CN115236851B - Planar superlens based on global regulation and control principle and design method thereof - Google Patents
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Abstract
The invention provides a design method of a planar superlens based on a global regulation principle, which comprises the steps of presetting topological charge number, expected focal length and diameter data of the planar superlens to design a structural pattern of the planar superlens; selecting a substrate and a metal layer, wherein the substrate is made of a light-transmitting material with complete transmission property for light, and the metal layer is made of a metal material with complete reflection or complete absorption property for light; manufacturing a substrate layer with a proper size according to the diameter of the planar superlens, and depositing a metal layer on the substrate layer; and etching the designed structural pattern of the planar superlens on the metal layer to ensure that the hole pattern grating array of the pattern on the metal layer is completely transparent and the rest part is completely light-blocking, thus obtaining the planar superlens.
Description
Technical Field
The invention relates to the technical field of photoelectrons, in particular to a planar superlens based on a global regulation and control principle and a design method thereof.
Background
The improvement of the design and performance of a lens, which is an important component in an optical system, is receiving a great deal of attention from researchers. However, conventional optical lenses and fresnel lenses are limited by their physical mechanism, have a large size, and have a single function, and cannot meet the requirements of miniaturization, integration, and multiple performances of optical devices in modern optical systems. To solve this problem, researchers have designed devices that can perform the function of a lens, i.e., superlenses (metalens), based on a superstructured surface. The superlens is composed of an artificial unit structure array with sub-wavelength scale, and the wave front of the incident light is divided into a plurality of sub-wave sources to realize phase modulation by the local interaction between the light and the unit structure, so that the functions of the lens such as focusing and the like are realized. The device has the advantages of ultrathin thickness, small volume, multidimensional regulation and control and the like. The super lens provides a new idea for regulating and controlling the light field in the micro-nano integrated optical system.
At present, the design of the superlens is the local phase regulation realized by resonance and non-resonance effects of the light by the sub-wavelength scatterer. The resonance effect type superlens is designed into a resonant cavity structure, and the abrupt change of the phase is caused by surface plasmon resonance or Mie resonance. However, the resonance effect is wavelength dependent, so superlenses designed based on this principle can only operate in a narrow band range; the super lens with non-resonance effect such as propagation phase or geometric phase method can effectively expand the working bandwidth. For a propagating phase superlens, it is desirable to design the cell structure with a large aspect ratio to meet the accumulated phase variation to alter the wavefront of the light wave. For the geometric phase type superlens, the wave front is regulated and controlled by the relation between the rotation angle and the phase of the nanostructure unit, and the performance of the geometric phase type superlens is strongly dependent on the precise rotation angle of the unit structure and the polarization state of incident light. The method has mode selection on the incident light, can only regulate and control the incident light with specific polarization or topological state, and needs to introduce an additional optical device to preprocess the light source; the mixed super-structure lens based on geometric phase and propagation phase modulation combines the advantages of two regulation and control mechanisms, however, the lens structure design is more complex, and the processing difficulty and the cost are higher.
In summary, in the design of planar superlenses, there are several disadvantages:
(1) The current superlens research is based on the local regulation of the unit structure from the supersurface, so that the superlens is a local regulation element and has no clear connection between a local unit and the whole. All the methods inevitably involve the design and combination of the geometric shape, the size and the spatial arrangement of the single micro-nano structure, and a reasonable period is required to be designed to correspond to the nano unit structure, so that the scattering of light waves in the structural unit is only a local effect, and the coupling and crosstalk effects among the scattering units are negligible. The design to manufacturing process is complex, and the processing technology is strict, so that the cost of the superlens is high, and the development of the application industrialization of the superlens is greatly limited;
(2) Each unit structure in the super-surface array plays an important role in regulating and controlling response of incident light, so that a single super-lens cannot regulate and control any light field with a complex structure, and only can play a role in light of a specific mode after fixing the structure;
(3) Although reconfigurable supersurfaces offer the possibility of implementing a multi-functional superlens, the functionality of current single superlenses is still relatively limited and it is difficult to implement a multi-functional planar superlens.
