CN113406729B - Bidirectional holographic modulation method and application based on broadband visible light nanometer metasurfaces - Google Patents

Abstract

The invention discloses a bidirectional holographic modulation method based on a broadband visible light nanometer super surface and application thereof. The super surface is formed by periodically arranging a plurality of unit structures, and each unit structure comprises a substrate and nano bricks arranged on the substrate. The method can realize bidirectional holographic image display of a single-layer nano structure in a broadband visible light range by designing the size parameters of the unit structure to construct the super surface. The single nano brick has independent freedom degrees in length and width, has different responses to two different linearly polarized light respectively, and realizes the display of two different images under different polarizations. The invention has small structure scale and easy integration, and can be used as a device for enhancing anti-counterfeiting application of nonreciprocal information processing, optical metering and encryption/decryption security.

Description

Bidirectional holographic modulation method and application based on broadband visible light nanometer metasurfaces

technical field

The invention relates to the fields of micro-nano optics and optical holography, in particular to a bidirectional holography method and application of a single-layer nano-structure in a broadband visible light range.

Background technique

Traditional optical devices based on geometrical optics or diffractive optics have rather limited means for controlling beam steering and splitting due to the limitation of structure size and inherent optical properties. For example, a blazed grating with a dispersive diffraction periodic effect splits light of different frequencies into different diffraction angles. Thanks to significant advances in nanofabrication technology, metasurfaces as a new class of planar optical elements have created unprecedented possibilities for arbitrarily manipulating optics and photonics. A wide range of optical applications have been rapidly demonstrated, including nanoprinting, beam deflection/splitting, metalens, holography, and orbital angular momentum (OAM) beams. Essentially, the underlying principle for obtaining various applications lies in the control of the frequency, phase, amplitude and polarization state of light. However, another general property of light, the wave vector direction (k-direction), has not been fully exploited in multifunctional metasurfaces. Conventional metasurfaces have exactly the same optical transmission and functionality for forward and reverse incidence of wave vectors. For a single-layer metasurface, the same response to bidirectionally incident transmitted light occurs due to the geometric symmetry. Breaking this symmetry is critical in applications such as optical isolators in optical communication systems, or for protecting high-power laser equipment. Therefore, a non-reciprocal metasurface is currently lacking to break the symmetry of transport and create a new degree of freedom that yields different bidirectional functions at forward and reverse incidence. This asymmetric behavior is mainly manifested only by the use of dispersion phenomena, which can analyze the frequency components of light waves, such as spectrometers and spectrometers.

SUMMARY OF THE INVENTION

In view of the deficiencies of the prior art, the present invention provides a bidirectional holographic modulation method of a single-layer nanostructure in a broadband visible light range. The invention provides a Janus metasurface structure in which the broadband is freely encrypted in the visible light range through the single-layer nano-texture bidirectionally.

The technical scheme provided by the present invention is as follows:

The first aspect of the present invention provides a metasurface-based bidirectional holographic image display method of a single-layer nanostructure in the broadband visible light range, the steps are as follows:

(1) Constructing a unit structure for forming a metasurface: wherein, the structure is a two-layer structure, and the two layers include a substrate and nanobricks arranged thereon; the metasurface is periodically arranged in the same metasurface by a plurality of unit structures It is composed of planes; the nano-bricks in the unit structure have independently set size parameters;

(2) Using electromagnetic simulation tools, set the working wavelength to scan the size of different unit structures, and obtain the relationship between the size and phase of the nanobricks under different linearly polarized light;

(3) Design two phase-type holographic patterns, convert their phase information into size parameters under two polarized lights, and arrange unit structures to construct metasurfaces;

(4) A half-wave plate film is covered on the metasurface, and the bidirectional holographic display of single-layer nanostructures in the broadband visible light range can be realized by changing the direction of the incident light.

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

Further, the base of the unit structure is constructed of low refractive index and transparent optical materials, the materials include MgF ₂ , Al ₂ O ₃ , and SiO ₂ , and the materials of the nanobricks include TiO ₂ , Si, Ag, Au, Cu, and Al.

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

Further, taking the right-angled sides of the top surface of the substrate as the x-axis and the y-axis, and the vertex as the origin, the xoy rectangular coordinate system is established. , the phase change is caused by L; when 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 nano-brick size information in the step (3) is as follows: at the working wavelength, the structural units with different sizes of nano-bricks have different phases, and the size and phase of the nano-bricks are obtained by scanning. Then establish a one-to-one correspondence between the pixels in the holographic image and the size of each unit structure, and finally realize that different polarizations store different holographic image information.

