Switchable surface plasmon system based on GST

Abstract

The utility model belongs to the technical field of terahertz dual-function switchable surface plasmon polaritons, provides a novel method for realizing a dual-function switchable SPs (Surface Plasmon polaritons) device on a GST metasurface, is low in cost, and provides a switchable surface plasmon polariton system based on GST. The structure is as follows: a spin correlation directional SPs coupler is formed by a Fresnel zone plate covered with a non-volatile phase change material GST, odd half-wave zones of the Fresnel zone plate are bright zones, and even half-wave zones of the Fresnel zone plate are covered with GST and are dark zones; the normal phase change from an amorphous state to a crystalline state of the GST is realized by setting an annealing temperature on a heating table to heat the GST, the inverse phase change from the crystalline state to the amorphous state is realized by irradiating the GST through nanosecond laser generated by a laser, and the excitation of plane waves SPs or the excitation of focused SPs is realized. The switchable surface plasmon device is mainly applied to design and manufacturing occasions of switchable surface plasmon devices.

Description

TECHNICAL FIELD

[0001] The technical field of the present application is THz dual-functional switchable surface plasmon technology, in particular, it relates to the mutual switching of THz spin-dependent directional planar wave surface plasmon (SPs) coupler and directional SPs F-P waveband sheet by changing the crystal state of non-volatile phase change material Ge2Sb2Te5 (GST). BACKGROUND

[0002] SPs is a special surface electromagnetic wave existing at the interface between medium and metal. Under the excitation of external electromagnetic wave, the collective oscillation of free electrons in the metal occurs, which leads to the fluctuation of electron density, thereby forming a surface electromagnetic wave propagating along the interface between metal and medium. In the past two decades, SPs has shown great potential in various applications, such as super strong light transmission [1, 2], high resolution and high sensitivity imaging [3, 4], biochemical sensing [5, 6] and integrated plasmonic circuit [7] etc. Metasurface is a two-dimensional surface composed of periodic microstructures, which shows unprecedented ability in manipulating free-space light, including phase [8-11], amplitude

[12] and polarization [13-15], thereby enabling the design of complex light propagation behaviors and related devices [16-18]. Recent studies have shown that this high degree of freedom regulation can also be introduced into SP systems

[19] .

[0003] So far, various metasurface-based SPs devices have been developed, including directional and asymmetric SPs couplers

[20] , SPs vortex generators and interferometers [21-23], and SPs holograms

[24] etc. However, most of them work in a passive way, which hinders their application in modern tunable chip systems. One of the most attractive ways to overcome this defect is to integrate functional materials into metasurfaces by referring to the regulation of free-space light. Such as VO2 [25-27], graphene [28, 29], high electron mobility transistor

[30] , referring to liquid crystal

[31] etc., which adjust the resonant response of the structure by changing the conductivity and refractive index. However, these materials need continuous external excitation to maintain their performance. This is conducive to fast modulation, but there are limitations in terms of power consumption, manufacturing cost and application environment.

[0004] Recently, a new type of phase change material GST has shown unique reconfigurable and non-volatile characteristics, which has attracted more and more attention in the application of active phase change material. The state can be controlled by heating

[33] , laser irradiation

[34] , applying a bias voltage

[35] and other ways, which is suitable for various scenarios. At present, there are various GST-based free-space metasurface devices, including dielectric metasurface devices [36, 37], color display devices

[38] and memory devices [39, 40]. However, there are few related researches on the regulation of SPs. SUMMARY

[0005] In order to overcome the shortcomings of the prior art, the present application aims to provide a new method for realizing a dual-function switchable SPs device by using a GST metasurface, which is low in cost. To this end, the technical scheme adopted by the present application is as follows: a switchable surface plasmon system based on GST, which is composed of a spin-dependent directional SPs coupler covered with a non-volatile phase change material GST, wherein the odd half-wave band of the Fresnel wave band is a bright area, and the even half-wave band of the Fresnel wave band covered with GST is a dark area; the positive phase change from the amorphous state to the crystalline state of GST is realized by setting the annealing temperature of the heating table to heat GST, and the reverse phase change from the crystalline state to the amorphous state of GST is realized by nanosecond laser generated by a laser to irradiate GST; the spin-dependent directional SPs coupler is a spin-dependent plane wave SPs coupler in the amorphous state of GST, which excites plane wave SPs, and the spin-dependent directional SPs coupler is a spin-dependent directional SPs wave band in the crystalline state of GST, which excites focused SPs.

