1. Introduction

Optical activity refers to an ability to both rotate the polarization sate of light (the so-called circular birefringence) and produce different absorptance levels for right- and left- circular polarizations (the so-called circular dichroism, CD) [1,2]. Optical activity occurs in natural chiral medium lacking mirror symmetry such as sugar, quartz crystals, cholesteric liquid crystals, and most biomolecules [3–6]. It is highly important in molecular biology, analytical chemistry, and medical sciences. However, the optical activity in the natural chiral material is very weak hence limiting its applications. Recently, chiral metamaterials (MMs) consisting of an array of chiral resonators have attracted intensive attentions [7]. Progress in chiral MMs has led to gigantic optical activity that exceeds the corresponding effects in nature by several orders of magnitude [8]. For example, MMs with helix elements [9,10], twisted U-shape split ring resonators [11], or sandwich like metal-dielectric-metal gammadions [12,13] are typical chiral MMs with circularly polarized eigenmodes. However, chiral MMs have a limitation on the detection of chiral molecules, due to their own structural chiral signals that could mess up chiral signals from the molecules [14]. Although the chiral MMs can enhance chiral fields to detect the chiral molecules [15], the need for distinguishing the chiral signals stemming from the molecules from that of a chiral structure introduces additional experimental complexity. Moreover, a fabrication of the chiral MMs in the high-frequency region remains challenging owing to the complicated geometry of the meta-atom. Recently, it has presented that the optical activity can be introduced even for non-chiral mirror-symmetrical MMs under an oblique incidence. These are termed as “extrinsic chiral” structures [16], such as periodically repeating plasma spheres [17], asymmetrical split rings [18] and nanorings [19]. The MMs consisting of achiral resonator were shown to be as efficient as those with chiral particles in diffraction experiments while having the simpler patterns to be fabricated [20,21].

Even so, the resonant responses in both chiral and achiral MMs are fixed by their structural geometry and could not be modulated, restraining their capability to manipulate electromagnetic (EM) waves. To this end, MMs with a controllable chirality have been widely demonstrated. Song et al. mechanically tune the operating frequency of a composite chiral MM [22]. While this mechanical system involves a high manufacturing complexity and it is restricted by a slow tuning speed. In this context, studies toward an active control of the optical activity are highly promoted. Kan et al. have tuned the optical activity in the terahertz (THz) region using spiral MMs, where the planar spirals can be transformed into three-dimensional (3D) helices through electrostatic actuation [7]. However, the integration of the electrodes for the structural reconfiguration can be difficult in the subwavelength MMs. Recently, achievements of tunable optical activity using all-photoinduced MMs are gaining traction. Kanda et al. presented that the chirality of MMs could be switched between “on” and “off” using photoexcited carriers in silicon with a subwavelength chiral-patterned metal mask [23]. Zhou et al. accomplished a wide tuning range of the optical activity using a chiral MM composed of an array of bilayer conjugated gammadion-shaped metal resonators, where a thin layer of the intrinsic silicon is integrated into the MM [24]. Zhang et al. experimentally observed a handedness switching in the MMs incorporating the photoactive silicon [2]. Kenanakis et al. numerically studied the tunable capabilities of uniaxial chiral MMs, where the parts of the metallic components are replaced by the silicon that changed from an insulating to a conducting state [8]. However, tunable chiral MMs integrated with the active silicon have only been demonstrated at the THz frequencies. Little research has been done on actively tuning the chiral responses at the higher frequencies. It is because the silicon does not sustain high densities of injected free electrons outside the THz frequency range, especially from the visible to mid-infrared (MIR) regions [25]. Furthermore, carrier lifetime of photocarriers in silicon (~μs) limits the tuning time. Recently, intensive efforts have been applied to actively control the plasmon resonances in the MMs for the infrared region by employing various tunable or switchable materials (e.g. VO₂ and graphene) driven by either thermal effects or biasing voltage [26–28]. These studies offer very effective approaches towards the tunable or reconfigurable devices and have been experimentally implemented for various practical applications such as light emission controllers, perfect absorbers, and optical modulators etc. Nevertheless, VO₂ needs constant heating above the transition temperature to keep the VO₂ in its high-temperature state; moreover, the integration of the electrodes for the material (i.e. graphene) tuning can be hard in the nanostructures. Thereby, an efficient method for fast controlling the chiroptical effect at the higher frequencies is desirable and necessary for practical applications.

