Gst-based terahertz surface plasmon dynamic wavefront modulation device

Abstract

The present application relates to a terahertz dynamic surface plasmon wavefront modulation device, and provides a new device for realizing terahertz surface plasmon wavefront dynamic modulation by using a GST metasurface.The present application is a terahertz surface plasmon dynamic wavefront modulation device based on GST, which comprises a silicon substrate and a lossy metal aluminum film with a subwavelength rectangular slit resonator array on the silicon substrate; the device comprises two columns of slit resonator arrays which are perpendicular to each other along the y-axis and form an active group or a passive group; the active group and the passive group differ in that the surface of the metal aluminum film of the active group is deposited with a germanium antimony tellurium (GST) film strip, while the surface of the passive group is not deposited; the active group and the passive group are arranged adjacently; by using the rotation angle of the subwavelength slit resonator, the relative distance between the subwavelength slit resonators and the GST phase transition characteristics, the switching of different functions of the surface plasmon can be realized.The present application is mainly applied to the design and manufacture of surface plasmon wavefront modulation devices.

Description

TECHNICAL FIELD

[0001] The present application relates to a terahertz dynamic surface plasmon wavefront modulation device, in particular to a terahertz surface plasmon dynamic wavefront modulation device based on GST by integrating phase change material Ge2Sb2Te5(GST) on the surface of the device and utilizing the metal-dielectric characteristics before and after the phase change of GST to realize the dynamic wavefront modulation of terahertz surface plasmons. BACKGROUND

[0002] Surface plasmons (SPs) formed by free-space light coupling to metal-dielectric surfaces are a special form of electromagnetic waves [1]. Due to their two-dimensional propagation characteristics, they have broad application prospects in many fields such as biosensing [2], nonlinear applications [3], waveguides [4] and communication [5], and thus are attracting much attention in the development of the next generation of compact and integrated on-chip plasmonic devices. However, there are certain limitations in previous research on most surface plasmons. Either they are generated and propagated passively in a passive manner, i.e. lacking active modulation means and relying on specific external conditions, or they are only dynamically controllable during propagation, and such controllability is often limited, making it difficult to meet the growing application demands. Therefore, it is urgent to find new and effective methods to excite and dynamically modulate the wavefront of surface plasmons to expand their application potential in the terahertz field.

[0003] In recent years, metasurfaces composed of subwavelength artificial microstructures have attracted widespread attention. Their permittivity and permeability can be arbitrarily adjusted by carefully designing the microstructures, and they exhibit excellent capabilities in controlling the amplitude, phase and polarization of electromagnetic waves, thereby realizing a variety of functional devices with ultra-thin dimensions, such as beam control [6], holography [7] and superlenses [8], and also providing a robust method for coupling and controlling surface plasmons [9]. Although metasurfaces are crucial for the development of optical devices, the functions are usually fixed after design and manufacture, hindering advanced photonic applications. Currently, metasurfaces for modulating surface plasmons are mostly passive and non-tunable, and the dynamic wavefront modulation after structure fixation is difficult and costly, greatly hindering the application of surface plasmonic devices in on-chip systems. With the development of society, there is an urgent need for active and tunable devices in modern electromagnetic and photonic systems, and the importance of using metasurfaces to dynamically modulate the wavefront of surface plasmons is increasing.

[0004] In recent years, the response of metasurfaces can be further modulated by external stimuli (such as heat, electricity or light) by integrating functional materials into the metasurface structure. So far, a variety of functional materials have been employed, including semiconductors [10-12], liquid crystals [13-16], MEMS [17-21], graphene [22-25], perovskites

[26] , VO2 [27-30] and transistors [31, 32] and so on, to dynamically manipulate THz waves. But these functional materials have the characteristics of being volatile, and need continuous external excitation to maintain the phase transition. When the excitation disappears, the phase transition also stops, which increases the complexity and energy consumption of the system. Therefore, it is urgent to develop an energy-saving active metasurface in the terahertz field. Among them, germanium antimony tellurium Ge2Sb2Te5(GST) is a kind of excellent chalcogenide phase change material, which shows significant differences between amorphous and crystalline states. The amorphous GST can be locally heated to its crystallization temperature by applying optical or electrical pulses, or GST crystallization can be induced by heating the sample as a whole [33-35]. In contrast, amorphization of GST can be achieved by applying high-intensity short pulses to heat the crystalline GST to a temperature above the melting point, and then rapidly cooling to prevent recrystallization

[36] . Once the state of GST is in the crystalline or amorphous state, its properties are very stable, and it can usually be stable at room temperature for years with zero power [37, 38]. For this reason, GST materials have received widespread attention from researchers over the past few decades. By combining GST with metasurfaces, the great potential of this material in the field of tunable photonics has gradually been tapped. Compared with other materials commonly used in THz wave tunable functional devices, there is no need to frequently excite to maintain performance, which greatly improves the reliability and stability of the system. But the use of GST metasurfaces to realize the regulation of surface plasmons is still in its infancy, and has broad application prospects. SUMMARY

[0005] In order to overcome the defects of the prior art, the present application aims to provide a new method for realizing dynamic control of terahertz surface plasmon wave front by GST super surface. To this end, the technical scheme adopted by the present application is as follows: a terahertz surface plasmon dynamic wave front control device based on GST, the bottom of which is a silicon substrate, and the silicon substrate is provided with a lossy metal aluminum film having a subwavelength rectangular slot resonator array; the device is composed of two columns of slot resonator arrays which are perpendicular to each other along the y axis and form an active group or a passive group, the active group and the passive group are different in that the surface of the metal aluminum film of the active group is deposited with a germanium antimony tellurium GST thin film strip, the surface of the passive group is not deposited, and the active group and the passive group are arranged adjacently; the slot resonators in each column are arranged in the x direction by sequentially rotating the angle according to the phase distribution; the overall surface plasmon excitation process is as follows: the incident terahertz wave is first incident vertically to the substrate, most of the light is reflected by the metal aluminum film after passing through the substrate, a small part of the light and the subwavelength rectangular slot resonator generate a local field, and the surface plasmon is formed by coupling the active free electrons on the metal surface, and the switching of different functions of the surface plasmon is realized by using the rotation angle of the subwavelength slot resonator, the relative distance between the subwavelength slot resonators and the GST phase change characteristics.