Disclosure of Invention
In view of the above, a first object of the present invention is to provide a design method of a planar superlens based on a global adjustment principle, which implements focusing adjustment of a spatial structure beam with arbitrary spatial phase and polarization structure by integrally designing a lens structure pattern.
Based on the same inventive concept, a second object of the present invention is to provide a planar superlens based on the global regulation principle.
The first object of the present invention can be achieved by adopting the following technical scheme:
A design method of a planar superlens based on a global regulation principle comprises the following steps:
Designing a structural pattern of the planar superlens according to a preset topological charge number, an expected focal length and diameter data of the planar superlens, wherein the structural pattern comprises a plurality of non-periodically distributed hole pattern grating arrays;
selecting a substrate and a metal layer, wherein the substrate is made of a light-transmitting material with complete transmission property for light, and the metal layer is made of a metal material with complete reflection or complete absorption property for light;
Manufacturing a substrate layer with a proper size according to the diameter of the planar superlens, and depositing a metal layer on the substrate layer;
And etching the designed structural pattern of the planar superlens on the metal layer to ensure that the hole pattern grating array of the pattern on the metal layer is completely transparent and the rest part is completely light-blocking, thus obtaining the planar superlens.
Further, the deposition thickness of the deposited metal layer is not more than one tenth of the operating wavelength.
Further, the substrate material is glass, and the metal layer material is gold.
Further, the designing the structural pattern of the planar superlens according to the topological charge number, the expected focal length and the diameter of the planar superlens, which is preset, includes:
Taking the angle cosine wave related to the topological charge number as reference light, interfering with spherical wave carrying expected focal length information, and obtaining an amplitude hologram;
Using a transmittance function to represent an amplitude hologram, and using a threshold value cut-off function related to the amplitude to process the transmittance function to obtain a transmittance function expression which is related to topological charge numbers and carries expected focal length information;
and calculating a pattern corresponding to the transmittance function expression to obtain the structural pattern of the planar superlens.
Further, the topological charge number related angle cosine wave has a cosine periodic profile (circular grating) along an azimuth angle, and the expression is cos (mθ), wherein m is a non-zero integer, the topological charge number representing the oscillation frequency of the angle cosine wave, and θ is the azimuth angle.
Further, the expression of the spherical wave carrying the expected focal length information is thatWherein/>Is the phase distribution of spherical wave front, and the expression is/> In the expression of (2), λ is the operating wavelength, x, y are the spatial coordinates of the spherical wavefront, and z f is the intended focal length.
Further, using a transmittance function to represent an amplitude hologram, and using a threshold value cut-off function related to the amplitude to process the transmittance function to obtain a transmittance function expression related to topological charge number and carrying expected focal length information, including:
let t (x, y) denote the transmittance function of the amplitude hologram at the (x, y) position, it can be expressed as:
An amplitude dependent threshold cut-off function q (x, y), q=arcsin (a (x, y))/pi is defined, where a (x, y) is a bias function that enables adjustment of the non-uniform distribution of the incident amplitude. Using the Signal sign function, the values of t (x, y) smaller and larger than the threshold are set equal to 0 and 1, respectively, namely:
Because the hologram is created by the interference of two waves in the transverse plane, based on paraxial approximation conditions, The simplification is as follows:
after finishing and deduction, the transmittance function expression is as follows:
The second object of the invention can be achieved by adopting the following technical scheme:
a planar superlens based on global steering principles, comprising:
A metal layer made of a metal material having a total reflection or total absorption characteristic for light and having a structural pattern including a plurality of non-periodically distributed hole pattern grating arrays; the structural pattern carries focal length information of the superlens; in the structural pattern, the hole pattern grating array is partially completely transparent, and the rest is completely light-blocking;
the substrate is made of a light-transmitting material with complete transmission property to light and is used for bearing the metal layer.
Further, the substrate is a glass substrate, the metal layer is a gold film, and the thickness of the gold film is not more than one tenth of the working wavelength.
Further, the structural pattern is a plurality of non-periodically distributed hole pattern grating arrays having rotational symmetry and axial symmetry.
