1. The bidirectional holographic modulation method based on the broadband visible light nanometer super surface is characterized by comprising the following steps of:

(1) constructing a unit structure for forming a super surface: wherein the structure is a two-layer structure, and the two layers comprise a substrate and a nano brick arranged on the substrate; the super surface is formed by a plurality of unit structures which are periodically arranged on the same plane; the nano bricks in the unit structure have independently set size parameters;

(2) scanning the sizes of different unit structures by setting working wavelength by adopting an electromagnetic simulation tool to obtain the relationship between the size and the phase of the nano-brick under different linearly polarized light;

(3) designing two phase type holographic patterns, converting phase information of the two phase type holographic patterns into size parameters under two polarized lights, and arranging unit structures to construct a super surface;

(4) and a layer of half-wave plate film is covered on the super surface, and the bidirectional holographic display of the single-layer nano structure in the broadband visible light range is realized by changing the direction of incident light.

2. The modulation method according to claim 1, characterized in that: in the step (1), the nano brick and the substrate are both in a cuboid structure; wherein the cross section of the substrate is square; the sizes of the substrates of the unit structures are the same.

3. The modulation method according to claim 1, characterized in that: the substrate of the unit structure is constructed of a transparent optical material with a low refractive index, the material including MgF₂、Al₂O₃、SiO₂The material of the nano brick comprises TiO₂、Si、Ag、Au、Cu、Al。

4. The modulation method according to claim 1, characterized in that: the size parameters in the steps (1) and (2) comprise the length L, the width W, the height H and the side length P of the cross section of the substrate.

5. The modulation method according to claim 4, wherein a xoy rectangular coordinate system is established with the rectangular sides of the top surface of the substrate as the x-axis and the y-axis and the vertex as the origin, and the nanoblock has a length L along the x-axis and a width W along the y-axis; when the x-polarized light is incident, the phase change is caused by L; when the y-polarized light is incident, the phase change is caused by W.

6. The modulation method according to claim 1, characterized in that: the method for converting the information of the two holographic images into the information of the size of the nano brick in the step (3) comprises the following steps: under the working wavelength, the structural units with the nano bricks with different sizes have different phases, and the relation between the sizes and the phases of the nano bricks is obtained through scanning; and then establishing a one-to-one correspondence relationship between the pixel in the holographic image and the size of each unit structure, and finally realizing different polarization storage of different holographic image information.

7. The modulation method according to claim 1, characterized in that: the super surface is covered with a layer of half-wave plate film, polarization multiplexing is converted into direction multiplexing, and emergent light forms two completely independent encrypted holographic images in the front and back directions only by changing the direction of incident light.

8. The super surface capable of realizing the bidirectional holographic image display of the single-layer nano structure in the broadband visible light range is characterized in that: prepared by the method of any one of claims 1 to 7.

9. Use of the metasurface of claim 8 for non-reciprocal information processing, optical metrology and security enhanced anti-counterfeiting of encryption/decryption.

Background

Conventional optical devices based on geometric optics or diffractive optics have a rather limited way to control the turning and splitting of the light beam due to limitations in structural size and inherent optical properties. For example, blazed gratings, which are the effect of dispersive diffraction periods, split light of different frequencies into different diffraction angles. Due to the great advances in nanofabrication technology, the super-surface has created unprecedented possibilities for arbitrarily manipulating optics and photonics as a new class of planar optical elements. A wide range of optical applications has rapidly been demonstrated, including nano-printing, beam deflection/splitting, superlenses, holography and Orbital Angular Momentum (OAM) beams. In essence, the inherent principle behind obtaining various applications is to control the frequency, phase, amplitude, polarization state, etc. of light. However, another general property of light, the wave-vector direction (k-direction), has not been fully exploited in multifunctional hypersurfaces. Conventional hypersurfaces have exactly the same optical transmission and function for both forward and reverse incidence of the wave vector. For a single layer super-surface, due to geometric symmetry, there is the same response for transmitted light incident in both directions. Breaking this symmetry is critical in applications such as optical isolators in optical communication systems or for protecting high power laser equipment. Therefore, a nonreciprocal hypersurface is not yet available to break the symmetry of transmission and create a new degree of freedom, i.e. to generate different bidirectional functions when incident in forward and reverse directions. Such asymmetric behavior is mainly exhibited in that only the dispersion phenomenon is exhibited, and the frequency components of the optical wave can be analyzed, such as a spectrometer, and the like.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a bidirectional holographic modulation method of a single-layer nano structure in a broadband visible light range. The invention provides a Janus super-surface structure with a wide band encrypted freely in a visible light range in a two-way mode through single-layer nano textures.