[0006] The device comprises two active groups and one passive group arranged between the active groups, when the GST is in the pre-phase change state, the GST acts as a medium, the two active groups U1 and U2 and the passive group U3 work at the same time, but due to the π phase shift of the surface plasmons excited by U2 and U3, destructive interference occurs, so the function of the device is only provided by U1; when the GST is in the post-phase change state, the GST acts as a metal-like medium to close the terahertz wave response of the subwavelength slot resonator, so that the active groups U1 and U2 do not work, and the function of the device is only provided by U3; the wave front of U1 and U3 is formed by using the rotation angle of the subwavelength slot resonator and the relative distance between the subwavelength slot resonators, so that the device can exhibit different functions before and after the GST phase change.

[0007] In the x axis direction, the passive group U3 satisfies the following conditions: θ2-θ1=3π / 2, s is the distance between the two columns of resonators and s=λ SP / 2, λ SP is the working wavelength of the surface plasmon, θ1 and θ2 are the angles between the electric field and the x axis, respectively, and the field components in the incident terahertz wave that can excite the left column and the right column of slot resonators are wherein the incident light electric field vector is (1, σi), i is the imaginary unit, and i 2= -1, σ ∈ {+1, -1} represent left-handed circularly polarized terahertz wave LCP and right-handed circularly polarized terahertz wave RCP, respectively, and the field of the excited surface plasmon is represented as:

[0008]

[0009] where A is the amplitude of the surface plasmon, i is the imaginary unit, i 2 = -1, θ1, θ2 are the angles between the electric field and the x-axis, respectively, and l is the distance between the center point of the two resonators and point N, N is a point on the aluminum film outside the active group and the passive group, s is the distance between the two columns of resonators, and the wave number of the surface plasmon is ksp= 2π / λ SP , λ SP is the working wavelength of the surface plasmon, and the field of the surface plasmon E U3 at point N excited by the passive group U3 is fixed in amplitude and freely controllable in phase, the sign of which is determined by the handedness of the circular polarization and changes with the rotation angle of the subwavelength slit resonator; since the subwavelength slit resonator can be rotated by 180°, the phase change can cover the entire range of 2π, so the use of the slit resonator can also achieve an arbitrary phase distribution; for the subwavelength slit resonator, when the distance s between the corresponding subwavelength slit resonators becomes mλ SP , m is an integer, and when the low-loss of the surface plasmon is considered, the phase response of the subwavelength slit resonator is close to constant, the active group and the passive group have the same size but different geometric phases and propagation phases, the geometric phase refers to the rotation angle of the subwavelength slit resonator, and the propagation phase refers to the horizontal distance between the subwavelength slit resonators, at point N, the field E N is the superposition of the surface plasmons excited by the active group U1, the passive group U2 and the passive group U3: E N = E U1N + E U2N + E U3N , E U1N , E U2N , E U3N are the surface plasmons excited by the active group U1, the passive group U2 and the passive group U3, respectively, wherein the geometric phase of the active group U1 and the passive group U2 is the same, but their propagation phases are different: s U1 = s + λ SP , s U1 , s are the horizontal distances between the subwavelength slit resonators of different groups, i.e. E U1N = e iπ E U2N , which means that the surface plasmon E U1N excited by the U1 structure is the same as the surface plasmon E U2Ncompared to having a pi phase shift, i.e. E U1N = -E U2N So, when the active groups U1 and U2 are working, destructive interference will occur, meaning E N = E U1N The function of the device is only provided by the active group U1; in addition, if the active groups U1 and U2 are closed by the GST in the crystalline state, the function of the device will only be provided by the passive group U3.

[0010] The device contains two active groups and a passive group sandwiched between the active groups, and the coupling of surface plasmons is achieved by designing different phases of the two metal slits: the coupling effect is represented as when the phase of the subwavelength slit resonator is adjusted to , the coupling can act as a secondary emission source, and the excited surface plasmons collectively produce an in-phase wave front. The subwavelength slit resonators are arranged in a linear phase distribution in the y direction, and the anomalous refraction angle of the surface plasmons is controlled by controlling the phase gradient where σ∈{+1,-1} represents the left-handed circularly polarized terahertz wave LCP and the right-handed circularly polarized terahertz wave RCP incident, respectively:

[0011]

[0012] The active groups and the passive group are both obtained by arranging subwavelength metal slits with different rotational directions along the y axis, and the rotation angle is calculated by formula (1):

[0013]

[0014] where y is the coordinate position of the slit resonator in the y axis, when the circular polarization CP chirality changes, the direction of the excited surface plasmons will also change, and when the circularly polarized terahertz wave is vertically incident, the corresponding phase of the three groups of structures is shown in formula (2).

[0015] The device contains two active groups U1, U2 and a passive group U3 arranged between the active groups, and the linear phase distribution of the subwavelength slit resonator is changed to a lens phase distribution to realize the function of a superlens; specifically, the active groups and the passive group are both obtained by arranging subwavelength metal slits with different rotational directions along the y axis, and the rotation angle distribution of the position y is calculated by formula (3):

[0016]

[0017] where f is the focal length of the superlens, and the rotational angle of the subwavelength slit resonator corresponding to the y position is calculated according to formula ​From formula (3), U1 and U3 have a negative sign difference in the corresponding super lens phase distribution, so the polarity of the super lens formed by U1 and U3 is opposite; U2 and U3 have the same polarity, and when right-handed circularly polarized light / left-handed circularly polarized light terahertz waves are incident, the convex lens / concave lens function is realized, but the phase of U2 and U3 has a phase shift of pi, and the opening and closing of U1 and U2 can be controlled by GST; through such a structure distribution, the device exhibits the functions of U1 and U3 before and after the phase transition of GST, respectively, and the function switching is realized: after being excited by right-handed circularly polarized light terahertz waves at room temperature, the device exhibits a diverging wave front, and the polarity of the super lens is a concave lens; as the annealing temperature increases, after the GST is induced to crystallize, the polarity of the super lens is reversed, and becomes a convex lens.