Phase-change materials (PCMs) based on chalcogenide glass was first investigated in the 1960s [29] and are now commonly employed in the non-volatile, rewritable digital storage and memory [30]. Recently a representative of the chalcogenide compound family, Ge₂Sb₂Te₅, offers the possibility for nonvolatile resonance tuning in the infrared region, having a good thermal stability, high cyclability and short switching [31]. Herein, energy is only needed for switching the Ge₂Sb₂Te₅ phase and not for maintaining the phase. Therefore, when the photonics device is turned, it will retain its optical properties until it is switched again. It apparently makes the Ge₂Sb₂Te₅ based plasmonic devices interesting from a green technology perspective. Meanwhile, 3D chiral MMs in the optical region have come into focus recently [32]. However, due to the nature of imprinting, a replication of 3D chiral MM designs is restricted to subsequently imprinting [33]. Multiple layers of perforated MDM stacks technique, including fabrication processes of planarization, alignment, and stacking, has shown a breakthrough for manufacturing 3D optical MMs [34] and stereometamaterials with flexible twists between layers [35]. Even so, very few attempts have been made up to study the tunability of enhanced chirality in the 3D multilayer stack achiral MMs. To bridge the gap between the high performance of the tunable optical activity and simple nanoscale fabrication in the infrared region, here we show that the multiband enhanced chirality in the multilayer stack MM can be ultrafast tuned using a phase transition of Ge₂Sb₂Te₅ at the NIR regime.

Our structure is composed of an elliptical nanoholes array (ENA) embedding through metal-dielectric-metal (MDM) multilayer stack, where the Ge₂Sb₂Te₅ is selected as the dielectric interlayer. An elliptical nanohole is a 2D anisotropic system that possesses two principle polarization directions along the two orthogonal principle diameters of the hole. Several peaks of the absorptance spectra linked to the multiband enhanced chirality can be obtained under an off-normal circularly polarized incidence. These peaks come from the excitation of surface plasmon polaritons (SPPs) modes with different diffraction orders at those wavelengths at which they couple to the incoming light via grating coupling on a multilayer holes array. Under an oblique incidence, the absorptance of the structure depends on the state of the polarization (left or right circular polarization) if only the two principle axes of the elliptical nanoholes are not contained in the plane of incidence. In such a configuration, the experimental arrangement is chiral. This fact is commonly known as pseudo or extrinsic chirality [36]. The multiband chiroptical response in the multilayers of perforated [(MD)v M] stacks (for v up to 4) is presented in the NIR region. In particular, a large tuning wavelength range of approximately 1000 nm for the shift of multiband CD is shown to be possible by switching between the amorphous and crystalline states, which can be useful in turning on/off chiroptical responses at a specified wavelength. A heat model is constructed to investigate the temporal variation of the temperature of the Ge₂Sb₂Te₅ layer. The model shows the temperature of the amorphous Ge₂Sb₂Te₅ layer can be raised from room temperature to > 433K (crystallization temperature of Ge₂Sb₂Te₅) [37] in just 4.3 ns with a low incident light intensity of 0.01 mW/μm². It can thus supply sufficient thermal energy to change the amorphous state to crystalline one for both LCP and RCP incidences. Furthermore, we show that the achiral [(MD)_v M]-ENA, which exhibits the strong magnetic dipolar resonator, can significantly enhance the chiral fields under an oblique circularly polarized light (CPL) for both the amorphous and crystalline states.