[0006] The odd and even half-wave bands are composed of rectangular sub-wavelength metal aluminum slit resonators (SR) at different coordinate positions, and the coordinates of the SR are fixed and unchanged.

[0007] The SRs are distributed in two columns along the y-axis perpendicular to each other, and under arbitrary circularly polarized incidence, the SPs excited to the right and left are represented as Er and E l :

[0008] E r = A [E1cosθe iφ +E2e iδ sinθ], (1)

[0009] E l = A [cosθ+E2e iδ sinθe iφ ], (1)

[0010] E1 and E2 are two orthogonal polarization components of incident light, whose directions are perpendicular to the SR in the two columns respectively, A is the coupling coefficient of SR, θ is the included angle between E1 and x axis, δ is the phase difference between E1 and E2, φ = k SP g is the propagation phase of SPs on the distance g between the two column structures, where k SP = 2π / λ SP is the wave number of SPs, λ SP is the wavelength of SPs;

[0011] Several regions of SR are covered by GST, which are determined according to the even half-waveband regions of the Fresnel zone plate, and the half-waveband radius r n is calculated by the following formula;

[0012]

[0013] Where n is a natural number describing the radius of each SPs Fresnel zone plate region, f is the designed focal length, the even half-waveband regions are classified as dark regions, and the GST can turn off the SPs excitation response of the SR below them, and the even half-waveband regions are classified as bright regions. When the GST is in the amorphous state, it acts as a medium, and all SRs work; at this time, the device is a directional plane wave SPs coupler, which excites left plane wave SPs under right-handed circularly polarized (RCP) incidence and excites right plane wave SPs under left-handed circularly polarized (LCP) incidence. By heating the GST above its phase transition temperature, it changes to a crystalline state and works as an approximate metal state, the SR in the dark region is short-circuited, and only the SR in the bright region works, and the SPs coupler becomes a SPs Fresnel zone plate to excite focused SPs to the left under RCP incidence and to the right under LCP incidence in a directional manner; by irradiating the crystalline GST with a nanosecond laser, the GST changes back to the amorphous state, and the SPs coupler then restores to the directional plane wave SPs coupler.

[0014] The method for manufacturing a GST metasurface switchable surface plasmonic device comprises the following steps: first, the silicon substrate is cleaned with acetone, isopropyl alcohol and deionized water in sequence; then, an aluminum layer with a thickness of 200 nm is deposited on the substrate using a thermal evaporation method; then, photoresist is spin-coated onto the aluminum surface, and then ultraviolet exposure is performed using a pre-designed mask; after development, a photoresist pattern is obtained; next, the sample is placed in an acidic solution for wet etching, and the metal parts not protected by the photoresist are etched away. The whole sample is then immersed in acetone to remove the photoresist, and the first layer of metal structure processing is completed; finally, further GST deposition and accurate alignment patterning are performed.

[0015] The detailed steps of the GST deposition and accurate alignment patterning are as follows: first, the photoresist is spin-coated on the device with the first layer of metal structure processed again, and a new photoresist pattern is obtained after ultraviolet exposure using a mask and further development; then, a GST film with a thickness of 100 nanometers is deposited thereon using a magnetron sputtering method; finally, the GST coating in the sample is stripped off with the photoresist below by immersing in acetone, leaving the required bifunctional switchable SPs device.

