CN107703579B - Metasurface lens for realizing lateral multifocal focusing and its realization method - Google Patents

Abstract

The invention discloses a metasurface lens and a realization method for realizing lateral multi-focus focusing. By dividing the metasurface lens into sub-wavelength square array units; each square unit is further divided into smaller square pixel units. Each sub-wavelength square unit is nested with a slit composed of certain pixels and having a certain aspect ratio. The doubling of the azimuthal orientation of the slit corresponds to the phase modulation value of the unit. The phase modulation value is composed of two parts superimposed, one part of the phase value modulation value produces a spherical wave that changes in the radial direction; the other part of the phase value modulation value that changes in the azimuth direction is determined by the Fourier transform phase shift theorem. The slits in each sub-wavelength square unit whose azimuth orientation is determined are made into correspondingly oriented titania cuboid array metasurface lenses. Multifocal focusing in the focal plane can be produced with circularly polarized light incidence.

Description

Metasurface lens for realizing lateral multifocal focusing and its realization method

technical field

The invention relates to a metasurface lens, in particular to a metasurface lens capable of realizing lateral multi-focus focusing, and also relates to a method for realizing lateral multi-focus focusing based on the lens.

Background technique

Traditional optical lenses achieve focusing by adjusting the phase of incident light through changes in the thickness of the glass. Such lenses are bulky and heavy. With the continuous development of integrated photonic devices, traditional optical devices including lenses obviously cannot meet the requirements of large-scale integration and device miniaturization and functional diversification.

Due to the phase modulation of the incident light by metal or dielectric nano-antennas, various nano-antenna arrays required for phase modulation can be processed on the surface of the substrate by using coating technology and modern micro-processing technology, so as to realize the phase and polarization modulation of the incident light. Optoelectronic devices, such as polarization converters, beam splitters, dispersive elements, achromatic focusing lenses, etc. Such devices are called metasurface elements, and their thickness and size can usually be on the order of microns. Therefore, it is extremely small in size, light in weight, and easy to integrate.

On the other hand, the vector beam focusing theory and phase modulation method of high numerical aperture objective lenses have enabled us to produce a variety of focusing spots, such as ring-shaped spots, chain-shaped spots, spiral spots, needle-shaped spots, tunnel-shaped spots, spherical spot, two-dimensional array spot and three-dimensional array spot. These spots have a very wide range of applications in super-resolution fluorescence imaging, confocal microscopy, ultra-precision laser parallel processing, lithography, super-resolution laser additive manufacturing, micro-nanoscale laser capture and manipulation, and optical data storage.

There is currently no metasurface lens that can generate lateral multifocals using nanoantenna arrays.

Contents of the invention

The present invention proposes a metasurface lens and a realization method for realizing lateral multi-focal focusing. The combination of modulation components meets the requirements of large-scale integration and device miniaturization with diversified functions.

Technical scheme of the present invention is as follows:

A metasurface lens for realizing lateral multifocal focusing is manufactured through the following steps:

(1) Select the material of the nano-antenna array according to the wavelength, and determine the side length size of the sub-wavelength square unit according to the wavelength;

(2) Divide the material surface of the nano-antenna array into MxM sub-wavelength size square units, M is an odd number;

(3) A slit is embedded in the center of each square unit, and the slit is parallel to the lateral sides of the square unit;

(4) establish a Cartesian coordinate system with the center of the vertical (M+1)/2 square unit on the horizontal (M+1)/2 row as the coordinate origin, and the plane where the square unit is located is the XY plane, and the coordinate system The x-axis is parallel to the lateral side of the square unit, and the axis perpendicular to the square unit is the z-axis;

(5) Construct a series of concentric circles centered on the origin, and divide the XY coordinate plane into a series of concentric ring regions;