2.Structure and design

The achiral MM is created by interpenetrating an array of elliptical holes through [(MD)_v M] stacks, where each MDM stack consists of two 30 nm thick Au layers spaced by a 190 nm thick Ge₂Sb₂Te₅, presented in Fig. 1(a). The unit cell is shown in Fig. 1(b), in which the incident wave vector k, the normal vector n, and one of the two primitive diameter vectors (a or b) form a chiral entity (shown in red). The lattice constant of ENA is L = 400nm, the diameters of the elliptical holes d₁ = 360nm and d₂ = 180nm, θ the incident angle, φ the rotation angle, E_p p-polarization, and E_s s-polarization. The structure is suspended in air and shined by the CPL. The electromagnetic response is calculated using a commercial 3D full-wave solver (CST MICROWAVE STUDIO®) based on the Finite Integration Technique. The proposed structure is developed from our previous design that was predominantly applied to tune the Fano resonances [38]. For experimental applicability, a 5-10 nm thick barrier layer (e.g. ZnS-SiO₂ or Si₃N₄) is required to insert between the metamaterial and chalcogenide to prevent Au diffusion into the Ge₂Sb₂Te₅ layer during the fabrication process, particularly at increased phase change temperatures. However, the introduction of ultrathin buffer layers may not sacrifice the performance of the plasmonic structures [31,39–41]. To simply the model, herein the buffer layer is not considered.

 

Fig. 1 (a) Schematic of an ENA penetrating through Au/ Ge₂Sb₂Te₅/Au multilayer stack, where the structure is suspended in air. (b) Illustration of the unit cell. (c) The transmittance of [(MD)₁ M]-ENA in the amorphous state for the LCP (blue solid line) and RCP (blue dashed line) incidences with θ = φ = 45°. Black solid curve presents the transmittances difference between the RCP and LCP incidences. (d) The CD_tran spectrum for an ENA penetrating through (MD)vM stacks (for v up to 4) with the amorphous state at θ = φ = 45°. The absorptances for both LCP (blue solid line), RCP (blue dashed line) and ΔA = A_R – A_L (red solid line) for the amorphous [(MD)₂ M]-ENA are shown in the inset.

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It is known that CD can primarily pass the light of circular polarization of one handedness while suppressing the transmittance of light of the other handedness [42,43]. Most of the materials do not discriminate between right-and left-circular polarizations in the transmittance, so that $\Delta T=T_R-T_L$where R and L stand for right- and left- circular polarizations, is zero for all frequencies. A nonzero$\Delta T$ indicates the existence of CD and enantiomeric asymmetries. In this work, the CD is defined as the result of the difference in the transmittance of right- and left-circularly polarized light: $\Delta T$ [44]. The dielectric constant (ε(ω)) of Ge₂Sb₂Te₅ at different phases can be obtained from the experimental data [45], in which for the NIR regime are shown in Appendix 1. As can be seen, the real part of permittivity (ε₁(ω)) is more than twice larger for the crystalline than the amorphous states in the wavelength range from 1500 to 3500 nm. This variation is due to a pronounced change in bonding between the two states. The ε(ω) of the amorphous phases is that expected of a covalent semiconductor; however, that of the crystalline states is significantly boosted by resonant bonding effects. Notably, resonance bonds form in the crystalline systems where the electronic orbitals of half-filled p-type bands are aligned over the next-nearest neighbors. Such extended delocalized states provide a significant dipole matrix-element enhancement of the optical properties, which is revealed in the rapid increase of ε₁(ω). This increase will disappear if the angular disorder between the extended p-states is existing. It thus distinctly varies the optical properties in spite of the local (nearest-neighbors) bonding staying relatively unperturbed, as is the circumstance of amorphous phase [45,46]. When Ge₂Sb₂Te₅ and multilayer stack metamaterial supporting SPPs are coupled properly, the significant variations in ε₁(ω) related to the metal-insulator transition enables a broadband tuning of resonance. It is because plasmon resonances in the metallic nanostructure are extremely sensitive to the dielectric environment, which has been investigated in the field of sensing [47]. In the NIR region, a substantial increase in the imaginary parts of permittivity ε₂(ω), associated with the absorption coefficient, is observed as transiting the state of Ge₂Sb₂Te₅ from amorphous to crystalline. This change lowers the transmission of the multilayer stack metamaterial, in turn, weakens the chiroptical responses. However, as red-shifting the wavelength to the MIR region (λ > 3000 nm), the Ge₂Sb₂Te₅ maintains a significant contrast in the ε₁(ω).It simultaneously offers an ultrasmall ratio of the imaginary to the real part of the dielectric constant, that is, ε₂(ω)/ε₁(ω)<0.001 indicating a very small loss [37]. Noteworthy, Ge₂Sb₂Te₅ has experimentally shown more than a billion cycles of reversible phase transition, which is clearly of practical importance when designing modulation devices. These very different optical properties are realistic and well known, but they have predominantly been applied to data storage applications.

