Unidirectional Self-Bending Beam Generator Based on Visible Light Binary Metasurface

Abstract

The present invention discloses a unidirectional self-bending beam generator based on a visible light binary metasurface. The unidirectional self-bending beam generator includes an upper optical metasurface and a lower optical metasurface that are parallelly placed and have a subwavelength thickness, wherein: both the upper optical metasurface and the lower optical metasurface are composed of 81 binary subwavelength units, and there is a certain air gap between the upper optical metasurface and the lower optical metasurface. The present invention encodes the units with opposite transmission phases using the numbers 0 and 1. By parallelly placing two subwavelength optical metasurfaces with different coding sequences, and utilizing the different transmission phase responses on both sides of the double-layer structure, a TM incident wave with a wavelength of 650 nm can be unidirectionally converted into a self-bending beam, while promoting the miniaturization and integration of optical devices, and greatly simplifying the design and fabrication process of the device.

Description

Technical Field

[0001] The invention relates to a passive optical device, and in particular to a unidirectional self-bending beam generator based on a visible light binary metasurface. Background Art

[0002] The use of artificial optical structures to achieve flexible control of the wavefront of light wave transmission is crucial in the field of optical functional device design. Researching and using a subwavelength-scale planar passive optical binary metasurface to achieve arbitrary control of the phase of the transmitted wave can greatly simplify the design and preparation process of the device while promoting the miniaturization and integration of optical devices. Although many research works have shown that the use of photonic crystals and phase gradient metasurfaces can achieve beam shaping of the transmitted light field, since no built-in circuits are involved, how to make the static structure have unidirectional transmission characteristics while achieving beam shaping remains to be explored, especially phenomena with complex generation mechanisms such as self-bending beams, which are crucial in ensuring the safety of optical communications and detection. Therefore, the design of a unidirectional self-bending beam generator using passive optical metasurface devices is an urgent problem to be solved. Summary of the invention

[0003] The purpose of the present invention is to provide a unidirectional self-bending beam generator based on a visible light binary metasurface, which uses the numbers 0 and 1 to encode units with opposite transmission phases. By placing two sub-wavelength optical metasurfaces with different coding sequences in parallel and utilizing the different transmission phase responses on both sides of the double-layer structure, a TM incident wave with a wavelength of 650nm can be unidirectionally converted into a self-bending beam, which greatly simplifies the design and preparation process of the device while promoting the miniaturization and integration of optical equipment.

[0004] The objective of the present invention is achieved through the following technical solutions:

[0005] A unidirectional self-bending beam generator based on a visible light binary metasurface comprises an upper optical metasurface and a lower optical metasurface which are placed in parallel and have subwavelength thickness, wherein: the upper optical metasurface and the lower optical metasurface are both composed of 81 binary subwavelength units, and there is a certain air gap between the upper optical metasurface and the lower optical metasurface.

[0006] In the present invention, the binary subwavelength unit includes a binary subwavelength unit A and a binary subwavelength unit B. The binary subwavelength unit A and the binary subwavelength unit B have the same size, and the length and height are respectively and λ / 2, where λ is the incident wavelength.

[0007] In the present invention, the binary sub-wavelength unit A and the binary sub-wavelength unit B have opposite transmission phase responses, that is: the binary sub-wavelength unit A and the binary sub-wavelength unit B have a phase difference of π.

[0008] In the present invention, in the upper optical metasurface, the binary sub-wavelength unit A and the binary sub-wavelength unit B adopt a periodic phase distribution, and the period length is (greater than the incident wavelength), and the phase encoding along the propagation direction is "001100110011001100110011001100110011001100110011001100110011001100110011001100110", where the encoding 0 represents a relative phase response of 0, and the encoding 1 represents a relative phase response of π.

[0009] In the present invention, in the lower optical metasurface, the binary sub-wavelength unit A and the binary sub-wavelength unit B adopt an aperiodic gradient phase distribution, and its phase encoding along the propagation direction is "000011110000111100000111110000011111000001111110000001111111000000000111111111111".

