Metastructure surface for realizing total-space asymmetric optical operation and optical device

Abstract

The invention provides a metasurface and an optical device for realizing total-space asymmetric optical operation, the metasurface comprises a first layer and a second layer, and the first layer and the second layer are connected; the first layer is uniformly provided with first metasurface units, the second layer is uniformly provided with second metasurface units, the first metasurface units are provided with first nano-columns, and the first nano-columns are provided with first inclination angles in an overlook plane of the first layer; and the second super-structure surface unit is provided with a second nano-column and a third nano-column. According to the scheme, when an x-polarized light beam enters the surface of the metasurface from the first layer, the x-polarized light beam directly penetrates through the first layer and is converted into a y-polarized light beam by the second layer. And when an x polarized light beam is incident on the metasurface from the second layer, the x polarized light beam is firstly converted into a y polarized light beam by the second layer and is reflected by the first layer, that is, the scheme can convert into multiplexing according to the incident direction according to the total space metasurface of incident polarization.

Description

Technical Field

[0001] The present invention relates to the field of optical technologies, and in particular, to a metasurface and an optical device for realizing all-space asymmetric optical operations. Background Art

[0002] An optical metasurface is a two-dimensional planar optical device composed of sub-wavelength scale (usually nanoscale) artificial microstructures, which realizes precise manipulation of multi-dimensional characteristics of the light field such as phase, amplitude, polarization, and frequency by regulating the geometric parameters of the microstructures (such as shape, size, arrangement method, etc.). Its core advantage lies in breaking through the physical limitations of traditional optical elements relying on the principles of refraction or diffraction, and innovating the optical system design with the characteristics of ultra-thin (thickness much smaller than the wavelength), lightweight, and high integration.

[0003] As a new type of sub-wavelength structure optical device, the optical metasurface has shown transformative application potential in multiple fields in recent years. Its core advantage lies in realizing precise regulation of multi-dimensional characteristics of the light field such as phase, amplitude, and polarization through micro-nano structure design. However, the existing optical metasurfaces are difficult to achieve multiplexing based on the incident polarization converted into the incident direction. Summary of the Invention

[0004] In view of this, embodiments of the present invention provide a metasurface and an optical device for realizing all-space asymmetric optical operations to eliminate or improve one or more defects existing in the prior art.

[0005] One aspect of the present invention provides a metasurface for realizing all-space asymmetric optical operations. The metasurface includes a first layer and a second layer, and the first layer and the second layer are connected;

[0006] The first layer is uniformly provided with first metasurface units, the second layer is uniformly provided with second metasurface units, the first metasurface units are provided with first nano-columns, and in the top view plane of the first layer, the first nano-columns are provided with a first inclination angle; the second metasurface units are provided with second nano-columns and third nano-columns.

[0007] Adopting the above solution, the first layer of this solution is used to convert x-polarized light into y-polarized light. When an x-polarized light beam is incident on the metasurface from the first layer, it will directly pass through the first layer and be converted into a y-polarized light beam by the second layer. And when an x-polarized light beam is incident on the metasurface from the second layer, it will first be converted into a y-polarized light beam by the second layer and be reflected by the first layer, that is, this solution can convert the all-space metasurface based on the incident polarization into multiplexing based on the incident direction.

[0008] In some embodiments of the present invention, the angle of the first inclination angle is 40 - 45°.

[0009] In some embodiments of the present invention, a first medium is disposed on the first layer, and the first medium wraps the first nanorods to obtain a first layer in the shape of a cuboid.

[0010] In some embodiments of the present invention, the first medium is made of silica, and the first nanorods and the second nanorods are made of silicon.

[0011] In some embodiments of the present invention, the length of the first nanorods is 375 - 385 nm, the width is 125 - 135 nm, and the height is 690 - 710 nm; the thickness of the first medium is 1150 - 1250 nm.

[0012] In some embodiments of the present invention, the bottom of the second nanorods or the third nanorods of the second layer is connected to the first medium. In the top view plane of the metasurface, the first metasurface unit and the second metasurface unit correspond to each other one by one.

[0013] In some embodiments of the present invention, the height of the second nanorods is 490 - 510 nm.

[0014] In some embodiments of the present invention, in the top view plane of the metasurface, the lengths of the first metasurface unit and the second metasurface unit are both a first length, and the width is a second length. The first length is twice the second length. The first metasurface unit is provided with two first nanorods, and the two first nanorods are respectively disposed at the center of the rectangle formed by the midpoint of the length and the width, and the second nanorods and the third nanorods are also respectively disposed at the center of the rectangle formed by the midpoint of the length and the width.