[0016] The characteristics and beneficial effects of the present application are:

[0017] 1. The strategy of the present application is based on the principle of Fresnel zone plate, and only the even half-wave zone of the GST covering the SPs Fresnel zone plate is used, which is very simple to realize;

[0018] 2. Based on the phase change characteristics of GST, the bifunctional switching between spin-dependent directional plane wave and focused SPs excitation is demonstrated experimentally;

[0019] 3. The non-volatility of GST makes the device function remain after each external excitation process, without the need for continuous energy input, so it is more energy-saving;

[0020] 4. The present application can be extended to realize more switching of SPs excitation functions, and has potential for diversified chip applications. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The control strategy of the GST metasurface bifunctional switchable SPs device provided for the embodiments of the present application is shown in (a). When the GST is in amorphous state, the device works as a spin-dependent directional plane wave SPs coupler. The inset shows the geometric size of the SRs. (b) When the GST is in crystalline state by annealing process, the device works as a spin-dependent directional focused SPs coupler. (c) When the GST is adjusted back to amorphous state by laser pumping, the device function switches back to the state in (a). (d) Schematic diagram of device design based on the principle of Fresnel zone plate. GST is placed in the even area. (e) Microscope image of part of the device made.

[0022] Figure 2 The performance of GST is characterized. The measured and simulated transmission amplitudes of the GST film at 0.75 THz and the measured real part of the conductivity are extracted. Please note that the scatter plots without heating and illumination are drawn before 150℃ for clear comparison, and do not mean that they have been annealed at the corresponding temperature.

[0023] Figure 3This is a schematic diagram of the NSTM system provided in an embodiment of the present invention. First, the terahertz wave generated by the transmitter is collimated using a lens. After passing through a linear polarizer (LP), it is projected onto the sample from the substrate side. Spatial polarizers (SPs) are measured using a terahertz probe, which detects the Ez field distribution above the sample.

[0024] Figure 4 Characterization of the dual-function switchable SPs device provided in this embodiment of the invention. (a) and (c) show the normalized SP intensity distribution measured under right-hand circularly polarized (RCP) and left-hand circularly polarized (LCP) incident radiation, respectively. (b) and (d) show the simulated normalized SP intensity distribution corresponding to RCP and LCP incident radiation in (a) and (c), respectively. The labels in the lower right corner indicate the GST processing procedure, from the initial unheated state, to the annealing temperature from 150°C to 300°C, and from top to bottom, 120 mJ / cm². 2 Nanosecond laser irradiation. Detailed Implementation

[0025] In the field of terahertz dual-function switchable surface plasmonic technology, this invention specifically relates to the mutual switching between terahertz spin-dependent oriented surface plasmon (SP) couplers and oriented SP Fresnel zone plates by not altering the crystal state of the volatile phase change material Ge2Sb2Te5 (GST).

[0026] Therefore, the technical solution adopted in this invention is a dual-function switchable surface plasmon resonance device based on GST metasurface. By setting the non-volatile phase change material GST in the even-numbered half-wave zone of the spin-correlated oriented SPs Fresnel zone plate, the Fresnel zone plate has a focusing function like a regular lens. The difference is that the zone plate utilizes the diffraction effect of light, that is, focusing is achieved by blocking the odd-numbered or even-numbered bands in the Fresnel half-wave zone. This invention chooses to block the even-numbered bands in the SPs Fresnel half-wave zone to achieve SPs focusing. When the gas slurry (GST) is in its amorphous state, it acts as a dielectric, and the even-numbered half-wave zones beneath it are unobstructed. In this state, the device functions as a spin-dependent plane-wave SPs coupler, exciting plane-wave SPs. When the GST is in its crystalline state, it acts as an approximate metal, and the even-numbered half-wave zones beneath it are obstructed, creating a spin-dependent directional SP Fresnel zone plate that excites focused SPs. When the spin direction changes, the direction of the excited SPs also changes. A positive phase transition from amorphous to crystalline state is achieved by setting different annealing temperatures on the heating stage, while an inverse phase transition from crystalline to amorphous state is achieved by irradiating the GST with a nanosecond laser. Since the state of the GST remains unchanged after each annealing process, the response of the SPs device is non-volatile. The experimental results agree well with the design. Our method paves the way for non-volatile and tunable on-chip devices.

