We numerically demonstrate a multiband circular dichroism (CD) by tilting achiral metamaterials (MMs) composed of an elliptical nanoholes array (ENA) penetrating through metal/ phase-change material (PCM) /metal multilayer stack, with respect to the incident light. The CD spectrum can be actively tuned across a wide range from the near-infrared (NIR) to mid-infrared (MIR) regime by transiting the state of the PCM (Ge2Sb2Te5) from amorphous to crystalline. Thus, it can switch on/off a multiband chiroptical response in the infrared region. Our simulation also elucidates that the achiral multilayer stack MMs, which have strong magnetic resonances, can enhance the optical chirality inside the elliptical apertures for both amorphous and crystalline states. The switching of the enhanced chirality may pave the way to manipulate electromagnetic waves, such as tunable circular polarizers, chiroptical spectroscopy, and chiral biosensors.
© 2017 Optical Society of America
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 . Progress in chiral MMs has led to gigantic optical activity that exceeds the corresponding effects in nature by several orders of magnitude . For example, MMs with helix elements [9,10], twisted U-shape split ring resonators , 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 . Although the chiral MMs can enhance chiral fields to detect the chiral molecules , 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 , such as periodically repeating plasma spheres , asymmetrical split rings  and nanorings . 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 . 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 . 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 . 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 . Zhang et al. experimentally observed a handedness switching in the MMs incorporating the photoactive silicon . 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 . 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 . 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  and are now commonly employed in the non-volatile, rewritable digital storage and memory . 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 . 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 . However, due to the nature of imprinting, a replication of 3D chiral MM designs is restricted to subsequently imprinting . Multiple layers of perforated MDM stacks technique, including fabrication processes of planarization, alignment, and stacking, has shown a breakthrough for manufacturing 3D optical MMs  and stereometamaterials with flexible twists between layers . 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 . 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)  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, Ep p-polarization, and Es 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 . 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.
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 where R and L stand for right- and left- circular polarizations, is zero for all frequencies. A nonzero 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: . The dielectric constant (ε(ω)) of Ge2Sb2Te5 at different phases can be obtained from the experimental data , 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 . 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 . 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 asFigure 1(c) shows the transmittance spectrum of [(MD)1 M]-ENA in the amorphous state for LCP (blue solid line) and RCP (blue dashed line) with θ = φ = 45°. It shows that three resonant peaks appear in the spectrum for each polarization. The corresponding reflectance and absorptance spectrums are shown in Appendix 1. These resonant peaks are caused by the excitation of the multiple surface plasmon polariton (SPP) modes in the MDM-ENA . At those frequencies, the SPP modes couple to the incoming light via grating coupling, which can, in turn, contribute to the multiband CDtran, shown as a black solid curve in Fig. 1(c). It implies that the [(MD)1 M]-ENA can be useful for obtaining the multiband chirality .
Figure 1(d) shows the CDtran of a [(MD)v M]-ENA (for v up to 4). It presents that the magnitude of CDtran 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 , 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 CDtran than the [(MD)1 M]-ENA. However, the magnitude of CDtran 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 . 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 . In the inset of Fig. 1(d), we present the absorptances for both LCP (blue solid line), RCP(blue dashed line) and a difference in the absorptance: (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 Eq. (2), the C is proportional to both the E- and H -field intensities. Therefore we propose [(MD)v]M-ENA, a well-known negative index metamaterial, to enhance the C. A straightforward proof of the chiral field enhancement by the strong magnetic and electric resonances of [(MD)2M]-ENA under the LCP incidence is presented in Fig. 2(a), where the dielectric interlayer is amorphous Ge2Sb2Te5. Figure 2(a) shows the spectra of at five distinct locations of center (black solid line), left (green solid line), right (purple solid line), lower (cyan solid line) and upper (solid blue line) positions inside the elliptical hole. Figure 2(b) is a zoom in picture of Fig. 2(a), showing that the can reach 10.5 at λ = 1975 nm (denoted as P1 mode) and 5.5 at λ = 1763 nm (indicated as P2 mode). Although theappears to be the lowest at the center of the elliptical hole (black solid line), it can still achieve 3.2 around λ = 1986 nm. Hence, our design can demonstrate a good tolerance of placing chiral molecules inside the apertures, shown in the inset of Fig. 2(b).
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 . 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 CDtran 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 CDtran 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 CDtran is decreased down to zero around λ = 1975 nm hence switching off the chiroptical response. In Fig. 2(d), we have simulated thefor both amorphous and crystalline phases, where the location of chirality enhancement () is at the upper inside the elliptical hole. We observe that the two major peaks of , allowing a tunable dual-band enhanced optical chirality. However as changing the Ge2Sb2Te5 phase from amorphous to crystalline, the 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 . 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 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 of [(MD)2 M]-ENA. Here, two pronounced peaks in the 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),
To explore the origin of the enhanced chiral field, we study the distribution of EM fields and the 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 , and 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 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 CDtran. Figure 4(b) presents the E-field intensities at the oblique incidences (θ = φ = 45°), where a substantial chiroptical response (CDtran = 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 CDtran. 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 CDtran 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 CDtran = 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 CDtran in the amorphous state shown in Fig. 2(c).
Ge2Sb2Te5 is a chalcogenide glass compound with a crystallization temperature TC of 433K and a melting temperature TM of 873K. To transit the state of Ge2Sb2Te5 from amorphous to crystalline, one needs to heat the sample above the TC, but below the TM [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 TM . 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 . Figure 5(a) shows the heat source power Qs(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 Ra. The Ra is 0.2316 for LCP and 0.2625 for RCP incidence respectively. The Ra 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.
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.
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 CDtran is clearly observed. A significant frequency shift of 53% for the CDtran 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.
Amorphous and crystalline states, reflectance and absorptance, and RCP incidences:
We simulate CDtran 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 CDtran can be excited as changing the φ from ± 15° to ± 60°. The signs of the CDtran are opposite for the positive and negative φ, whereas the magnitudes of CDtran are almost identical. At θ=φ=45°, CDtran can achieve the highest magnitude of 0.18 for the amorphous state. It clearly indicates the CDtran 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 CDtran at φ=45°. It shows the CDtran 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 CDtran does not reverse for the opposite θ that satisfies the definition of extrinsic chirality . In Fig. 6(c), we have illustrated a 2D diagram of CDtran against θ and φ with a step of 1° at λ=1937 nm, where the CDtran obtains the maximum value of 0.18 at θ = φ = 45°.
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 fr = 25 kHz and pulse duration of 2.6 ns . 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, fr =25 kHz and pulse duration of 2.6 ns. The light fluence shining on the sample from a single pulse is written as65]. The heat source power is then described by a Gaussian pulse function
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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