Two-dimensional plane grating and three-dimensional magneto-optical trap system

Abstract

The invention discloses a two-dimensional plane grating which is provided with a two-dimensional array formed by periodically and repeatedly arranging a plurality of cylindrical structures in space, and each cylindrical structure sequentially comprises a silicon substrate layer, a grating structure, a bonding layer, a metal film layer and a metal oxide layer from bottom to top. The invention further discloses a three-dimensional magneto-optical trap system comprising the two-dimensional plane grating, the three-dimensional magneto-optical trap system comprises a single beam of incident light projected to the two-dimensional plane grating in the normal direction, the grating period of the two-dimensional plane grating is smaller than twice the central wavelength of the incident light, and the central wavelength of the incident light is 2.5 times the depth of a grating groove; the system can solve the problems that an existing magneto-optical trap system adopts six beams of laser to perform two-to-two correlation, fluctuation mutual interference exists among the six beams of laser, the system structure is complex, miniaturization is difficult and the like, and the system can be applied to quantum precision measurement systems such as small-sized atom interference gyroscopes and gravimeters and future integrated atom chip systems.

Description

Technical Field

[0001] The present invention belongs to the technical field of magneto-optical traps, and in particular relates to a two-dimensional plane grating for single beam light incidence and a three-dimensional magneto-optical trap system comprising the two-dimensional plane grating. Background Art

[0002] Three-dimensional magneto-optical trap is a basic means to achieve atomic cooling and trapping, and is an essential unit of cold atom interferometry system.

[0003] The existing three-dimensional magneto-optical trap structure requires six laser beams to pass through an optical window for three-dimensional optical input and ensure the effect of two-to-two irradiation and three-axis orthogonality. The structure is complex and difficult to miniaturize, which limits its application in integrated atomic chips (the chip blocks the three-dimensional light path in space). Therefore, a new magneto-optical trap design and structure is needed. Summary of the invention

[0004] In view of the technical problems existing in the prior art, one of the objectives of the present invention is to provide a two-dimensional planar grating for a three-dimensional magneto-optical trap system with single beam of light incidence, so as to solve the problems that the existing magneto-optical trap system uses six laser beams for two-by-two irradiation, there is mutual interference of waves among the six beams of light, the system structure is complex, and it is difficult to miniaturize. It can be applied to quantum precision measurement systems such as small atomic interferometer gyroscopes and gravimeters and future integrated atomic chip systems.

[0005] The technical solution adopted by the present invention to solve its technical problem is: a two-dimensional plane grating, which has a two-dimensional array of multiple cylindrical structures periodically and repeatedly arranged in space, and the parameters of the single cylindrical structure and the array structure are completely consistent. The cylindrical structure includes a bottom silicon base layer, a grating structure located on the silicon base layer, and an adhesive layer located on the silicon base layer and the grating structure, and also includes a high-reflectivity metal film layer located on the silicon base layer, the grating structure and the adhesive layer, and the topmost layer on the metal film layer is provided with a metal oxide layer to prevent the metal film layer from being oxidized.

[0006] Furthermore, the material of the bonding layer is Cr, the material of the metal film layer is Au, and the material of the metal oxide layer is Al2O3.

[0007] Furthermore, the silicon substrate layer has a thickness of 3 mm, the grating structure groove depth is 195 nm, the adhesive layer has a thickness of 10 nm, the metal film layer has a thickness of 100 nm, and the metal oxide layer has a thickness of 20 nm.

[0008] Furthermore, the diameter of the cylindrical structure is 540 nm, the diameter of the etched area circle is 20 mm, the grating period is 1080 nm, and the duty cycle is 0.5:0.5.

[0009] The second object of the present invention is to provide a three-dimensional magneto-optical trap system including the above-mentioned two-dimensional plane grating, comprising the above-mentioned two-dimensional plane grating and a single beam of incident light projected onto the two-dimensional plane grating along the normal direction of the two-dimensional plane grating, wherein the grating period of the two-dimensional plane grating is less than times the central wavelength of the incident light, and the central wavelength of the incident light is 2.5 times the depth of the grating groove.