Further, the structural scheme is obtained by the following method:
taking the angle cosine wave related to the topological charge number as reference light, interfering with spherical wave carrying expected focal length information, and obtaining an amplitude hologram; the expression of the angle cosine wave is cos (mθ), wherein m is a non-zero integer, the topological charge number of the oscillation frequency of the angle cosine wave is represented, and θ is an azimuth angle; the expression of the spherical wave carrying the expected focal length information is that Wherein the method comprises the steps ofIs the phase distribution of spherical wave front, and the expression is/> In the expression of (2), lambda is the working wavelength, x and y are the spatial coordinates of the spherical wavefront, and z f is the expected focal length;
Using a transmittance function to represent an amplitude hologram, and using a threshold value cut-off function related to the amplitude to process the transmittance function to obtain a transmittance function expression which is related to topological charge numbers and carries expected focal length information;
and calculating a pattern corresponding to the transmittance function expression to obtain the structural pattern of the planar superlens.
Further, the transmittance function expression is:
And calculating a pattern corresponding to the transmittance function expression, wherein the obtained structural pattern of the planar superlens has |m| heavy rotational symmetry and |m| symmetry axes.
Compared with the prior art, the invention has the following beneficial effects:
1. The invention is different from the superlens based on local unit regulation in the traditional planar superlens, and provides an ultrathin planar superlens based on a global regulation mechanism and with the thickness smaller than the sub-wavelength magnitude, and the functions of Fourier transformation, imaging, broadband response, scalar and vector light field regulation and the like are realized on a single lens structure.
2. The planar superlens provided by the invention can increase the convergent spherical wave front on incident light with any space structure by designing the structural pattern, and when the incident light is diffracted by the planar superlens structure, the output light can be focused at an expected focus and the initial phase and polarization characteristics can be maintained. Therefore, the mode-free selection of the incident light is realized, and the focusing requirements of plane waves (scalar light field) and light beams with arbitrary spatial phases and spatial structures of polarization structures can be met.
3. Compared with the traditional optical lens, the planar superlens provided by the invention has the characteristics of ultra-thin (the thickness can be less than one tenth of the working wavelength), and can be applied to miniaturized modern optical application scenes; compared with the superlens, the planar superlens has a simple structure, reduces the processing and manufacturing cost, and greatly expands the application prospect of the planar superlens.
Drawings
Fig. 1 is a planar superlens structure pattern according to embodiment 1 of the present invention.
Fig. 2 is a diagram of a plane wave focusing experimental apparatus in embodiment 2 of the present invention.
Fig. 3 is a graph showing the result of a plane wave focusing experiment of the planar superlens according to embodiment 2 of the present invention.
Fig. 4 is a graph of the result of focusing experiments of the structural light field of the planar superlens according to embodiment 2 of the present invention, wherein (a) is the light field intensity distribution diagram of the x-z plane after the vector light field passes through the planar lens sample, (b) is the light field distribution diagram of the focal plane corresponding to (a), (c) is the x-component diagram of the focal plane corresponding to (b) obtained by using the analyzer, (d) is the light field intensity distribution diagram of the x-z plane after the vortex light field passes through the planar lens sample, (e) is the light field distribution diagram of the focal plane corresponding to (d), and (f) is the interference pattern obtained by using the vortex light of the focal plane and the plane wave.
Fig. 5 is a diagram of a fourier transform performance verification experiment apparatus of example 3 of the present invention.
Fig. 6 is a graph showing the experimental results of fourier transform performance verification of the planar superlens according to example 3 of the present invention.
Fig. 7 is a graph of experimental results of imaging performance of the planar superlens according to embodiment 3 of the present invention, wherein two graphs a and c are imaging results, and b is a schematic diagram of an imaging target.
Fig. 8 is a graph of broadband performance experiment results of the planar superlens of embodiment 3 of the present invention, in which a graph a shows the optical field intensity distribution of the x-z plane after plane waves of different wavelengths are incident on the planar lens sample, and b illustrates the optical field distribution of the focal plane.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments, and all other embodiments obtained by persons of ordinary skill in the art without making any inventive effort based on the embodiments of the present invention are within the scope of protection of the present invention.