[0018] The device comprises two active groups and a passive group sandwiched between the two active groups, and the active group and the passive group are obtained by arranging subwavelength metal slits with different rotation directions along the y axis, and the rotation angle is calculated by formula (4); the active group and the passive group are lens phase distributions, the polarity of structure U1 is the same as that of U2 and U3, but the focal length of U1 is f1=6mm, and the focal lengths of U2 and U3 are both f2=4mm; the phase distribution The relationship with the position y is:

[0019]

[0020] When right-handed circularly polarized light is incident, as the annealing temperature increases, the super lens is a convex lens, and the focal length changes from 6mm to 4mm, realizing the zoom function of the super lens; due to the spin correlation, when left-handed circularly polarized light is incident, the super lens is always a concave lens, and under nanosecond laser irradiation, the function of the device is restructured to the initial state.

[0021] The characteristics and beneficial effects of the present application are:

[0022] 1. A surface plasmon excitation phase control technology based on a double-hole structure is proposed, which breaks the strong correlation between the traditional excitation area shape and the wave surface shape, realizes the local regulation of any phase distribution in any excitation area, and can freely design the shape of the surface plasmon wave surface;

[0023] 2. GST is deeply integrated with metasurface, and the phase change characteristics of the phase change material break the limitations of the traditional metasurface function solidification, and provide high-performance solutions for reconfigurable photonic devices, dynamic holographic display, intelligent sensing and other fields;

[0024] 3. By combining the dynamic adjustable characteristics of the active metasurface with the control principle of the propagation phase and the geometric phase of the surface plasmon, a general method for actively regulating the surface plasmon is proposed. BRIEF DESCRIPTION OF DRAWINGS:

[0025] Figure 1 The working principle of active metasurface. (a), (b) Schematic of the angle-tunable anomalous refraction surface plasmonic coupler composed of three groups of structures U1, U2 and U3. By GST crystallization and amorphization, any two functions can be reversibly switched; (c), (d) Schematic of the polarity-tunable surface plasmonic metalens; (e), (f) Schematic of the focal length-tunable surface plasmonic metalens; (g) Schematic for deriving the surface plasmon phase excited by metasurface, active groups U1, U2, passive group U3. The dark and light stripes in the figure represent the excited surface plasmons.

[0026] Figure 2 The GST performance characterization results, NSTM system schematic and partial micrographs of three metasurface devices are shown. (a) shows the transmittance and the real part of conductivity of GST after different annealing temperatures and nanosecond laser irradiation at 0.75 THz operating frequency. (b) depicts the schematic of the NSTM system; the micrograph of the angle-tunable surface plasmonic coupler in the upper right corner is the structure corresponding to the part in the white dashed box in (c). (c) partial micrograph of the angle-tunable surface plasmonic coupler of sample 1. (d) partial micrograph of the polarity-tunable surface plasmonic metalens of sample 2. (e) partial micrograph of the focal length-tunable surface plasmonic metalens of sample 3.

[0027] Figure 3 The experimental and simulation characterization of the angle-tunable surface plasmonic coupler. (a) and (c) show the real part of Ez obtained experimentally under right-handed circularly polarized and left-handed circularly polarized THz wave incidence, respectively. (b) and (d) show the real part of Ez obtained by simulation under RCP and LCP incidence, respectively, corresponding to (a) and (c). The labels in the lower right corner are the states of GST, listed in order as the initial unheated state, the state after annealing in the temperature range of 150°C to 260°C, and the state after exposure to 120 mJ / cm2 nanosecond laser reconstruction. 2

[0028] Figure 4 The experimental and simulation characterization of the polarity-tunable surface plasmonic metalens. (a) and (c) show the real part of Ez obtained experimentally under right-handed circularly polarized and left-handed circularly polarized THz wave incidence, respectively. (b) and (d) show the real part of Ez obtained by simulation under RCP and LCP incidence, respectively, corresponding to (a) and (c). The labels in the lower right corner are the states of GST, listed in order as the initial unheated state, the state after annealing in the temperature range of 150°C to 260°C, and the state after exposure to 120 mJ / cm2 nanosecond laser reconstruction. 2

[0029] ​​Figure 5 Experimental and simulation characterization of the angle-tunable surface plasmonic coupler. (a) and (c) show the real part of Ez obtained experimentally under right-circularly polarized and left-circularly polarized terahertz wave incidence, respectively. (b) and (d) show the real part of Ez obtained by simulation corresponding to (a) and (c) under RCP and LCP incidence, respectively. The labels in the lower right corner are the states of GST, listed in order as the initial unheated state, the state after annealing in the temperature range of 150℃ to 260℃, and the state after exposure to 120mJ / cm 2 The state after nanosecond laser reconstruction. DETAILED DESCRIPTION

[0030] The technical field of terahertz surface plasmon dynamic wavefront modulation, in particular to a general method of actively modulating surface plasmons by combining the dynamically adjustable characteristics of GST with the control principles of the propagation phase and geometric phase of surface plasmons, and three active devices are made to verify.

[0031] The technical scheme adopted by the application is a GST-based terahertz surface plasmon dynamic wavefront modulation device, the bottom is a silicon substrate, and the silicon substrate is a lossy aluminum film with a subwavelength rectangular slot resonator array; the active group or passive group is composed of two columns of slot resonator arrays distributed along the y-axis perpendicular to each other, the difference between the active group and the passive group is that the surface of the aluminum film of the active group is deposited with a GST thin film strip, and the surface of the passive group is not deposited, and the active group and the passive group are arranged adjacent to each other; the slot resonators in each column are arranged in the x direction with a phase distribution and a successive deflection angle; the overall surface plasmon excitation process is as follows: the incident terahertz wave is first incident vertically to the substrate, most of the light is reflected by the metal aluminum mode after passing through the substrate, a small part of the light and the subwavelength rectangular slot resonator generate a local field, and the active free electrons on the metal surface are coupled to form surface plasmons, and the rotation angle of the subwavelength slot resonator, the relative distance between the subwavelength slot resonators and the GST phase change characteristics are used to realize the switching of different functions of surface plasmons.

[0032] The device contains two active groups and one passive group arranged between the active groups, when the GST is in the pre-phase change state, the GST acts as a medium, the two active groups U1, U2 and the passive group U3 work at the same time, but due to the π phase shift of the surface plasmons excited by U2 and U3, destructive interference occurs, so the function of the device is only provided by U1; when the GST is in the post-phase change state, the GST acts like a metal to close the terahertz wave response of the subwavelength slot resonator, resulting in the inactivity of the active groups U1 and U2, and the function of the device is only provided by U3; the rotation angle of the subwavelength slot resonator and the relative distance between the subwavelength slot resonators are used to form the wavefront of U1 and U3, and the device can exhibit different functions before and after the GST phase change.