[0010] In the present invention, the air gap width between the upper optical metasurface and the lower optical metasurface is set to λ.

[0011] In the present invention, the incident modes of the upper optical metasurface and the lower optical metasurface are that TM waves with a wavelength of 650 nm are normally incident from both sides of the device respectively.

[0012] Compared with the prior art, the present invention has the following advantages:

[0013] The unidirectional self-bending beam generator of the present invention can convert the TM wave with a wavelength of 650 nm incident from one side into a self-bending beam by using an array of binary sub-wavelength units to construct a double-layer metasurface, while strong reflection is generated when incident from the other side, so as to achieve the effect of unidirectional transmission. Since it does not involve any driving circuit and adopts a planar binary metasurface configuration with sub-wavelength scale, this passive optical device greatly simplifies the design process and fabrication process, enables the device to have the advantages of small volume, planarization, multi-function and easy integration, solves the problem that it is difficult for passive optical metasurfaces to achieve unidirectional self-bending beams, and has wide applications in the fields of optical communication, optical detection and optical imaging. Description of the Drawings

[0014] Figure 1Schematic diagram of the working principle of a unidirectional self-bending beam generator based on a visible light binary metasurface. 1 - Upper optical metasurface; 2 - Binary sub-wavelength unit A; 3 - Binary sub-wavelength unit B; 4 - Lower optical metasurface.

[0015] Figure 2 Specific structural configuration of the binary sub-wavelength unit.

[0016] Figure 3 Effect of the height change of the silica medium inside the binary sub-wavelength unit on the transmittance and phase response of the unit.

[0017] Figure 4 Transmission field distribution diagram of the double-layer optical metasurface composed of binary sub-wavelength units under the normal incidence of TM waves at a working wavelength of 650 nm. On the left, it is incident from the bottom, and on the right, it is incident from the upper end.

[0018] Figure 5 Transmission field distribution diagram of the device obtained by longitudinally mirroring the double-layer optical metasurface under the normal incidence of TM waves at a working wavelength of 650 nm. On the left, it is incident from the bottom, and on the right, it is incident from the upper end. Detailed implementation mode

[0019] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention shall be covered by the protection scope of the present invention.

[0020] The present invention provides a unidirectional self-bending beam generator based on a visible light binary metasurface, as Figure 1 shown, including an upper optical metasurface and a lower optical metasurface that are parallelly placed and have a sub-wavelength thickness. Both the upper optical metasurface and the lower optical metasurface are composed of 81 binary sub-wavelength units. The binary sub-wavelength unit includes a binary sub-wavelength unit A and a binary sub-wavelength unit B. There is a certain air gap between the upper optical metasurface and the lower optical metasurface.

[0021] In the above-mentioned upper optical metasurface, the binary sub-wavelength unit A and the binary sub-wavelength B adopt a periodic phase distribution, and the coding sequence is "001100110011001100110011001100110011001100110011001100110011001100110011001100110", and the period length is (greater than the incident wavelength). At this time, the generalized Snell's law can be used for analysis:

[0022] ktsinθt - kisinθi = dφ(x) / dx (1)

[0023] where, θ i and θ t represent the incident and transmission angles respectively, represents the surface phase gradient. At normal incidence, the transmission angle can be determined by the following formula:

[0024]

[0025] Since the period length is less than the incident wavelength, the transmitted wave cannot exist in the form of a propagating wave at this time, but will form a surface-bound wave. In order to avoid the generation of evanescent waves without changing the incident wavelength, it is necessary to change the incident angle so that the refraction angle satisfies the following formula:

[0026]

[0027] It can be found at this time that at oblique incidence, there is an opportunity to make the device generate strong transmitted energy, while at normal incidence, high-intensity acoustic wave energy transmission cannot be generated, which provides the basis for the unidirectional transmission of self-bending beams.