[0027] The specific technical solutions are as follows:

[0028] The GST-based switchable surface plasmon system has the following structure: a spin-dependent directional SPs coupler is formed by a Fresnel zone plate covered with a non-volatile phase change material GST, the odd half wave band of the Fresnel zone plate is a bright zone, and the even half wave band of the Fresnel zone plate is covered with GST to form a dark zone; the positive phase change from the amorphous state to the crystalline state of GST is realized by heating GST at an annealing temperature on a heating table, and the reverse phase change from the crystalline state to the amorphous state of GST is realized by nanosecond laser irradiation of GST generated by a laser; the spin-dependent directional SPs coupler is a spin-dependent plane wave SPs coupler in the amorphous state of GST, and plane wave SPs are excited, while the spin-dependent directional SPs coupler is a spin-dependent directional SPs lens in the crystalline state of GST, and focused SPs are excited.

[0029] The odd and even half wave bands are composed of rectangular subwavelength metal aluminum slit resonators SR at different coordinate positions, and the coordinates of the SR are fixed and unchanged.

[0030] The SRs are distributed in two columns along the y-axis perpendicular to each other, and under arbitrary circularly polarized incidence, the SPs excited to the right side Er and the left side El are represented as:

[0031] E r = A [E1cosθe iφ +E2e iδ sinθ], (1)

[0032] E l = A [cosθ+E2e iδ sinθe iφ ], (1)

[0033] E1 and E2 are two orthogonal polarization components of the incident light, the directions of which are perpendicular to the SRs in the two columns, A is the coupling coefficient of the SR, θ is the included angle between E1 and the x-axis, δ is the phase difference between E1 and E2, and φ=k SP g is the propagation phase of SPs on the distance g between the two column structures, where k SP = 2π / λ SP is the wave number of SPs, and λ SP is the wavelength of SPs;

[0034] Several regions of the SR are covered with GST, and these regions are determined according to the even half wave band region of the Fresnel zone plate, and the half wave band radius r n of the Fresnel zone is calculated by the following formula;

[0035]

[0036] where n is a natural number describing the radius of each SPs zone of the Fresnel zone plate, f = 3 mm is the designed focal length, even numbered half-zone is classified as dark zone, GST can shut down the SPs excitation response of SRs under them, even numbered half-zone is classified as bright zone. When GST is in amorphous state, it acts as a medium, all SRs work; at this time, the device is a directional plane wave SPs coupler, under right circularly polarized RCP incidence, it excites leftward plane wave SPs, under left circularly polarized LCP incidence, it excites rightward plane wave SPs, by heating GST above its phase transition temperature, it turns into crystalline state and works as an approximate metal state, the SRs of dark zone are short-circuited, only the SRs of bright zone work, the device becomes an SPs Fresnel zone plate, in a directional way, under RCP incidence, it excites leftward focused SPs, under LCP incidence, it excites rightward focused SPs, by irradiating crystalline GST with nanosecond laser, it turns back to amorphous state, the device then restores the function of directional plane wave SPs coupler.

[0037] The method for manufacturing a GST metasurface switchable surface plasmonic device comprises the following steps: firstly, a silicon substrate is cleaned with acetone, isopropyl alcohol and deionized water in sequence; then, an aluminum layer with a thickness of 200 nm is deposited on the substrate by using a thermal evaporation method; then, photoresist is spin-coated onto the aluminum surface, and then ultraviolet exposure is performed by using a pre-designed mask; after development, a photoresist pattern is obtained; next, the sample is placed in an acidic solution for wet etching, wherein the metal parts not protected by the photoresist are etched away. The whole sample is then immersed in acetone to remove the photoresist, and the first layer of metal structure processing is completed; finally, GST deposition and accurate alignment patterning are further performed.

[0038] The detailed steps of GST deposition and accurate alignment patterning are as follows: firstly, the photoresist is spin-coated onto the device after the first layer of metal structure processing is completed, and then ultraviolet exposure is performed by using a mask, and a new photoresist pattern is obtained through further development; then, a GST film with a thickness of 100 nm is deposited on it by using a magnetron sputtering method; finally, the GST coating sample is peeled off by immersing it in acetone, leaving the desired dual-functional switchable SPs device with the underlying photoresist.

[0039] The application will be further described in detail below in combination with the drawings and specific examples.

