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Abstract
A multifunctional metasurface capable of dynamic control for polarization selectivity and absorption is proposed by controlling the phase of Ge2Sb2Te5 (GST) in the near-infrared region. At amorphous state of GST (a-GST), the proposed GST strip array realized polarization selectivity in transmission-reflection integrated modes. The high-efficiency asymmetric transmission (AT = 0.92) and asymmetric reflection (AR = -0.82) are both obtained by selectively exciting Mie multipole resonances. With the transition from a-GST to crystalline (c-GST) state, the giant polarization selectivity almost disappeared, and the absorptions climb from < 0.1 to > 0.9. The maximum modulation depth reaches 94%. The mechanism of the dynamic switching between polarization selectivity and absorption is quantitively analyzed via multipole expansion. The GST based metasurfaces simultaneously possess excellent switchable capability for AT, AR, and absorption without refabricating structures, which is promising to the applications for next generation optical devices.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metasurfaces have enabled unprecedented capabilities to flexibly manipulate electromagnetic waves in subwavelength scale [1,2]. For polarization control, metasurfaces have shown superior performance in terms of size and efficiency compared with conventional methods. Over the past few years, many symmetry-broken metasurfaces have achieved polarization recognition, polarization filter, and polarization conversion for linearly and circularly polarized waves at different wavebands [3–9]. Unfortunately, these passive metasurfaces suffer from the constraints of fixed wavelength and single function, which hinders their further development and applications.
Recently, to meet the requirement of dynamic polarization manipulation in real-time biosensing, dynamic display, spectroscopy and numerous other areas, many reconfigurable metasurfaces [10] are proposed by integrating various tuning materials, like graphene [11,12], ITO [13], PIN diodes [14], stretchable substrates [15], and phase change material VO2 [16–18]. However, there are some limitations from tuning constituents for some specific applications. For instance, the tunability of graphene from intra-band and inter-band transitions are limited in mid-infrared and THz wavebands. The carrier density of ITO could be tuned in optical and near-infrared wavebands but in a narrow range. The PIN diode-based devices rely on big-size structures. The switching speed and operative difficulty for stretching substrates are not satisfactory. VO2 exhibits a very fast insulator-to-metal phase transition but has a low (68°C) phase switching temperature.
Phase change materials GST can be easily, rapidly, and reversibly switched between ordered-crystalline and disordered-amorphous states under thermal, electrical, or optical stimuli [19], which has been commercialized for a variety of optoelectrical applications [20]. Owing to the good stability, high-speed and reversible phase transition, and a broad operating temperature range, GST becomes a good candidate to construct reconfigurable metasurfaces. Cao et al. reported a wide wavelength tuning of circular dichroism [21]. Li et al. achieved frequency-tunable waveplate in a GST based metasurface [22]. Dong et al. demonstrated tunable chiroptical responses in an asymmetric transmissive split ring array [23]. In addition, the strength of Fano-resonance [24] and circular dichroism [25,26] are also tailored by controlling the phase change of GST.
Unlike many reported frequency- and strength-switchable metasurfaces, only a few function-switchable metasurfaces are proposed [27,28]. Relying on continuous phase variation, multifunctional polarization conversion is obtained by switching response strengths [24]. In fact, due to the fixed symmetry in a metasurface structure, it is very hard to gain polarization-dependent and -independent properties simultaneously [29]. Surprisingly, the multifunctional metasurfaces with polarization conversion and absorption are reported by electrically controlling PIN diodes [14,28]. Also, the functions of asymmetric transmission and absorption are switched in VO2 integrated metamaterials in the terahertz region [30]. In another aspect, most of polarization metasurfaces work in a single transmission or reflection mode, while the transmission-reflection integrated metasurface are relatively few [31].
In this paper, a reconfigurable metasurface capable of giant polarization selectivity in transmission-reflection integrated modes and high-efficiency absorption is demonstrated by controlling the phase of GST. At a-GST state, the metasurface behaves as polarization selective devices, exhibiting giant AT and AR effects simultaneously. With the transition from a-GST to c-GST state, the function of the GST array is switched into polarization less-dependent absorption. Before and after the phase change of GST, the absorption is changed from < 0.1 to > 0.9. In addition, the mechanism of the multifunctional metasurface is quantitatively analyzed by using the general multipole scattering theory.
