CN103941316A - Polarization-insensitive high-index-of-refraction metamaterial and preparation method thereof - Google Patents

Abstract

The invention provides an ultra-high refractive index material, the main body of which is a metamaterial. The structural unit of the metamaterial is as follows, which includes a dielectric material and two I-shaped metal materials, and the two I-shaped metal materials are embedded in the medium In the material, two I-shaped metal materials are vertically symmetrically arranged, and the structural units are periodically arranged on the plane to form a single-layer two-dimensional metamaterial, and three layers of two-dimensional metamaterials are stacked together to form an ultra-high refractive index Super material. A method for manufacturing an ultra-high refractive index material, the specific steps of which are detailed in the main text. The present invention achieves the following effect, multi-frequency response: the structure exhibits the property of high refractive index at two frequencies, while existing materials can only have high refractive index at one frequency. Isotropic: While maintaining the broadband and high refractive index of the material, the structure truly makes it isotropic. Ease of application: The material has good toughness and is easy to bend, which reduces its limitations in practical use.

Description

Polarization-insensitive high-refractive index metamaterial and preparation method thereof

technical field

The invention relates to optical materials, in particular to ultra-high refractive index materials and a manufacturing method thereof.

Background technique

The refractive index of natural materials is mostly a small positive value, and only a few semiconductors and insulators will have high refractive index in the mid-infrared and far-infrared bands.

Existing high-refractive-index metamaterials have a lower peak refractive index and a narrower response frequency range. At the same time, the polarization direction of light has a significant impact on its performance.

At present, people's research on high refractive index materials is still in its infancy, and the relevant theory was only proposed in 2009. According to Maxwell's equations, the refractive index of a material is determined by its (relative) permittivity and magnetic permeability. Therefore, if you want to increase the refractive index of the material, the first task is to increase ε and μ as much as possible, especially for metamaterials, whose (relative) permittivity and permeability are dependent on the capacitance of its metal dielectric structure and inductance. Therefore, for designing high-refractive-index materials, the first task is to realize a large capacitive unit structure. Secondly, it is also necessary to suppress the diamagnetic effect. The design and proposal of high refractive index material theory has profound significance for the development of imaging and lithography. As shown in Figure 1, using high refractive index materials as the filling medium at 200 can increase the numerical aperture of the microscope (NA=n*sinα), thereby greatly increasing the resolution of the microscope. In the figure, 100 is an objective lens, and 300 is an object stage.

It is difficult for current ultra-high refractive index metamaterials to have the properties of isotropy, high refractive index peak and wide response frequency domain at the same time. When the polarization direction of the incident light changes, the two metamaterials shown in Figure 2 maintain a relatively stable refractive index curve, but the metamaterial with a honeycomb structure can only reach the peak value of the refractive index in a very narrow range. , cannot achieve broadband high refractive index, and although the window structure metamaterial maintains a stable refractive index in the wide frequency domain, its peak refractive index is twice as small as that of the honeycomb structure metamaterial, that is, it cannot reach a Broadband high refractive index.

When the metamaterial can maintain a wide band of high refractive index at the same time, it has a great dependence on the polarization direction of the incident light. As shown in Figure 3, the structure of the metamaterial is I-shaped. It can be seen that when the incident light is polarized When the direction is turned by 90 degrees, the peak value of its refractive index and its response frequency are greatly changed, which proves that the high refractive index of this two-dimensional metamaterial has a great dependence on the polarization of incident light.

Contents of the invention

In view of the deficiencies of the existing high refractive index materials, in order to increase the peak value of the refractive index, broaden its response frequency domain, and maintain isotropy, the following design of high refractive index materials is proposed to truly realize "broadband, isotropic super Materials with high refractive index".

An ultra-high refractive index material, the main body of which is a metamaterial. The structural unit of the metamaterial is as follows, which includes a dielectric material and two I-shaped metal materials, and the two I-shaped metal materials are embedded in the dielectric material. Two I-shaped metal materials are vertically symmetrically arranged, and the structural units are periodically arranged on the plane to form a single-layer two-dimensional metamaterial, and three layers of two-dimensional metamaterials are stacked together to form an ultra-high refractive index metamaterial.