[0010] Furthermore, the plane where the two-dimensional plane grating is located is the XY plane, and its normal direction is the Z axis. When incident vertically on the two-dimensional plane grating surface along the negative direction of the Z axis, after diffraction by the cylindrical structure array area, the four first-order diffraction lights and They are emitted at a diffraction angle θ from the Z axis respectively; in the XYZ three-dimensional space coordinates, the wave vector and Satisfy in and The included angles of the projections in the XY plane are 90° to each other.

[0011] Furthermore, the incident light and four first-order diffracted beams The optical adhesive balance condition relationship must be met That is, the sum of the product of light intensity and wave vector is zero, I j (j=0, 1, 2, 3, 4) corresponds to the intensities of the incident light and the first-order diffracted light respectively.

[0012] Furthermore, the incident light intensity I0, the first-order diffraction beam intensity I1, and the corresponding beam width w i and w d And the first-order diffraction efficiency η1 satisfies the relationship I1 / I0=η1w i / w d =η1secθ, where θ is the first-order diffraction angle obtained under Bragg conditions, satisfying α=arcsin(λ / d), secθ=1 / cosθ means that after the light is diffracted at an angle of θ, the cross-section of the first-order diffraction beam is narrowed to cosθ times of the incident beam, the light intensity increases by 1 / cosθ times, and the optical power density increases.

[0013] Furthermore, when the diffracted light is emitted at an angle of 46° to the Z axis, the beam cross section is narrowed to cos46° of the incident beam, the light intensity increases by 1 / cos46°, and the light intensity along the z-axis direction satisfies the optical adhesive balance condition. The light intensities of the four diffracted light beams are the same, which is I1=η1I0 / cosθ, that is, I1=1 / 4I0secθ.

[0014] Furthermore, the overlapping area of ​​the incident light and the four diffracted light beams in space, i.e., the beam overlapping volume V, and the number of trapped atoms N satisfy the relationship N=4×10 7 V 1.2 .

[0015] The beneficial effects of the present invention are as follows: a single circularly polarized input light beam simplifies system alignment; the planar structure of the grating is small in size and is conducive to integration, and the grating can introduce a three-dimensional optical lattice into the chip to expand its function; the trapped atomic clusters are above the grating and are easy to control; each beam of captured light only passes through the atomic cloud once, reducing the intensity imbalance caused by absorption; the grating is a passive device, the system configuration is stable, and the stability of atomic trapping only depends on the fluctuation of a single cooling light beam, which is significantly better than the traditional magneto-optical trap structure that is disturbed by the fluctuations of six light beams.

[0016] The present invention can solve the problems of the existing magneto-optical trap system which uses six laser beams for two-by-two irradiation, mutual interference of waves among the six beams, complex system structure, and difficulty in miniaturization. The present invention can be applied to quantum precision measurement systems such as small atomic interferometer gyroscopes and gravimeters and future integrated atomic chip systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic structural diagram of the two-dimensional plane grating of the present invention in the XY plane;

[0018] Figure 2 It is a schematic diagram of the structure of the two-dimensional plane grating of the present invention in the YZ plane;

[0019] Figure 3 It is a distribution diagram of the wave vectors of four first-order diffracted lights and incident light diffracted by the two-dimensional plane grating of the present invention;

[0020] Figure 4 It is a schematic diagram of the geometric relationship between the incident light beam and the first-order diffraction light beam.

[0021] The reference numerals of the figures are: 1—silicon base layer, 2—grating structure, 3—adhesion layer, 4—metal film layer, 5—metal oxide layer. DETAILED DESCRIPTION

[0022] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0023] Example 1

[0024] Figure 1 , Figure 2The figure shows a basic example of a two-dimensional plane grating of a three-dimensional magneto-optical trap system for single-beam light incidence of the present invention, which has a two-dimensional periodically repeated structure in space, and its two-dimensional array pattern is a cylindrical structure. The parameters of a single cylindrical structure and the array structure are completely consistent. The cylindrical structure includes a bottom silicon base layer 1, a grating structure 2 located on the silicon base layer 1, and an adhesive layer 3 located on the silicon base layer 1 and the grating structure 2. It also includes a high-reflectivity metal film layer 4 located on the silicon base layer 1, the grating structure 2 and the adhesive layer 3, and the top layer on the metal film layer 4 is provided with a metal oxide layer 5 for preventing the metal film layer 4 from oxidizing.