(6) Taking the positive direction of the x-axis as the starting line of the azimuth angle, a radial ray is made from the origin of the coordinate system to divide the concentric annular area into P partitions, P is an even number, and each partition is further drawn from the origin of the coordinate system. Divide into Q sub-regions with equal central angles, Q is equal to the number of focal points to be generated, and the i-th sub-region of each partition corresponds to the i-th focal point;

(7) Rotate the nested slits in each square unit by different angles, the rotation angle of the slits in each square unit is ψ ₀ =ψ ₁ +ψ ₂ , where ψ ₁ is the focus spherical wave to generate focal length f Half of the required phase modulation value, the size of this value is determined by the radius of the ring area where the center point of the square unit is _; Half of , the lateral displacement refers to the displacement of the projection of the focal point on the material surface of the nano-antenna array relative to the origin of the coordinate system;

(8) Determine the final angle of the slit according to the calculated rotation angle of each slit, and then process the slits in each square unit.

Further: For the slit in the square unit whose center is in the i-th sub-area in the partition, the required rotation angle:

In the above formula, λ is the wavelength of the incident light, f is the focal length corresponding to the focal plane, R is the radius of the annular area where the center point of the square unit is located, and x and y are the abscissa of the center of the square unit where the slit is located in the coordinate system coordinates and ordinates, NA is the numerical aperture of the lens, Δxi and Δy _i are the abscissa and ordinate of the projection of the focal point corresponding to the _ith sub-region on the material surface of the nano-antenna array in the coordinate system.

Further: in step (2), divide each square unit into NxN pixels, N is an odd number, and the slit is composed of several pixels.

Further: when constructing concentric circles, the radius is taken as an integer multiple of the side length of the square unit.

Further: processing the slit by using coating and micro-processing methods.

Further: the position of the multi-focus can be set arbitrarily.

The method for realizing lateral multi-focal focusing based on the above-mentioned metasurface lens: circularly polarized light is incident vertically on the metasurface element, and lateral Q focal points are generated at the back focal plane of the metasurface lens.

Further: the method for generating the circularly polarized light is to convert the laser beam into circularly polarized light through a linear polarizer of corresponding wavelength and a quarter-wave plate.

Compared with the prior art, the present invention has the following advantages: first: the entire multifocal lens is a planar two-dimensional element with a very small size, thickness and volume, which can be on the order of hundreds of microns, and is very easy to integrate; second: The entire multi-focus phase modulation and focusing functions are unified by the nano-antenna array, without the need for additional phase modulation components, and without the use of spatial light modulators and Fourier transform imaging systems to achieve lateral multi-focus focusing.

Description of drawings

Fig. 1 is a schematic diagram of the rotation of the square unit array, the concentric ring area, the partition, the sub-area and the nested slit in a single square unit of the metasurface lens of the present invention;

Fig. 2 is a schematic diagram of the realization of specific phase modulation distribution by the metasurface lens of the present invention through slit arrays with different rotation angles;

3 is a schematic diagram of a circular metalens composed of 60x60 square array elements designed by the present invention;

Fig. 4 is a schematic diagram of 10 focal points generated at the focal plane when circularly polarized light is incident on a transverse multifocal metasurface lens.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the present invention. Apparently, the described embodiments are some, but not all, embodiments of the present invention.

First determine the radius of the lens to be designed, the material of the lens substrate, the focal length, and the number of focal points. In this embodiment, the lens radius is 100 microns, the base material is quartz material with a thickness of 0.17 mm, the focal length is 90 microns, the number of focal points is 10, and the laser wavelength used is 633 nanometers. The lens is designed, manufactured and verified according to the following steps.

Step 1: As shown in accompanying drawing 1, select the material of suitable nano-antenna array according to wavelength, usually material can be noble metal such as gold and silver, also can be dielectric material, present embodiment chooses titanium dioxide; Determine sub-wavelength size according to wavelength The side length of the square unit is generally smaller than the wavelength of the incident light, and the side length in this embodiment is 400 nanometers.