As a further improvement of the present invention, the structural unit period p=60±5 microns, and the thickness H=1±0.05 microns. Explanation: 60±5 microns means that the positive and negative tolerance is 5 microns, and the following similar expressions all mean tolerances.

As a further improvement of the present invention, the upper and lower lengths of the I-shaped metal material are equal, which is a=59±0.1 microns.

As a further improvement of the present invention, the width of the I-shaped metal material is d=8±0.1 microns.

As a further improvement of the present invention, gold is used as the metal material, and the thickness of the gold film is wd=100±10 nanometers.

As a further improvement of the present invention, polyimide is used as the dielectric material.

As a further improvement of the present invention, the two I-shaped metal materials are respectively embedded in a dielectric material with a thickness of 250±10 nm.

A method for manufacturing an ultra-high refractive index material, the specific steps are as follows:

Step 1. Spin-coat a polyimide solution with a thickness of 250±10nm on a clean silicon wafer, and dry it at 180±10°C for 30±5 minutes to make it a substrate with high toughness;

Step 2. Heating the polyimide layer to 350±10°C in an inert gas for aging, so that the polyamic acid is converted into a water-insoluble aromatic polyimide;

Step 3. Clean the obtained sample, then spin-coat the photoresist, carry out post-baking after photolithography, and then develop it, and perform hard-baking treatment after completion to obtain the first layer of I-shaped pattern;

Step 4. Utilize electron beam vacuum evaporation to plate a layer of gold film with a thickness of 100 ± 10 nanometers according to the obtained pattern, and obtain the first layer of I-shaped structure after washing off the photoresist;

Step 5. Based on the bottom of the gold film, continue to spin-coat a polyimide solution with a thickness of 250 ± 10nm, and repeat the drying and aging operations of steps 1 and 2;

Step 6. After cleaning the sample, spin-coat the photoresist, and perform photolithography again, align the center of the I-shaped pattern of this photolithography with the center of the I-shaped pattern of the first photolithography, and complete Finally, carry out post-baking, and then develop it, and then carry out hard-baking treatment to obtain the second layer of I-shaped pattern;

Step 7. Then use electron beam vacuum evaporation to plate a layer of gold film with a thickness of 100 ± 10nm according to the obtained pattern, and obtain the second layer of I-shaped structure after washing off the photoresist;

Step 8. Continue to spin-coat a polyimide solution with a thickness of 300±10nm, and repeat the drying and curing operations of steps 1 and 2, thus completing the preparation of the two-dimensional metamaterial;

Step 9. Repeat the preparation steps of the above-mentioned single-layer two-dimensional metamaterial twice to obtain a three-layer metamaterial. Finally, the three-layer metamaterial is peeled off from the silicon substrate, and the high-performance high-refractive index material is completed. preparation.

As a further improvement of the present invention, in step 2, the polyimide layer is heated in an inert gas in an open quartz tube furnace.

The beneficial effects of the present invention are:

Multi-frequency response: The structure exhibits high refractive index properties at two frequencies, whereas existing materials can only have a high refractive index at one frequency.

Isotropic: While maintaining the broadband and high refractive index of the material, the structure truly makes it isotropic.

Ease of application: The material has good toughness and is easy to bend, which reduces its limitations in practical use.

Description of drawings

Fig. 1 is a schematic diagram of a high refractive index material used as a filling substance between an objective lens and a stage in a microscopic system;

Fig. 2 is the real part and the imaginary part spectrogram of the refractive index of two kinds of metamaterials;

Fig. 3 is the permittivity of the I-shaped metamaterial under two kinds of polarization directions, magnetic permeability, and refractive index spectrogram;

Fig. 4 is a three-dimensional schematic diagram of a metamaterial structural unit of the present invention;

5 is a top view of a metamaterial structural unit of the present invention;

Figure 6x (left), y (right) transmission, reflection, and loss spectra under polarized light;

Fig. 7 is the refractive index under the x-direction polarized light;

Figure 8 is the refractive index under polarized light in the y direction;

Fig. 9 is a schematic diagram of a three-dimensional metamaterial.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