[0015] In some embodiments of the present invention, the phase information required for holographic imaging is encoded into the second layer to obtain the sizes of the second nanorods and the third nanorods of each second metasurface unit in the second layer.

[0016] Another aspect of the present solution provides an optical device, and the optical device includes the above - mentioned metasurface.

[0017] The additional advantages, objects, and features of the present invention will be partially described below, and will become partially apparent to those of ordinary skill in the art after studying the following text, or can be learned from the practice of the present invention. The objects and other advantages of the present invention can be pointed out and obtained specifically in the description and the drawings.

[0018] Those skilled in the art will understand that the objects and advantages that can be achieved by the present invention are not limited to the above - mentioned specifically, and the above - mentioned and other objects that the present invention can achieve will be more clearly understood according to the following detailed description. Brief Description of the Drawings

[0019] The drawings described herein are provided to further understand the present invention, form a part of this application, and do not limit the present invention.

[0020] Figure 1 It is a longitudinal space schematic diagram of a part of the metasurface for achieving all - space asymmetric optical operations of the present invention;

[0021] Figure 2 It is a comparison schematic diagram of each scheme under illumination with different incident directions;

[0022] Figure 3 It is a schematic diagram of the first metasurface unit of the first layer;

[0023] Figure 4 It is a schematic diagram of the second metasurface unit of the second layer;

[0024] Figure 5 It is a schematic diagram of the phase distribution of transmitted (reflected) light under x(y) - polarized incidence;

[0025] Figure 6 It is a schematic diagram of one of the optical functions of this scheme;

[0026] Figure 7 It is a schematic diagram of another optical function of this scheme. Detailed Description of the Preferred Embodiments

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with the embodiments and the drawings. Herein, the illustrative embodiments of the present invention and their descriptions are used to explain the present invention, but do not limit the present invention.

[0028] Here, it should also be noted that in order to avoid obscuring the present invention due to unnecessary details, only the structures and / or processing steps closely related to the solution of the present invention are shown in the drawings, while other details less related to the present invention are omitted.

[0029] In recent years, due to its ultra - thin thickness and extremely high degree of freedom, metasurfaces can arbitrarily manipulate incident light beams in compact devices. For example, metasurfaces with multiple functions, such as polarization multiplexing, wavelength multiplexing, and incident - angle multiplexing metasurfaces, have important application values in optical tweezers, holographic projection, and optical imaging.

[0030] To meet the growing demand for compact asymmetric optical devices (such as isolators, circulators, etc.), the concept of Janus metasurface was proposed. Due to the satisfaction of Lorentz reciprocity in traditional metasurfaces, the optical response of the metasurface is usually independent of the incident direction, that is, when the incident direction is reversed, the optical properties will remain unchanged as Figure 2 shown. Introducing additional degrees of freedom (such as polarization state) into the metasurface can achieve asymmetric phase modulation without violating Lorentz reciprocity. When the incident light beam is restricted to a specific polarization state, the response of the transmitted light in the orthogonal polarization channels will vary with the incident direction. For linearly polarized incidence (x- or y-polarized substrate), the forward-incidence Jones matrix of the metasurface can be expressed as When the incident direction is reversed, the reverse-incidence Jones matrix will be transformed into thus generating an asymmetric optical field modulation ability in the cross-polarization channels.

[0031] Janus metasurfaces have been realized in the optical band and shown remarkable applications in fields such as hologram generation and optical encryption. However, most previous Janus metasurface designs ignored an important property: the all-space property. Although some metasurfaces have been proven to be able to simultaneously generate all-space beams with Janus characteristics, the transmitted and reflected beams usually either appear simultaneously or are separated into different polarization or wavelength channels. Herein, the present solution defines a metasurface that can switch between transmission and reflection states by only changing the incident direction as a Janus all-space metasurface. Compared with the asymmetric metasurfaces that only work in the reflection or transmission region, the present solution not only greatly improves the space utilization rate and information density, but more importantly, can modulate the incident light beams from different directions with extremely low crosstalk and be received by the same detector.

[0032] As Figure 1 , 3 and shown in Fig. 4, the present invention proposes a metasurface and an optical device for realizing all-space asymmetric optical operations. The metasurface includes a first layer (MS1) and a second layer (MS2), and the first layer and the second layer are connected;

[0033] The first layer is uniformly provided with first metasurface units, the second layer is uniformly provided with second metasurface units, the first metasurface units are provided with first nanocolumns, and in the top view plane of the first layer, the first nanocolumns are provided with first tilting angles; the second metasurface units are provided with second nanocolumns and third nanocolumns.