3. Results and discussions

Circular dichroism in transmittance is defined as

Figure 1(d) shows the CD_tran of a [(MD)_v M]-ENA (for v up to 4). It presents that the magnitude of CD_tran increases with the number of stacked film v from 1 to 2. It is most likely because as the v increases from the unity, the stronger magneto-inductive coupling between neighboring functional layers becomes [50], which is directly connected to an antisymmetric charge-oscillation eigenmode. This antisymmetric eigenmode gives the combined plasmon mode a twist in the propagation direction of light to mimic 3D chirality. As such, the [(MD)₂ M]-ENA has a stronger CD_tran than the [(MD)₁ M]-ENA. However, the magnitude of CD_tran decreases when v>2. It is because the 3D bulk MMs with a large number of multilayer stack exhibit a significant loss, particularly in the optical region [51]. This [(MD)_v M]-ENA (v>2) operating in the NIR region has a high loss coefficient that could be reduced using lower-loss metallic films or compensated by adding a gain medium between the two metallic layers [52]. In the inset of Fig. 1(d), we present the absorptances for both LCP $A_L$(blue solid line), RCP$A_R$(blue dashed line) and a difference in the absorptance: $\Delta A=A_R-A_L$ (red solid line) for the amorphous [(MD)₂ M]-ENA with θ = φ = 45°. The details for optimizing the θ and φ can be found in Appendix 2.

The chirality of EM wave is defined as [53]

 

Fig. 2 (a) Chirality enhancement ($C/C_{CPL}$) at the center (black solid line), left (green solid line), right (purple solid line), upper (solid blue line) and lower (cyan solid line) positions inside the elliptical hole. (b) The zoom in picture of Fig. 2(a). Inset presents a vertical cross section of the elliptical hole containing a chiral entity. (c) The CD_tran spectra for both amorphous and crystalline states under θ = φ = 45°. (d) The$C/C_{CPL}$ spectra for both amorphous and crystalline states under θ = φ = 45°, where the location of $C/C_{CPL}$ is at the upper inside the elliptical hole under the LCP incidence.

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As the amorphous Ge₂Sb₂Te₅ has a smaller imaginary part of dielectric constant (ε₂(ω)) than that of the crystalline Ge₂Sb₂Te₅ (Appendix 1), the switch state is “ON (transparent)” for the amorphous Ge₂Sb₂Te₅ and “OFF (opaque)” for the crystalline Ge₂Sb₂Te₅ [54]. A large difference in the dielectric function between the amorphous and crystalline structural phase can be used to enhance the “ON/OFF”. Figure 2(c) presents the CD_tran of [(MD)₂ M]-ENA with the different Ge₂Sb₂Te₅ states at θ = φ = 45°.The most notable feature observed in Fig. 2(c) is the large frequency tunability. It has been found that the resonances red-shift towards the longer wavelength as transiting the phase of Ge₂Sb₂Te₅ from amorphous to crystalline, which is a 53% tuning range. This result highlights that a widely tunable spectrum of the CD_tran can be obtained by switching between the amorphous and crystalline states. It can be useful in switching on/off the chiroptical response. For example, as transiting the Ge₂Sb₂Te₅ phase from amorphous to crystalline, the CD_tran is decreased down to zero around λ = 1975 nm hence switching off the chiroptical response. In Fig. 2(d), we have simulated the$C/C_{CPL}$for both amorphous and crystalline phases, where the location of chirality enhancement ($C/C_{CPL}$) is at the upper inside the elliptical hole. We observe that the two major peaks of $C/C_{CPL}$, allowing a tunable dual-band enhanced optical chirality. However as changing the Ge₂Sb₂Te₅ phase from amorphous to crystalline, the $C/C_{CPL}$ decreases, due to the weaker magnetic resonance at λ = 2970 (denote as P₃ mode) and λ = 2748 nm (denoted as P₄ mode) in the crystalline state. The infrared region is a remarkable spectral regime for plasmonic devices, owing to the existence of molecular vibrational fingerprints and the atmospheric transparency window [55–57]. Thus, the presented tunable enhanced chirality in the infrared region is critical and may be particularly useful for sensing a broad range of chemical and biological agents [58]. For example, our structure possesses highly enhanced optical chirality inside the elliptical apertures, where the enhanced chiral fields have the same handedness (either left or right circularly polarized state). We may then use the elliptical hole as a nanosize cuvette exhibiting the chiral Purcell effect and measure the enhanced chirality of molecules inside the holes, shown in the inset of Fig. 2(b).