The technical scheme provided by the invention is as follows:

the invention provides a bidirectional holographic image display method of a single-layer nano structure in a super-surface-based broadband visible light range, which comprises the following steps:

(1) constructing a unit structure for forming a super surface: wherein the structure is a two-layer structure, and the two layers comprise a substrate and a nano brick arranged on the substrate; the super surface is formed by a plurality of unit structures which are periodically arranged on the same plane; the nano bricks in the unit structure have independently set size parameters;

(2) scanning the sizes of different unit structures by setting working wavelength by adopting an electromagnetic simulation tool to obtain the relationship between the size and the phase of the nano-brick under different linearly polarized light;

(3) designing two phase type holographic patterns, converting phase information of the two phase type holographic patterns into size parameters under two polarized lights, and arranging unit structures to construct a super surface;

(4) and a layer of half-wave plate film is covered on the super surface, and the bidirectional holographic display of the single-layer nano structure in the broadband visible light range is realized by changing the direction of incident light.

Further, the nano brick and the substrate in the step (1) are both in a cuboid structure; wherein the cross section of the substrate is square; the sizes of the substrates of the unit structures are the same.

Further, the substrate of the unit structure is constructed of a low refractive index and transparent optical material including MgF₂、Al₂O₃、SiO₂The material of the nano brick comprises TiO₂、Si、Ag、Au、Cu、Al。

Further, the size parameters in the steps (1) and (2) comprise the length L, the width W, the height H and the side length P of the cross section of the substrate.

Further, a xoy rectangular coordinate system is established by taking the right-angle sides of the top surface of the substrate as an x axis and a y axis and the vertex as an origin, wherein the dimension of the nano brick along the x axis is L, and the dimension of the nano brick along the y axis is W; when the x-polarized light is incident, the phase change is caused by L; when the y-polarized light is incident, the phase change is caused by W.

Further, the method for converting the information of the two holographic images into the information of the size of the nano brick in the step (3) is as follows: under the working wavelength, the structural units with the nano bricks with different sizes have different phases, and the relation between the sizes and the phases of the nano bricks is obtained through scanning; and then establishing a one-to-one correspondence relationship between the pixel in the holographic image and the size of each unit structure, and finally realizing different polarization storage of different holographic image information.

Further, a layer of half-wave plate film covers the super surface, polarization multiplexing is converted into direction multiplexing, and emergent light forms two completely independent encrypted holographic images in the positive and negative directions only by changing the direction of incident light.

A second aspect of the invention provides a super-surface modulated using the method of the first aspect.

The working principle of the invention is as follows:

1. structural size parameter of scanning unit

The dielectric nano brick array super surface is formed by periodically arraying a plurality of nano brick unit structures on a plane;

the unit structure comprises a two-layer structure which is a substrate and a top layer from bottom to top in sequence;

wherein the content of the first and second substances,

the substrate is a square with a rectangular top surface;

the top layer is a nano brick;

the top sides of the substrates are the same in length;

establishing a xoy rectangular coordinate system by taking the right-angle sides of the top surface of the substrate as an x axis and a y axis, wherein the dimension of the nano brick along the x axis is L, and the dimension along the y axis is W; l and W are in the range of 0-300 nm;

the period P of the unit structure is the side length of the top surface of the dielectric layer;

and scanning the relation between the size and the phase of the nano brick by an electromagnetic simulation method. For the substrate-nanoblock structure, the structural parameters include the length L, width W, height H and period P of the nanoblock, and the operation mode is transmissive.

2. Pattern information translation

When the polarization multiplexing super surface is realized, the length L and the width W of the nano bricks in the array respectively correspond to the holographic code under one polarization. The length and width arrangement size of the nano brick array is optimized, the phase control of an emergent light field can be realized, and corresponding four-step Fourier holography is designed. After the construction of the super surface is completed, a layer of half-wave plate film is covered on the super surface, polarization multiplexing is converted into direction multiplexing, and emergent light can form encrypted holographic imaging with completely independent front and back directions only by changing the direction of incident light.

The element of the invention has the following advantages and beneficial effects:

(1) two-channel holography which converts polarization multiplexing into direction multiplexing.

(2) The bidirectional encrypted information transmission is realized in the whole visible frequency range.

(3) Has an unprecedented wide bandwidth in the visible range and great simplicity in structural design and fabrication of single-layer nanostructures, rather than requiring multilayer coupling or alignment as in previous work.

(4) The unit structure of the element of the invention has ultramicro size, can promote to increase the information coding capacity, and the holographic multiplexing channel can be widely applied to the fields of telecommunication, encryption, information processing, communication and the like.

