CN113504585A - Polarization-independent superlens - Google Patents

Abstract

The invention discloses a polarization-independent super-lens, comprising a base layer and a super-unit for constructing a metasurface array on the base layer. The super-unit includes a single cylinder, an annular cylinder and a concentric cylinder. The lens controls the wavefront of incident polarized light and eliminates chromatic aberration through a combination of propagating phase and compensating phase. The incident ray polarized light adopted in the present invention vertically irradiates the metasurface structure, according to the selected incident center wavelength, the structural parameters of the dielectric column are scanned and optimized, the required compensation phase can be obtained by changing the size of the cylindrical silicon, and the high The transmittance ensures that the combination of structure selection and propagation phase can control the wavefront of incident light and eliminate chromatic aberration.

Description

A Polarization-Independent Metalens

technical field

The invention belongs to the technical field of composite material metasurfaces, in particular to a polarization-independent superlens.

Background technique

Wavelength dispersion is an important property of optical materials and has always played an important role in the design of optical components and systems. In most media, like glass, the refractive index decreases with increasing wavelength. This is called normal dispersion. With this material, refractive lenses will have a larger focal length at longer wavelengths than at shorter wavelengths, and prisms will deflect at a smaller angle, this chromatic aberration severely degrades the performance of panchromatic optical applications such as communications, Detection, imaging, display, etc. Currently, there are metalens that use multilayer structures to achieve dual-wavelength and triple-wavelength chromatic aberration cancellation. Although this strategy has been successful, it increases the weight, complexity, and cost of optical systems, greatly limiting their use; and these Metalens are all limited by polarization dependence and can only focus circularly polarized light.

Therefore, recent studies have mainly focused on the design of polarization-insensitive achromatic metalens in the visible and near-infrared, however, how to design a polarization-independent achromatic metalens to eliminate the chromatic aberration effect in the mid-infrared band is still a problem. huge challenge.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a polarization-independent superlens to overcome the above-mentioned technical problems.

The above-mentioned technical purpose of the present invention is achieved through the following technical solutions:

A polarization-independent superlens, comprising a base layer and a super-unit for constructing a metasurface array on the base layer, the super-unit comprising a single cylinder, an annular cylinder and a concentric cylinder;

The polarization-independent superlens controls the wavefront of the incident polarized light and eliminates chromatic aberration by combining the propagation phase and the compensation phase;

Among them, using the propagation phase as the focusing phase, the incident ray polarized light irradiates the super unit vertically, and the incident wavelength range is 3.7-4.7 μm, and the super unit is selected by the principle of the propagation phase to control the wave front of the incident ray polarized light, so that the selected Focus at the center wavelength of ;

The required different compensation phase values are obtained through three different types of structures in the superunit, to ensure that within the selected incident wavelength range, the compensation phase is linearly related to the inverse of the incident wavelength, so as to eliminate incident wavelengths outside the central wavelength. chromatic aberration effect.

Further, the single column, the annular column and the concentric column are all Si nanostructures, and the height H of the single column, the annular column and the concentric column is 4.5 μm.

Further, the material of the base layer is CaF ₂ .

Further, the outer diameters of the single cylinder, the annular cylinder and the concentric cylinder in the super unit are kept the same.

Further, the formula of the phase distribution of the polarization-independent metalens is φ(x, λ)=(2πλ)ne _ff H, where n _eff represents the effective refractive index of the nanostructure, and H represents the high.

Further, the distance between the super-units for constructing the meta-surface array is 1.8 μm.

Beneficial effects:

The achromatic achromat realized by the metasurface designed in the present invention is within the continuous bandwidth range. In addition to the metasurface can be used alone to realize achromatic achromat, it can also be combined with traditional optical devices, thereby improving the performance and size of the metasurface achromatic device. scope of application;

The incident ray polarized light adopted in the present invention vertically irradiates the metasurface structure, according to the selected incident center wavelength, the structural parameters of the dielectric column are scanned and optimized, the required compensation phase can be obtained by changing the size of the cylindrical silicon, and the high The transmittance ensures that the combination of structure selection and propagation phase can control the wavefront of incident light and eliminate chromatic aberration;

The polarization-independent broadband achromatic device based on the dielectric metasurface in the present invention, on the other hand, realizes achromatic achromaticity in a continuous bandwidth range, which greatly reduces the volume and cost compared with the previous devices.

Description of drawings

Fig. 1 is the structural representation of the present invention;

2 is a schematic diagram of dispersion of the present invention;

3 is a phase distribution diagram of the present invention;

Figure 4 and Figure 5 are physical analysis of three prototype structures in the present invention;

6 is a phase distribution diagram of polarization-insensitive BAML in the present invention;

In the figure: 1. Base layer; 2. Single cylinder; 3. Ring-shaped column; 4. Concentric column.