Example 1:
the embodiment provides a design method of a planar superlens based on a global regulation principle, which comprises the following steps:
s10, designing a structural pattern of the planar superlens according to the preset topological charge number, the expected focal length and the diameter data of the planar superlens, wherein the structural pattern comprises a plurality of non-periodically distributed hole pattern grating arrays, and the method comprises the following steps:
s11, in this embodiment, the expected focal length z f =50 μm, the diameter is 70 μm, and the topological charge number m=8 is set.
S12, using an angle cosine wave with the expression cos (mθ) as reference light, wherein the phase distribution of the angle cosine wave and the spherical wavefront is as followsSpherical wave/>Interference is performed to obtain an amplitude hologram.
S13, let t (x, y) denote the transmittance function of the amplitude hologram at the (x, y) position, and can be expressed as:
An amplitude dependent threshold cut-off function q (x, y), q=arcsin (a (x, y))/pi is defined, where a (x, y) is a bias function that enables adjustment of the non-uniform distribution of the incident amplitude. Using the Signal sign function, the values of t (x, y) smaller and larger than the threshold are set equal to 0 and 1, respectively, namely:
Because the hologram is created by the interference of two waves in the transverse plane, based on paraxial approximation conditions, The simplification is as follows:
after finishing and deduction, the transmittance function expression is as follows:
S14, calculating a pattern corresponding to the transmittance function expression, and obtaining the structural pattern of the planar superlens as shown in figure 1.
S20, selecting a material of a substrate and a metal layer, wherein the material of the substrate is a light-transmitting material with complete transmission property for light, and the material of the metal layer is a metal material with complete reflection or complete absorption property for light. In this embodiment, the substrate material is transparent glass, and the metal layer material is gold.
S30, manufacturing a substrate layer with an adaptive size according to the diameter of the planar superlens, and depositing a metal layer on the substrate layer; in this example, a gold film having a thickness of not more than one tenth of the operating wavelength was deposited on a 0.3mm glass substrate.
And S40, etching a designed structural pattern of the planar superlens on the gold film by using a focused ion beam technology, so that the white part of the pattern on the metal layer is completely transparent, and the black part is completely light-blocking, thereby obtaining the planar superlens.
Example 2:
The embodiment provides a planar superlens based on a global regulation principle, which is used for focusing plane waves and a structural light field, and specifically comprises the following steps:
s10, manufacturing a planar superlens sample, which comprises the following steps:
S11, setting an expected focal length z f =50 mu m, setting the diameter to be 70 mu m, and designing structural patterns of the planar superlens according to topological charge numbers m=4, 6, 8 and 10;
S12, depositing a gold film with the thickness not more than one tenth of the working wavelength on a 0.3mm glass substrate, and etching a structural pattern of the designed planar superlens on the gold film to enable a hole-type grating array of the pattern on the metal layer to be completely transparent and the rest to be completely light-blocking, so as to obtain 4 planar superlenses corresponding to m=4, 6, 8 and 10.
S20, constructing a plane wave focusing optical system, as shown in fig. 2. In this embodiment, a helium-neon laser (with a working wavelength of 632.8 nm) is selected as the light source, and is collimated and expanded, light is incident on a planar superlens sample, and a diffraction field transmitted through the sample is collected by using a 150×objective lens and a cylindrical lens in a matching manner and projected onto a camera (CCD) to realize focusing of plane waves.
S30, building a structural light field focusing optical system, namely taking an experimental device shown in fig. 2 as a basis, respectively making vector light and vortex rotation normally incident on a planar superlens sample corresponding to m=8 after preparing the vector light and the vortex rotation, and collecting light field intensity distribution after penetrating through the sample by utilizing a 150 multiplied objective lens and a barrel lens in a matching way and projecting the light field intensity distribution on a CCD (charge coupled device) to realize focusing of structural light beams.
In this embodiment, the focusing experiment result of the plane wave focusing optical system is shown in fig. 3. It can be seen that the light modulated by the planar superlens has a focusing effect near z=50μm, the optical field distribution of the focal plane is shown in fig. 3 (b), the half-width of the focal point measured by the experiment is about 540nm, and the focusing effect of sub-wavelength magnitude is achieved.