2. Design and simulations
Reversible phase transitions of GST between amorphous and crystalline states can be induced via thermal, electrical, and optical stimuli [32,33]. By controlling the temperatures and the heating times, the crystallization degree of GST over a hot plate could be well adjusted [34]. The optical properties of a-GST and c-GST are given in Fig. 1(b). As predicted, with the phase change from amorphous to crystalline, the complex refractive index changes dramatically. The difference of optical properties between crystalline and amorphous GST originates from different bonding modes. The crystalline phase experiences resonant bonding, whereas the amorphous phase undergoes shifts to covalent bonding [35,36]. Due to resonant bonding, crystalline GST have higher real part of complex refractive index than the amorphous one [35]. At low frequencies, crystalline GST has more free charge carriers, causing optical absorption. Thus, the imaginary part of complex refractive index for c-GST is higher than that of a-GST [36].
Fig. 1. (a) Schematic diagram of GST phase change between amorphous and crystalline phases. (b) Complex refractive index of crystalline and amorphous GST [37].
Considering the damping loss difference between two states, we use a-GST to design the basic polarization selective metasurfaces by breaking the symmetry of structures, and then weaken the polarization selectivity extremely by changing the phase of GST. Due to the higher imaginary part of complex refractive index for c-GST, the metasurface will absorb most of the incident left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light when a-GST is transformed into c-GST. Thus, the polarization less-dependent absorption will be switched from near zero to near one.
Figure 2(a) illustrates the schematic diagram of the dual functions in the GST based metasurface. The GST strip array is excited by the circularly polarized light (CPL) propagating in -z direction. Most of the LCP light could transmit the a-GST based structure, while the RCP light is reflected. The high-efficient polarization selective transmission and reflection denotes giant AT and AR effects, because opposite incident directions will induce the exchange of polarization spectra LCP and RCP light [38]. However, when the phase of GST is changed from amorphous to crystalline state, both of LCP and RCP light will be absorbed (Fig. 2(b)). The polarization selectivity is reduced greatly. Therefore, the reconfigurable metasurface with the functions of switchable polarization selectivity and polarization less-dependent absorption are realized by changing the phase of GST but not by refabricating constructions.
Fig. 2. The schematic diagram of (a) the switchable polarization selectivity and (b) the polarization-independent absorption of the GST based metasurfaces. The (c) top and (d) cross-section views of a unit cell for the GST based metasurfaces, where l1 = 460, w1 = 210 nm, t1 = 1310 nm, l2 = 710 nm, w2 = 190 nm, t2 = 370 nm, and p = 950 nm in x and y directions.
To gain high-efficiency polarization selectivity at a-GST state, the symmetry of the unit structure needs to be broken. As shown in Fig. 2(c), the unit structure consists of two-layer rotationally twisted GST strips, which are placed on SiO2 substrate. After optimizing the geometric parameters, the period of the GST array is set as 950 nm in both x and y directions. The thicknesses of the two twisted strips are 1310 and 370 nm, respectively. The spacer between two-layer strips is 410 nm.
All spectra are simulated by using the FDTD Solutions, which is based on a finite difference time domain (FDTD) method using periodic boundary conditions. Perfectly matched layers (PMLs) are adopted in the ± z direction. The dispersive optical parameters of a-GST and c-GST are from Ref. [37]. The refractive index of SiO2 substrate is 2.08. Additionally, the polarization spectra are also calculated using the frequency domain finite element method (FEM) in CST Microwave Studio, which are in good agreement with the ones in FDTD Solutions.
3. Results and discussion
3.1 Switchable polarization selectivity
The proposed metasurface achieved polarization selectivity with different polars in transmission and reflection modes, respectively. Under the excitations of LCP and RCP waves, the transmission spectra at both a-GST and c-GST states are calculated. In Fig. 3(a), the significant difference of transmission between LCP and RCP light is apparent at a-GST state. The transmission of LCP light (${T_ - }$) is as high as 0.92, but the transmission of RCP light (${T_ + }$) is near zero at 1547 nm. The transmission difference of CPL in one propagating direction represents the difference of propagating directions for one-helicity CPL. In other words, the transmission of RCP (LCP) light forwards is equal to the transmission of LCP (RCP) light backwards ($\overrightarrow {{T_ \pm }} = \overleftarrow {{T_ \mp }} $,→and← represent the directions of propagation forward and backward respectively) [38]. This is because opposite-direction propagation of CPL will induce the reversal of enantiomeric arrangements. Essentially, the resonance response excited by RCP (LCP) light propagating forwards is same as the one of LCP (RCP) propagating backwards. So, the polarization selectivity in transmission mode can be estimated by asymmetric transmission parameter (AT), which is defined as $AT_{cir}^ \pm{=} {T_ \pm } - {T_ \mp } ={-} AT_{cir}^ \mp $. As shown in Fig. 3(c), the asymmetry parameters of LCP waves ($AT_{cir}^ - $) reaches 0.92 at 1547 nm, which represents that most of LCP waves could propagate through the a-GST metasurface, while the RCP waves are blocked.