Detailed ways

In the description of the present invention, unless otherwise specified, the orientation or positional relationship indicated by the terms "upper", "lower", "left", "right", "front", "rear", etc. is only for describing the present invention and simplifying the description, rather than Indication or implication that the device or structure referred to must have a specific orientation should not be construed as a limitation of the present invention. Furthermore, the terms "first," "second," etc. are used for descriptive purposes only and should not be construed to indicate or imply relative importance.

As shown in FIG. 1 , a polarization-independent superlens according to the present invention includes a base layer and a super-unit for constructing a metasurface array on the base layer, wherein the super-unit includes a single cylinder, an annular cylinder and a concentric cylinder , the material of the base layer is CaF ₂ , the single cylinder, the annular column and the concentric column are all Si nanostructures, and the height H of the single cylinder, the annular column and the concentric column is 4.5 μm, in the super unit The outer diameters of the single cylinder, annular cylinder and concentric cylinder are kept the same, and the distance between the super-units for constructing the metasurface array is 1.8 μm.

The polarization-independent superlens controls the wavefront of the incident ray polarized light and eliminates chromatic aberration by combining the propagation phase and the compensation phase. First, using the propagation phase as the focusing phase, the incident ray polarized light irradiates the superunit vertically, and the incident wavelength The range is 3.7-4.7 μm, and the superunit is selected to control the wavefront of the incident ray polarized light through the principle of propagation phase, so as to focus at the selected central wavelength; secondly, through the three different types of structures in the superunit Obtain different compensation phase values required to ensure that within the selected incident wavelength range, the compensation phase has a linear relationship with the reciprocal of the incident wavelength, so as to eliminate the chromatic aberration effect at incident wavelengths outside the central wavelength; further, the polarization independent The formula for the phase distribution of the superlens is φ(x, λ)=(2πλ)ne _ff H, where n _eff represents the effective refractive index of the nanostructure, which is closely related to the radius of the nanostructure, and H represents the high.

Principle description:

First, as shown in Figure 3, to achieve broadband achromatic metalens, the formula for the phase distribution is:

in,

和

同时被满足时才能实现；第一项

是在参考波长λ₀处的无色散相位，即如图2所示；第二项

是工作波长的函数，与1/λ线性相关，称为补偿相位。Among them, λ ₀ in equations (1) and (2) is the limited center wavelength, that is, λ ₀ is selected to be 4.2 μm in this embodiment, so the broadband achromatic superlens can only be used when

and

It can only be achieved when both are satisfied; the first item

is the dispersion-free phase at the reference wavelength λ ₀ , as shown in Figure 2; the second term

is a function of the operating wavelength and is linearly related to 1/λ, called the compensation phase.

和

两个相位设计沿着x轴摆放的宽带消色差超透镜的超单元，然此法仅对具有偏振依赖性的超透镜是可行的，限制了消色差超表面器件的发展。In short, according to

and

Two-phase design of metaunits of broadband achromatic metalens arranged along the x-axis, however, this method is only feasible for metalens with polarization dependence, which limits the development of achromatic metasurface devices.

To this end, in this embodiment, a new nanostructure is used in the superunit to realize a polarization-insensitive broadband achromatic superlens. Specifically, a silicon (Si) nanostructure is placed on a calcium fluoride (CaF ₂ ) substrate. Dielectric metasurface platform, each Si nanostructure with a height of H = 4.5 μm; where CaF ₂ was chosen as the substrate due to its low refractive index (n = 1.4) and low absorption loss at the design wavelength; for the design , the phase at a given spatial coordinate x is φ(x,λ)=(2πλ)n _eff H, where n _eff represents the effective refractive index of the nanostructure, which is closely related to the radius of the nanostructure; in the above, the super The subclass of cells consists of three prototype shapes, namely single cylinder, annular cylinder and concentric cylinder. The plane geometric parameters of each prototype are variable, and the distance between each supercell is p = 1.8 μm. In this embodiment, the optical response of each prototype can be controlled by changing the relative radius of each prototype, which ensures that the achromatic metalens are polarization insensitive due to the symmetry of each structure.

