Adjustable beam splitter based on superconducting NbN and design method

Abstract

The invention discloses an adjustable beam splitter based on superconducting NbN and a design method, the splitter is of a three-layer reflective structure composed of metal-medium-Gold or NbN, the bottom metal is Gold and is used for reflecting all electromagnetic waves, the medium layer is PI, and the top patch metasurface is of an annular structure composed of Gold or NbN; wherein the Gold is a non-adjustable unit, and the NbN is a unit of which the reflection amplitude is adjustable along with the temperature; the device is based on an amplitude-adjustable material, has universality and is flexible in design.

Description

Technical Field

[0001] The present invention relates to the technical field of beam splitters, and in particular to an adjustable beam splitter based on superconducting NbN and a design method thereof. Background Art

[0002] As a key component in optical systems, beam splitters can split a beam of electromagnetic waves into two or more beams. Compared with common methods such as waveguides and gratings, one advantage of using metasurfaces for beam splitters is that they can achieve adjustable performance. In the microwave frequency band, integrated diodes are usually used to make the reflected waves phase shift. With the increase of frequency, in the terahertz frequency band, materials such as liquid crystal, graphene, and vanadium dioxide are used to achieve changes in the reflection phase or amplitude. These materials are usually used in room temperature environments. With the development of low-temperature terahertz systems, it is necessary to develop beam splitting devices that can be adjusted with temperature and devices with low losses under low temperature conditions. However, the existing beam splitters and adjustable beam splitters mainly have problems such as high electromagnetic losses in the terahertz band, inability to be used in low-temperature environments, and lack of design methods for amplitude-adjustable devices. Summary of the invention

[0003] Purpose of the invention: The purpose of the present invention is to provide an adjustable beam splitter based on superconducting NbN and a design method to solve the problems in the prior art such as high electromagnetic loss in the terahertz band, inability to be used in low-temperature environments, and lack of a design method for amplitude adjustable devices.

[0004] Technical solution: The adjustable beam splitter based on superconducting NbN described in the present invention is a three-layer reflective structure composed of metal-dielectric-Gold or NbN, the bottom metal is Gold, which is used to reflect all electromagnetic waves, the dielectric layer is PI, and the top patch is a circular ring structure composed of Gold or NbN; wherein Gold is a non-adjustable unit and NbN is an adjustable unit whose reflection amplitude varies with temperature.

[0005] Furthermore,

[0006] The metasurface is composed of any combination of at most two non-adjustable units based on metal Gold patches and at most two reflection amplitude adjustable units based on niobium nitride NbN patches; wherein the metal Gold patches and the niobium nitride NbN patches are both composed of "0" or "1" coding.

[0007] Furthermore, the dielectric constant is 3.0-3.5, and the loss tangent is 0.005-0.05.

[0008] Furthermore, the unit period is 50 μm-150 μm, and the dielectric thickness is 30 μm-50 μm.

[0009] Furthermore, NbN is a temperature-adjustable unit with a reflection amplitude in the range of 4.2K-14K.

[0010] Furthermore, the size of the ring is 2 μm-20 μm in width and 5 μm-45 μm in outer radius.

[0011] Furthermore, the same patch units have a phase difference of 160°-220°, and the units corresponding to different patches have a similar phase within 60°.

[0012] The method for designing an adjustable beam splitter based on superconducting NbN of the present invention comprises the following steps:

[0013] (1) Establish a theoretical model for the change of conductivity of NbN material with temperature. Specifically, the conductivity and impedance of NbN film at 0.85 THz are calculated based on the BCS theory and used in the design of unit structure.

[0014] (2) Establish the basic structure of the metasurface unit: Specifically, it is a three-layer reflective structure of metal-dielectric-metal, the bottom metal backplane is gold, which is used to reflect all electromagnetic waves, the dielectric layer is PI, the dielectric constant is 3.0-3.5, the loss tangent is 0.005-0.05, and the top layer is a circular ring structure of NbN or Gold;

[0015] (3) Optimize the unit structure, mainly including unit period, dielectric thickness, and patch size;

[0016] (4) Encode the circular ring structure of NbN or Gold.

