CN113885115B - Terahertz beam deflector based on dielectric metasurface and its design method - Google Patents

Abstract

The invention provides a terahertz light beam deflector based on a dielectric super surface and a design method thereof, wherein silicon dioxide is used as a base material and a grating unit material of the light beam deflector; the unit structure of the beam deflector is constructed by using the periodic arrangement of two one-dimensional gratings; setting the polarization state, working frequency and incident angle of incident light; modulating the phase of incident light by designing the grating and substrate dimensions of a fixed thickness with the arrangement period as an optimization variable, and selectively scattering most of the incident light to the +1 diffraction order while suppressing the intensities of other diffraction orders; and adding a thickness optimization variable, and optimizing the size, the periodic arrangement and the substrate thickness of two gratings with different widths to finally obtain the terahertz light beam deflector which can guide incident light to a very large deflection angle and has high deflection efficiency in a wide deflection angle range. The deflector can realize large-angle deflection, has small loss, is easy to process and has high deflection efficiency.

Description

Terahertz beam deflector based on dielectric metasurface and its design method

technical field

The invention belongs to the technical fields of terahertz and light field regulation, and in particular relates to a terahertz beam deflector based on a dielectric metasurface and a design method thereof.

Background technique

Terahertz waves generally refer to electromagnetic waves with a band between millimeter waves of 0.1 THz and far-infrared rays of 10 THz. Terahertz waves are different from electromagnetic waves such as microwaves, infrared and X-rays. Terahertz waves have significant research value in broadband mobile communications, satellite communications, military radar, object imaging, environmental monitoring, and medical diagnosis.

In recent years, light field modulation technology has attracted the strong interest of many scientific researchers. In fact, any photonic technology is essentially a variety of adjustments to various physical parameters of the light field, including frequency, wavelength, time, amplitude, phase, polarization, etc., so as to realize the loading, processing and extraction of optical information. Among them, the deflection control of the light beam is in a crucial position and has always been the focus of research in the field of optics. It has a wide range of applications in many fields such as industry, communication, biomedicine, astronomical observation, and national defense.

At present, the traditional metasurface beam deflectors used in the terahertz band mostly use a series of subwavelength units with different sizes or directions to form a metasurface structure, each unit forms a specific phase mutation, and the overall structure forms a phase gradient of 0-2π . When the light is vertically incident, the phase gradient introduced by the unit array can realize the deflection of the outgoing light angle. Most traditional beam deflectors can only achieve relatively small deflection, and the device efficiency will be significantly reduced at large deflection angles, and traditional metasurface devices use metal as the unit material, which will lead to low device efficiency due to metal loss . Therefore, it is of great significance to design a large deflection angle and high efficiency terahertz beam deflector.

Contents of the invention

In view of this, in order to overcome the defects and deficiencies in the prior art, the object of the present invention is to provide a terahertz beam deflector based on a dielectric metasurface and its design method, using silicon dioxide as the substrate of the beam deflector Materials and grating unit materials; use two one-dimensional gratings to periodically arrange the unit structure of the beam deflector; set the incident light polarization state, operating frequency, and incident angle; by designing a fixed thickness grating and substrate size, the arrangement period is Optimize the variable to modulate the phase of the incident light, and selectively scatter most of the incident light to the +1 diffraction order, while suppressing the intensity of other diffraction orders; add the thickness optimization variable, by adjusting the size and period of the two gratings with different widths The arrangement and substrate thickness are optimized, and finally a terahertz beam deflector that can guide the incident light to a very large deflection angle and has high deflection efficiency in a wide range of deflection angles is obtained. The deflector in the invention can realize large-angle deflection, has low loss, is easy to process, and has high deflection efficiency.

The present invention specifically adopts the following technical solutions:

A terahertz beam deflector based on a dielectric metasurface is characterized in that: firstly, the unit structure of the beam deflector is constructed by using the periodic arrangement of two one-dimensional gratings; based on the incident light polarization state, operating frequency, and incident angle, By designing the grating with a fixed thickness and the size of the substrate, the phase of the incident light is modulated with the arrangement period as the optimal variable.

Further, silicon dioxide is used as the base material and grating unit material. The material has high transmittance in the terahertz band; the refractive index n=1.98; the terahertz wave loss is small and easy to process.

Further, the one-dimensional grating structure is a reflective or transmissive diffraction grating. Au can be arranged under the silicon dioxide substrate as a reflective material to prepare a reflective diffraction grating.

