Plasmonic wideband GST‐based switch in the near‐infrared region
2022; Institution of Engineering and Technology; Volume: 16; Issue: 5 Linguagem: Inglês
10.1049/ote2.12073
ISSN1751-8776
Autores Tópico(s)Optical Wireless Communication Technologies
ResumoIET OptoelectronicsVolume 16, Issue 5 p. 201-206 ORIGINAL RESEARCHOpen Access Plasmonic wideband GST-based switch in the near-infrared region Saman Heidari, Saman Heidari Department of Electrical Engineering, Shiraz University of Technology, Shiraz, IranSearch for more papers by this authorNajmeh Nozhat, Corresponding Author Najmeh Nozhat [email protected] orcid.org/0000-0002-5652-0123 Department of Electrical Engineering, Shiraz University of Technology, Shiraz, Iran Correspondence Najmeh Nozhat, Department of Electrical Engineering, Shiraz University of Technology, Shiraz, Iran. Email: [email protected]Search for more papers by this author Saman Heidari, Saman Heidari Department of Electrical Engineering, Shiraz University of Technology, Shiraz, IranSearch for more papers by this authorNajmeh Nozhat, Corresponding Author Najmeh Nozhat [email protected] orcid.org/0000-0002-5652-0123 Department of Electrical Engineering, Shiraz University of Technology, Shiraz, Iran Correspondence Najmeh Nozhat, Department of Electrical Engineering, Shiraz University of Technology, Shiraz, Iran. Email: [email protected]Search for more papers by this author First published: 01 June 2022 https://doi.org/10.1049/ote2.12073AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract In this paper, we present a GST-based wideband plasmonic switch. With the excitation of localised surface plasmons and concentration of electric field in the structure, a near-perfect and wideband absorption is achieved. The switch consists of a GST layer, which acts as a Fabry–Perot cavity. Increasing the temperature and changing the state of GST lead to a high difference between the absorption spectra in amorphous and crystalline states in a wide range of wavelength. Therefore, the switch has a high extinction ratio of 13.71 dB at the wavelength of 1343 nm. Also, the response time of the switch is obtained as 46 fs. The structure has near-perfect absorption up to the incident angle of about 20°. Moreover, due to the symmetry of structure, the absorption spectrum is independent of polarisation. To show the validity of simulation results, the analytical method of equivalent circuit model is presented. The proposed polarisation-insensitive switch with high extinction ratio, fast response time and wide bandwidth can be used in photodetectors, plasmonic modulators and logic gates. 1 INTRODUCTION Phase change materials (PCMs) are suitable materials for use in adjustable plasmonic devices such as switches and multilevel memories [1]. GST (germanium-antimony-tellurium) is one of the phase change materials with amorphous and crystalline states. The refractive index of GST is temperature dependent and with increasing the temperature to above 160°C, its state changes from amorphous to crystalline and shows different optical properties [2]. For re-amorphisation, the material must be heated at the temperature of 640°C. Vanadium dioxide (VO2) is another suitable PCM for designing adjustable structures. VO2 exhibits insulating behaviour at temperatures below 68ºC and metal behaviour at temperatures above 68ºC [3]. There are different phase change methods such as thermal, electrical and all-optical transitions. Switches are one of the most important devices used in optical communication systems, modern medicine and colour switching. The two main categories of switches are free space and waveguide structures. In both categories, switching performance depends on changing the optical properties of materials used in the structure [4]. The most common way to achieve switching capability is changing the refractive index of material, which can be achieved by using adjustable PCMs such as GST, VO2 and other tunable materials such as graphene and liquid crystals. Changing the polarisation of incident light is another way to achieve switching mechanism [5]. The most important factors of a switch are extinction ratio, response time and optical bandwidth. The extinction ratio is defined as η = 10 log ( A ON A OFF ) $\eta =10\,\mathrm{log}(\frac{{A}_{\text{ON}}}{{A}_{\text{OFF}}})$ , where A ON ${A}_{\text{ON}}$ and A OFF ${A}_{\text{OFF}}$ are the absorption of the structure at ON and OFF states, respectively [6]. Optical switches are used in passive devices such as polarisation-division-multiplexer (PDM) [7], power splitter [8], and polariser [9], and also used in active devices such as modulator [10]. Plasmonics is a part of nanophotonics, which is based on the interaction between electromagnetic wave and free electrons on the metal surface [11]. Plasmonics studies the optical properties and propagation of electromagnetic waves in structures with dimensions smaller than the wavelength. Field reinforcement, overcoming the diffraction limit and working at high speeds are important capabilities of plasmonic structures [11]. In recent years, plasmonic devices such as switch [12], waveguide [13], absorber [14], sensor [15], laser [16], and modulator [17, 18] have been designed accordingly. So far, several switches have been designed based on PCMs. A wideband switch based on GST has been designed in the