[0028] In the above-mentioned lower-layer optical metasurface, the binary sub-wavelength unit A and the binary sub-wavelength unit B are formed by a non-periodic gradient phase distribution, and the coding sequence is "000011110000111100000111110000011111000001111110000001111111000000000111111111111", which can match and approximately generate the coding phase profile required for self-bending beams, so that the transmitted field forms a high-intensity self-bending beam with an inclined radiation direction. When a TM wave with a working wavelength of 650 nm is incident normally on the lower-layer metasurface, a transmitted beam with a curved propagation trajectory will be generated first, and then pass through the upper-layer metasurface in an oblique incidence manner to achieve a high-intensity self-bending beam in this incident direction. When a TM wave with a working wavelength of 650 nm is incident normally on the upper-layer metasurface, due to the generation of surface evanescent waves, the intensity of the transmitted field in this incident direction will be relatively weak, so as to achieve the unidirectional excitation of the self-bending beam.

[0029] Embodiment:

[0030] In this embodiment, as Figure 2 shown, a combination of silver metal, silica dielectric, and air cavity is used to construct the binary sub-wavelength unit. Among them: the length of the silica dielectric is w = 180 nm, and the dielectric constant is 3.9. By scanning the height parameter of the silica dielectric from 10 nm to 450 nm, as Figure 3As shown, it can be found that different height information corresponds to different transmittance and transmission phase. When the heights are selected as 135 nm and 420 nm (the positions marked by points A, B and the dashed line), opposite phase responses can be achieved under nearly perfect transmission, so that it can be used to play the role of a binary sub-wavelength unit. At this time, by regarding the unit configurations with heights of 135 nm and 420 nm as encoding 0 and 1, and constructing the double-layer optical metasurface according to the encoding sequence as Figure 1 shown, as Figure 4 shown, when the TM wave with a working wavelength of 650 nm is incident normally from the lower metasurface, a high-intensity self-bending beam can be generated. When it is incident normally from the upper metasurface, the intensity of the transmitted field is relatively weak, thus realizing the unidirectional self-bending beam generator based on the visible-light binary metasurface described in the present invention. In addition, as Figure 5 shown, by performing a longitudinal mirror symmetry operation on the double-layer optical metasurface, interference superposition effects can also be generated for two self-bending beams from completely opposite directions, so as to realize the generation of unidirectional self-focusing beams, further improving the functionality of the device mentioned in the present invention and broadening its application scope.

Claims

1. A unidirectional self-bending beam generator based on a visible light binary metasurface, characterized in that The unidirectional self-bending beam generator includes an upper optical metasurface and a lower optical metasurface that are placed in parallel and have a subwavelength thickness, where: both the upper optical metasurface and the lower optical metasurface are composed of 81 binary subwavelength units, and there is a certain air gap between the upper optical metasurface and the lower optical metasurface; the binary subwavelength unit includes binary subwavelength unit A and binary subwavelength unit B, and binary subwavelength unit A and binary subwavelength unit B have the same size, with the length and height being and λ / 2, where λ is the incident wavelength; binary subwavelength unit A and binary subwavelength unit B have opposite transmission phase responses; binary subwavelength unit A and binary subwavelength unit B adopt a periodic phase distribution in the upper optical metasurface, and the period length is phase-encoded as "001100110011001100110011001100110011001100110011001100110011001100110011001100110" along the propagation direction, where the code 0 represents a relative phase response of 0 and the code 1 represents a relative phase response of π; binary subwavelength unit A and binary subwavelength unit B adopt an aperiodic gradient phase distribution in the lower optical metasurface, and its phase-encoding along the propagation direction is "000011110000111100000111110000011111000001111110000001111111000000000111111111111", where the code 0 represents a relative phase response of 0 and the code 1 represents a relative phase response of π.

2. The unidirectional self-bending beam generator based on a visible light binary metasurface according to claim 1, characterized in that the air gap width between the upper optical metasurface and the lower optical metasurface is set to λ.

3. The unidirectional self-bending beam generator based on a visible light binary metasurface according to claim 1, characterized in that the incident modes of the upper optical metasurface and the lower optical metasurface are that TM waves with a wavelength of 650 nm are normally incident from both sides of the device respectively.

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