[0033] In the direction of x-axis, the passive group U3 satisfies the following condition: θ2-θ1=3π / 2, s is the distance between the two columns of resonators and s=λ SP / 2, λ SP is the working wavelength of surface plasmon, θ1, θ2 are the angles between the electric field and x-axis, respectively, are the field components of the incident terahertz wave that can excite the left and right column of slit resonators, when the incident light is circularly polarized light, i.e. where is the incident light electric field vector, (1, σi) is the component of the Jones vector, i is the imaginary unit, i 2 =-1, σ∈{+1,-1} represent left-handed circularly polarized terahertz wave LCP and right-handed circularly polarized terahertz wave RCP respectively, the excited surface plasmon field is represented as:

[0034]

[0035] where A is the amplitude of surface plasmon, i is the imaginary unit, i2=-1, θ1, θ2 are the angles between the electric field and x-axis, l is the distance between the center of the two resonators and point N, N is a point on the aluminum film outside the active group and passive group, s is the distance between the two columns of resonators, and the wave number of surface plasmon is ksp=2π / λ SP , λ SP is the working wavelength of surface plasmon. It is obtained from the formula that the field amplitude of the surface plasmon excited by the passive group U3 at point N is fixed, and the phase can be freely controlled, the sign of which is determined by the handedness of circular polarization and changes with the rotation angle of the subwavelength slit resonator; since the subwavelength slit resonator can be rotated by 180°, the phase change can cover the entire 2π range, so the use of slit resonators can also achieve any phase distribution. For subwavelength slit resonators, when the distance s between the corresponding subwavelength slit resonators becomes mλ SP , m is an integer, and considering the low loss of surface plasmon, the phase response of the subwavelength slit resonator is close to constant, the active group and the passive group have the same size but different geometric phase and propagation phase, the geometric phase refers to the rotation angle of the subwavelength slit resonator, and the propagation phase refers to the horizontal distance between the subwavelength slit resonators. At point N, the field E N is the superposition of the surface plasmons excited by the active group U1, the passive group U2 and the passive group U3: E N =E U1N +E U2N +E U3N , E U1N , E U2N , E U3NSurface plasmons excited by active / passive groups U1, U2 and U3, respectively, wherein the active groups U1 and U2 have the same geometric phase but different propagation phases: s U1 = s + λ SP , s U1 , s are the horizontal distances between different groups of subwavelength slit resonators, i.e. E U1N = e iπ E U2N This means that the surface plasmon E U1N excited by the U1 structure has a phase shift of π compared with the surface plasmon E U2N excited by the U2 structure, i.e. E U1N = -E U2N Therefore, when the active groups U1 and U2 are working, destructive interference occurs, meaning that E N = E U1N The function of the device is only provided by the active group U1; in addition, if the active groups U1 and U2 are turned off by the GST in the crystal state, the function of the device will only be provided by the passive group U3.

[0036] The device contains two active groups and a passive group sandwiched between the active groups, and the coupling of surface plasmons is achieved by designing different phases of the two metal slits: the coupling effect is represented as when the phase of the subwavelength slit resonator is adjusted to , the coupling can act as a secondary emission source, exciting surface plasmons to generate a wave front in phase, arranging the subwavelength slit resonators in the y direction into a linear phase distribution, and controlling the anomalous refraction angle of surface plasmons by controlling the phase gradient wherein σ∈{+1,-1} represents left-handed circularly polarized terahertz waves LCP and right-handed circularly polarized terahertz waves RCP incident, respectively:

[0037]

[0038] The active group and the passive group are both arranged along the y axis by subwavelength metal slits with different rotational directions, and the rotation angle is calculated by formula (2):

[0039]

[0040] wherein y is the coordinate position of the slit resonator in the y axis, when the circular polarization CP chirality changes, the direction of the excited surface plasmon will also change, and when the circularly polarized terahertz wave is vertically incident, the corresponding phase of the three groups of structures is shown in formula (2).

[0041] ​The linear phase distribution of the sub-wavelength slit resonator is changed into a lens phase distribution by comprising two active groups U1 and U2 and a passive group U3 arranged between the active groups, so as to realize the function of a superlens; the active group and the passive group are both obtained by arranging sub-wavelength metal slits with different rotation directions along the y axis, and the rotation angle distribution is The relationship with the position y is calculated by formula (3):

[0042]

[0043] Wherein f is the focal length of the superlens, and the rotation angle of the sub-wavelength slit resonator corresponding to the y position is calculated according to formula (3) From formula (3), the sign of the superlens phase distribution corresponding to U1 and U3 is different by one negative sign, so the polarities of the superlenses formed by the two groups of U1 and U3 are opposite; U2 and U3 have the same polarity, and realize the functions of a convex lens and a concave lens when right circularly polarized light / left circularly polarized light terahertz waves are incident, but the two groups of U2 and U3 have a phase shift of π, and the opening and closing of U1 and U2 can be controlled by GST; through such a structure distribution, the device respectively exhibits the functions of U1 and U3 before and after the phase transition of GST, and realizes the switching of the functions: after being excited by right circularly polarized light terahertz waves at room temperature, the device exhibits a divergent wave front, and the polarity of the superlens is a concave lens; as the annealing temperature increases, after the GST is induced to crystallize, the polarity of the superlens is reversed, and becomes a convex lens.

[0044] The device comprises two active groups and a passive group sandwiched between the two active groups, the active group and the passive group are both obtained by arranging sub-wavelength metal slits with different rotation directions along the y axis, the rotation angle is calculated by formula (4), the active group and the passive group are both lens phase distributions, the polarity of structure U1 is the same as that of U2 and U3, but the focal length of U1 is f1=6mm, and the focal lengths of U2 and U3 are both f2=4mm; the phase distribution is The relationship with the position y is:

[0045]

[0046] When right circularly polarized light is incident, as the annealing temperature increases, the superlens is a convex lens, and the focal length changes from 6mm to 4mm, realizing the zoom function of the superlens; due to the spin correlation, when left circularly polarized light is incident, the superlens is always a concave lens, and under nanosecond laser irradiation, the function of the device is restructured to the initial state.

[0047] The application will be further described in detail below in combination with the drawings and specific examples.