The two peaks of $C/C_{CPL}$spectra are related to the two internal-SPP modes with the different diffraction orders that satisfy the phase matching conditions of the wave vectors to the period of resonators. The coupling of the incident light to the various inner modes running between the metals is based on the existence of the diameters of the ellipse (d₁ and d₂) providing the parallel momentums equal to the different orders of SPP modes. These internal-SPP modes can, in turn, contribute to the magnetic dipolar resonance, which interacts with the electric dipolar resonance to pronouncedly enhance the chiral fields. The SPP modes in the [(MD)₂M]-ENA are similar to those of the same (MD)₂ M multilayer without elliptical apertures, where the SPP mode resonating at the metal-dielectric interface is defined as an internal mode. Hence, the SPP dispersion relation of the [(MD)₂M]-ENA can be approximated by that of the (MD)₂ M multilayers.

In Fig. 3(a), we have simulated the dispersion relation of the (MD)₂M laminar films with the 30 nm thick Au film and 190 nm thick amorphous Ge₂Sb₂Te₅ film, presented together with the $C/C_{CPL}$ of [(MD)₂ M]-ENA. Here, two pronounced peaks in the $C/C_{CPL}$spectrum denoted as P₁ and P₂ modes, shown in the right column of Fig. 3(a), corresponds to the (1,0) and (1,1) internal-SPP modes shown in the left column. The order of SPP mode can be determined using Eq. (3),

 

Fig. 3 (a) The dispersion map of (MD)₂M multilayer (left) and $C/C_{CPL}$of the [(MD)₂M]-ENA (right) for the amorphous state. The distributions of $\left|E/E_0\right|$, $\left|H/H_0\right|$, and $C/C_{CPL}$ along both a horizontal plane at an interface between the top Au layer and Ge₂Sb₂Te₅ layer as well as a cross-section plane of the [(MD)₂M]-ENA at (b) P₂ mode (λ = 1763 nm) and (c) P₁ mode (λ = 1975 nm). White dotted lines indicate the elliptical hole’s boundaries. (d) The dispersion relation (left) and $C/C_{CPL}$(right) for the crystalline state. The distributions of $\left|E/E_0\right|$, $\left|H/H_0\right|$, and $C/C_{CPL}$at (e) P₄ mode (λ = 2748 nm) and (f) P₃ mode (λ = 2970 nm).

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To explore the origin of the enhanced chiral field, we study the distribution of EM fields and the $C/C_{CPL}$at the internal-SPP modes for both the amorphous and crystalline states. The [(MD)₂M]-ENA is illuminated by an oblique LCP incidence (θ = φ = 45°). For the amorphous state, the field distributions of $\left|E/E_0\right|$, $\left|H/H_0\right|$and $C/C_{CPL}$ are plotted along both a horizontal plane at an interface between the top Au layer and Ge₂Sb₂Te₅ layer, as well as a cross-section plane for the P₂ mode (λ = 1763 nm) in Fig. 3(b) and P₁ mode (λ = 1975 nm) in Fig. 3(c). The left columns of Figs. 3(b)-3(c) show that the strong enhancements of E-field appear in the dielectric interlayers and elliptical apertures, indicating the strong electric resonances. Meanwhile, as can be seen in the central columns, the strong enhancements of H-fields are also obtained in the Ge₂Sb₂Te₅ interlayer. It is expected for the internal SPP resonances, which is owing to a concomitant coupling between surface plasmons counter-propagating on the two closely spaced interfaces. The H-field can be confined between the two Au layers to support magnetic resonance at which light is trapped and absorbed. The interaction of the electric dipolar resonance with the magnetic dipolar resonance in the same spectral region leads to an improvement of the chirality, presented in the right column. The chirality at the P₁ mode shown in Fig. 3c can be enhanced more significantly than the P₂ mode shown in Fig. 3b. Nevertheless, the patterns of field distributions are asymmetric over both of the planes due to the oblique incidence. As transiting the state from amorphous to crystalline, the internal-SPP modes redshift to P₃ (λ = 2970nm) and P₄ (λ = 2748 nm) modes shown in Fig. 3d. Figures 3(e) and 3(f) show that the enhancements of the E-field (left column), H-field (central column), and C (right column) at P₃ and P₄ modes are less than the amorphous ones, accordingly.

