A tunable electromagnetic metasurface for vortex beam conversion

Abstract

An adjustable electromagnetic metasurface for vortex beam conversion relates to the field of adjustable electromagnetic metadevices. The present invention aims to solve the problem that the metasurface pattern is not adjustable after it is determined, while the use of a refractive index gradient is limited by the thickness of the liquid crystal. The adjustable electromagnetic metasurface for vortex beam conversion described in the present invention includes a metal metasurface, a fixed-phase liquid crystal phase shifter, and an electrically controlled liquid crystal phase shifter, which are stacked in sequence; the metal metasurface is used to modulate left-handed circularly polarized light into right-handed circularly polarized light; the fixed-phase liquid crystal phase shifter is used to modulate the right-handed circularly polarized light into y-polarized light; and the electrically controlled liquid crystal phase shifter is used to deflect the liquid crystal with the electric field when driven by a voltage, thereby adjusting the ratio of the first-order vortex beam to the perfect vortex beam in the beam generated after the y-polarized light passes through the electrically controlled liquid crystal phase shifter.

Description

Technical Field

[0001] The invention belongs to the field of adjustable electromagnetic metastructure devices. Background Art

[0002] Electromagnetic metamaterial technology is a means of engineering the electromagnetic response of passive micro- and nanostructured materials, enabling single or multiple control of electromagnetic wave field properties such as amplitude, phase, and polarization. The application of tunable materials has greatly enriched and promoted the development of tunable terahertz devices. In recent years, the rapid development of micro- and nanofabrication technologies and new effects have greatly promoted the miniaturization and integration of terahertz modulators, enabling more complex device functions. On the one hand, the unique diffraction characteristics and light field distribution of these beams remain to be studied, which can effectively compensate for the shortcomings of terahertz technology in practical applications. On the other hand, people need high-quality and diverse structured beam generation devices, especially customized engineering design solutions and tunable structured beam devices.

[0003] The traditional vortex beam generating device combined with the geometric phase metasurface is the most commonly used vortex beam generating method, but there is a problem of lack of adjustability. When the metasurface pattern is finally determined, the light field distribution that can be modulated by the metasurface is also determined and cannot be changed. The “A dual perfect vortex beam metasurface generator” with patent number CN202111561164.8 realizes the generation of perfect vortex beams in the optical band, but is not tunable, and cannot achieve switching between vortex beams and perfect vortex beams. In addition, the all-medium metasurface solution is difficult to process. The “Design method for a phase-controllable ultra-thin sub-terahertz vortex vector beam generator” with patent number CN202010648859.9 uses a double-layer metal layer plus a dielectric layer to generate terahertz vortex vector beams, but requires two lithography steps and there are alignment problems. The processing process needs to be further optimized, and the solution is not tunable. This makes the system redundancy of the customized engineering design solution low, and the device needs to be redesigned to achieve other functions. If you want to achieve multi-functions, you need a large number of devices and the system will be complex.

[0004] The refractive index gradient engineering of liquid crystal can be used to construct a variety of terahertz band phase control devices, but the phase modulation range limits the lower limit of the liquid crystal thickness. The larger thickness combined with the elastic properties and electrical effects of the liquid crystal directly limits the phase sampling accuracy of the liquid crystal layer, making it difficult to use a single patterned electrode to drive the liquid crystal layer to form a high-precision structured light beam. The use of a uniform electrode liquid crystal layer has limited modulation capabilities for metasurfaces and cannot complete the modulation of complex light field metasurfaces. Patent No. CN201821491010.X, "A liquid crystal phase shifter and an electromagnetic wave control lens based thereon", uses a refractive index gradient for phase control, which requires a relatively complex multi-channel liquid crystal electric control circuit to match it. The error is large during actual control, which will cause distortion of the controlled light field. Summary of the Invention

[0005] The present invention aims to solve the problem that the metasurface pattern has no adjustability after being determined, and the use of refractive index gradient is limited by the thickness of liquid crystal. An adjustable electromagnetic metasurface for vortex beam conversion is provided.