Furthermore, a layer of half-wave plate film is covered on the metasurface to convert polarization multiplexing into direction multiplexing. Only by changing the direction of the incident light, the outgoing light forms two completely independent encrypted holographic images in two directions.

A second aspect of the present invention provides a metasurface modulated using the method of the first aspect.

The working principle of the present invention:

1. Structural size parameters of scanning unit

The dielectric nano-brick array metasurface is composed of a plurality of nano-brick unit structures periodically arrayed on a plane;

The unit structure includes a two-layer structure, which are a base and a top layer in order from bottom to top;

in,

The base is a square with a rectangular top surface;

The top layer is nano bricks;

The top side of the base has the same length;

Taking the right-angled sides of the top surface of the substrate as the x-axis and the y-axis, an xoy rectangular coordinate system is established. The dimension of the nanobrick along the x-axis is the length L, and the dimension along the y-axis is the width W; the range of L and W is 0～300nm;

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

The relationship between nanobrick size and phase was scanned by electromagnetic simulation. For the substrate-nanobrick structure, the structural parameters include the length L, width W, height H and period P of the nanobrick, and the working mode is transmission type.

2. Pattern information conversion

When the polarization multiplexed metasurface is realized, the length L and width W of the nanobricks in the array correspond to the holographic code under one polarization, respectively. By optimizing the length and width of the nano-brick array, the phase control of the outgoing light field can be realized, and the corresponding four-step Fourier hologram can be designed. After completing the construction of the metasurface, a half-wave plate film is covered on the metasurface, and the polarization multiplexing is converted into direction multiplexing. Only by changing the direction of the incident light, the outgoing light can form a completely independent positive and negative direction. Encrypted holographic imaging.

The elements of the present invention have the following advantages and benefits:

(1) Two-channel holography that converts polarization multiplexing into direction multiplexing.

(2) Two-way encrypted information transmission is realized in the entire visible frequency range.

(3) Unprecedented broadband in the visible range and great simplicity in structural design and fabrication of single-layer nanostructures without requiring multi-layer coupling or alignment as in previous work.

(4) The unit structure of the element of the present invention has an ultra-micro size, which can promote the increase of information encoding capability, and the holographic multiplexing channel can be widely used in the fields of telecommunication, encryption, information processing and communication.

Description of drawings

Fig. 1 is the unit structure schematic diagram in the present invention

Fig. 2 is the SEM image of the cell structure array arrangement in the present invention

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

4 is a schematic diagram of the optical measurement setup of the non-reciprocal hologram in the embodiment of the present invention, and forward and reverse incidence is performed by sample rotation

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

In the figure, 1-nano-brick; 2-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 ways

In order to more clearly describe the embodiments of the present invention and/or the technical solutions in the prior art, the specific embodiments of the present invention will be described below with reference to the accompanying drawings. Obviously, the drawings described below are only embodiments of the present invention. For those of ordinary skill in the art, other drawings and other implementation. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention.

Example

This embodiment is a bidirectional holographic display method of a single-layer nanostructure in a broadband visible light range and its application.

FIG. 1 shows a cell structure, which is a two-layer structure, including a substrate and a nanorotor disposed thereon. Cell structures with independent size parameters are periodically arranged along the x- and y-directions, as shown in Figure 2, to form a silicon geometry array. Figure 3 shows the bidirectional meta-holography function for image display of a real traffic scene (traffic sign), that is, when a beam of polarized light is vertically incident on the array surface from the front of the array, the light passes through the substrate, nanobricks and half-wave plates in sequence film, the outgoing light will form a hologram of the "STOP" sign; a beam of light with the same polarization direction is vertically incident on the array surface in the opposite direction, and the light passes through the half-wave plate film, nano-bricks and substrate in turn, and emerges light A hologram of the speed limit sign with "90" will be formed. Figure 4 shows the schematic diagram of the optical measurement setup for a bidirectional hologram, with sample rotation for forward and reverse incidence. Figure 5 shows holographic images of selected wavelengths (blue, green, and red) in forward and reverse incidence in the visible range.

In order to facilitate the understanding of the technical solution 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 will be described in detail below:

The geometry of the nanounit structure on the glass substrate along the light polarization direction determines its resonant phase shift. Based on this, the single-layer metasurface designed in this embodiment includes a plurality of unit structures with different size parameters, the length and width of which are between 80 nm and 270 nm. This rectangular array can be seen as the integration of two independent arrays. Taking the right-angled sides of the top surface of the substrate as the x-axis and the y-axis, an xoy rectangular coordinate system is established. The dimension of the nanobrick along the x-axis is the length L and the dimension along the y-axis is the width W. On the one hand, the interfacial phase gradient experienced at x-polarized incidence is caused by the gradual change in the nanobrick length L; on the other hand, the perceived phase gradient at y-polarized incidence is caused by the gradual change in the nanobrick width W. Therefore, due to the different trends of phase change experienced by incident light with different polarizations, the metasurface will form different optical properties when passing through two incident lights with different polarizations, that is, a polarization-multiplexed metasurface.