[0017] Furthermore, in step (1), based on the BCS theory, the calculation formula of the complex conductivity is:

[0018]

[0019] Among them, the DC conductivity σ of the 50nm film is dc 1.2×10 6 S / m, scattering rate γ is 40THz, energy gap frequency Δ is 1.2THz, k B is the Boltzmann constant, the critical temperature T of NbN grown on PI c is 11.5K, and T is the real-time temperature. The impedance of the NbN film is calculated based on the complex conductivity, and the calculation formula is:

[0020]

[0021] Where μ0 is the magnetic permeability of vacuum and d is the film thickness. Figure 1As shown in Figure 2, the real part of the complex conductivity and the resistance value are close to 0 at 4.2 K, showing excellent superconductivity. As the temperature increases, after reaching the critical temperature, NbN returns to its normal state.

[0022] Furthermore, step (4) includes: a multi-channel independent coding method for independent beam control and a composite structure coding method based on phase coding.

[0023] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The adjustable beam splitter based on superconducting NbN proposed in the present invention can further reduce the loss of terahertz waves under low temperature conditions of 4.2K, and is expected to be applied to low-temperature terahertz detection systems. (2) The adjustable beam splitter proposed in the present invention can realize the switching of the number of beams and the control of beam intensity under different temperature environments, and can be used in environmental sensing and real-time detection. (3) The design method of the adjustable beam splitter based on NbN proposed in the present invention is based on amplitude-adjustable materials, is universal, and has flexible design. (4) The method proposed in the present invention to expand the far-field beam from a one-dimensional arrangement to a two-dimensional space simplifies the design process of complex beam splitters. (5) The adjustable beam splitter designed in the present invention is a polarization-insensitive structure, which is suitable for a variety of application scenarios, and the metasurface unit can be used for other beam control devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The conductivity and impedance of the 50 nm NbN film on PI of the present invention;

[0025] Figure 2 It is the basic unit of the adjustable beam splitter of the present invention;

[0026] Figure 3 The effect of the NbN patch unit of the present invention on the far-field reflected beam;

[0027] Figure 4 The phase distribution and far-field scattering diagram of the present invention;

[0028] Figure 5 The one-dimensional sequence phase distribution and far-field scattering diagram of the present invention;

[0029] Figure 6 The two-dimensional phase distribution and far-field scattering diagram of the present invention;

[0030] Figure 7 The unit distribution and far-field scattering diagram of the one-dimensional material direction of the present invention;

[0031] Figure 8 The two-dimensional adjustable beam splitter and far-field scattering diagram of the present invention. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0033] like Figure 1 As shown, an embodiment of the present invention provides an adjustable beam splitter based on superconducting NbN, which is a three-layer reflective structure composed of metal-dielectric-Gold or NbN, wherein the bottom metal is Gold, which is used to reflect all electromagnetic waves, the dielectric layer is PI, and the top patch metasurface is a circular ring structure composed of Gold or NbN; wherein Gold is a non-adjustable unit and NbN is an adjustable unit whose reflection amplitude varies with temperature.

[0034] Among them, the specific composition of the super surface is as follows: Figure 4 , Figure 6 , Figure 8 As shown, it is composed of any combination of two non-adjustable units based on metal Gold patches and two adjustable reflection amplitude units based on niobium nitride NbN patches; wherein, the metal Gold patches and the niobium nitride NbN patches are both composed of "0" or "1" coding.

[0035] Or Figure 5 As shown, it is composed of any combination of two non-adjustable units based on metal Gold patches and one reflection amplitude adjustable unit based on niobium nitride NbN patches; wherein, the metal Gold patch is composed of "0" or "1" coding, and the niobium nitride NbN patch is composed of "1" coding.

[0036] The dielectric constant is 3.0-3.5, and the loss tangent is 0.005-0.05. The unit period is 50μm-150μm, and the dielectric thickness is 30μm-50μm. NbN is an adjustable unit with reflection amplitude with temperature, and the temperature range is 4.2K-14K. The ring size is 2μm-20μm in width and 5μm-45μm in outer radius. The same patch unit of the ring has a phase difference of 160°-220°, and the units corresponding to different patches have a similar phase within 60°.