[0034] In the specific implementation process, the first layer and the second layer respectively undertake two functions such as full-space light field regulation and polarization conversion, based on the birefringence effect of Si nanocolumns. The first metasurface units of the first layer all have exactly the same geometric shape, and at the same time have an additional first inclination angle of 45°. As Figure 4 shown, when the phase of the Si nanocolumn satisfies , it can convert the incident x / y polarized light into cross polarization; the second layer part of the double-layer metasurface designed in this scheme is used to realize full-space light field regulation, and its principle is based on the anomalous diffraction of induced reflection. For a periodic metasurface unit array with a phase gradient of ξ.

[0035] Specifically, the state of the output light can be described by using the Generalized Snell's Law (GSL):

[0036] k out = k in + ξ

[0037] In the formula, k out and k in are the transverse momenta of the outgoing light and the incident light respectively, and ξ is the phase gradient between the metasurface units.

[0038] However, when the phase gradient is greater than the wave vector in free space (ξ > k0), GSL will not be applicable to this metasurface. This condition is called the critical angle condition, and at this time, the formula describing the outgoing light field needs to be replaced with:

[0039] k out = k in + ξ + (n - 1)G

[0040] This formula is called the diffraction law of parity inversion. In this formula, n represents the diffraction order of the outgoing light field, and G is the reciprocal lattice vector of the metasurface unit structure, which is usually equivalent to ξ numerically.

[0041] As Figure 1 , 3 and 4 shown, each basic unit in the second layer part has two nanocolumns. At this time, when the critical condition is satisfied, the number of times the light travels back and forth in the metasurface unit is set to Then can be expressed as where m represents the number of nanocolumns in a metasurface unit, that is, m = 2. Therefore, direct transmission cannot occur Correspondingly, reflection is induced, and at this time, the reflected light occupies the vast majority of the energy.

[0042] Based on the above characteristics, as Figure 2As shown, the first-layer part is designed as a group of half-wave plates with high efficiency (96%), and the second-layer part is designed as a full-space metasurface. The first layer is composed of Si nanocolumns wrapped in SiO2. The length and width of the nanocolumns are both 380 nm and 130 nm, and they have a rotation angle of θ = 45°. The basic unit of the second-layer part consists of two amorphous silicon nanocolumns grown on the first layer. The length (L) and width (W) of each nanocolumn are adjustable, and the height is uniformly set to H = 500 nm. The period of a single nanocolumn is P = 450 nm. Amorphous silicon has a high refractive index and low absorption rate at the operating wavelength (1030 nm). Therefore, when x-polarized light is incident, MS2 behaves as a transmissive metasurface. When y-polarized light is incident on MS2, due to the breaking of the critical angle condition, the metasurface exhibits the characteristics of a reflective metasurface. First, 64 groups of metasurface units of MS2 were screened, as Figure 5 shown. To ensure that the metasurface of MS2 can achieve high transmission efficiency under x-polarized incidence and as high reflection efficiency as possible under y-polarized incidence, the phase relationship of the metasurface units is designed to satisfy the phase φ x2 = φ x1 inside the metasurface units under x-polarized incidence, and the phase φ y2 - φ y1 = π under y-polarized incidence. At this time, when an x-polarized light beam is incident normally on the metasurface, it will directly pass through the first layer and be converted into a y-polarized light beam by the second layer. When an x-polarized light beam is incident on the metasurface in the reverse direction, it will first be converted into a y-polarized light beam by the second layer and be reflected by MS1. In other words, this scheme can cleverly convert the full-space metasurface based on the incident polarization into multiplexing based on the incident direction.

[0043] According to the Gerchberg-Saxton (GS) algorithm, this scheme encodes the phase information required for holographic imaging into MS2. The Gerchberg-Saxton algorithm can obtain the target phase close to the optimal solution of the phase arrangement through multiple iterations and by constraining the calculated target phase with the target holographic image. This scheme obtains the phase information by inputting the far-field amplitude information of the required holographic image into the GS algorithm, and finally maps the phase information to the sizes of the two units on the second layer of the metasurface. This scheme first sets that for x-polarized light, illuminating the metasurface from the front will generate an unmodulated ordinary bright spot in the transmission space, and illuminating the metasurface from the back will reconstruct the holographic image of "BUPT" in the reflection space, as Figure 6 (c) shown, where the reflectivity and transmittance of this scheme are 58% and 84% respectively.

[0044] With the above solution, the first layer of this solution is used to convert x-polarized light into y-polarized light. When an x-polarized light beam is incident on the metasurface from the first layer, it will directly pass through the first layer and be converted into a y-polarized light beam by the second layer. When an x-polarized light beam is incident on the metasurface from the second layer, it will first be converted into a y-polarized light beam by the second layer and be reflected by the first layer, that is, this solution can be converted from a metasurface based on the incident polarization in the entire space to multiplexing based on the incident direction.