[0025] The silicon base layer 1 at the bottom of the present invention has a thickness of 3 mm; the groove depth of the grating structure 2 located on the silicon base layer 1 is 195 nm; the bonding layer 3 located on the silicon base layer 1 and the grating structure 2 has a thickness of 10 nm, and the material is chromium Cr, which is mainly used to enhance the adhesion and firmness of the metal film layer 4; the high-reflectivity metal film layer 4 located on the silicon base layer 1, the grating structure 2 and the bonding layer 3 has a thickness of 100 nm, and the material is gold Au, and its function is to improve the diffraction efficiency of the diffracted light beam; the topmost layer is a metal oxide layer 5 for preventing oxidation of the metal film layer 4, with a thickness of 20 nm, and the material is aluminum oxide Al2O3, and its purpose is to introduce an anti-corrosion film layer between the surface of the metal film layer 4 of the grating and the air.

[0026] In the embodiment of the present invention, the grating period is 1080nm, the duty cycle is 0.5:0.5, the diameter of the cylindrical structure is 540nm, the grating groove depth is 195nm, the corresponding grating first-order diffraction angle is 46°, and the diameter of the etched area circle is 20mm. When a beam of circularly polarized light is vertically incident on the two-dimensional plane grating, the diffraction efficiency deviation of the S light and the P light in the first-order diffracted light is less than 5%, and the circular deflection of the generated diffracted light reaches more than 0.98.

[0027] Example 2

[0028] Reference Figure 3 , Figure 4 As shown, the present invention discloses a three-dimensional magneto-optical trap system with a single beam of light incident, comprising the above-mentioned two-dimensional plane grating and a single beam of incident light projected toward the two-dimensional plane grating along the normal direction of the two-dimensional plane grating, wherein the grating period of the two-dimensional plane grating is less than times the central wavelength of the incident light, and the central wavelength of the incident light is 2.5 times the depth of the grating groove.

[0029] The novel magneto-optical trap structure proposed by the present invention has a light channel limited to one window, which is suitable for integrated atomic chips and other vacuum systems. In the three-dimensional magneto-optical trap, the four first-order diffraction beams diffracted toward the center play a role in trapping atoms. In order to concentrate the energy on the first-order diffraction beam, it is necessary to suppress other high-order diffraction beams except the zeroth order.

[0030] According to the grating diffraction Bragg equation dsinθ=mλ, where θ is the diffraction angle and d is the grating period, the second-order and higher-order diffracted beams must satisfy |m|<2 to be suppressed, that is, The four first-order diffracted beams of the two-dimensional grating are (0, +1), (0, -1), (+1, 0) and (-1, 0). From the above formula, we can get That is, the grating period is less than times the central wavelength, which can ensure that after the vertically incident light is diffracted by the grating, all higher-order diffracted light except the zero-order light and the positive and negative first-order diffracted light are suppressed.

[0031] When the central wavelength λ=780nm, the array grating period d<1103nm, corresponding to the diffraction angle θ>45°. The zero-order light has the opposite polarization direction to the incident light, which has a counter-effect on the atomic trapping cooling. The reflectivity of the zero-order light should be reduced as much as possible. According to the scalar diffraction theory, when the grating groove depth is 1 / 4 wavelength, the zero-order light efficiency is theoretically close to zero.

[0032] The cooling light source used in the three-dimensional magneto-optical trap system is ideal circularly polarized light. The grating is a polarization-sensitive device. The diffraction efficiency of S and P polarized light is inconsistent. The circular deflection of the diffracted light beam generated after the circularly polarized cooling light is diffracted by the grating will decrease. A cylindrical grating structure is adopted with a grating duty ratio of 0.5:0.5 to ensure that when the incident light is circularly polarized, the diffraction efficiency of S light and P light in the first-order diffracted light is consistent, and the circular deflection of the diffracted light can reach above 0.95.

[0033] The plane where the two-dimensional plane grating is located is the XY plane, and its normal direction is the Z axis. When incident vertically on the two-dimensional plane grating surface along the negative direction of the Z axis, four beams of first-order diffraction light are generated after diffraction by the cylindrical structure array area of ​​the two-dimensional plane grating. and They are emitted at a diffraction angle θ with the Z axis respectively; since the grating structure parameters along the X and Y directions are completely consistent, the diffraction angle, first-order diffraction efficiency, diffraction light circular deflection and zero-order light efficiency corresponding to the four first-order diffraction lights generated by diffraction are equal, and the scattering forces in the three-dimensional direction provided by each first-order diffraction light and the incident light are projected onto the three-dimensional space to meet the light balance condition, so that the atoms reach force balance, and cooperate with the four-level magnetic field formed by the anti-Helmholtz coil to realize three-dimensional cooling and trapping of atoms.