In this embodiment, the focusing experiment result of the structured light field focusing optical system is shown in fig. 4. It can be seen that the planar superlens sample focused the structured light field to a focal plane z=50μm, the peak-to-peak value of the focal spot was 1.2 μm, the full width at half maximum of the central dark ring was close to 600nm, and the planar superlens did not change the phase and polarization properties of the incident light field.
Example 3:
The embodiment provides a planar superlens based on a global regulation principle and used for Fourier transformation, which specifically comprises the following steps:
s10, manufacturing a planar superlens sample, which comprises the following steps:
S11, setting an expected focal length z f =18 cm, wherein the diameter is 5mm, the topological charge number m=8, and designing a structural pattern of the planar superlens;
S12, depositing a gold film with the thickness not more than one tenth of the working wavelength on the 0.3mm glass substrate, and etching a designed structural pattern of the planar superlens on the gold film to enable a hole-type grating array of the pattern on the metal layer to be completely transparent and the rest to be completely light-blocking, so as to obtain the planar superlens.
S20, building an experimental system, as shown in fig. 5. Light emitted by the laser is collimated and then enters a reflective Spatial Light Modulator (SLM) loaded with an Airy beam phase diagram, and then is normally incident on a planar superlens sample. The diffraction field after passing through the lens is collected by means of a CCD. A cross-sectional view of the Airy beam taken at different propagation distances is shown in FIG. 6.
Fig. 7 illustrates the experimental results of imaging, wherein both a and c are imaging results, and b is a schematic view of the imaging target, and the planar lens sample clearly shows the image of the highlighted area in the resolution plate.
The experimental results of broadband response are shown in fig. 8, and the planar lens can generate modulation focusing effect on incident light under the illumination of light sources with different wavelengths. Wherein plot a in fig. 8 shows the optical field intensity distribution in the x-z plane after a plane wave of different wavelengths is incident on a planar lens sample, visible light can be focused to different axial propagation positions (i.e., z f) of 28.9cm-15cm when the wavelength is scanned from 430nm to 780 nm; the optical field distribution of the focal plane is shown in graph b of fig. 8, and the half width of the focal point measured experimentally is about 30 μm.
Therefore, the planar superlens of the embodiment successfully converts the amplitude information of the Airy light field near the focal plane, and the Airy light beam converted by the phase information has the properties of diffraction-free transmission and self-acceleration.
In summary, the ultra-thin planar superlens provided by the invention realizes multiple functions of fourier transform, imaging, broadband response, scalar and vector light field regulation and control on a single lens structure, and can meet the focusing requirements of plane waves (scalar light field) and light beams with spatial structures of any spatial phase and polarization structure. Compared with the traditional optical lens and the traditional superlens, the lens has respective advantages, and greatly expands the application prospect of the planar superlens.
It is apparent that the above-described embodiments are only some embodiments of the present invention, but not all embodiments, and the present invention is not limited to the details of the above-described embodiments, and any appropriate changes or modifications made by those skilled in the art will be deemed to be within the scope of the present invention.