[0045] In some embodiments of the present invention, the angle of the first inclination angle θ is 40 - 45°.

[0046] In some embodiments of the present invention, the first layer is provided with a first medium, and the first medium wraps the first nanorod to obtain a cuboid first layer.

[0047] In some embodiments of the present invention, the first medium is made of silica, and the first nanorod and the second nanorod are made of silicon.

[0048] In some embodiments of the present invention, the length L1 of the first nanorod is 375 - 385 nm, the length W1 is 125 - 135 nm, and the height h1 is 690 - 710 nm; the thickness h3 of the first medium is 1150 - 1250 nm.

[0049] In some embodiments of the present invention, the bottom of the second nanorod or the third nanorod of the second layer is connected to the first medium. In the top view plane of the metasurface, the first metasurface unit and the second metasurface unit correspond one by one.

[0050] In some embodiments of the present invention, the height h2 of the second nanorod is 490 - 510 nm.

[0051] In some embodiments of the present invention, in the top view plane of the metasurface, the lengths of the first metasurface unit and the second metasurface unit are both the first length, and the width is the second length. The first length is twice the second length. The first metasurface unit is provided with two first nanorods, and the two first nanorods are respectively arranged at the center point of the length and the center of gravity point of the rectangle formed by the width. The second nanorod and the third nanorod are also respectively arranged at the center point of the length and the center of gravity point of the rectangle formed by the width.

[0052] In the specific implementation process, the first length is set to 900 nm.

[0053] In some embodiments of the present invention, the phase information required for holographic imaging is encoded into the second layer to obtain the sizes of the second nanorods and the third nanorods of each second metasurface unit in the second layer.

[0054] In this metasurface, information is hidden within a specific observation plane in a given incident direction, providing an optical encryption function, such as Figure 6 (a) shows. More importantly, this scheme can combine an unmodulated forward illumination beam with a modulated reflected beam generated by back illumination, and these two beams can be observed by the same CMOS camera. This design enables this scheme to couple and output real information (transmitted beam) and virtual information (reflected beam) simultaneously, as Figure 6 (b) shows, acting as an optical combinational element.

[0055] Specifically, Figure 6 (a) When the metasurface of this scheme is illuminated from the front, only the polarization state of the transmitted beam changes. When illuminated from the back, the metasurface of this scheme reconstructs the holographic image of "BUPT" in the reflection space; as Figure 6 (b) is a conceptual schematic diagram of the metasurface of this scheme acting as an optical combinational element and serving as an output coupler; as Figure 6 (c) is a schematic diagram of the simulation results of the metasurface of this scheme.

[0056] To further demonstrate the function of the asymmetric full-space optical operation of this scheme, this scheme is designed such that the forward-illuminated beam transmits the image of the letter "T", while the back-illuminated beam reflects the image of the letter "R". Under the full-space illumination of the x-polarized beam, this scheme successfully generates independently modulated transmitted and reflected light fields and can be simultaneously received by the same receiver as Figure 7 (a-c) shows. The reflectivity and transmittance of this scheme are 61% and 77% respectively. The simulation results show that this scheme can independently and simultaneously modulate the reflected and transmitted beams, realizing the coupling of different virtual information in a single system. Considering the ultra-thin and compact characteristics of the metasurface, this design indicates that this scheme has great potential as an ultra-thin and highly efficient integrated element for an augmented reality (AR) optical combiner.

[0057] Specifically, Figure 7 (a) shows the holographic image of the letter "T" transmitted by the metasurface of this scheme under forward illumination. Figure 7 (b) shows the holographic image of the letter "R" reflected by the metasurface of this scheme under back illumination. Figure 7 (c) shows that when the beams in both directions illuminate simultaneously, the reflected and transmitted beams of the metasurface of this scheme are successfully coupled into one channel and received by the detector simultaneously.

[0058] This solution presents a general design framework. The designed metasurface demonstrates bidirectional and full-space functions while maintaining a relatively simple structural configuration. The results show that this solution can achieve full-space optical field manipulation. This solution has the ability to manipulate the wavefront with independent, crosstalk-free, and full-phase (2π) control in all incident spaces. This solution not only realizes spatial multiplexing but also enables the reception of bidirectional optical fields using a single receiver, breaking through the traditional detection limitations, thereby greatly expanding the design possibilities and application scope of Janus metasurfaces. Such metasurfaces have broad application prospects in fields such as holographic display and security encryption.