[0037] The embodiment of the present invention further provides a method for designing an adjustable beam splitter based on superconducting NbN, comprising the following steps:

[0038] (1) Establish a theoretical model for the change of conductivity of NbN material with temperature. Specifically, the conductivity and impedance of NbN film at 0.85 THz are calculated according to the BCS theory and used in the design of unit structure. Based on the BCS theory, the calculation formula of complex conductivity is:

[0039]

[0040] Among them, the DC conductivity σ of the 50nm film is dc 1.2×106 S / m, scattering rate γ is 40THz, energy gap frequency Δ is 1.2THz, k B is the Boltzmann constant, the critical temperature T of NbN grown on PI c is 11.5K, and T is the real-time temperature. The impedance of the NbN film is calculated based on the complex conductivity, and the calculation formula is:

[0041]

[0042] Where μ0 is the magnetic permeability of vacuum and d is the film thickness. Figure 1 As shown in Figure 2, the real part of the complex conductivity and the resistance value are close to 0 at 4.2 K, showing excellent superconductivity. As the temperature increases, after reaching the critical temperature, NbN returns to its normal state.

[0043] (2) Establish the basic structure of the super surface unit: Figure 2 As shown, it is a three-layer reflective structure of metal-dielectric-metal. The bottom metal backplane is Gold, which is used to reflect all electromagnetic waves. The dielectric layer is PI with a dielectric constant of 3.0-3.5 and a loss tangent of 0.005-0.05. The top layer is a circular ring structure of NbN or Gold.

[0044] (3) Optimizing the unit structure, mainly including the unit period, dielectric thickness, and patch size; optimizing the unit structure size, the dielectric thickness is 45 μm, the unit period is 100 μm, and the change of the patch size can change the intensity of the reflected beam. For example, the four unit sizes are shown in Table 1.

[0045] Table 1 Four different patch sizes

[0046]

[0047] The far-field scattering diagram calculated at 14K after periodically arranging the four units is as follows: Figure 3 As shown, it can be seen that the patch size has a significant effect on the amplitude adjustable depth. The far-field scattering intensity of NbN unit 4 when the temperature changes is further calculated. It can be seen that changing the patch size can achieve an amplitude control depth close to 0.9.

[0048] (3) For the coding metasurface, the optimized unit structure patch size is NbN'0' unit: R = 30μm, r = 20μm, NbN'1' unit: R = 45μm, r = 2μm, Gold'0' unit: R = 30μm, r = 20μm, Gold'1' unit: R = 45μm, r = 2μm. The two NbN units have a 180° phase difference at 0.85THz, and the phase of the Gold unit is close to that of the NbN unit. Therefore, the coding positions of '0' and '1' can be swapped.

[0049] (4) An adjustable beam splitter consisting of 80 × 80 units is constructed, of which 40 × 80 units are Gold units and the other 40 × 80 are NbN units. The units are arranged in a periodic structure of 1010101010 sequence with an angle of 10° along the x and y directions. The units are expanded in the form of super subunits. The unit distribution is as follows: Figure 4 (a)

[0050] (5) Using open boundaries along the x, y, and z directions and a plane wave with an incident frequency of 0.85 THz, the far-field scattering diagrams at 4.2 K and 14 K are simulated and calculated, as shown in Figure 5. Figure 4 (b)(c) At 4.2K low temperature, there are 4 beams, and at 14K, there are 2 beams.

[0051] Embodiment 2: An adjustable beam splitter designed in a composite coding manner comprises the following steps:

[0052] (1) The phase encoding sequence is designed to be 111000111000111, and is expanded into a super sub-unit consisting of 3×3 units, with a total of 45×45 units constituting an adjustable beam splitter. The distribution of the materials is the same as in the phase encoding direction, 000111010111000, '0' is an NbN unit, and '1' is a Gold unit. The unit structure is the same as in Example 1, as shown in FIG. Figure 5 The unit arrangement order and far-field scattering diagrams in different states are shown. At 4.2K low temperature environment, there are 2 main beams in the far field, and at 14K, there are 3 beams in the far field.

[0053] (2) Design as Figure 6 The unit distribution shown is composed of 48×48 units, and the unit structure is the same as that in Example 1. The far-field scattering diagrams at 4.2K and 14K are simulated and calculated, with 16 beams at 4.2K and 8 beams at 14K.

[0054] Embodiment 3: An adjustable beam splitter designed by expanding from one dimension to two dimensions includes the following steps:

[0055] (1) Using the one-dimensional coding sequence in Example 2, the addition operation of the phase coding sequence can be combined with the convolution operation of the coding hyperplane to achieve multi-beam in any direction. The calculation method of extending the digital convolution operation to the complex code domain is:

[0056]

[0057] The expression in 1-bit phase-coding metasurface is

[0058]

[0059] In the 4.2K superconducting state, all units have extremely high reflectivity, and the beam splitter after phase addition operation is reflected as a 2×2 beam in the far field.