Step 2: Divide the material surface of the nano-antenna array into MxM square units of sub-wavelength size, M is an odd number and must be large enough, preferably 300 to 1000; then divide each square unit into NxN pixels, and N is an odd number. In this embodiment, M=401, N=25.

Step 3: Embed a slit consisting of a certain number of pixels and having a certain aspect ratio in the center of each square unit, the slit is parallel to the lateral side of the square unit, and the width is less than a quarter of the wavelength or One-fifth, in this embodiment, the width of the slit is set to 80 nanometers, and the length is 400 nanometers. Accompanying drawing 2 is a partial enlarged view of a square unit array with a certain orientation of slits.

Step 4: Establish a Cartesian coordinate system with the center of the 201st vertical square unit on the 201st horizontal column as the coordinate origin, take the plane where the square unit is located as the XY plane, and the x-axis of the coordinate system is parallel to the lateral side of the square unit, Take the axis of the vertical square unit as the z-axis.

Step 5: Construct a series of concentric circles with the origin as the center and an integer multiple of the side length of the square unit as the radius, and divide the XY coordinate plane into a series of concentric annular areas, that is, the radii of the concentric circles are 400 nanometers, 800 nanometers, and 1200 nanometers , 1600 nanometers, etc., the radius difference between two adjacent concentric circles is 400 nanometers.

Step 6: Take the positive direction of the x-axis as the starting line of the azimuth angle, and make a radial ray from the origin of the coordinate system to divide the concentric annular area into P partitions, where P is an even number, such as 60, 90 or 120, etc., in this embodiment In P=60, each partition is further divided into Q sub-regions with equal central angles by taking rays from the origin of the coordinate system, Q is equal to the number of focal points to be generated, and the i-th sub-region of each partition corresponds to the i-th focus , Q=10 in this embodiment.

Step 7: According to the requirement of generating multi-focus, rotate the nested slits in each square unit by different angles ψ ₀ through MATLAB program programming. The angle ψ ₀ of the slit rotation of each square unit is composed of two parts: ψ ₀ =ψ ₁ +ψ ₂ , where ψ ₁ is half of the phase modulation value required to produce the focal length f by focusing the spherical wave, and the magnitude of this value Determined by the size of the radius of the annular area where the center point of the square unit is located; wherein ψ ₂ is half of the phase modulation value required to make the generated focus generate a lateral displacement in the focal plane of the metasurface lens, and the lateral displacement refers to the focal point in the nanometer The displacement of the projection of the material surface of the antenna array relative to the origin of the coordinate system is determined by the Fourier transform phase shift theorem, specifically:

For a slit in a square cell whose center is in the i-th subregion of the partition, the required rotation angle is:

In the above formula, λ is the wavelength of the incident light, f is the focal length corresponding to the focal plane, R is the radius of the annular area where the center point of the square unit is located, and x and y are the abscissa of the center of the square unit where the slit is located in the coordinate system coordinates and ordinates, NA is the numerical aperture of the lens, Δxi and Δy _i are the abscissa and ordinate of the projection of the focal point corresponding to the _ith sub-region on the material surface of the nano-antenna array in the coordinate system. The position of the multi-focus can be set arbitrarily.

Accompanying drawing 3 is a top view of a metasurface lens composed of a 60x60 square unit array.

Step 8: Determine the final angle of the slit according to the calculated angle of rotation of each slit, take the length and width of the slit as the length and width of the cuboid, and use the coating technology and micro-machining technology to make the designed titanium dioxide dielectric material Cuboid Array Metasurface Lens. The cuboid height of the titanium dioxide dielectric material is 600 nm.

Step 9: The laser beam is converted into circularly polarized light by a linear polarizer and a quarter-wave plate of the corresponding wavelength, and is vertically incident on the aligned metasurface lens.

Step 10: As shown in Figure 4, generate 10 lateral focal points at the rear focal plane of the metasurface lens. Use another objective lens, tube lens, charge-coupled imaging device, computer and software to observe and verify the focus.