Figure 4(a) shows the total E-field intensities $E=\sqrt{{\left|E_x\right|}^2+{\left|E_y\right|}^2+{\left|E_z\right|}^2}$on the top Au- Ge₂Sb₂Te₅ (amorphous) under the LCP (left column) and RCP (right column) normal incidences (θ = φ = 0°) with λ = 1975nm. The -field intensities for the RCP and LCP lights are exact mirror images of each other. Their differences cancel out hence leading to a zero CD_tran. Figure 4(b) presents the E-field intensities at the oblique incidences (θ = φ = 45°), where a substantial chiroptical response (CD_tran = 0.18) is observed. A clear asymmetry field pattern appears over the nanoholes array under the oblique incidences since the time of the pulse propagating through different regions of the structure is unequal. The difference of the asymmetric E-field intensities of the two CPLs leads to a large CD_tran. The phase of the resonant modes excited by these two circular polarizations gives rise to strong interferences of the modes at the nanoholes. It results in the different transmittances between the LCP and RCP incidences to obtain an extrinsic chirality. Figure 4(c) shows the E-field intensities on the top Au-Ge₂Sb₂Te₅ (crystalline) interface with the LCP and RCP normal incidences (θ = φ = 0°) at λ = 2970nm, where CD_tran appears to be zero. Figure 4(d) shows the E-field intensities for the different circular polarizations on the top Au- Ge₂Sb₂Te₅ (crystalline) interface at λ = 2970nm with θ = φ = 45°, where CD_tran = 0.1. It can be seen that the differences of E-field intensities between LCP and RCP oblique incidences associated with the P₁ mode for the amorphous Ge₂Sb₂Te₅ shown in Fig. 4(b) is more significant than the crystalline one linked to the P₃ mode shown in Fig. 4(d) hence implying the larger CD_tran in the amorphous state shown in Fig. 2(c).

 

Fig. 4 (a-b) Snapshots of the normalized E-field intensities of P₁ mode at the top Au- Ge₂Sb₂Te₅ (amorphous) interface when the light propagates through the [(MD)₂ M]-ENA. The left and right columns present the response to LCP and RCP incidences respectively, obtained from the same time steps along the light propagation. (a) E-field intensities under θ = φ = 0°, where the patterns of both LCP and RCP incidences have a mirror symmetry. (b) E-field intensities are asymmetric under θ = φ = 45°. (c-d) E-field intensities of P₃ mode at the top Au-Ge₂Sb₂Te₅ (crystalline) interface. It presents (c) mirror symmetric patterns under θ = φ = 0°, and (d) asymmetric field distribution under θ = φ = 45°.