Claims (5)
1. The design method of the planar superlens based on the global regulation principle is characterized by comprising the following steps of:
Designing a structural pattern of the planar superlens according to a preset topological charge number, an expected focal length and diameter data of the planar superlens, wherein the structural pattern comprises a plurality of non-periodically distributed hole pattern grating arrays;
selecting a substrate and a metal layer, wherein the substrate is made of a light-transmitting material with complete transmission property for light, and the metal layer is made of a metal material with complete reflection or complete absorption property for light;
Manufacturing a substrate layer with a proper size according to the diameter of the planar superlens, and depositing a metal layer on the substrate layer;
Etching a designed structural pattern of the planar superlens on the metal layer to ensure that the hole pattern grating array of the pattern on the metal layer is completely transparent and the rest part is completely light-blocking, so as to obtain the planar superlens;
The design of the structural pattern of the planar superlens according to the preset topological charge number, the expected focal length and the diameter of the planar superlens comprises the following steps:
Taking the angle cosine wave related to the topological charge number as reference light, interfering with spherical wave carrying expected focal length information, and obtaining an amplitude hologram;
Using a transmittance function to represent an amplitude hologram, and using a threshold value cut-off function related to the amplitude to process the transmittance function to obtain a transmittance function expression which is related to topological charge numbers and carries expected focal length information;
Calculating a pattern corresponding to the transmittance function expression to obtain a structural pattern of the planar superlens;
The topological charge number related angle cosine wave has a cosine periodic contour along an azimuth angle, the expression is cos (mθ), wherein m is a non-zero integer, the topological charge number of the oscillation frequency of the angle cosine wave is represented, and θ is the azimuth angle;
The expression of the spherical wave carrying the expected focal length information is that Wherein/>Is the phase distribution of spherical wave front, and the expression is/> In the expression of (2), lambda is the working wavelength, x and y are the spatial coordinates of the spherical wavefront, and z f is the expected focal length;
let the transmittance function t (x, y) of the amplitude hologram at the (x, y) position be:
Defining a threshold cut-off function q (x, y) related to amplitude, q=arcsin (a (x, y))/pi; wherein A (x, y) is a bias function for adjusting the non-uniform distribution of the incident amplitude; using the Signal sign function, the value of t (x, y) less than the threshold is set equal to 0, and the value of t (x, y) greater than the threshold is set equal to 1, namely:
Phase distribution of spherical wave fronts The simplification is as follows:
after finishing and deduction, the transmittance function expression is as follows:
2. The method for designing a planar superlens based on the global steering principle according to claim 1, wherein the deposition thickness of the deposited metal layer is not more than one tenth of the operating wavelength.
3. The method for designing a planar superlens based on the global regulation principle according to claim 1, wherein the substrate material is glass and the metal layer material is gold.
4. A planar superlens based on global regulation and control principle, comprising:
A metal layer made of a metal material having a total reflection or total absorption characteristic for light and having a structural pattern including a plurality of non-periodically distributed hole pattern grating arrays; the structural pattern carries focal length information of the superlens; in the structural pattern, the hole pattern grating array is partially completely transparent, and the rest is completely light-blocking;
a substrate made of a light-transmitting material having a completely light-transmitting property for carrying a metal layer;
the structural pattern is a plurality of non-periodically distributed hole pattern grating arrays with rotational symmetry and axial symmetry; the structural pattern is obtained by the following method:
Taking the angle cosine wave related to the topological charge number as reference light, interfering with spherical wave carrying expected focal length information, and obtaining an amplitude hologram;
Using a transmittance function to represent an amplitude hologram, and using a threshold value cut-off function related to the amplitude to process the transmittance function to obtain a transmittance function expression which is related to topological charge numbers and carries expected focal length information;
Calculating a pattern corresponding to the transmittance function expression to obtain a structural pattern of the planar superlens;
The topological charge number related angle cosine wave has a cosine periodic contour along an azimuth angle, the expression is cos (mθ), wherein m is a non-zero integer, the topological charge number of the oscillation frequency of the angle cosine wave is represented, and θ is the azimuth angle; the expression of the spherical wave carrying the expected focal length information is that Wherein/>Is the phase distribution of spherical wave front, and the expression is/> In the expression of (2), lambda is the working wavelength, x and y are the spatial coordinates of the spherical wavefront, and z f is the expected focal length; let the transmittance function t (x, y) of the amplitude hologram at the (x, y) position be:
Defining a threshold cut-off function q (x, y) related to amplitude, q=arcsin (a (x, y))/pi; wherein A (x, y) is a bias function for adjusting the non-uniform distribution of the incident amplitude; using the Signal sign function, the value of t (x, y) less than the threshold is set equal to 0, and the value of t (x, y) greater than the threshold is set equal to 1, namely:
Phase distribution of spherical wave fronts The simplification is as follows:
after finishing and deduction, the transmittance function expression is as follows:
5. The planar superlens based on global steering principle according to claim 4, wherein the substrate is a glass substrate, the metal layer is a gold film, and the thickness of the gold film is not more than one tenth of an operating wavelength.
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