[0048] The technical scheme adopted by the present application is a GST-based terahertz surface plasmon dynamic wavefront modulation device, which introduces the concept of phase discontinuity in free space into a metasurface composed of subwavelength slit resonators in the terahertz waveband, and integrates GST on the metasurface. By controlling the phase change state of GST, the excitation and dynamic wavefront modulation of terahertz surface plasmons are realized. By appropriately designing the spatial distribution of the pair of subwavelength slit resonators, almost any phase distribution can be achieved, thereby realizing different functions. The method is composed of three groups of structures, two groups of active groups (U1, U2) of subwavelength slit resonators integrated with GST, and one group of passive group (U3) composed of subwavelength slit resonators. By changing the different phase states of GST, the effective working structure of the device is switched. When GST is in an amorphous state, the function of the surface plasmon device is determined by the joint action of the three groups of structures. When GST is in a crystalline state, the function of the device is determined by the passive group (U3) alone. By reasonably designing the phase distribution of the three groups of structures, the dynamic wavefront modulation of the device can be realized.

[0049] Specifically, different wavefront modulations are realized by the interaction of the three groups of subwavelength slit resonators, wherein two active groups U1 and U2 have phase change material GST integrated on the surface, and the remaining one passive group U3 and the surface plasmon excited by the active group U2 have a propagation phase difference of π. When GST is in a pre-phase change state, GST acts as a medium, and the active group and the passive group work simultaneously. However, due to the π phase shift of the surface plasmons excited by U2 and U3, destructive interference occurs, so the function of the device is only provided by U1. When GST is in a post-phase change state, GST acts like a metal, and its large conductivity can turn off the terahertz wave response of the subwavelength slit resonator, so the active groups U1 and U2 do not work, and the function of the device is only provided by U3. At this time, by reasonably designing the wavefront of U1 and U3, the function of the device can exhibit different functions before and after the phase change of GST.

[0050] The sub-wavelength slit resonators of surface integrated GST are active groups U1 and U2, and the non-integrated GST is passive group U3; the positive phase change of GST from amorphous state to crystalline state is realized by setting the annealing temperature to heat GST, and the reverse phase change from crystalline state to amorphous state is realized by nanosecond laser generated by a laser; before the phase change of GST, the transmission coefficient of GST in the medium state relative to the silicon substrate in the terahertz wave band is close to 1, so it will not cause the short circuit effect of the active groups U1 and U2, and the three groups of structures of the super surface work normally at the same time, and when U2 and U3 are in the working state at the same time, the excited surface plasmons have a π phase shift, so the device exhibits the function of structure U1. After the phase change of GST, the metallic GST greatly enhances the short circuit effect of structure U1 and U2, U1 and U2 are closed, and the device exhibits the function of structure U3. Through reasonable design and arrangement of the three groups of structures, any two functions can be realized before and after the phase change of GST, and the realized functions can be reconfigured, which provides a theoretical basis for realizing the surface plasmon tunable device.

[0051] The overall structure is designed as three layers, which can realize wavefront shaping and surface plasmon excitation at the same time. The bottom is a silicon substrate with a thickness of 650 μm (ε = 11.9), and the top is a 200 nm thick lossy metal aluminum film (σ = 3.56 x 10 7 S / m) with a sub-wavelength rectangular slit resonator array. Two active groups and one passive group are composed of sub-wavelength metal aluminum slit resonators with different angles, and a 100 nm thick GST film strip with a width of 300 μm is deposited on the surface of the two active groups on the aluminum film, and the passive group surface is not deposited. The selected sub-wavelength slit resonator has a length of a = 66 μm and a width of b = 12 μm, and under this size, the working frequency of the device is 0.75 THz (λ SP The designed tunable super surface simultaneously utilizes the circular polarization related geometric phase (rotation angle of sub-wavelength slit resonator), circular polarization independent propagation phase (relative distance between sub-wavelength slit resonators) and GST phase change characteristics, realizes the switching of different functions of surface plasmon device. The slit resonators are perpendicular to each other and distributed along the y axis in two columns, and the different angles are calculated and arranged according to different phase distributions. The overall surface plasmon excitation process is that the incident terahertz wave is first vertically incident on the substrate, most of the light is reflected by the metal aluminum mode after passing through the substrate, a small part of the light and the sub-wavelength rectangular slit resonator generate a strong local field, and the active free electrons on the metal surface couple to form surface plasmons.

[0052] First, take a passive group U3 as an example, in the x-axis direction, U3 satisfies the following conditions: θ2-θ1=3π / 2, s=λ SP / 2, when the incident light is circularly polarized light, that is where The incident optical electric field vector, (1, σi) is the component of the Jones vector. i is the imaginary unit (i 2 = -1). σ ∈ {+1, -1} represents left-handed circularly polarized terahertz wave (LCP) and right-handed circularly polarized terahertz wave (RCP), respectively. The field of the excited surface plasmon can be expressed as:

[0053]

[0054] where A is the amplitude of the surface plasmon, i is the imaginary unit (i 2 = -1), θ 1,2 are the angles between the electric field and the x-axis, respectively, and l is the distance between the center of the two resonators and point N, s is the distance between the two rows of resonators, and the wave number of the surface plasmon is ksp= 2π / λ SP , λ SP is the working wavelength of the surface plasmon. It can be obtained from equation 1 that the field amplitude of the surface plasmon excited by structure U3 at point N is fixed, while the phase can be freely controlled, and its sign is determined by the handedness of the circular polarization and changes with the rotation angle of the subwavelength slit resonator. Since the subwavelength slit resonator can be rotated by 180°, the phase change can cover the entire 2π range, so almost arbitrary phase distribution can also be achieved using slit resonators. Fortunately, for subwavelength slit resonators, when the distance s between the corresponding subwavelength slit resonators becomes mλ SP (where m is an integer) and considering the low loss of the surface plasmon, the phase response of the subwavelength slit resonator can be considered to be nearly constant. This allows the subwavelength slit resonators to be further separated, allowing the concentric arrangement of subwavelength slit resonators with different intervals while sharing the same phase center. Therefore, the metasurface uses three groups of SR structures, all with the same size but with different geometric phases (rotation angles θ) and propagation phases (distances s between SRs). At point N, the field E N is the superposition of the surface plasmons excited by structures U1, U2 and U3: E N = E U1N + E U2N + E U3N , where the geometric phase of structures U2 and U3 is the same, but their propagation phases are different (s U2 = s + λ SP ). That is, E U2N = e iπ E U3N , which means that the surface plasmon excited by the U2 structure has a π phase shift compared to the surface plasmon excited by the U3 structure, i.e. E U2N = -E U3NTherefore, U2 and U3 will produce destructive interference when GST is in the amorphous state, which means that E N = E U1N The function of the device is only provided by the active group U1. In addition, if the active groups U1 and U2 are closed by GST in the crystalline state, the function of the device will only be provided by the passive group U3. At this time, a method that can be dynamically controlled before and after the GST phase change is found, and three active devices are designed and characterized based on this scheme.