near-infrared range by Hu et al. The extinction ratio of 8.8 dB at the wavelength of 1410 nm has been obtained for this switch according to the tunability of GST [6]. Also, a switch that consists of micro-ring resonators partially covered with GST has been studied with the extinction ratio of 10 dB [19]. Moreover, a plasmonic switch that consists of three thin wings of VO2 placed on a silica substrate has been reported in Ref. [20] with the extinction ratio of 11.14 dB and response time of 600 fs at the wavelength of 1550 nm. Sun et al. have proposed a VO2-based switch with the low extinction ratio of 5.3 dB in the near-infrared region. The structure consists of holes embedded in the aluminium layer [21]. In this aper, a polarisation-insensitive wideband plasmonic switch in the near-infrared region has been designed and simulated. The structure consists of metallic square and circular resonators placed on Mgf2 and GST layers. Initially, the switching capability of the proposed structure has been shown. The absorption spectrum of the structure changes between near-perfect absorption and near-total reflection states, and the extinction ratio of 13.71 dB has been obtained at the resonance wavelength of 1343 nm. Also, the response time of the switch is 46 fs. Then, the effects of geometric parameters and polarisation and incident angles on the absorption spectrum of the structure have been studied. Moreover, the analytic method of equivalent circuit model has been extracted and shown there is good agreement between the simulated and analytic results. Finally, the proposed switch is compared with previous studies in terms of basic switching parameters and analytic method. 2 GEOMETRY OF THE STRUCTURE AND SIMULATION RESULTS Figure 1 shows the schematic and top views of a unit cell of the proposed switch. The structure consists of square and circular annular resonators made of silver. The Mgf2 and GST layers are located under the silver layer, respectively, and the whole structure is placed on the silver substrate. The relative permittivity of Mgf2 is equal to 1.96 [22] and the dielectric constants of GST and silver are based on the experimental data [19] and Babar-Weaver data [23], respectively. The values of the structural parameters of the structure are listed in Table 1. FIGURE 1Open in figure viewerPowerPoint (a) The schematic diagram and (b) top view of a unit cell of the suggested switch TABLE 1. The values of geometric parameters of the proposed GST-based switch Parameter Value (nm) Parameter Value (nm) t1 195 g 34 t2 125 r1 150 t3 10 r2 140 tm 100 p 500 w 34 The structure is stimulated with an electromagnetic wave in the -z direction. The finite element method (FEM) is used to simulate the structure. The boundary conditions are open in the z direction and periodic along x and y directions. Figure 2 depicts the absorption spectrum of the structure in amorphous and crystalline states of GST. The absorption coefficient is calculated according to A ( λ ) = 1 − T ( λ ) − R ( λ ) $A(\lambda )=1-T(\lambda )-R(\lambda )$ , where T ( λ ) $T(\lambda )$ and R ( λ ) $R(\lambda )$ are the transmission and refection coefficients, respectively [15]. The structure has a wideband spectrum in amorphous state with the full-width at half-maximum (FWHM) of 242 nm. In amorphous state, the absorption is about 94% at the wavelength of 1343 nm. When the GST state changes from amorphous to crystalline, the absorption decreases from 94% to 4% and the structure switches from ON state to OFF state. Therefore, the structure has the maximum extinction ratio of 13.71 dB at this wavelength. FIGURE 2Open in figure viewerPowerPoint Absorption spectra of the GST-based switch in amorphous and crystalline states To calculate the response time, a continuous wave (CW) at the resonance wavelength stimulates the structure. As demonstrated in Figure 3, by monitoring the time changes at the output port, when the output signal reaches to steady state, the response time is obtained. The response time of the proposed switch is 46 fs at 1343 nm. It is worth noting that in calculating the response time, the time duration of the phase change of GST is not considered. However, by utilising femtosecond laser pulses with the widths of 50 and 60 fs, the phase change of GST occurs [24, 25]. FIGURE 3Open in figure viewerPowerPoint Input and output signals for calculating the response time at 1343 nm To have a better insight about the switching function of the structure, the electric field distributions at the resonance wavelength are shown in Figure 4. In the amorphous state, localised surface plasmons (LSPs) are formed at the silver and Mgf2 boundary and also concentrated in the GST layer. However, in the crystalline state, the LSPs at the silver and Mgf2 boundary are slightly excited. Also, the confinement of LSPs in the GST layer is very low, so the absorption of the structure is reduced. In other words, since the silver layer is at the bottom of the structure with the thickness greater than its skin depth, the transmission is zero. In the crystalline state of GST, most of the incident wave is reflected and so the absorption is close to zero. Therefore, as obvious in the electric field distribution of Figure 4b the concentration of electric field in the structure is low. FIGURE 4Open in figure viewerPowerPoint The distributions of electric