[0040] The spin-dependent directional SPs coupler is a Fresnel zone plate, the odd half-zone corresponds to the previous bright zone, and the even half-zone corresponds to the dark zone, and the even half-zone is covered with GST. The odd and even half-zones are composed of SRs at different coordinate positions, and the coordinates of the SRs are fixed. The positional relationship between the SRs and the GST can be regarded as the positional relationship between the odd and even half-zones and the GST.

[0041] The basic building block of the proposed metasurface is a rectangular subwavelength metallic aluminum slit resonator (SR) with width a = 12 μm and length b = 66 μm, which is a common structure for coupling free-space light into SPs, see Figure 1 The inset in (a). Only the incident light with polarization direction perpendicular to the metallic hole can be excited into strong SPs and radiate like in-plane dipoles to both sides. According to Huygens principle, when many SRs with the same handedness are arranged in a line with a distance smaller than the SPs wavelength, planar wave SPs will be excited with a propagation direction perpendicular to the whole structure; when they are arranged in a line according to FZP distribution, focused SPs will be excited. Here, to increase the functionality, the present application adopts a two-line design, see Figure 1 .

[0042] By setting the non-volatile phase-change material GST in the even half waveband of the spin-dependent directional SPs Fresnel zone plate, the Fresnel zone plate has the focusing function as the ordinary lens, but the difference is that the zone plate uses the diffraction effect of light, that is, focusing is achieved by blocking the odd or even band in the Fresnel half waveband, and the present application selects to block the even band in the SPs Fresnel half waveband to achieve SPs focusing. When the GST is in the amorphous state, the GST acts as a medium, and the even half waveband under the GST is not blocked, at this time the device is a spin-dependent planar wave SPs coupler, exciting planar wave SPs; and in the crystalline state of GST, after heating, the conductivity of GST rises, the transmittance of THz decreases, and the short-circuit effect of SR is enhanced, so that GST acts as an approximate metal, the even half waveband under the GST is blocked, and it is a spin-dependent directional SPs Fresnel zone plate, exciting focused SPs; when the spin direction changes, the direction of the excited SPs also changes, and the positive phase change of GST from amorphous state to crystalline state is realized by setting different annealing temperatures on the heating table, and the negative phase change from crystalline state to amorphous state is realized by nanosecond laser irradiation of GST.

[0043] Figure 1 In the above, the SRs in the two lines are perpendicular to each other. Under any circularly polarized incidence, the SPs excited to the right E r and left E l can be expressed as:

[0044]

[0045] E1 and E2 are two orthogonal polarization components of the incident light, their directions are perpendicular to the SRs in the two lines respectively. A is the coupling coefficient of the SR, θ is the angle between E1 and the x-axis, δ is the phase difference between E1 and E2, φ = k SP g is the propagation phase of SPs on the distance g between the two lines of structures, where k SP = 2π / λSP is the wave number of SPs, λ SP is the wavelength of SPs. By analyzing equation (1), it can be found that when θ = 45°, g = λ SP / 4, and the incident light is circularly polarized, under the right circularly polarized (RCP, δ = -π / 2) incident condition, there are |E r | = 0 and |E l | = 2 1 / 2 AE1, under the left circularly polarized (LCP, δ = π / 2) incident condition, there are |E r | = 2 1 / 2 AE1and |E l | = 0. The present application is designed based on this strategy, aiming to realize spin-dependent directional SPs coupler.

[0046] Figure 1 (a) to (c) show the schematic diagram of the bifunctional switchable SPs device proposed by the present application. Two columns of SRs are arranged along the y-axis. Several regions of SRs are covered by GST, which are determined according to the even regions of the Fresnel zone plate, see Figure 1 (d). The radius r n of the Fresnel zone can be calculated by equation 2;

[0047]