Furthermore, the design goal of modulating the phase of the incident light with the arrangement period as the optimal variable is to scatter most of the incident light to the +1 diffraction order while suppressing the intensity of other diffraction orders.

More preferably, by adjusting the thickness and width of the grating, the phase delay of the incident light passing through the grating array is further controlled, and by optimizing the size, periodic arrangement, and substrate thickness of two gratings with different widths, a terahertz beam with a large deflection angle can be obtained deflector.

Further, the one-dimensional grating structure is a reflective or transmissive diffraction grating; the arrangement period is: p=λ/sinθ, where λ is the wavelength of the incident light, and θ is the deflection angle. For example, when the wavelength of incident light is 400 μm and the corresponding working frequency is 0.75 THz, through the above design, the deflection angle can reach 20°-70°.

Compared with the prior art, the present invention and its preferred solution can realize large-angle deflection, low loss, easy processing and high deflection efficiency. Most of the existing traditional beam deflectors can only achieve relatively small deflection, and the efficiency of the device will be significantly reduced at a large deflection angle, and the traditional metasurface device uses metal as the unit material, which will cause the device to lose due to metal loss. Inefficiencies hinder the development of high-performance imaging applications such as molecular microscopy. Compared with such designs, the present invention uses two one-dimensional gratings periodically arranged to replace a series of discrete units used in traditional metasurface beam deflectors. The thickness and width of the grating can control the phase delay of the incident light passing through the grating array. By optimizing the size of the grating and the size of the substrate to modulate the phase of the incident light, a metasurface terahertz beam deflector with specific angle deflection can be obtained, which can obtain large angle deflection and high Deflection efficiency of terahertz beam deflectors.

Description of drawings

Below in conjunction with accompanying drawing and specific embodiment the present invention is described in further detail:

Fig. 1 is a schematic diagram of the structure and principle of a transmissive terahertz beam deflector according to an embodiment of the present invention;

Fig. 2 is a schematic diagram of the structure and principle of a reflective terahertz beam deflector according to an embodiment of the present invention;

Fig. 3 is a schematic diagram of the relationship between diffraction efficiency and deflection angle of the transmissive beam deflector according to the embodiment of the present invention under fixed thickness and optimized thickness.

4 is a schematic diagram of the relationship between the diffraction efficiency and the deflection angle of the reflective beam deflector of the embodiment of the present invention under fixed thickness and optimized thickness;

Fig. 5 is a schematic diagram of the wave front of the transmissive beam deflector according to the embodiment of the present invention when the deflection angles are 20° and 70°;

FIG. 6 is a schematic diagram of wave fronts of a reflective beam deflector according to an embodiment of the present invention when the deflection angles are 20° and 70°.

Detailed ways

In order to make the features and advantages of this patent more obvious and easy to understand, the following special examples are described in detail as follows:

In this embodiment, the design of the beam deflector mainly adopts the following steps:

Step S1: using two one-dimensional gratings with different widths to be periodically arranged to construct the unit structure of the beam deflector;

Step S2: The designed polarization state of the incident light is TM polarized light, the wavelength of the incident light is 400 μm, the corresponding working frequency is 0.75 THz, and the incident angle is 0°;

Step S3: Design the fixed thickness of the grating to be 200 μm, and the fixed thickness of the substrate to be 300 μm. The phase of the incident light is modulated with the arrangement period as the optimal variable, and can selectively scatter most of the incident light to the +1 diffraction order while suppressing Intensities of other diffraction orders;

Step S4: add thickness optimization variables, and use Matlab programming to calculate the diffraction efficiency at different deflection angles according to the rigorous coupled wave analysis method. Under the constraint conditions, the designed structure is optimized. The incident light is normal incidence, and the light needs to fall on the +1 diffraction order, then the grating period p=λ/sinθ. Optimizing the structural dimensions of the corresponding terahertz beam deflectors with a deflection angle of 20°-70° respectively, and finally obtaining the ability to guide the incident light to a very large deflection angle with high deflection efficiency in a wide range of deflection angles terahertz beam deflector.

Fig. 1 and Fig. 2 are design schematic diagrams of the transmissive and reflective terahertz beam deflectors in the design examples of the present invention, respectively.