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Ge₂Sb₂Te₅ is a chalcogenide glass compound with a crystallization temperature T_C of 433K and a melting temperature T_M of 873K. To transit the state of Ge₂Sb₂Te₅ from amorphous to crystalline, one needs to heat the sample above the T_C, but below the T_M [39,40]. Meanwhile, a reversible transition from the crystalline to amorphous can be driven by a melt-quenching procedure initiated by a short, high-intensity pulse sufficient to increase the local temperature quickly above the T_M [59]. Since the reversible amorphous - crystalline phase transition of Ge₂Sb₂Te₅ can be induced through optical heating, it is important to understand the heat induced switching behavior of the [(MD)₂ M]-ENA. To show this, a heat transfer model is used to investigate the temporal variation of temperatures of the Ge₂Sb₂Te₅ layers using the Finite Element Method (FEM) solver within COMSOL [60]. Figure 5(a) shows the heat source power Q_s(r, t) and the temperatures of the two Ge₂Sb₂Te₅ layers for both LCP and RCP incidences, where the structure is located at the center of the incident beam. The numerical simulation shows that the temperature of the bottom Ge₂Sb₂Te₅ layer (shown in dashed line) is lower than the upper one (shown in dotted line) for both LCP and RCP incidences. Moreover, the temperature of each Ge₂Sb₂Te₅ layer under the LCP incidence (shown in red lines) is smaller than the RCP incidence (shown in blue lines), owing to its lower absorption coefficient R_a. The R_a is 0.2316 for LCP and 0.2625 for RCP incidence respectively. The R_a is calculated by integrating the power density of input light with the absorptance of the amorphous [(MD)₂M]-ENA, shown in the inset of Fig. 1(d). Particularly, the bottom Ge₂Sb₂Te₅ layer under the LCP incidence (shown in red dashed line), exhibiting the lowest temperature distribution, can still reach 433K at 4.3 ns with an incident light intensity of 0.01mW/μm². It can thus change the phase of both Ge₂Sb₂Te₅ layers from the amorphous to crystalline. Due to a heat dissipation to the surroundings, the temperature starts dropping after 5.6 ns before the next pulse comes.

 

Fig. 5 (a) 3D-FEM simulation of heat power irradiating on a [(MD)₂ M]-ENA located at the beam center, where the red and blue solid lines present the heat power irradiating on the structures, the red and blue dashed lines show the temperature of the bottom Ge₂Sb₂Te₅ layer, the red and blue dotted lines demonstrate the temperature of the top Ge₂Sb₂Te₅ layer for the LCP and RCP incidences accordingly. (b) The temperature distribution of [(MD)₂ M]-ENA along a vertical cross-section plane at 4.3 ns, where the color image indicates the temperature distribution and the arrows indicate the heat flux for the LCP (top column) and RCP (bottom column) incidences.

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The temperature distributions at 4.3 ns along the vertical cross-section of structure are shown in Fig. 5(b), where the upper panel is for LCP and the lower panel for RCP incidence. One can observe that the temperature within each Ge₂Sb₂Te₅ layer is uniform and the dominant temperature gradient is towards the top and bottom Au films. Importantly, the strong SPP resonance can concentrate the electromagnetic field more efficiently on the Ge₂Sb₂Te₅ dielectric interlayer. It will enable the incident light to heat up the Ge₂Sb₂Te₅ sufficiently, thus shortening the switching time. Therefore, the multilayer stack achiral phase-change metamaterials (PCMMs) have a potential for ultrafast switching on/off circular polarizer and polarization filter in the optical region. The material thermal properties used for the model are summarized in Table 1. The detailed description of the thermal model can be found in Appendix 3.

Table 1. Material thermal properties used in the heat transfer model

View Table

4.Conclusion

In conclusion, we have theoretically presented a new scheme for active control of a multiband optical activity using multilayer stack achiral PCMMs. The structure consists of an elliptical nanoholes array embedding through an Au/Ge₂Sb₂Te₅/Au multilayer stack. Tilting the structure with respect to the incident light, the experimental arrangement is chiral, and thus, the multiband CD_tran is clearly observed. A significant frequency shift of 53% for the CD_tran is seen in the infrared region as changing the phase from amorphous to crystalline. A heat transfer model is investigated to predict that the amorphous Ge₂Sb₂Te₅ can reach 433K in only 4.3 ns under a low light intensity of 0.01mW/μm² hence being crystallized for both LCP and RCP incidences. We have also demonstrated that the multilayer stack achiral PCMMs can enhance the chiral fields, which origins from the simultaneous excitation of strong electric and magnetic fields in the multilayer structure. The bridging of strong chiral response and tunable achiral PCMM, along with their simple geometry, may provide an ideally functional element for the highly efficient polarization modulation devices, circular polarizers and polarization filters in the infrared regime.