[0055] The following are three specific examples of the present application.

[0056] The first device is an angle-variable surface plasmon coupler, which is composed of two active groups and one passive group. The active group and the passive group are obtained by arranging sub-wavelength metal slits with different rotation directions along the y-axis, and the rotation angle is calculated by formula 2. According to the theory of generalized Snell's law, the coupling of surface plasmons can be realized by designing the different phases of the two metal slits, which is represented as When the phase of the sub-wavelength slit resonator is controlled to make , the coupling can act as a secondary emission source, and the excited surface plasmons together produce a same-phase wave front. By arranging the sub-wavelength slit resonators in a linear phase distribution in the y direction, the anomalous refraction angle of surface plasmons is controlled by controlling the phase gradient where σ∈{+1,-1} represents the left-handed circularly polarized terahertz wave LCP and the right-handed circularly polarized terahertz wave RCP incident, respectively.

[0057]

[0058] It can be observed that when the circular polarization (CP) chirality changes, the direction of the excited surface plasmons also changes. The schematic diagram of the device is shown in Figure 1 (a) and (b), and the sub-wavelength slit resonators are arranged equidistantly along the y-axis. The rotation angle θ1 between the two adjacent pairs of SRs in the y direction is 30° , and the pitch d is 160 μm. Therefore, the length of one phase period is 960 μm. The finally designed device contains four phase periods. When the circularly polarized terahertz wave is vertically incident, the corresponding phase distribution of the three groups of structures is:

[0059]

[0060] where y is the coordinate position of the slit resonator in the y-axis. In order to measure the excitation response of surface plasmons, the probe of the near-field scanning optical microscope (NSTM) system is placed about 50 μm above the sample to detect the electric field component Ez of the excited surface plasmons. Figure 3(a) and (c) respectively show the Ez real part of the surface plasmons excited by the anomalous refraction angle tunable surface plasmon coupler under the incidence of right-handed circularly polarized light and left-handed circularly polarized light. It can be seen from the experimental results that the surface plasmons are inclined to propagate downward (upward) when the right-handed circularly polarized light (left-handed circularly polarized light) is incident without heating, because the GST is in an amorphous state after sample processing, which acts as a medium, and U2 and U3 have the same geometric phase in the active device, but the propagation phase of the two has a phase shift of π, and when the two groups of structures work at the same time, the excited surface plasmons will be coherent and cancelled, and the geometric phase of U1 determines the function of the device, and when the right-handed circularly polarized light is incident, the surface plasmons are inclined to propagate downward, and the anomalous refraction angle is about -24.6°, and due to the spin-dependent characteristics of the geometric phase, when the left-handed circularly polarized light is incident, the surface plasmons will be inclined to propagate upward, and the anomalous refraction angle is about 24.6°. As the temperature rises to 260℃, when the GST is in a crystalline state, the surface plasmon excitation response of the subwavelength slit resonator covered by GST is turned off, and only the passive group U3 provides the function of the device, and when the right-handed circularly polarized light is incident, the surface plasmons are inclined to propagate upward, and the anomalous refraction angle is about 24.6°, and when the left-handed circularly polarized light is incident, the surface plasmons are inclined to propagate downward, and the anomalous refraction angle is about -24.6°. And after irradiation by laser pulses with a wavelength of 1064 nm, a pulse width of 7 ns, a spot diameter of 1 cm, and an energy density of 120 mJ / cm2, the function of the device returns to the initial state without heating, verifying the reconfiguration characteristics of the device. Therefore, the two functions of the device are determined by U1 and U3 respectively, and the phase gradient of U1 and U3 in the design of the device differs by a negative sign, so that the propagation direction of the device before and after the GST phase transition is opposite. At the same time, the chiral phase of different circular polarizations also differs by a negative sign, so the switching of the device function by changing the GST phase transition state is consistent with the switching of the device function by circular polarization chirality change, which can be well verified from the experimental results. In addition, CST software is used to simulate the sample. The y direction is set to be a periodic boundary condition, and the x and z directions are set to be open. The incident light is a plane wave, which is vertically incident on the sample from the substrate. The surface plasmon field distribution information is obtained by the 0.75 THz frequency field monitor. The simulation results are as follows Figure 3 (b, d) show that the anomalous refraction angle is in good agreement with the experimental results.

[0061] The second device is a polarimetric surface plasmon superlens, consisting of two active groups and one passive group. Both the active and passive groups are formed by arranging subwavelength metal slits with different rotation directions along the y-axis, with the rotation angle calculated using Equation 3. By transforming the linear phase distribution of the subwavelength slit resonator into the lens phase distribution, the superlens function can be achieved. Based on this, a polarimetric surface plasmon superlens was designed at 0.75 THz. Without changing the incident circular polarization state, the polarity of the superlens can be switched simply by changing the amorphous and crystalline states of the GST, such as... Figure 1 As shown in (c) and (d), it consists of three sets of structures, where U1 and U3 are designed with specific phase distributions to achieve a polarity reversal of the lens, and U2 has a phase delay of π compared to U3, which leads to destructive interference. Phase distribution The relationship with position y is:

[0062]