Drawings

FIG. 1 is a schematic diagram of the structure of a unit according to the present invention

FIG. 2 is an SEM image of the array arrangement of the unit structures in the present invention

FIG. 3 is a schematic diagram of different holographic images formed by normal incidence and reverse incidence in an embodiment of the present invention

FIG. 4 is a schematic diagram of an optical measurement setup for a non-reciprocal hologram with forward and reverse incidence by sample rotation in an embodiment of the invention

FIG. 5 is a hologram for measuring different illumination directions and wavelengths according to an embodiment of the present invention

In the figure, 1-nano brick; 2-a substrate; l is the length of the nano brick, H is the height of the nano brick, and W is the width of the nano brick.

Detailed Description

In order to more clearly illustrate the embodiments of the present invention and/or the technical solutions in the prior art, the following description will explain specific embodiments of the present invention with reference to the accompanying drawings. It is obvious that the drawings described below are only examples of the invention, and that for a person skilled in the art, other drawings and embodiments can be obtained from them without inventive effort. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Examples

The embodiment is a bidirectional holographic display method of a single-layer nano structure in a broadband visible light range and application thereof.

Fig. 1 shows a unit structure, which is a two-layer structure including a substrate and a nano-roller disposed thereon. Cell structures with independent dimensional parameters are periodically arranged in the x-direction and y-direction, as shown in fig. 2, to form a silicon geometry array. Fig. 3 shows a two-way element hologram function for displaying an image of an actual traffic scene (traffic sign), that is, when a polarized light beam is vertically incident on an array surface from the front of the array, the light beam sequentially passes through a substrate, a nano-brick and a half-wave plate film, and the emergent light beam forms a hologram of a STOP sign; a beam of light with the same polarization direction is perpendicularly incident to the surface of the array in the opposite direction, the light sequentially passes through the half-wave plate film, the nano-brick and the substrate, and emergent light forms a hologram painted with a 90-degree speed limit sign. Fig. 4 shows a schematic diagram of an optical measurement setup for a two-way hologram, with both sides incident by means of sample rotation. Fig. 5 shows a holographic image of selected wavelengths (blue, green and red) in the visible range with forward and reverse incidence.

In order to facilitate understanding of the technical scheme of the present invention, the technical principle that the structure of the present invention can realize the bidirectional holography of the single-layer nanostructure in the broadband visible light range is described in detail as follows:

the geometry of the nano-unit structures on the glass substrate, along the light polarization direction, determines their resonance phase shift. Based on this, the single-layer super-surface designed by this embodiment includes a plurality of unit structures with different dimensional parameters, and the length and width thereof are between 80nm and 270 nm. Such a rectangular array can be viewed as an integration of two independent arrays. And establishing a xoy rectangular coordinate system by taking the right-angle sides of the top surface of the substrate as an x axis and a y axis, wherein the dimension of the nano brick along the x axis is L, and the dimension along the y axis is W. On one hand, the interface phase gradient felt when x-polarized incidence is caused by the gradual change of the length L of the nano brick; on the other hand, the perceived phase gradient at the incidence of the y-polarization is caused by a gradual change in the width W of the nanoblock. Therefore, due to the different phase variation tendencies experienced by incident light of different polarizations, the super-surface will develop different optical properties, i.e., a polarization-multiplexed super-surface, when incident light of two different polarizations passes through the super-surface.

To achieve bidirectional holography of a single-layer nanostructure in the broadband visible range, this embodiment combines a polarization multiplexing super-surface with a commercial half-wave plate film. When the x-polarized light is used for positive incidence, emergent light forms a hologram through the polarization multiplexing super surface firstly, and then is converted into y polarization through the half-wave plate; when the back incidence is carried out by using the x-polarized light, the emergent light is firstly changed into y polarization through a half-wave plate, and the y-polarized light forms another hologram after passing through the polarization multiplexing super surface.

In order to fully show the degree of freedom of bidirectional encryption, the embodiment shows the direction-independent holography, and may have a wide application scene in the aspects of expanding information storage and wave vector multiplexing. The conceptual view of the two-way asymmetric holography of this embodiment shows that it can display two different traffic signs in two directions simultaneously. In an actual two-way traffic on highway scene, the illuminated holographic image will be projected onto the windshield of the forward driving vehicle and will show the "STOP" symbol, while the other driver driving on the other side will be projected and shown the speed limit symbol "90".

In summary, the present invention provides a novel bi-directional super-surface design that includes only a single super-surface layer and a commercial half-wave plate film. The modulation method provided by the invention skillfully converts polarization multiplexing into direction multiplexing by means of hybridization of two independent nano bricks. Therefore, the encrypted holographic imaging which is completely independent in the front direction and the back direction can be formed simultaneously. Therefore, the anti-counterfeiting optical fiber can be used as a device for enhancing anti-counterfeiting application of non-reciprocal information processing, optical metering and encryption/decryption security.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any modification, equivalent replacement, and improvement made by those skilled in the art within the technical scope of the present invention should be included in the scope of the present invention.