[0006] An adjustable electromagnetic metasurface for vortex beam conversion, comprising a metal metasurface, a fixed-phase liquid crystal phase shifter, and an electrically controlled liquid crystal phase shifter stacked in sequence;

[0007] The metal metasurface is used to modulate left-handed circularly polarized light into right-handed circularly polarized light;

[0008] The fixed-phase liquid crystal phase shifting portion is used to modulate the right-handed circularly polarized light into y-polarized light;

[0009] The electrically controlled liquid crystal phase shift portion is used to deflect the liquid crystal along the electric field when driven by a voltage, thereby adjusting the ratio of the l-order vortex beam to the perfect vortex beam in the light beam generated after the y-polarized light passes through the electrically controlled liquid crystal phase shift portion, where l is the topological charge number.

[0010] Furthermore, the electrically controlled liquid crystal phase shift portion includes: a patterned transparent electrode layer, a uniform transparent electrode layer, two first-layer polyimide alignment layers and an electrically controlled liquid crystal layer.

[0011] The two first-layer polyimide alignment layers are respectively fixed on both sides of the electrically controlled liquid crystal layer, and the patterned transparent electrode layer and the uniform transparent electrode layer are respectively fixed on the outer sides of the two first-layer polyimide alignment layers.

[0012] Furthermore, the patterned transparent electrode layer includes: patterned electrodes, electrode wires and rectangular electrodes.

[0013] The patterned electrode includes a plurality of concentrically nested and fixed ring electrodes and a circular electrode located at the center of the concentric rings, and the electrode wires connect all the ring electrodes and the circular electrodes with the rectangular electrode.

[0014] Furthermore, the patterned transparent electrode layer is a PEDOT:PSS terahertz transparent electrode layer with a thickness of 30nm to 100nm;

[0015] The uniform transparent electrode layer is a PEDOT:PSS terahertz transparent electrode layer with a thickness of 30nm to 100nm.

[0016] Furthermore, the fixed-phase liquid crystal phase shifting part includes: two No. 2 polyimide alignment layers and a fixed liquid crystal layer, and the two No. 2 polyimide alignment layers are respectively fixed on both sides of the fixed liquid crystal layer.

[0017] Furthermore, the electrically controlled liquid crystal layer and the fixed liquid crystal layer are both nematic liquid crystal layers.

[0018] When the birefringence is 0.35, the thickness of the electrically controlled liquid crystal layer is 200 um, and the liquid crystal birefringence phase shift is π; the thickness of the fixed liquid crystal layer is 100 um, and the liquid crystal birefringence phase shift is π / 2.

[0019] Furthermore, the above-mentioned adjustable electromagnetic metasurface for vortex beam conversion also includes three layers of quartz substrate, one of which is located between the fixed-phase liquid crystal phase-shifting part and the electrically controlled liquid crystal phase-shifting part, and the remaining two layers of quartz substrate are located outside the metal metasurface and the electrically controlled liquid crystal phase-shifting part.

[0020] Furthermore, the quartz substrate is a fused quartz substrate with a thickness of 500 μm.

[0021] Furthermore, the metal metasurface is a resonant strip structure, the length of the resonant strip is 30um to 40um, the width is 10um, and the unit period is 50um to 60um.

[0022] Furthermore, the phase distribution of the above resonant bar structure is as follows:

[0023]

[0024] Where d is the phase parameter, phase represents the phase distribution, exp(·) represents the exponential function, and i is the imaginary unit. is the polar coordinate angle.

[0025] The beneficial effects of the adjustable electromagnetic metasurface for vortex beam conversion described in the present invention are as follows:

[0026] This invention addresses the need for miniaturization and integration of terahertz structured beam modulation devices. It can be realized using only a double-layer liquid crystal cell structure and an additional metasurface. This simplifies the manufacturing process, improves the device's integration, provides tunability, and generates highly accurate vortex beams. This simple structure achieves vortex beam conversion, simplifying the associated optical system.

[0027] The present invention uses a patterned etching process to prepare a terahertz patterned transparent electrode layer, and achieves a specific light field distribution after power is applied. The required driving circuit is simple, the device preparation process is compatible with semiconductor processes, and the cost is low while the switching efficiency is high.