field magnitude of the suggested switch in (a) amorphous and (b) crystalline states of GST at λ = 1343 nm In the following, the absorption behaviour of the switch for various structural parameters is investigated. The absorption spectra for different values of GST (t1) and Mgf2 (t2) thicknesses are demonstrated in Figures 5 and 6, respectively. The GST and Mgf2 layers can be considered as an insulating material with an effective refractive index. Therefore, these two layers act as Fabry–Perot (FP) cavity. By increasing t1 and t2, the number of resonances increases and the absorption is enhanced. FIGURE 5Open in figure viewerPowerPoint Absorption spectra by changing the thickness of GST layer (t1) FIGURE 6Open in figure viewerPowerPoint Absorption spectra by changing the thickness of Mgf2 layer (t2) Figure 7 shows the effect of square resonator width (w) on the absorption spectrum. By increasing w, the coupling between the excited LSPs caused by the square resonators in the adjacent unit cells decreases, which leads to decreasing the absorption value. Moreover, the absorption spectrum for different values of the large radius of the circular resonator (r2) is depicted in Figure 8. When r2 increases, the coupling between the LSPs formed at the two edges of ring decreases. Therefore, the absorption is reduced. FIGURE 7Open in figure viewerPowerPoint Absorption spectra for different values of square resonator width (w) FIGURE 8Open in figure viewerPowerPoint Absorption spectra for different values of large radius of circular resonator (r2) Finally, the absorption value of the structure for different polarisation and incident angles at different wavelengths are plotted in Figures 9 and 10, respectively. It is obvious that the structure is polarisation-insensitive due to the symmetry of the structure. Also, the structure is independent on the incident angle up to about 20°. As the angle increases to more than 20°, the absorption decreases due to the lack of impedance matching condition. FIGURE 9Open in figure viewerPowerPoint Switch absorption spectrum for different polarisation angles FIGURE 10Open in figure viewerPowerPoint Switch absorption spectrum for different incident angles 3 ANALYTICAL METHOD The analytical method of equivalent circuit model is investigated for the proposed structure to evaluate the simulation results. The circuit model of the structure of Figure 1 is shown in Figure 11. The metal parts of the structure, such as resonators and substrate, behave like good conductor in the near-infrared regime. Therefore, the surface impedance of silver is obtained by the following equation [26]: Z Ag = R Ag + j ω L Ag = ( 1 + j ) ω μ 2 σ ${Z}_{\text{Ag}}={R}_{\text{Ag}}+j\omega {L}_{\text{Ag}}=(1+j)\sqrt{\frac{\omega \mu }{2\sigma }}$ (1)in which μ $\mu $ and σ $\sigma $ are permeability and conductivity of silver, respectively. The insulator layers of Mgf2 and GST are modelled with the transmission line. The characteristic impedance and electrical length of transmission line are represented by Z d = 120 π ε r ${Z}_{d}=\frac{120\pi }{\sqrt{{\varepsilon }_{r}}}$ and E d = 2 β × t ${E}_{d}=2\beta \times t$ , where β = 2 π λ 0 ε r $\beta =\frac{2\pi }{{\lambda }_{0}}\sqrt{{\varepsilon }_{r}}$ is propagation constant, λ 0 ${\lambda }_{0}$ , t $t$ and ε r ${\varepsilon }_{r}$ are the free space wavelength, the thickness and permittivity of dielectric, respectively. The values of the circuit model parameters are listed in Table 2. The results of the simulation and circuit model are depicted in Figure 12 that there is a good match between them. FIGURE 11Open in figure viewerPowerPoint The equivalent circuit model of the GST-based switch TABLE 2. The values of the parameters of the circuit model Parameter Value ( Ω $\mathbf{\Omega }$ ) Parameter Value RAg 37.42 LAg 0.3 (fH) Zd1 269.72 Ed1 93.78° Zd2 83.77 Ed2 470.26° Z0 377 FIGURE 12Open in figure viewerPowerPoint Comparison between the absorption spectra obtained from the analytical and numerical results 4 COMPARISON BETWEEN THE PROPOSED SWITCH WITH SIMILAR SWITCHES In Table 3, the function of the proposed GST-based switch in terms of switching parameters and also using the analytical method has been compared with other similar switches. All references except Ref. [30] not only have lower extinction ratio than our work but also the response time and analytical method have not been reported in most of them. Also, Ref. [30] has high extinction ratio but without reporting the response time and theoretical method. Our proposed work is a wideband switch with the FWHM = 242 nm, good switching capability, representing the equivalent circuit model. TABLE 3. Comparison of the proposed wideband switch and other switches in the near-infrared region References λ (nm) ɳ (dB) Response time (fs) Analytical method [6] 1410 8.8 - No [19] 1200 10 - No [20] 1550 11.14 600 No [21] 2000 5.3 - Yes [27] 1550 8.1 - No [28] 1550 10.08 - No [29] 1550 8.7 - No [30] 1550 20 - No [31] 1470 7 - No [32] 1520 6 - No This work 1343 13.71 46 Yes 5 CONCLUSION In summary, a wideband polarisation-independent plasmonic switch in the near-infrared region is presented. Utilising GST makes the structure tunable and by changing the crystallisation factor of GST a good switching capability is achieved in a wide range of wavelength. 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