In this design, metamaterials with special properties are utilized to achieve high capacitive coupling and low magnetic loss to achieve high refractive index. Metamaterials with special optical properties, especially optical and electromagnetic responses, are generally composed of metal nanostructures. Specially designed metamaterials can provide very high electrical responses locally, and at the same time, through proper design, the magnetic loss of the entire material can be reduced. According to Maxwell's equations, the refractive index of a material is determined by its (relative) permittivity and permeability, Therefore, a high refractive index of the material is achieved when the permittivity ε is very high and the magnetic permeability μ is close to 1. Existing designs and experiments have confirmed that the dielectric constant ε of metamaterials with special properties can reach 5000, while the magnetic permeability is close to 1, that is to say, the refractive index of the material can reach more than 70. In the present invention, the structure of the metamaterial is integrated to improve its symmetry. Since there will always be a corresponding internal structure in the material to respond to the incident light when the polarization direction of the incident light changes, the specially designed metamaterial can have isotropic properties, combined with the original broadband and high refractive index of the metamaterial properties, so the ultra-high refractive index material of the present invention can simultaneously meet the performance requirements of broadband, high refractive index peak and isotropy.

The specific implementation plan is as follows:

The main body of the high-performance ultra-high refractive index material is composed of metamaterials. The structural units of the metamaterials are shown in Figure 4, where 1 is a dielectric material, 2 and 3 are I-shaped metal materials, and they are embedded in dielectric material 1 , its structure is vertically symmetrical. In the present invention, gold is used as the metal material because the loss of gold is small in the terahertz band. Polymide (polymide) is used as a dielectric material, and polyimide (polymide) is one of the organic polymer materials with the best comprehensive performance at present. Salient traits are called "problem solvers".

Periodically arrange the structural units on a plane to form a single-layer two-dimensional metamaterial, and then stack three layers of two-dimensional metamaterials together to obtain the high-performance ultra-high refractive index metamaterial.

The specific steps to make this material are as follows:

1. Spin-coat a 250±10nm thick polyimide solution (PI-2610, HDMicroSystem) on a clean silicon wafer, and dry it at 180±10°C for 30±5 minutes to make it a high toughness base.

2. In an open quartz tube furnace, heat the polyimide layer in an inert gas to 350±10°C for aging, so that the polyamic acid is transformed into a water-insoluble aromatic polyimide.

3. Clean the obtained sample, then spin-coat the photoresist, perform photolithography, post-baking, and then develop it. After completion, perform hard-baking treatment to obtain the first layer of I-shaped pattern.

4. Utilize the method of electron beam vacuum evaporation to plate a layer of 100nm thick gold film according to the obtained pattern. After washing off the photoresist, the first layer of I-shaped structure is obtained.

5. Taking the bottom of the gold film as a reference, continue to spin-coat polyimide solution with a thickness of 250±10nm, and repeat steps 1 and 2 for drying and aging.

6. Spin-coat the photoresist after cleaning the sample, and then perform photolithography on it again, and align the center of the I-shaped pattern of this photolithography with the center of the I-shaped pattern of the first photolithography. After completion, post-baking is carried out, and then it is developed, and after completion, hard-baking is carried out to obtain the second layer of I-shaped pattern.

7. Then use the method of electron beam vacuum evaporation to plate a layer of gold film with a thickness of 100±10nm according to the obtained pattern. After washing off the photoresist, the second layer of I-shaped structure is obtained.

8. Continue to spin-coat polyimide solution with a thickness of 300±10nm, and repeat the drying and curing operations of steps 1 and 2. In this way, the preparation of the two-dimensional metamaterial is completed.

9. Repeat the preparation steps of the above-mentioned single-layer two-dimensional metamaterial twice to obtain a three-layer metamaterial, and finally peel off the three-layer metamaterial from the silicon substrate to complete the preparation of a high-performance high-refractive index material .

The parameters used in the design are gold film thickness wd=100±10 nanometers, structural unit period p=60±5 microns, thickness H=1±0.05 microns. a=59±0.1 microns, d=8±0.1 microns. The transmission and reflection spectra of the structure for incident light polarized in two directions are shown in Figure 6.

The resonant frequencies of the structure are 0.35 terahertz and 0.9 terahertz, which also correspond to the resonant wavelengths of the two structures inside the material. Further calculation of its refractive index can obtain its refractive index map under two kinds of polarized light.