[0028] The control method of the present invention is simple, does not require complex circuit design, and can complete the modulation of complex light field metasurfaces using a single power supply voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the structure of an adjustable electromagnetic metasurface for vortex beam conversion based on liquid crystal, where 1 is quartz substrate No. 1, 2 is patterned transparent electrode layer, 3 is polyimide alignment layer No. 1, 4 is electrically controlled liquid crystal layer, 5 is polyimide alignment layer No. 2, 6 is a uniform transparent electrode layer, 7 is quartz substrate No. 2, 8 is polyimide alignment layer No. 3, 9 is fixed liquid crystal layer, 10 is polyimide alignment layer No. 4, 11 is a metal metasurface, and 12 is quartz substrate No. 3;

[0030] Figure 2 Schematic diagram of a patterned transparent electrode layer, wherein 13 is a patterned electrode, 14 is an electrode wire, and 15 is a rectangular electrode. DETAILED DESCRIPTION

[0031] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other in the absence of conflict.

[0032] Specific implementation method 1: refer to Figure 1 and Figure 2 Specifically describing this embodiment, the adjustable electromagnetic metasurface for vortex beam conversion described in this embodiment includes: a quartz substrate No. 1 1, a patterned transparent electrode layer 2, a polyimide orientation layer No. 1 3, an electrically controlled liquid crystal layer 4, a polyimide orientation layer No. 2 5, a uniform transparent electrode layer 6, a quartz substrate No. 2 7, a polyimide orientation layer No. 3 8, a fixed liquid crystal layer 9, a polyimide orientation layer No. 4 10, a metal metasurface 11 and a quartz substrate No. 3 12.

[0033] The metal metasurface 11 is used to modulate left-handed circularly polarized light into right-handed circularly polarized light. The metal metasurface 11 is a resonant bar structure with a length of 30 to 40 μm, a width of 10 μm, and a unit period of 50 to 60 μm. The phase distribution of the resonant bar structure follows the following formula:

[0034]

[0035] Where d is the phase parameter, l is the topological charge, phase represents the phase distribution, exp(·) represents the exponential function, and i is the imaginary unit. is the polar coordinate angle.

[0036] The third polyimide orientation layer 8, the fixed liquid crystal layer 9 and the fourth polyimide orientation layer 10 are stacked in sequence, so that the third polyimide orientation layer 8 and the fourth polyimide orientation layer 10 are respectively fixed on both sides of the fixed liquid crystal layer 9, forming a fixed phase liquid crystal phase shift part, and are used to modulate the right circularly polarized light into y polarized light.

[0037] The patterned transparent electrode layer 2, the first polyimide alignment layer 3, the electrically controlled liquid crystal layer 4, the second polyimide alignment layer 5 and the uniform transparent electrode layer 6 are stacked in sequence, so that the first polyimide alignment layer 3 and the second polyimide alignment layer 5 are respectively fixed on both sides of the electrically controlled liquid crystal layer 4, the patterned transparent electrode layer 2 is fixed on the outside of the first polyimide alignment layer 3, and the uniform transparent electrode layer 6 is fixed on the outside of the second polyimide alignment layer 5, constituting an electrically controlled liquid crystal phase shifter portion, and is used to deflect the liquid crystal with the electric field when driven by a voltage, thereby adjusting the ratio of the first-order vortex beam to the perfect vortex beam in the light beam generated after the y-polarized light passes through the electrically controlled liquid crystal phase shifter portion.

[0038] Further, such as Figure 2 As shown, the patterned transparent electrode layer 2 is a 30nm to 100nm thick dimethyl sulfoxide-doped PEDOT:PSS (3,4-ethylenedioxythiophene:styrenesulfonate) terahertz transparent electrode layer. It includes: a patterned electrode 13, an electrode wire 14, and a rectangular electrode 15. The maximum radius of the circular electrode is 2cm. The patterned electrode 13 includes multiple concentrically nested and fixed circular electrodes and a circular electrode located at the center of the concentric rings. The electrode wire 14 connects all the circular and circular electrodes to the rectangular electrode 15. The uniform transparent electrode layer is a 30nm to 100nm thick PEDOT:PSS terahertz transparent electrode layer.