[0059] Another aspect of this solution provides an optical device, and the optical device includes the above-mentioned metasurface.

[0060] In summary, this solution proposes a double-layer dielectric metasurface capable of achieving full-space asymmetric optical operations. Based on a simple structural unit, this double-layer metasurface can transmit (reflect) the optical field when illuminated by an x-polarized beam in the forward (backward) direction, and at the same time perform fully decoupled optical field regulation on the reflected and transmitted beams. Through reasonable design, the first-layer metasurface converts the switching of the incident direction into the switching of the incident polarization through a half-wave plate structure, while the second-layer metasurface introduces the degree of freedom of operating the metasurface beam from separate transmission into the full space through anomalous diffraction. Thus, this solution can independently operate the reflected (transmitted) beam according to the direction of the incident light. This concept provides a novel method for the design of asymmetric metasurfaces, and the designed metasurface can be applied to occasions such as holographic display and optical encryption.

[0061] Those of ordinary skill in the art should understand that the various exemplary components, systems, and methods described in conjunction with the embodiments disclosed herein can be implemented in hardware, software, or a combination of both. Specifically, whether to execute in hardware or software depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of the present invention. When implemented in hardware, it can be, for example, an electronic circuit, an application-specific integrated circuit (ASIC), appropriate firmware, a plug-in, a functional card, and so on. When implemented in software, the elements of the present invention are programs or code segments used to perform the required tasks. The program or code segment can be stored in a machine-readable medium or transmitted through a data signal carried in a carrier wave on a transmission medium or a communication link.

[0062] It should be clear that the present invention is not limited to the specific configurations and processes described above and shown in the figures. For the sake of brevity, detailed descriptions of known methods are omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method process of the present invention is not limited to the specific steps described and shown, and those skilled in the art can make various changes, modifications, and additions, or change the order between steps after understanding the spirit of the present invention.

[0063] In the present invention, features described and / or illustrated for one embodiment can be used in the same or similar manner in one or more other embodiments, and / or combined with the features of other embodiments or replace the features of other embodiments.

[0064] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, various changes and modifications can be made to the embodiments of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A metasurface for achieving all-space asymmetric optical manipulation, characterized in that, The metasurface includes a first layer and a second layer, and the first layer and the second layer are connected to each other; The first layer is uniformly provided with first metasurface units, and the second layer is uniformly provided with second metasurface units. The first metasurface units are provided with first nanocolumns. In the top view plane of the first layer, the first nanocolumns are provided with a first inclination angle; the second metasurface units are provided with second nanocolumns and third nanocolumns.

2. The metasurface for realizing all-space asymmetric optical manipulation according to claim 1, wherein The angle of the first inclination angle is 40-45°.

3. The metasurface for realizing all-space asymmetric optical manipulation according to claim 1, wherein The first layer is provided with a first medium, and the first medium wraps the first nanocolumns to obtain a cuboid first layer.

4. The metasurface for achieving all-space asymmetric optical manipulation according to claim 3, wherein The first medium is made of silica material, and the first nanocolumns and the second nanocolumns are made of silicon material.

5. The metasurface for realizing all-space asymmetric optical manipulation according to claim 3, characterized in that The length of the first nanocolumns is 375-385 nm, the width is 125-135 nm, and the height is 690-710 nm; the thickness of the first medium is 1150-1250 nm.

6. The metasurface for achieving all-space asymmetric optical manipulation according to claim 3, wherein The bottom of the second nanocolumns or the third nanocolumns of the second layer is connected to the first medium. In the top view plane of the metasurface, the first metasurface units and the second metasurface units correspond to each other one by one.

7. The metasurface for realizing all-space asymmetric optical manipulation according to claim 1, characterized in that, The height of the second nanocolumns is 490-510 nm.

8. The metasurface for achieving all-space asymmetric optical operations according to any one of claims 1 to 7, characterized in that, In the top view plane of the metasurface, the lengths of the first metasurface units and the second metasurface units are both a first length, and the width is a second length. The first length is twice the second length. The first metasurface units are provided with two first nanocolumns, and the two first nanocolumns are respectively arranged at the center point of the rectangle formed by the middle point of the length and the width. The second nanocolumns and the third nanocolumns are also respectively arranged at the center point of the rectangle formed by the middle point of the length and the width.

9. The metasurface for achieving all-space asymmetric optical manipulation according to claim 1, wherein By encoding the phase information required for holographic imaging into the second layer, the sizes of the second nanocolumns and the third nanocolumns of each second metasurface unit in the second layer are obtained.

10. An optical device, characterized in that, The optical device includes the metasurface according to any one of claims 1-9.

Images