[0048] where n is a natural number describing the radius of each Fresnel zone plate region, and f is the designed focal length. When n is even, the radius r n of the Fresnel zone is the even region radius, corresponding to the even region; when n is odd, the radius r n of the Fresnel zone is the odd region radius, corresponding to the odd region. In this way, the interval between bright and dark regions is cyclic, forming a Fresnel zone plate covered by non-volatile phase change material GST. Here, the even region is classified as a dark region because GST can turn off the SPs excitation response of the SRs below it. In contrast, the odd region is classified as a bright region. When GST is in amorphous state, it acts as a medium and all SRs work. This device excites leftward plane wave SPs under right circularly polarized (RCP) incidence and rightward plane wave SPs under left circularly polarized (LCP) incidence, which is called directional plane wave SPs coupler, see Figure 1 (a). By heating GST above its phase transition temperature, it is converted to crystalline state and works as a metal state. Therefore, the SRs of the dark region are short-circuited, and only the SRs of the bright region work. The device becomes a SPs Fresnel zone plate, which excites focused SPs to the left under right circularly polarized (RCP) incidence and to the right under left circularly polarized (LCP) incidence in a directional manner, see Figure 1(b) By irradiating the crystalline GST with a nanosecond laser, it is converted back to the amorphous state. The device then regains the functionality of a directional planar wave SPs coupler, see Figure 1 (c) The bright-dark zone coordinate distribution of the device is shown in Table 1, which gives the coordinates of the odd and even half-wave bands. The coordinates of the even half-wave bands (r2, r4, r6) are the coordinates of the GST, and the bright-dark zones are represented by "0" and "1", respectively.

[0049] y (mm) -3.2 -3.1 -3 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 Distribution of light and dark areas 0 0 0 1 1 1 0 0 0 1 y (mm) -2.2 -2.1 -2 -1.9 -1.8 -1.7 -1.6 -1.5 -1.4 -1.3 Distribution of light and dark areas 1 1 1 0 0 0 0 1 1 1 y (mm) -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 Distribution of light and dark areas 1 0 0 0 0 0 0 0 0 0 y (mm) -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Distribution of light and dark areas 0 0 0 0 0 0 0 0 0 0 y (mm) 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Distribution of light and dark areas 0 0 0 0 1 1 1 1 0 0 y (mm) 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Distribution of light and dark areas 0 0 1 1 1 1 0 0 0 1 y (mm) 2.8 2.9 3.0 3.1 3.2 Distribution of light and dark areas 1 1 0 0 0

[0050] The sample was made by depositing a 200 nm thick aluminum film (σ = 3.56 x 10 7 S / m) on a 650 μm thick silicon substrate (ε Si = 11.9). The SRs have dimensions a = 12 μm and b = 66 μm, and a resonance frequency of 0.75 THz, at which the SPs excitation efficiency is the highest. Since the metal behaves almost like a perfect conductor at terahertz frequencies, the SPs dispersion is almost equal to that in free space. Therefore, g was set to 100 μm, so that the device operates at 0.75 THz (λ SP = 400 μm). Each column contains 65 SRs with d = 100 μm.

[0051] To experimentally characterize the performance of the SPs device, a fully fiberized NSTM system was used to directly detect the SPs field on the sample surface. This system uses a photoconductive antenna to generate a broadband terahertz wave, which is collimated by a terahertz lens and is incident perpendicularly to the substrate through a linear polarizer to excite the SPs on the sample surface. A terahertz near-field probe was used to probe the SP field about 50 μm above the sample surface. The probe is sensitive to the z-polarized electric field component and can detect the z component of the electric field. To measure the SP distribution under circularly polarized incident waves in a wide frequency range, we measured the SP distribution under two orthogonal linearly polarized incident waves (Ez, +45° and Ez, -45°) by rotating the linear polarizer to angles of +45° and -45° with respect to the polarization direction of the terahertz wave emitted by the photoconductive antenna. The results under circularly polarized incident waves were synthesized from the results under +45° and -45° linearly polarized incident waves. E z,RCP = E z,+45° -iE z,-45° ; E z,LCP = E z,+45° +iE z,-45° .

[0052] In the present application, the phase transition characteristics of GST are the key to realizing the dual-function switching application. To study the modulation effect of GST on terahertz waves, a 100 nm thick GST film was deposited on a silicon substrate using a magnetron sputtering technique, and a fully fiberized terahertz time-domain spectroscopy (THz-TDS) system was used for measurement. Figure 2The functions of transmittance and conductivity at 0.75 THz versus annealing temperature are plotted.