和

的消色差超透镜的所有超单元；而与单圆柱相比，环形柱和同心柱都有相对较低的有效折射率从而能实现更多的补偿相位；因此，本申请中同时选择上述三种原型来设计超透镜。Of these, a single cylinder provides the largest compensated phase for each phase value because it has the highest effective refractive index compared to the other two prototypes with the same outer radius; however, a single cylinder only allows each phase to be compensated individually phase value, i.e. it is difficult to find by only a single cylinder that simultaneously satisfies

and

Compared with the single cylinder, both the annular cylinder and the concentric cylinder have relatively low effective refractive index and can achieve more compensation phase; therefore, in this application, the above three are selected at the same time Prototype to design metalens.

和频率(1/λ)在设计波长范围之间的线性关系。Further, the optical properties of the three superunits under the incidence of x (colored dots) and y (black dots) polarized light were calculated. The incident optical response is identical; and Figure 4 demonstrates the phase

and frequency (1/λ) linearly over the design wavelength range.

To further analyze the polarization insensitivity of the three prototypes, the near-field distribution of the electric field in the three prototypes was calculated, and the results are shown in Fig. 5. The selected wavelengths are 4.3 μm, 3.9 μm and 4.0 μm, as shown in Fig. Shown by the dashed lines in 4, corresponding to the three prototypes, with the top and bottom plots being top and side views, respectively, these same near-field results above for x- and y-polarized incidence indicate that the selected prototype is polarization-insensitive.

与空间位置的函数和补偿相位

这两个关键量，每个超单元的设计波长为4.2μm；结果表明，选定超单元实现的相位和补偿相位与所需值有良好的一致性。As such, in this embodiment, the proposed metalens have a diameter of 77.4 μm and a focal length of f=25 μm (NA ≈ 0.84). Figure 6 plots the phases respectively

as a function of spatial position and compensated phase

For these two key quantities, each supercell is designed with a wavelength of 4.2 μm; the results show that the phase and compensated phase achieved by the selected supercell are in good agreement with the desired values.

To sum up, based on the principle of propagation phase compensation for dispersion accumulation phase, three different prototype structures are selected to realize achromatic metalens in the 3.7-4.7μm bandwidth. The three prototype structures achieve different compensation phase ranges, among which the cylindrical The realized compensation phase is the largest and the coaxiality is the smallest. This conclusion can also be deduced from the equivalent medium theory. Since the unit structures used are all centrosymmetric structures, the geometric phase can no longer be used. The focusing phase is determined by the propagation phase at the center wavelength of 4.2μm. This is the key to realizing the polarization-independent achromatic metasurface. By comparing with the chromatic lens, it can be clearly seen that the achromatic metalens can well suppress the chromatic aberration effect.

In order to make the purpose, technical solutions and advantages of the present invention more concise and clear, the present invention is described by the above specific embodiments, which are only used to describe the present invention and should not be construed as limiting the scope of the present invention. It should be pointed out that any modifications, equivalent replacements and improvements made within the spirit and principle of the present invention should be included within the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention should be subject to the appended claims.

Claims (6)

1. A polarization-independent superlens is characterized by comprising a substrate layer (1) and superunits for constructing a super-surface array on the substrate layer, wherein the superunits comprise single cylinders (2), annular cylinders (3) and concentric cylinders (4);

the polarization-independent super lens controls the wave front of incident linearly polarized light and eliminates chromatic aberration by combining a propagation phase and a compensation phase;

the transmission phase is used as a focusing phase, incident linearly polarized light vertically irradiates the superunit, the incident wavelength range is 3.7-4.7 mu m, the superunit is selected through the principle of the transmission phase to control the wave front of the incident linearly polarized light, and the focusing is carried out at the selected central wavelength;

the required different compensation phase values are obtained through three different types of structures in the superunit, and the linear relation between the compensation phase and the reciprocal of the incident wavelength is ensured in the selected incident wavelength range, so that the chromatic aberration effect at the incident wavelength outside the central wavelength is eliminated.

2. The polarization-independent superlens of claim 1, wherein the single cylinder, annular cylinder, and concentric cylinder are all Si nanostructures, and the height H of the single cylinder, annular cylinder, and concentric cylinder is 4.5 μ ι η.

3. The polarization-independent superlens of claim 1, wherein the substrate layer is CaF₂。

4. The polarization-independent superlens of claim 1, wherein the outer diameters of the single, annular, and concentric cylinders in the superunit remain uniform.

5. The polarization-independent superlens of claim 1, wherein the formula of the phase distribution of the polarization-independent superlens is phi (x, lambda) ne (2 pi lambda)_ffH, wherein n_effRepresenting the effective refractive index of the nanostructure and H represents the height of each structure in the superunit.

6. The polarization-independent superlens of claim 1, wherein a distance between superunits constructing the supersurface array is 1.8 μm.

Images