrate, divide the substrate into multiple unit structure arrays, draw three dividing lines outward with the center of the metasurface substrate as the origin. The dividing lines divide the metasurface substrate into three parts. Adjacent two dividing lines form a part. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. Adjacent two dividing lines form an included angle, which are α, β, and γ respectively. The unit structures with decreasing phase are arranged along the direction of the angle bisector of the included angle pointing to the origin in the three parts respectively; establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the center position coordinates of each unit structure in the unit structure array ; Calculate the theoretical phase φ at each position coordinate in sequence and select the unit structure with the actual transmission phase φ idea which has the smallest difference from the ideal phase φ idea from the database; trans ​ Arrange the selected unit structures according to the designed phase gradient to form the final phase gradient metasurface device. The specific arrangement of the unit structures in step S3 includes the following steps: S31: After dividing the substrate into multiple unit arrays, taking the center of the metasurface substrate as the origin, draw three dividing lines outward. The dividing lines divide the metasurface substrate into three parts. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. Adjacent two dividing lines form a part. Adjacent two dividing lines form an included angle. Denote the upper left region as region A, with the corresponding included angle α, the upper right region as region B, with the corresponding included angle β, and the lower region as region C, with the corresponding included angle γ. The phase in region A is set to decrease along the direction pointed by the angular bisector of angle α towards the origin. The phase in region B is set to decrease along the direction pointed by the angular bisector of angle β towards the origin. The phase in region C is set to decrease along the direction pointed by the angular bisector of angle γ towards the origin. S32: Establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the central position coordinates of each unit structure in the unit structure array ; S33: According to the phase gradient dφ / dX calculated in step S2, calculate the theoretical phase φ at each position coordinate in turn The calculation method is as follows: idea ​ Integrate formula (9) to obtain the calculation formula for the ideal phase φ of the unit structure idea as follows: (10) Where X is the distance between the unit structure in the three regions and the origin along the direction of phase decrease. In region A, on the angular bisector of angle α or a ray parallel to the angular bisector of angle α, the distance between the unit structure and the origin is denoted as X. In region B, on the angular bisector of angle β or a ray parallel to the angular bisector of angle β, the distance between the unit structure and the origin is denoted as X. In region C, on the angular bisector of angle γ or a ray parallel to the angular bisector of angle γ, the distance between the unit structure and the origin is denoted as X. S34: Retrieve in the database established in step S1 according to the ideal phase φ calculated in step S33, and select the unit structure with the smallest difference from the ideal phase φ idea , and place the corresponding unit structure at the corresponding position coordinates idea of the actual transmission phase φ with the smallest difference from the ideal phase φ trans , and repeat step S3 to form a metasurface device with a phase gradient. ​ 3. A magneto-optical trap chip, comprising the metasurface device for splitting and overlapping laser beams according to claim 1 or 2, characterized in that, There is a rear-end laser beam behind the metasurface device. After passing through the metasurface device, the rear-end laser beam forms three deflected light beams. The three beams of light are all deflected towards the center direction of the metasurface device and overlap with each other. There is a front-end laser beam opposite to the rear-end laser beam in front of the metasurface device. The rear-end laser beam and the front-end laser beam form a light field center in the overlapping region. A planar coil is arranged around the metasurface device to make the magnetic field zero point of the planar coil coincide with the light field center.

4. The magneto-optical trap chip according to claim 3, characterized in that The design method of the magneto-optical trap chip includes the following steps: S1: Change the unit structure size. Through simulation, establish a database of unit structures. The database includes the size data of the unit structures and the actual transmittance and actual transmission phase φ corresponding to the unit structure sizes trans ; S2: Set the deflection angle θ t and the working wavelength λ0 of the magneto-optical trap, and use the formula to calculate the required phase gradient dφ / dX for each region. The calculation formula is: （9）； S3: Establish a metasurface substrate, divide the substrate into a plurality of unit structure arrays, draw three dividing lines outward from the center of the metasurface substrate as the origin. The dividing lines divide the metasurface substrate into three parts. Adjacent two dividing lines form a part. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. Adjacent two dividing lines form an included angle, which are α, β, γ respectively. The unit structures with decreasing phase are arranged in the directions pointing to the origin along the angle bisectors of the included angles for the three parts respectively; establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the central position coordinates of each unit structure in the unit structure array ; Calculate the theoretical phase φ at each position coordinate in sequence and select the unit structure with the actual transmission phase φ idea which has the smallest difference from the ideal phase φ idea from the database; trans ​ Arrange the selected unit structures according to the designed phase gradient to form the final phase gradient metasurface device; The arrangement of the unit structures in step S3 specifically includes the following steps: S31: After dividing the substrate into multiple unit arrays, draw three dividing lines outward with the center of the metasurface substrate as the origin. The dividing lines divide the metasurface substrate into three parts. The unit structures of the three parts are arranged according to the same phase gradient magnitude and different gradient change directions. Adjacent two dividing lines form a part, and adjacent two dividing lines form an included angle. Denote the upper left region as region A, the corresponding included angle as α, the upper right region as region B, the corresponding included angle as β, and the lower region as region C, the corresponding included angle as γ; In region A, the phase is set to decrease along the direction of the angular bisector of angle α pointing to the origin, in region B, the phase is set to decrease along the direction of the angular bisector of angle β pointing to the origin, and in region C, the phase is set to decrease along the direction of the angular bisector of angle γ pointing to the origin; S32: Establish a rectangular coordinate system with the center of the metasurface substrate as the origin, and determine the central position coordinates of each unit structure in the unit structure array ; S33: According to the phase gradient dφ / dx calculated in step S2, calculate the theoretical phase φ at each position coordinate in turn The calculation method is as follows: idea ​ Integrate formula (9) to obtain the calculation formula for the ideal phase φ of the unit structure idea as follows: (10) where X is the distance between the unit structure in the three regions and the origin in the direction of phase decrease; S34: Retrieve in the database established in step S1 according to the ideal phase φ calculated in step S33, and select the unit structure with the smallest difference from the ideal phase φ idea , and place the corresponding unit structure at the corresponding position coordinates idea of the actual transmission phase φ with the smallest difference from the ideal phase φ trans , and repeat step S3 to form a metasurface device with a phase gradient; ​ S4: Simulate the phase-gradient metasurface device obtained in step S3 in CST to obtain the electric field distribution data after the laser beam propagates through the metasurface device. The part with the highest electric field intensity is the optical field center, and obtain the position coordinates (x c , y c , z c ); S5: Input the radius, current, and number of turns parameters of the inner coil and the outer coil in the planar coil into Maxwell software, use Maxwell software to perform electromagnetic simulation on the coil, and denote the point where the magnetic field component is zero as the working point (x0, y0, z0) of the planar coil; Adjust the parameters of the inner coil and the outer coil by observing the position of the working point, so that the working point of the planar coil coincides with the optical field center in step S4, and determine the radius, current, and number of turns of the inner coil and the outer coil in the planar coil.

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