Figure 3 and Figure 4 respectively show the relationship between the diffraction efficiency and the deflection angle of the transmissive and reflective beam deflectors at fixed thickness and optimized thickness. Under the condition that the working frequency f=0.75THz, the incident light is TM polarized light incident vertically, and the deflection angle range is set to 20°-70°. It can be seen from the figure that when it is a transmission type, the diffraction efficiency of the fixed-thickness beam deflector is generally low, but it is also higher than the diffraction efficiency of the traditional beam deflector at a large deflection angle, and the thickness of the grating and the thickness of the substrate When participating in the optimization as an optimization variable, the overall diffraction efficiency is greatly improved, especially when the deflection angle is 30°, the diffraction efficiency reaches a maximum of 0.99, and the diffraction efficiency of the deflection angle in the range of 20°-70° is maintained above 0.8, The efficiency is much higher than that of the traditional beam deflector when the deflection angle is large; when it is reflective, the deflection angle is less than 50°, and the diffraction efficiency of the two schemes has reached above 0.8, especially when the thickness is involved in the optimization. In the deflection angle range of 20°-70°, the diffraction efficiency of the designed beam deflector is close to 1.

Fig. 5 and Fig. 6 are the wavefronts in the x-z plane when the deflection angles are 20° and 70° respectively for the transmissive and reflective beam deflectors in the design examples of the present invention, wherein the abscissa and ordinate are respectively represented in x and The length in the z direction is expressed in multiples of the wavelength; the arrows indicate the incident direction and the deflection direction; the contour line is the field intensity; the box is the geometric position of the terahertz beam deflector. It can be seen from the figure that when it is transmissive, the incident light is deflected in the direction of propagation after the designed device. Substituting the optimized structural parameters into p=λ/sinθ to deduce the simulated deflection angle, the conclusion is consistent with the deflection angle set at the beginning, and the transmitted beam is indeed efficiently deflected to the preset angle, which shows that the optimization process is effective It also shows that the designed beam deflector can indeed achieve high efficiency and large deflection angle of the beam; when it is reflective, it can be seen that the reflected light is deflected at a certain angle, and the measured deflection angle is reversed and simulated It is consistent with the set deflection angle, which proves that the designed reflective beam deflector meets the requirements.

Table 1 shows the optimized structural parameters of the transmissive beam deflector in the design example of the present invention. It can be found that the periods under the same deflection angle are basically equal, because the period p=λ/sinθ, and the period is only related to the wavelength of the incident light and the deflection angle.

Table 2 shows the optimized structural parameters of the reflective beam deflector in the design example of the present invention. The grating period at each deflection angle is substantially the same for the transmissive and reflective beam deflectors. And it is found that the grating thickness of the reflective beam deflector is much thinner than that of the transmissive beam deflector in the deflection angle range of 20°-70°, because the light interacts with the metasurface twice.

This patent is not limited to the above-mentioned optimal implementation mode, anyone can draw other various forms of terahertz beam deflectors and design methods based on dielectric metasurfaces under the inspiration of this patent, and any patent application according to the present invention The equivalent changes and modifications made within the scope of the invention should all fall within the scope of this patent.

Claims (3)

1. A terahertz light beam deflector based on a medium super surface is characterized in that: firstly, constructing a unit structure of a beam deflector by utilizing the periodic arrangement of two one-dimensional gratings; based on the polarization state, working frequency and incident angle of the incident light, modulating the phase of the incident light by designing the grating with fixed thickness and the size of the substrate and taking the arrangement period as an optimized variable;

silicon dioxide is used as a base material and a grating unit material;

the design targets for modulating the phase of the incident light with the arrangement period as the optimization variable are: and scattering most of incident light to a +1 diffraction order, inhibiting the intensity of other diffraction orders, controlling the phase delay of the incident light passing through the whole array of the gratings by adjusting the thickness and the width of the gratings, and optimizing the size, the periodic arrangement and the substrate thickness of two gratings with different widths to obtain the terahertz light beam deflector with a large deflection angle.

2. The terahertz beam deflector based on the super surface of a medium according to claim 1, wherein: the one-dimensional grating structure is a reflective or transmissive diffraction grating.

3. The method for designing the terahertz beam deflector based on the medium super surface according to claim 1, wherein the method comprises the following steps: the one-dimensional grating structure is a reflective or transmissive diffraction grating; the arrangement period is as follows: p=l/sinq, where l is the wavelength of the incident light and q is the deflection angle.