[0039] Quartz substrate No. 1 is fixed to the outside of patterned transparent electrode layer 2, quartz substrate No. 2 is fixed between uniform transparent electrode layer 6 and polyimide alignment layer No. 3, and quartz substrate No. 3 is fixed to the outside of metal metasurface 11. All three quartz substrates are fused quartz substrates with a thickness of 500 μm.

[0040] Both the electrically controlled liquid crystal layer 4 and the fixed liquid crystal layer 9 are highly birefringent nematic liquid crystal layers. When the birefringence is 0.35, the electrically controlled liquid crystal layer has a thickness of 200 μm and a birefringence phase shift of π; the fixed liquid crystal layer has a thickness of 100 μm and a birefringence phase shift of π / 2. In practical applications, other nematic liquid crystals can also be used. The thicknesses of the electrically controlled liquid crystal layer 4 and the fixed liquid crystal layer 9 need to be adjusted accordingly to meet the birefringence phase shift of the corresponding liquid crystal layers.

[0041] In the adjustable electromagnetic metasurface for vortex beam conversion described in this embodiment, after the externally incident terahertz left-handed circularly polarized light passes through the third quartz substrate 12 and the metal metasurface 11, a portion is converted into right-handed circularly polarized light to form a vortex beam phase, and the other portion remains left-handed circularly polarized light and maintains an unchanged phase. Subsequently, these two portions of light pass through the fourth polyimide orientation layer 10 and the fixed liquid crystal layer 9, so that the right-handed circularly polarized light is converted into linearly polarized light in the y direction, and the left-handed circularly polarized light is converted into linearly polarized light in the x direction.

[0042] The light then passes through the third polyimide alignment layer 8, the second quartz substrate 7, the uniform transparent electrode layer 6, the second polyimide alignment layer 5, and the electrically controlled liquid crystal layer 4 oriented in the y-direction for modulation, before finally exiting through the first polyimide alignment layer 3, the patterned transparent electrode layer 2, and the first quartz substrate 1. During this process, when no driving voltage is applied to the electrically controlled liquid crystal layer 4, the vortex beam of linearly polarized light in the y-direction is not phase-modulated by the fixed liquid crystal layer 9, and the exiting light is a normal vortex beam. When a driving voltage is applied to the electrically controlled liquid crystal layer 4, causing the liquid crystal to fully deflect, the vortex beam of linearly polarized light in the y-direction is converted into a perfect vortex beam through the focused phase modulation formed by the fixed liquid crystal layer 9.

[0043] The patterned transparent electrode layer 2 is fabricated using lift-off UV lithography to etch a spin-coated film. Four polyimide alignment layers are spin-coated and oriented by friction. Polyimide alignment layers 3 and 5 are oriented in the y direction, while polyimide alignment layers 8 and 10 are oriented in the xy direction.

[0044] The geometric phase metasurface layer introduced in this embodiment is prepared by ultraviolet lithography and is made of gold. It can be replaced with other geometric phase metal metasurface layers such as silver and aluminum, or other dielectric metasurface layers that can achieve the same phase distribution. The principles for achieving phase distribution include but are not limited to geometric phase, propagation phase, and other transmission metasurface phases.

[0045] This embodiment utilizes a tunable liquid crystal phase shifter layer in conjunction with a fixed-phase liquid crystal phase shifter layer to reduce the effects of stray light. It also enables the generation of a 2.52THz circularly polarized vortex beam (OVB) and a perfect vortex beam (POV), and the conversion between these two beams can be achieved by adjusting the driving voltage of the liquid crystal layer.

[0046] In summary, this embodiment is aimed at the needs of miniaturization and integration of terahertz structured beam modulation devices, and combines liquid crystal materials and metasurface platforms to realize an adjustable electromagnetic metasurface for vortex beam conversion. The terahertz patterned transparent electrode layer can be prepared by patterned etching process, and a specific light field distribution can be achieved after power is turned on. Based on this, through structural design and thickness selection, a concentric ring phase distribution with gradually increasing radius is designed in the liquid crystal layer. When no power is turned on, the liquid crystal layer has no contribution to the phase of the device; when power is turned on, a focused phase distribution will appear in the liquid crystal layer. This embodiment has the advantages of high switching efficiency, controllable beam type, simple device structure, high integration, low cost, and compatibility with semiconductor processes. The precise control of the switching between vortex beams and perfect vortex beams can be achieved through a simple single-channel external power supply system, and the superposition ratio of the two can be selected by the external driving voltage.