[0060] (2) Using one-dimensional material encoding such as Figure 7 As shown, combined with phase encoding, the unit distribution is obtained, and the far-field scattering diagram at 14K is simulated and calculated. The number of beams changes from 2×2 beams at 4.2K to 2×3 beams;

[0061] (3) The two material calculation methods obtained far-field multi-beam scattering diagrams with different switching effects. Among them, the method of retaining the NbN patch realizes the adjustable number of beams in two-dimensional space. Figure 8 As shown, the number of main beams is expanded from 2 to 3 beams in one dimension to 2×2 to 3×3 beams in two dimensions.

Claims

1. An adjustable beam splitter based on superconducting NbN, characterized in that: The three-layer reflective structure consists of metal-dielectric-Gold or NbN. The bottom metal is Gold, which is used to reflect all electromagnetic waves. The dielectric layer is PI. The top patch metasurface is a circular ring structure composed of Gold or NbN. Among them, Gold is a non-adjustable unit and NbN is an adjustable unit whose reflection amplitude varies with temperature.

2. The adjustable beam splitter based on superconducting NbN according to claim 1, characterized in that: The metasurface is composed of any combination of at most two non-adjustable units based on metal Gold patches and at most two reflection amplitude adjustable units based on niobium nitride NbN patches; wherein the metal Gold patches and the niobium nitride NbN patches are both composed of "0" or "1" coding.

3. The adjustable beam splitter based on superconducting NbN according to claim 1, characterized in that: The dielectric constant is 3.0-3.5 and the loss tangent is 0.005-0.

05.

4. The adjustable beam splitter based on superconducting NbN according to claim 1, characterized in that: The unit period is 50μm-150μm, and the dielectric thickness is 30μm-50μm.

5. The adjustable beam splitter based on superconducting NbN according to claim 1, characterized in that: NbN is a temperature-adjustable unit with a reflection amplitude in the range of 4.2K-14K.

6. The adjustable beam splitter based on superconducting NbN according to claim 2, characterized in that: The ring size is 2μm-20μm in width and 5μm-45μm in outer radius.

7. The adjustable beam splitter based on superconducting NbN according to claim 2, characterized in that: The units of the same patch have a phase difference of 160°-220°, and the units corresponding to different patches have a similar phase within 60°.

8. A design method for an adjustable beam splitter based on superconducting NbN, characterized in that: The following steps are involved: (1) Establish a theoretical model for the change of conductivity of NbN material with temperature. Specifically, the conductivity and impedance of NbN film at 0.85 THz are calculated based on the BCS theory and used in the design of unit structure. (2) Establish the basic structure of the metasurface unit: Specifically, it is a three-layer reflective structure of metal-dielectric-metal, the bottom metal backplane is gold, which is used to reflect all electromagnetic waves, the dielectric layer is PI, the dielectric constant is 3.0-3.5, the loss tangent is 0.005-0.05, and the top layer is a circular ring structure of NbN or Gold; (3) Optimize the unit structure, mainly including unit period, dielectric thickness, and patch size; (4) Encode the circular ring structure of NbN or Gold.

9. The adjustable beam splitter based on superconducting NbN according to claim 8, characterized in that: In step (1), based on the BCS theory, the calculation formula of complex conductivity is: Among them, the DC conductivity σ of the 50nm film is dc 1.2×10 6 S / m, scattering rate γ is 40THz, energy gap frequency Δ is 1.2THz, k B is the Boltzmann constant, the critical temperature T of NbN grown on PI c is 11.5K, and T is the real-time temperature. The impedance of the NbN film is calculated based on the complex conductivity, and the calculation formula is: Where μ0 is the magnetic permeability of vacuum and d is the film thickness. As shown in Figure 1, the real part of the complex conductivity and the resistance value are close to 0 at 4.2K, showing excellent superconductivity. As the temperature increases, after reaching the critical temperature, NbN returns to its normal state.

10. The adjustable beam splitter based on superconducting NbN according to claim 8, characterized in that: Step (4) includes: a multi-channel independent coding method for independent beam control and a composite structure coding method based on phase coding.

Images