In order to realize the bidirectional holography of the single-layer nanostructure in the broadband visible light range, the polarization multiplexing metasurface is combined with a commercial half-wave plate film in this embodiment. When using x-polarized light for forward incidence, the outgoing light first forms a hologram through the polarization multiplexing metasurface, and then converts it into y-polarization via a half-wave plate; when using x-polarized light for backward incidence, the outgoing light first It becomes y-polarized through the half-wave plate, and the y-polarized light forms another hologram after passing through the polarization multiplexing metasurface.

In order to fully demonstrate the freedom of two-way encryption, this embodiment demonstrates direction-independent holography, which may have broad application scenarios in expanding information storage and wave-vector multiplexing. The conceptual diagram of the two-way asymmetric holography of this embodiment shows that it can display two different traffic signs in two directions at the same time. On a highway scene with actual two-way traffic, the holographic image is projected onto the windshield of the forward-driving vehicle, and a "STOP" symbol is displayed, while for other drivers on the other side, the speed limit is projected. Symbol "90".

In summary, the present invention proposes a novel bidirectional metasurface design that only includes a single-layer metasurface and a commercial half-wave plate thin film. The modulation method provided by the present invention skillfully converts polarization multiplexing into directional multiplexing by relying on the hybridization of two independent nano-bricks. Therefore, encrypted holographic imaging with completely independent forward and reverse directions can be constructed at the same time. Therefore, it can be used as a device for non-reciprocal information processing, optical metrology, and encryption/decryption security-enhancing anti-counterfeiting applications.

The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited to this. Any modifications made by any person skilled in the art within the technical scope disclosed by the present invention are equivalent Substitutions and improvements, etc., should all be included within the protection scope of the invention.

Claims (7)

1. based on the bidirectional holographic modulation method of broadband visible light nanometer metasurface, it is characterized in that, step is as follows: (1) Constructing a unit structure for forming a metasurface: wherein, the structure is a two-layer structure, and the two layers include a substrate and nanobricks arranged thereon; the metasurface is periodically arranged in the same metasurface by a plurality of unit structures It is composed of planes; the nano-bricks in the unit structure have independently set size parameters; (2) Using electromagnetic simulation tools, set the working wavelength to scan the size of different unit structures, and obtain the relationship between the size and phase of the nanobricks under different linearly polarized light; (3) Design two phase-type holographic patterns, convert their phase information into size parameters under two polarized lights, and arrange the unit structure to construct a metasurface; the method for converting the two holographic image information into nano-brick size information is as follows: At the working wavelength, the structural units with different sizes of nano-bricks have different phases, and the relationship between the nano-brick size and the phase is obtained by scanning; then a one-to-one correspondence between the pixels in the holographic image and the size of each unit structure is established. Realize different polarizations to store different holographic image information; (4) Cover a layer of half-wave plate film on the metasurface, convert polarization multiplexing into direction multiplexing, and only change the direction of the incident light to make the outgoing light form two completely independent encrypted holographic images in two directions. . 2 . The modulation method according to claim 1 , wherein in the step (1), the nanobricks and the substrate are both cuboid structures; wherein the cross section of the substrate is a square; and the substrates of the unit structures have the same size. 3 . 3 . The modulation method according to claim 1 , wherein the base of the unit structure is constructed of a low refractive index and transparent optical material, the material comprises MgF ₂ , Al ₂ O ₃ , and SiO ₂ , and the material of the nano-brick comprises: 4 . _TiO2 , Si, Ag, Au, Cu, Al. 4 . The modulation method according to claim 1 , wherein the size parameters in the steps (1) and (2) include the nano-brick length L, width W, height H and the side length P of the cross-section of the substrate. 5 . 5. modulation method according to claim 4, is characterized in that, with the right angle side of base top surface as x-axis and y-axis, vertex is origin, establish xoy Cartesian coordinate system, nano-brick is long L along x-axis size, The dimension along the y-axis is wide W; when x-polarized light is incident, the phase change is caused by L; when y-polarized light is incident, the phase change is caused by W. 6. A metasurface capable of realizing bidirectional holographic image display of single-layer nanostructures in the broadband visible light range, characterized in that: it is prepared by the method described in any one of claims 1-5. 7. The application of the metasurface of claim 6 in non-reciprocal information processing, optical metrology and encryption/decryption security enhanced anti-counterfeiting.

Images