[0034] Specifically, Figure 3 and Figure 4 As shown, when a beam of incident light When incident vertically on the grating surface along the negative direction of the Z axis, after diffraction by the two-dimensional plane grating area, the four first-order diffracted lights and They are emitted at a diffraction angle of 46° with the Z axis. In the XYZ three-dimensional space coordinates, the wave vector and Satisfy Where θ = 46°, and The included angles of the projections in the XY plane are 90° to each other.

[0035] Incident light When incident on the grating surface vertically along the negative direction of the Z axis, the incident light Four first-order diffracted beams Optical adhesive balance conditions must be met That is, the sum of the product of light intensity and wave vector is zero, I j (j=0, 1, 2, 3, 4) corresponds to the intensities of the incident light and the first-order diffracted light respectively.

[0036] According to the projection decomposition of the diffracted light vector in three dimensions, there is only projection decomposition of the diffracted light vector in the horizontal direction. The four diffracted light beams have equal intensities, the projection components of the wave vector in the X and Y directions are symmetrical, and the light balance condition is satisfied in the horizontal direction.

[0037] In the vertical direction, there are four diffracted light vector projection components along the positive direction of the Z axis and the incident light vector projection along the negative direction of the Z axis. The relationship between the vertical incident light beam intensity I0 and the first-order diffraction light beam intensity I1 is respectively determined by the corresponding beam width w i and w d Determine the first-order diffraction efficiency η1: I1 / I0=η1w i / w d =η1secθ, where θ is the first-order diffraction angle obtained under the Bragg condition, satisfying α=arcsin(λ / d). secθ=1 / cosθ means that after the light is diffracted at an angle of θ, the cross section of the first-order diffraction beam is narrowed to cosθ times of the incident beam, the light intensity increases by 1 / cosθ times, and the light power density increases. In this embodiment, when the diffracted light beam is emitted at an angle of 46° to the Z axis, the beam cross section is narrowed to cos46° of the incident beam, and the light intensity increases by 1 / cos46°.

[0038] It can be obtained that the light intensity along the z-axis direction satisfies the optical viscosity balance condition: The light intensities of the four diffracted light beams are the same, which is I1=η1I0 / cosθ, that is, I1=1 / 4I0secθ.

[0039] The diffraction efficiency of the four diffracted light beams along the positive direction of the Z axis is 1 / 4, which ensures that the sum of the products of the light intensity and the wave vector components of the four diffracted light beams along the positive direction of the Z axis is equal to the product of the light intensity and the wave vector of the incident light along the negative direction of the Z axis, and the vertical direction also meets the light balance condition.

[0040] The three-dimensional optical viscosity balance enables the atoms to achieve force balance in three-dimensional space, and cooperates with the four-level magnetic field formed by the anti-Helmholtz coil to achieve three-dimensional cooling and trapping of atoms.

[0041] When the incident light is incident vertically on the grating, the diffraction angles of the four diffracted light beams generated vary with the grating period. The overlapping area of ​​the incident light and the four diffracted light beams in space, i.e., the beam overlap volume V, satisfies the relationship N=4×10 7 V 1.2 .

[0042] From the above formula, we can know that the larger the overlap volume, the more trapped atoms there are. The central wavelength of the incident light is 780nm. When the grating period changes between 900nm and 1400nm, the diffraction angle gradually decreases from 60° to 45°, and the beam overlap volume increases as the diffraction angle decreases. When the incident beam diameter is 20mm, the overlap volume increases from 60mm to 45°. 3 ~283mm 3 Gradually increases, corresponding to the number of trapped atoms from 2×10 7 ~1.4×10 8 Increase gradually.

[0043] The incident light has a central wavelength of 780 nm. When the grating period is 1080 nm and the incident beam diameter is 20 mm, the diffraction angle is 46° and the beam overlap volume is 200 mm. 3 Theoretically, the number of trapped atoms can reach 9×10 7 .