[0053] The two discrete points in the upper left corner represent the results of the GST film as initially fabricated (without heating) and after being irradiated with a nanosecond laser after heating, which is almost transparent to terahertz waves, so the corresponding conductivity values are very low. The continuous points in the figure show the results when the annealing temperature is increased from 150°C to 340°C. The annealing process is simply performed by placing the sample on a hot plate set to the target temperature for 1 hour, and then naturally cooling to room temperature for measurement. Due to the non-volatile nature of GST, the material properties of GST remain unchanged, and no continuous heating process is required. It can be seen that as the annealing temperature increases, the transmittance amplitude gradually decreases from about 0.97 to about 0.41. It can be seen that it changes sharply around 280°C, and shows a saturation effect when the annealing temperature is further increased above 340°C. Then, the GST film is irradiated with a nanosecond laser with a center wavelength of 1064 nm, a pulse width of 10 ns, a spot size of 9 mm, and an energy density of 120 mJ / cm2. The corresponding transmittance amplitude suddenly increases to about 1, which is almost the same as that without heating, which means that the GST has returned to the initial state.

[0054] In order to examine the change in conductivity of the GST film, the thin film approximation theory

[45] is used, and the complex conductivity σ GST (ω) of the GST film can be expressed as: r (ω) + iσ i (ω).

[0055]

[0056] In the formula, n si = 3.42 is the refractive index of the silicon substrate, Z0 = 377 Ω is the vacuum impedance, d GST = 100 nm is the thickness of the GST film, t GST (ω) is the complex transmittance of the GST film. Figure 2 The five-pointed star in the figure is the calculated real part σ r (ω) of the conductivity of the GST film, which shows a trend opposite to the transmittance, that is, the greater the conductivity, the greater the shielding effect on terahertz waves. This indicates that the GST gradually changes to a crystalline state. At an annealing temperature of 340°C, the conductivity can reach 1.68 x 10 5 S / m, which is sufficient to shorten the SRs. After irradiation with a nanosecond laser, the conductivity returns to the unheated state, and the conductivity suddenly decreases to 9.30 x 10 2 S / m. This indicates that the GST is switched back to the amorphous state. The imaginary part σ i (ω) of the conductivity of the GST film is smaller than σ r(ω) is two orders of magnitude smaller. To demonstrate the validity of the measured conductivity, the present invention further calculates the relative permittivity ε of the GST thin film approximately in the following manner. GST (ω):

[0057]

[0058] The data was then imported into CST simulation software for simulation. Here, ε0 is the dielectric constant of vacuum. Figure 2 The solid spheres represent the corresponding simulated transmittance. They are in excellent agreement with the measured results (boxes). This also demonstrates that the present invention can directly apply the measured dielectric constant of GST to numerically simulate the modulation effect of GST.

[0059] The fabrication process for a dual-function switchable SPs device is as follows: First, the silicon substrate is sequentially cleaned with acetone, isopropanol, and deionized water. Then, a 200 nm thick aluminum layer is deposited on the substrate using thermal evaporation. Next, photoresist is spin-coated onto the aluminum surface, followed by UV exposure using a pre-designed mask. After development, a photoresist pattern is obtained. Next, the sample is immersed in an acidic solution for wet etching, during which the metal portions not protected by the photoresist are etched away. The entire sample is then immersed in acetone to remove the photoresist. At this point, the first metal structure is complete, resulting in Sample 1. Further GST deposition and precise alignment patterning are required on Sample 1. First, photoresist is spin-coated onto another prepared Sample 1. After UV exposure using another pre-designed mask, a new photoresist pattern is obtained through further development. Then, a 100 nm thick GST film is deposited on it using magnetron sputtering. Finally, the portion of the GST-coated sample containing the underlying photoresist was peeled off by immersion in acetone, leaving the desired dual-function switchable SPs device. The final sample was a 200 nm thick aluminum film (σ = 3.56 × 10⁻⁶). 7 S / m) is laid on a 650 mm thick silicon substrate (ε Si =11.9) is used to construct the SR. The dimensions of the SR are a = 12 μm and b = 66 μm, and the resonant frequency is 0.75 THz, with the highest SPs excitation efficiency. Since the metal behaves almost like a perfect conductor at terahertz frequencies, the SPs dispersion is almost equal to the dispersion in free space. Therefore, setting g to 100 μm makes the device operate at 0.75 THz (λ SP =400μm).