The advantages of this structure are as follows:

1. While maintaining the broadband and high refractive index of the material, the structure truly makes it isotropic.

2. The structure has two peaks of refractive index, and the resonance position can be adjusted by adjusting the structural parameters of the metamaterial, and the thickness of the dielectric material can be adjusted according to the needs to adjust the peak of the refractive index

3. The structure has the advantages of small size and thin thickness. At the same time, due to the special properties of the dielectric material, the material has good toughness and easy bending properties.

The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments, and it cannot be assumed that the specific implementation of the present invention is limited to these descriptions. For those of ordinary skill in the technical field of the present invention, without departing from the concept of the present invention, some simple deduction or replacement can be made, which should be regarded as belonging to the protection scope of the present invention.

Claims (9)

1. a superelevation refraction materials, it is characterized in that: its main body is super material, the structural unit of this super material is as follows, it comprises dielectric material and two I shape metal materials, and two I shape metal materials are embedded in dielectric material, and two I shape metal material vertical symmetry arrange, this structural unit is periodically arranged in the plane, form the super material of two dimension of individual layer, three layers of super material stacks of two dimension together, form the super material of superelevation refractive index.

2. superelevation refraction materials according to claim 1, is characterized in that: structural unit period p=60 ± 5 micron, thickness H=1 ± 0.05 micron.

3. superelevation refraction materials according to claim 1, is characterized in that: the lower-upper length of I-shaped metal material equates, is a=59 ± 0.1 micron.

4. superelevation refraction materials according to claim 1, is characterized in that: the width of I-shaped metal material is d=8 ± 0.1 micron.

5. superelevation refraction materials according to claim 1, is characterized in that: golden as metal material, and golden film thickness wd=100 ± 10 nanometer.

6. superelevation refraction materials according to claim 1, is characterized in that: polyimide is as dielectric material.

7. superelevation refraction materials according to claim 1, is characterized in that: two I shape metal materials are to be respectively embedded in the dielectric material that 250 ± 10nm is thick.

8. a method for making for superelevation refraction materials, is characterized in that, concrete steps are as follows:

Step 1. is the thick polyimide solution of spin coating 250 ± 10nm on clean silicon chip, and it is dried 30 ± 5 minutes under 180 ± 10 DEG C of conditions, makes it to become the substrate with high tenacity;

Step 2. is heated to polyimide layer 350 ± 10 DEG C and carries out slaking in inert gas, makes polyamic acid be transformed into water-fast aromatic polyimide;

Step 3. is cleaned obtaining sample, and then spin coating photoresist, carries out carrying out rear baking after photoetching to it, then developed, and carries out hard baking and process acquisition ground floor I shape pattern after completing;

Step 4. utilizes the way of electron beam vacuum evaporation to plate the golden film of one deck 100 ± 10 nanometer thickness according to gained figure, obtains ground floor I-shaped structure after washing photoresist off;

Step 5., taking the bottom of golden film as benchmark, continues the thick polyimide solution of spin coating 250 ± 10nm, and it is repeated to the oven dry of 1,2 step, slaking operation;

Sample is cleaned rear spin coating photoresist by step 6., again it is carried out to photoetching treatment, the center of the I shape pattern of this photoetching is aimed at the I shape pattern center of photoetching for the first time, after completing, carry out rear baking, developed again, after completing, carry out hard baking and process acquisition second layer I shape pattern;

The way that step 7. recycles electron beam vacuum evaporation plates according to gained figure the golden film that one deck 100 ± 10nm is thick, obtains second layer I-shaped structure after washing photoresist off;

Step 8. continues the polyimide solution that spin coating 300 ± 10nm is thick, repeats the oven dry slaking operation of 1,2 steps, has so just completed the preparation of two-dimentional super material;

Step 9. repeats the preparation process of twice super material of above-mentioned individual layer two dimension, can obtain the super material of three layers, finally these three layers of super materials is peeled off from silicon base, has completed the preparation of high-performance high-index material.

9. the method for making of superelevation refraction materials according to claim 8, is characterized in that: in step 2, in open quartz tube furnace, polyimide layer is heated in inert gas.