[0063] Where f is the focal length of the superlens, the focal length of which is designed to be f = 6mm. To make the focusing and diverging effects of the superlens more pronounced, a total of 61 structures are arranged in the y-direction, with a span of 9.76mm. According to the formula, the rotation angle of the subwavelength slit resonator at the corresponding y position can be determined. Formula 3 also shows that the signs of the phase distributions of the corresponding superlenses U1 and U3 differ by a negative sign, indicating that the polarities of the two superlenses are opposite. U2 and U3 have the same polarity, functioning as convex (concave) lenses when right-handed (left-handed) circularly polarized (or left-handed) terahertz waves are incident. However, there is a π phase shift between the phases of U2 and U3, leading to coherent destructive phase transitions when both groups operate simultaneously. This can be considered as the two structures being "closed." Furthermore, the opening and closing of U1 and U2 can be controlled by the GST. This structural distribution allows the device to exhibit the functions of U1 and U3 before and after the GST phase transition, respectively, achieving functional switching. Experimental characterization using the NSTM system yields the following results: Figure 4 As shown in (a) and (c), the superlens exhibits a diverging wavefront after being excited by right-handed circularly polarized terahertz waves at room temperature. The superlens is initially concave. With increasing annealing temperature, after GST is induced to crystallize, the polarity of the superlens reverses, transforming it into a convex lens with a focal length of 6 mm. Under nanosecond laser irradiation, the polarity of the superlens returns to its pre-annealing state. Due to spin dependence, the polarity of the superlens is reversed under left-handed circularly polarized terahertz wave excitation compared to when right-handed circularly polarized light is incident. Therefore, the designed superlens can change its polarity by adjusting the chirality of the incident terahertz circularly polarized wave or increasing the annealing temperature of GST, possessing two degrees of freedom for more flexible switching of lens polarity.

[0064] The simulation verification is carried out, and the simulation settings are similar to the previous ones. The x, y and z directions are all open boundary conditions. The surface plasmon field distribution information is obtained through the field monitor at 0.75 THz frequency. As shown in Figure 4 As shown in (b) and (d), it can be seen that under the incidence of right-handed circularly polarized light (left-handed circularly polarized light), with the gradual increase of the annealing temperature, the polarity of the superlens changes from concave (convex) lens to convex (concave) lens, and the simulation results are consistent with the experimental results.

[0065] The third device is a variable-focus surface plasmonic superlens. The device is composed of two active groups and one passive group. The active group and the passive group are both obtained by arranging subwavelength metal slits with different rotation directions along the y axis. The rotation angle is calculated by formula 4. Similar to the design principle of the polarity-variable surface plasmonic superlens, it is composed of three groups of lens phase distribution structures. The difference is that, compared with the polarity-variable surface plasmonic superlens, the polarity of structure U1 is the same as that of U2 and U3, but the focal length of U1 is f1 = 6 mm, and the focal length of U2 and U3 is f2 = 4 mm. The phase distribution The relationship with the position y is:

[0066]

[0067] Here, a total of 61 structures are arranged in the y direction, with a span of 9.76 mm. The test results using the NSTM system are as follows Figure 5 As shown in (a) and (c), according to the experimental results, when right-handed circularly polarized light is incident, with the increase of the annealing temperature, the superlens is a convex lens, and the focal length changes from 6 mm to 4 mm, realizing the zoom function of the superlens. Due to the spin correlation, when left-handed circularly polarized light is incident, the superlens is always a concave lens. Under nanosecond laser irradiation, the function of the device is restructured to the initial state, which is highly consistent with the design results. The simulation of the zoom superlens is consistent with the simulation settings of sample 2, and the results are as follows Figure 5 As shown in (b) and (d), the simulation results are highly consistent with the experimental results.

[0068] In the above three devices, the subwavelength slit resonator is a small hole etched on the metal aluminum film, and the metal aluminum film etched with small holes is a metasurface. Since three devices are designed, each device is 1 passive group and 2 active groups, and the overall structure can be regarded as a passive group (surface not covered with GST) with enlarged spacing and concentric arrangement. Each passive group and active group is composed of two columns of slits, and the two columns of slits are perpendicular. Figure 1 is a schematic diagram, and the accurate structure position is shown in Figure 2 micrograph. The main change is that different rotation angles of the small holes can exhibit different functions. Different rotation angles are calculated by formulas (2), (3) and (4). The rotation angle GST is directly deposited on the surface, corresponding to Figure 2 The black position in the micrograph, the GST acts as a switch, before the GST phase transition, the terahertz wave incident from the bottom can normally pass through the hole, at this time, the three groups of structures are normally excited surface plasmons, and the destructive interference device function is only provided by U1. After the GST phase transition, the terahertz wave cannot pass through the hole covered with GST on the surface, which is equivalent to the hole being closed, and the function of the device is only provided by the passive group U3 of the surface not covered with the GST film.

[0069] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited to this, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A GST-based terahertz surface plasmon dynamic wavefront modulation device, characterized in that, The bottom is a silicon substrate, on which is a lossy aluminum film with a subwavelength rectangular slit resonator array. Two columns of slit resonators, perpendicularly distributed along the y-axis, form either an active or passive group. The difference between the active and passive groups is that the aluminum film in the active group has a germanium-antimony-tellurium (GST) thin film strip deposited on its surface, while the passive group does not. The active and passive groups are arranged adjacent to each other. The slit resonators in each column are arranged according to their phase distribution, rotating sequentially along the y-axis. The overall surface plasmon excitation process is as follows: the incident terahertz wave is first incident perpendicularly from the substrate. After passing through the substrate, most of the light is reflected by the aluminum film, while a small portion of the light generates a local field with the subwavelength rectangular slit resonators. This field couples with the active free electrons on the metal surface to form surface plasmons. By utilizing the rotation angle of the subwavelength slit resonators, the relative distance between the subwavelength slit resonators, and the GST phase transition characteristics, the different functions of the surface plasmons are switched.

2. The terahertz surface plasmon dynamic wavefront modulation device based on GST as described in claim 1, characterized in that, The device comprises two active groups and a passive group positioned between the active groups. When the GST is in the pre-phase transition state, the GST acts as a dielectric, and the two active groups U1 and U2 and the passive group U3 operate simultaneously. However, due to the π phase shift of the surface plasmons excited by U2 and U3, destructive interference occurs, so the device's function is provided solely by U1. When the GST is in the post-phase transition state, the GST acts like a metal, thereby shutting off the terahertz wave response of the subwavelength slit resonator, causing the active groups U1 and U2 to cease operation, and the device's function is provided solely by U3. By utilizing the rotation angle of the subwavelength slit resonator and the relative distance between the subwavelength slit resonators to form the wavefronts of U1 and U3, the device can exhibit different functions before and after the GST phase transition.