[0047] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely illustrative of the principles and applications of the invention. It should be understood that many modifications may be made to the illustrative embodiments, and that other arrangements may be devised, without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in ways other than those described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in conjunction with other described embodiments.

Claims

1. An adjustable electromagnetic metasurface for vortex beam conversion, characterized in that: It includes a metal metasurface, a fixed-phase liquid crystal phase shifter, and an electrically controlled liquid crystal phase shifter that are stacked in sequence; The metal metasurface is used to modulate left-handed circularly polarized light into right-handed circularly polarized light; The fixed-phase liquid crystal phase shifting portion is used to modulate the right-handed circularly polarized light into y-polarized light; The electrically controlled liquid crystal phase shifter is used to deflect the liquid crystal with the electric field when driven by a voltage, thereby adjusting the ratio of the l-order vortex beam to the perfect vortex beam in the light beam generated after the y-polarized light passes through the electrically controlled liquid crystal phase shifter, where l is the topological charge number; The electrically controlled liquid crystal phase shift part includes: a patterned transparent electrode layer, a uniform transparent electrode layer, two polyimide alignment layers and an electrically controlled liquid crystal layer. The two polyimide alignment layers are respectively fixed on both sides of the electrically controlled liquid crystal layer, and the patterned transparent electrode layer and the uniform transparent electrode layer are respectively fixed on the outer sides of the two polyimide alignment layers.

2. The tunable electromagnetic metasurface for vortex beam conversion according to claim 1, characterized in that: The patterned transparent electrode layer includes: a patterned electrode, an electrode wire and a rectangular electrode. The patterned electrode includes a plurality of concentrically nested and fixed ring electrodes and a circular electrode located at the center of the concentric rings, and the electrode wires connect all the ring electrodes and the circular electrodes with the rectangular electrode.

3. The tunable electromagnetic metasurface for vortex beam conversion according to claim 2, characterized in that: The patterned transparent electrode layer is a PEDOT:PSS terahertz transparent electrode layer with a thickness of 30nm to 100nm; The uniform transparent electrode layer is a PEDOT:PSS terahertz transparent electrode layer with a thickness of 30nm to 100nm.

4. The tunable electromagnetic metasurface for vortex beam conversion according to claim 1, characterized in that: The fixed-phase liquid crystal phase shifting part comprises: two polyimide alignment layers and a fixed liquid crystal layer, wherein the two polyimide alignment layers are respectively fixed on both sides of the fixed liquid crystal layer.

5. The tunable electromagnetic metasurface for vortex beam conversion according to claim 4, characterized in that: The electrically controlled liquid crystal layer and the fixed liquid crystal layer are both nematic liquid crystal layers, When the birefringence is 0.35, the thickness of the electrically controlled liquid crystal layer is 200 um, and the liquid crystal birefringence phase shift is π; the thickness of the fixed liquid crystal layer is 100 um, and the liquid crystal birefringence phase shift is π / 2.

6. The tunable electromagnetic metasurface for vortex beam conversion according to claim 1 or 4, characterized in that: It also includes three layers of quartz substrates, one of which is located between the fixed-phase liquid crystal phase-shifting part and the electrically controlled liquid crystal phase-shifting part, and the remaining two layers of quartz substrate are located on the metal supersurface and outside the electrically controlled liquid crystal phase-shifting part.

7. The tunable electromagnetic metasurface for vortex beam conversion according to claim 6, characterized in that: The quartz substrate is a fused quartz substrate with a thickness of 500 μm.

8. The tunable electromagnetic metasurface for vortex beam conversion according to claim 1 or 4, characterized in that: The metal metasurface is a resonant bar structure, the length of the resonant bar is 30um to 40um, the width is 10um, and the unit period is 50um to 60um.

9. The tunable electromagnetic metasurface for vortex beam conversion according to claim 8, characterized in that: The phase distribution followed by the resonant bar structure is as follows: Where d is the phase parameter, phase represents the phase distribution, exp(·) represents the exponential function, and i is the imaginary unit. is the polar coordinate angle.

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