Although the present invention has been described in detail with general descriptions and specific examples above, it is obvious to those skilled in the art that some modifications or improvements can be made on the basis of the present invention. Therefore, the modifications or improvements made on the basis of not departing from the spirit of the present invention all belong to the protection scope of the present invention.

Claims (7)

1. a kind of super structure surface lens for realizing lateral multiple-point focusing, it is characterised in that: made by following step:

(1) according to the material of wavelength selection nanotube antenna array, and the side of sub-wavelength dimensions square shaped cells is determined according to wavelength Long size；

(2) material surface of nanotube antenna array is divided into MxM sub-wavelength dimensions square shaped cells, M is odd number；

(3) a slit is embedded at each square shaped cells center, the slit is parallel with the widthwise edge of square shaped cells；

(4) right angle is established as coordinate origin using longitudinal square shaped cells center (M+1)/2 on laterally (M+1)/2 column to sit Mark system, using plane where square shaped cells as X/Y plane, the x-axis of the coordinate system and the widthwise edge of square shaped cells are parallel, Using the axis of vertical square shaped cells as z-axis；

(5) a series of concentric circles are constructed centered on origin, and XY coordinate plane is divided into a series of concentric annular regions；

(6) again using x-axis forward direction as azimuth start line, make radial ray from coordinate origin and be divided into concentric annular regions P subregion, P are even number, and each subregion is further divided into the equal sub-district of Q central angle from coordinate origin as ray again Domain, Q is equal with the focus number to be generated, and i-th of subregion of each subregion corresponds to i-th of focus；

(7) slit that each square shaped cells are nested with is rotated to different angles, the slit rotation of each square shaped cells Angle ψ₀=ψ₁+ψ₂, wherein ψ₁The half that focal length is phase modulation values required for f is generated to focus spherical wave, this value Size is determined by the annular region radius size where square shaped cells central point；ψ₂To keep the focus generated saturating on super structure surface The half of phase modulation values required for lateral displacement is generated in mirror focal plane, the lateral displacement refers to focus in nano-antenna Displacement of the projection of the material surface of array relative to coordinate origin；

(8) it needs the angle rotated to determine the final angle of slit according to calculated each slit, it is single then to process each square Slit in member.

2. the super structure surface lens as described in claim 1 for realizing lateral multiple-point focusing, it is characterised in that: exist for center Slit in subregion in the square shaped cells of i-th of subregion, the angle of required rotation:

In above formula, λ is lambda1-wavelength, and f is the corresponding focal length in focal plane, and R is square the annulus where unit center point The radius in domain, x, y are the abscissa and ordinate of the center of the square shaped cells where the slit in a coordinate system, and NA is lens Numerical aperture, Δ x_i、Δy_iFor focus corresponding to i-th of subregion the material surface of nanotube antenna array projection Abscissa and ordinate in a coordinate system.

3. the super structure surface lens as described in claim 1 for realizing lateral multiple-point focusing, it is characterised in that: in step (2) In, then each square shaped cells are divided into NxN pixel, N is odd number, and the slit is made of several pixels.

4. the super structure surface lens as described in claim 1 for realizing lateral multiple-point focusing, it is characterised in that: building concentric circles When using the integral multiple of the side length of square shaped cells as radius.

5. the as described in claim 1 super structure surface lens for realizing lateral multiple-point focusing, it is characterised in that: using plated film and Micro-processing method processes the slit.

6. based on the method that super structure surface lens as claimed in claim 1 to 5 realize lateral multiple-point focusing, feature It is: with super structure surface element described in circularly polarized light vertical incidence, transverse direction Q is generated at super structure surface lens back focal plane Focus.

7. the method as claimed in claim 6 for realizing lateral multiple-point focusing, it is characterised in that: the generation of the circularly polarized light Method is that laser beam is converted into circularly polarized light by the linear polarizer and quarter-wave plate of respective wavelength.