3. The terahertz surface plasmon dynamic wavefront modulation device based on GST as described in claim 2, characterized in that, Along the x-axis direction, the passive group U3 satisfies the following conditions: θ2-θ1 = 3π / 2, s is the distance between the two columns of resonators and s = λ SP / 2, λ SP is the working wavelength of surface plasmons, θ1 and θ2 are the angles between the electric field , and the x-axis, , respectively, and the field components that can excite the left and right column of slit resonators in the incident terahertz wave are respectively, where is the incident light electric field vector, (1, σi) is the component of the Jones vector, i is the imaginary unit, i² = -1, represent left-handed circularly polarized terahertz wave LCP and right-handed circularly polarized terahertz wave RCP respectively, and the field of the excited surface plasmons is represented as: Where A is the amplitude of the surface plasmon resonance, i is the imaginary unit, and i² = -1. , They are electric fields , The angle with the x-axis, l is the distance between the center point of the two resonators and point N, N is a point on the aluminum film outside the active and passive groups, s is the distance between the two rows of resonators, and the wavenumber of the surface plasmon resonance is ksp = 2π / λ. SP , λ SP It is the operating wavelength of surface plasmon polaritons, determined by the surface plasmon field. The formula shows that the field amplitude of the surface plasmon excited by the passive group U3 at point N is fixed, while the phase is freely controllable. Its sign is determined by the direction of circular polarization and varies with the rotation angle of the subwavelength slit resonator. Since the subwavelength slit resonator can rotate 180°, the phase change can cover the entire 2π range. Therefore, arbitrary phase distributions can be achieved using slit resonators. For subwavelength slit resonators, when the distance s between the corresponding subwavelength slit resonators becomes mλ... SP Where m is an integer, and considering the low loss of surface plasmon resonances, the phase response of the subwavelength slit resonator is nearly constant. The active and passive groups have the same dimensions but different geometric phases and propagation phases. The geometric phase refers to the rotation angle of the subwavelength slit resonator, and the propagation phase refers to the horizontal distance between the subwavelength slit resonators. At point N, the field E... N It is a superposition of surface plasmons excited by the active / passive groups U1, U2, and U3: E N = E U1N +E U2N +E U3N E U1N E U2N E U3N The surface plasmons excited by the active / passive groups U1, U2, and U3 are respectively. The active groups U1 and U2 have the same geometric phase, but their propagation phases are different: s U1 =s+λ SP s U1、 s represents the horizontal distance between the subwavelength slit resonators, i.e. This means that surface plasmons excited by the U1 structure Surface plasmons excited by U2 structure Compared to having a π phase shift, i.e., E U1N = -E U2N Therefore, when active groups U1 and U2 are working, destructive interference will occur, meaning E N = E U1N , the functionality of the device is provided only by the active group U1; furthermore, if the active groups U1 and U2 are closed by the GST in the crystalline state, the functionality of the device will be provided only by the passive group U3.

4. The terahertz surface plasmon dynamic wavefront modulation device based on GST as described in claim 1, characterized in that, The regulating device comprises two active groups and a passive group sandwiched between the active groups, and the coupling of surface plasmons is realized by designing different phases of two metal slits: when the phase of the regulating subwavelength slit resonator is k SP = dφ / dy, the coupling can serve as a secondary emission source, and the excited surface plasmons jointly generate a same-phase wave front, the subwavelength slit resonators are arranged into a linear phase distribution in the y direction, and the anomalous refraction angle of the surface plasmons is controlled by controlling the phase gradient dφ / dy wherein respectively represent left-handed circularly polarized terahertz waves LCP and right-handed circularly polarized terahertz waves RCP incident: (1) Both the active and passive groups are obtained by arranging subwavelength metal slits with different rotation directions along the y-axis, and the rotation angle is calculated by formula (2): (2) Where y is the coordinate position of the slit resonator on the y-axis. When the chirality of the circularly polarized CP changes, the direction of the excited surface plasmons will also change. When the circularly polarized terahertz wave is incident perpendicularly, the corresponding phases of the three sets of structures are shown in formula (2).

5. The terahertz surface plasmon dynamic wavefront modulation device based on GST as described in claim 1, characterized in that, The control device includes two active groups U1 and U2 and a passive group U3 set between the active groups, which transforms the linear phase distribution of the subwavelength slit resonator into a lens phase distribution to achieve the function of a superlens; specifically, the active and passive groups are both obtained by arranging subwavelength metal slits with different rotation directions along the y-axis, and the relationship between the rotation angle distribution φ and the position y is calculated by formula (3): (3) Where f is the focal length of the superlens, and according to the formula, the rotation angle of the subwavelength slit resonator at position y is θ = φ(y) / 2; From formula (3), the signs of the phase distributions of the superlenses corresponding to U1 and U3 differ by a negative sign, so the polarities of the superlenses formed by the two groups of U1 and U3 are opposite; U2 and U3 have the same polarity, and realize the function of convex lens / concave lens when right-hand circularly polarized light / left-hand circularly polarized light terahertz wave is incident, but the phases of the two groups of U2 and U3 have a π phase shift, and coherent cancellation will occur when the two groups work at the same time, and the opening and closing of U1 and U2 are controlled by GST; Through such structural distribution, the device can exhibit the functions of U1 and U3 before and after the phase transition of GST, respectively, and realize the function switching: after being excited by right-hand circularly polarized light terahertz wave at room temperature, it exhibits a diverging wavefront, and the polarity of the superlens is a concave lens. As the annealing temperature increases, after GST is induced to crystallize, the polarity of the superlens is reversed and becomes a convex lens.

6. The terahertz surface plasmon dynamic wavefront modulation device based on GST as described in claim 1, characterized in that, The control device comprises two active groups and a passive group sandwiched between the two active groups. Both the active and passive groups are formed by arranging subwavelength metal slits with different rotation directions along the y-axis. The rotation angle is calculated by formula (4). Both the active and passive groups are lens phase distributions. The polarity of structure U1 is the same as that of U2 and U3, but the focal length of U1 is designed to be f1 = 6 mm, while the focal lengths of U2 and U3 are both f2 = 4 mm. The relationship between the phase distribution φ and the position y is: (4) When right-hand circularly polarized light is incident, as the annealing temperature increases, the superlens becomes a convex lens, and the focal length changes from 6 mm to 4 mm, realizing the zoom function of the superlens. Due to spin correlation, when left-hand circularly polarized light is incident, the superlens is always a concave lens. Under nanosecond laser irradiation, the function of the device is reconstructed back to the initial state.

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