[0044] The present invention utilizes a single beam of cooling light to be vertically incident on a two-dimensional plane grating. The generated four beams of first-order diffraction light and the incident light form a five-beam cooling light intensity balanced configuration. Only one optical channel is required to provide a complete optical structure requirement for the three-dimensional magneto-optical trap. Combined with the four-level magnetic field formed by the anti-Helmholtz coil, atomic cooling and trapping can be effectively achieved.

[0045] The planar grating of the three-dimensional magneto-optical trap system for single-beam light incidence can be applied to existing quantum precision measurement systems such as small atomic interferometer gyroscopes and gravimeters, and also lays a technical foundation for the development of future atomic chip systems.

[0046] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A two-dimensional plane grating, characterized in that: The invention has a two-dimensional array of a plurality of cylindrical structures which are periodically and repeatedly arranged in space, wherein the cylindrical structures include a silicon substrate layer (1), a grating structure (2) located on the silicon substrate layer (1), and an adhesive layer (3) located on the silicon substrate layer (1) and the grating structure (2), and also includes a high-reflectivity metal film layer (4) located on the silicon substrate layer (1), the grating structure (2) and the adhesive layer (3), wherein a metal oxide layer (5) is arranged on the metal film layer (4).

2. A two-dimensional plane grating according to claim 1, characterized in that: The material of the bonding layer (3) is Cr, the material of the metal film layer (4) is Au, and the material of the metal oxide layer (5) is Al2O3.

3. A two-dimensional plane grating according to claim 1, characterized in that: The silicon substrate layer (1) has a thickness of 3 mm, the groove depth of the grating structure (2) is 195 nm, the thickness of the adhesive layer (3) is 10 nm, the thickness of the metal film layer (4) is 100 nm, and the thickness of the metal oxide layer (5) is 20 nm.

4. A two-dimensional plane grating according to claim 1, 2 or 3, characterized in that: The diameter of the cylindrical structure is 540 nm, the diameter of the etched area circle is 20 mm, the grating period is 1080 nm, and the duty ratio is 0.5:0.

5.

5. A three-dimensional magneto-optical trap system, characterized in that: The invention comprises the two-dimensional plane grating as claimed in claim 1 and a single beam of incident light projected toward the two-dimensional plane grating along a normal direction, wherein the grating period of the two-dimensional plane grating is less than times the central wavelength of the incident light, and the central wavelength of the incident light is 2.5 times the depth of the grating groove.

6. A three-dimensional magneto-optical trap system according to claim 5, characterized in that: The plane where the two-dimensional plane grating is located is the XY plane, and its normal direction is the Z axis. When incident on the two-dimensional plane grating surface along the negative direction of the Z axis, after diffraction by the cylindrical structure array area, the four first-order diffraction lights and They are emitted at a diffraction angle θ from the Z axis respectively; in the XYZ three-dimensional space coordinates, the wave vector and Satisfy in and The included angles of the projections in the XY plane are 90° to each other.

7. A three-dimensional magneto-optical trap system according to claim 6, characterized in that: The incident light and diffracted light Satisfy the relationship I j (j=0, 1, 2, 3, 4) corresponds to the intensities of the incident light and the first-order diffracted light respectively.

8. A three-dimensional magneto-optical trap system according to claim 6, characterized in that: The incident light intensity I0, the first-order diffraction beam intensity I1, and the corresponding beam width w i and w d And the first-order diffraction efficiency η1 satisfies the relationship I1 / I0=η1w i / w d =η1secθ, where θ is the first-order diffraction angle obtained under Bragg conditions, satisfying α=arcsin(λ / d), and secθ=1 / cosθ, which means that after the light is diffracted at an angle of θ, the cross-section of the first-order diffraction beam is narrowed to cosθ times of the incident beam.

9. A three-dimensional magneto-optical trap system according to claim 6, characterized in that: When the diffracted light is emitted at an angle of 46° to the Z axis, the beam cross section is narrowed to cos46° of the incident beam, the light intensity increases by 1 / cos46°, and the light intensity along the z-axis direction satisfies the optical adhesive balance condition. The light intensities of the four diffracted light beams are the same, which is I1=η1I0 / cosθ.

10. A three-dimensional magneto-optical trap system according to claim 6, characterized in that: The overlapping area of ​​the incident light and the four diffracted light beams in space, i.e., the beam overlapping volume V, and the number of trapped atoms N satisfy the relationship N = 4 × 10 7 V 1.2 .

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