[0060] The fabricated device was experimentally characterized using an all-fiber near-field scanning terahertz microscope (NSTM) system. The NSTM system probe was positioned approximately 50 μm above the sample to detect the Ez field of the excited SPs. Figure 4(a) and (c) show the normalized SPs intensity (|Ez 2 ) distribution on the x-y plane measured at RCP and LCP incidence, respectively. The GST is initially amorphous after fabrication, and its dielectric property makes the device a directional coupler for exciting planar SPs. When the annealing temperature is raised to 200 °C, the excitation response of SPs almost remains unchanged, because the temperature is not high enough to cause a significant phase transition of GST, and the conductivity is still very low. As the annealing temperature continues to rise to 260 °C, the GST undergoes a phase transition, and the conductivity of GST increases sharply. The shorting effect of dark regions on SRs becomes obvious, but not enough. Therefore, both planar SPs and focused SPs are excited, in which the planar SPs become weaker, and the focused SPs become stronger. Under the conditions of left circular polarization LCP and right circular polarization RCP incidence, the focal points are observed obviously on both sides of the structure. Continue to raise the annealing temperature to 280 °C, the GST is almost converted to a crystalline state. The high conductivity almost completely shorts the SRs, and the device switches to a SPs Fresnel zone plate. Therefore, the focused SPs dominate. At the annealing temperature of 300 °C, the GST is in a crystalline state, and the focal points become more clear. After nanosecond laser irradiation, the device switches back to a directional planar SPs coupler. The simulation results (b), (d) and experimental results have good consistency. The above results well prove the dual-function switchability and reconfigurability of the device. Note that due to the limitations of the experiment, such as the state of the system and the alignment difference of the relative position of the sample and the probe, Figure 4 there is no comparability between the absolute intensities measured at different rows. This does not affect the internal physical properties of the present application, because the switching behavior is clearly observed.

[0061] The above merely describes the specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any changes or replacements easily thought of by those skilled in the art within the technical scope disclosed by the present application should be covered within the protection scope of the present application.

Claims

1. A GST-based switchable surface plasmon system, characterized in that, The structure is as follows: a spin-dependent directional SPs coupler is composed of a Fresnel zone plate covered with a non-volatile phase change material GST, odd half-wave bands of the Fresnel zone plate are bright areas, and even half-wave bands of the Fresnel zone plate covered with the GST are dark areas; a heating table is used for heating the GST to realize positive phase change; and a laser is further included, and nanosecond laser generated by the laser irradiates the GST to realize negative phase change.

2. The GST-based switchable surface plasmon system of claim 1, wherein, The odd and even half-wave bands are composed of rectangular subwavelength metal aluminum slit resonators SR at different coordinate positions, and the coordinates of the SR are fixed and unchanged.

3. The GST-based switchable surface plasmon system of claim 1, wherein, The SRs are distributed along two columns perpendicular to each other along the y-axis. Under arbitrary circularly polarized incidence, the SPs excited to the right and left sides are denoted as Erand E l : E r = A [E1cosθe iφ +E2e iδ sinθ] E l = A [cos θ + E2e iδ sin θe iφ ] E1 and E2 are two orthogonal polarization components of the incident light, whose directions are perpendicular to the SR in the two columns, respectively, A is the coupling coefficient of the SR, θ is the included angle between E1 and the x-axis, δ is the phase difference between E1 and E2, φ = k SP g is the propagation phase of the SPs on the distance g between the two column structures, where k SP = 2π / λ SP is the wave number of the SPs, λ SP is the wavelength of the SPs; Several regions of the SR are covered by the GST, which are determined from the even half-zone regions of the Fresnel zone plate, the half-zone radius r n This is calculated by the following formula; Wherein n is a natural number describing the radius of each SPs Fresnel zone plate area, f is the designed focal length, and the even half-wave band area is classified as a dark area.

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