Appendix 1

Amorphous and crystalline states, reflectance and absorptance, and RCP incidences:

Appendix 2

We simulate CD_tran at different φ with a fixed θ=45° for the amorphous [(MD)₂ M]-ENA (Fig. 6(a)). As can be seen, the circular difference effects do not exist for the structure orientation φ=0° and 90°. It is because the anisotropic axis of the structure is in the incident plane, leading to a mirror plane of the experimental geometry. However, the multiband CD_tran can be excited as changing the φ from ± 15° to ± 60°. The signs of the CD_tran are opposite for the positive and negative φ, whereas the magnitudes of CD_tran are almost identical. At θ=φ=45°, CD_tran can achieve the highest magnitude of 0.18 for the amorphous state. It clearly indicates the CD_tran is sensitive to the φ and the enantiomer chirality switching can be realized by changing the sign of φ. In Fig. 6(b), we investigate the θ effect on the CD_tran at φ=45°. It shows the CD_tran is absent under normal incidence and can achieve the maximum value of 0.18 at θ=φ=45°. Therefore, the chiroptical response is optimized at θ=φ= 45° (see Fig. 7). Furthermore, the spectrum of CD_tran does not reverse for the opposite θ that satisfies the definition of extrinsic chirality [61]. In Fig. 6(c), we have illustrated a 2D diagram of CD_tran against θ and φ with a step of 1° at λ=1937 nm, where the CD_tran obtains the maximum value of 0.18 at θ = φ = 45°.

 

Fig. 6 (a) Dielectric constant ε₁(ω) and ε₂(ω)vs wavelength for both the amorphous and crystalline states in Ge₂Sb₂Te₅. (b) Reflectance and (c) absorptance of (MD)₁ M-ENA with the amorphous state for both LCP (blue solid line) and RCP (blue dashed line) incidences at θ = φ = 45°. $\Delta A=A_R-A_L$ and $\Delta R=R_R-R_L$ are shown in red curve.

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Fig. 7 The CD_tran of (a) various φ at θ = 45°, (b) different θ at φ = 45° for the amorphous [(MD)₂ M]-ENA. (c) A 2D diagram of CD_tran against θ and φ at λ = 1937 nm.

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Appendix 3

A nanosecond pulsed light (i.e. supercontinuum light) is an important approach to excite the plasmonic nanostructures, with an advantage of the broad wavelength range from the visible to the MIR region [62,63]. The general supercontinuum light source has a repetition rate f_r = 25 kHz and pulse duration of 2.6 ns [64]. To mimic the supercontinuum light employed in the experiment, in the thermal model, a Gaussian pulse is used as the excitation source to evaluate the required time to switch the Ge₂Sb₂Te₅ state from amorphous to crystalline. It has a repetition rate, f_r =25 kHz and pulse duration of 2.6 ns. The light fluence shining on the sample from a single pulse is written as

(4)$$F_l(r)=\frac{2P_{{}^0}}{\pi w^2{f}_r}\exp (-\frac{2r^2}{w^2})$$

where P₀ = 2.6 mW is the total incident power, r the distance from the beam center, w = 10 μm Gaussian beam waist. The thermal energy absorbed by one unit cell is

(5)$$E_{th}(r)=R_a\times L^2\times F_l(r)$$

where L is the lattice constant of the [(MD)₂ M]-ENA, R_a the absorption coefficient of the structure. The thermal conductivity of Ge₂Sb₂Te₅ changing with the temperature is obtained from the experiment data in [65]. The heat source power is then described by a Gaussian pulse function

(6)$$Q_s(r,t)=E_{th}(r)\frac{1}{\sqrt{\pi }\tau }\exp (-\frac{{(t-t_0)}^2}{{\tau }^2})$$

where τ = 1.5 ns is the time constant of a pulse, t₀ = 3 ns the time delay of pulse peak.

Funding

National Natural Science Foundation of China (NSFC) (Grant Nos. 61172059, 51302026), International Science and Technology Cooperation Program of China (Grant No. 2015DFG12630), and Program for Liaoning Excellent Talents in University (Grant No. LJQ2015021).

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