Fig. 3. Switchable polarization selectivity. (a), (b) Transmission and (c), (d) reflection spectra of CPL at a-GST and c-GST states, respectively. (e) AT and (f) AR spectra.
With the transform from a-GST to c-GST state, the transmissions of both LCP and RCP light become below 0.1 near 1547 nm in Fig. 3(b). The $AT_{cir}^ - $ is switched from 0.92 to −0.01 in Fig. 3(c), which means the giant polarization selectivity in transmission mode (AT) at a-GST is extremely weakened by changing the phase of GST. As a result, the large-range switchable asymmetric transmission is realized. By elaborately controlling the crystallization degree of GST, the AT could be continuously switched. When the GST becomes into partial crystalline state, the AT value is in the range from 0.92 to −0.01 as shown in Fig. 3(c). As a result, the large-range asymmetric transmission could be dynamically switched.
According to the analysis above, the incident LCP wave could transmit the a-GST based metasurface while the RCP is blocked. To ensure that the RCP light is reflected but not absorbed, the reflection spectra of LCP and RCP waves are given out in Figs. 3(d) and (e). Obviously, the reflections of RCP light are above 0.8 near 1547 nm, but the LCP ones are relatively low. Therefore, the giant polarization selectivity in reflection mode is also achieved. Similar with the aforementioned AT, the asymmetric reflection parameter (AR) is defined as $AR_{cir}^ \pm{=} {R_ \pm } - {R_ \mp } ={-} AR_{cir}^ \mp $. As shown in Fig. 3(f), the $AR_{cir}^ - $ at a-GST state reaches −0.82 at 1547 nm. At partial crystalline state of GST, the metasurface shows much weaker AR value in the whole waveband. With the phase transition coming into c-GST, the absolute value of $AR_{cir}^ - $ falls to near zero, which means the polarization selectivity in reflection mode (AR) is almost disappeared. In general, the 92% LCP waves could transmit the a-GST based metasurface, while the 84% RCP waves are reflected.
In fact, the giant polarization selectivity in transmission and reflection integrated mode results from the different selective excitations of multipole resonance modes at a-GST state. The high transmission of LCP light at a-GST state originates from strong toroidal dipole resonance, whereas the high reflection of RCP waves results from the common contributions of magnetic dipole, electric quadrupole, magnetic quadrupole and toroidal dipole resonance modes. (Quantitively discussed later.)
3.2 Switchable absorption
Not only gaining large-range switchable polarization selectivity in transmission-reflection integrated mode, the giant switch of absorption is also generated by changing the phase of GST. At a-GST state, the absorptions of both LCP and RCP waves are relatively low in a broadband of 1470-1650 nm as shown in Fig. 4(a). When GST is changed from amorphous to crystalline state, the absorptions of LCP and RCP waves are switched in a large range from < 0.1 to > 0.9 in the whole waveband in Fig. 4(b). The large-range switching of absorption is caused by the change of resonances before and after the phase change of GST. At a-GST state, the multipole resonances are excited selectively under the excitations of LCP and RCP waves, while the multipole resonances are both relatively weak under the excitations of both CPL in the c-GST based metasurfaces. Exactually, the high absorptions of LCP and RCP waves result from the higher imaginary part of refractive index for c-GST. In addition, it is noted that, before and after the phase change of GST, the absorptions of both LCP and RCP waves have the same variation trends from near zero to near one. The function of the GST based metasurface is switched from polarization sensitivity in transmission and reflection integrated mode (AT, AR) to polarization less-dependent absorption in an unchanged structure, which represents a major advance compared with previously reported metasurfaces.
Fig. 4. The absorption spectra of CPL at (a) a-GST and (b) c-GST states. (c) MD spectra before and after the phase change of GST.
To estimate the switch capability of the reconfigurable metasurface relying on the phase change of GST, a modulation depth (MD) of the switchable absorptions for both LCP and RCP waves are given out, which is defined as
Before and after the phase change of GST, the MDs of LCP and RCP waves are plotted in Fig. 4(c). The MDs of LCP and RCP are as high as 94% and 84% at 1547 nm, respectively. The high MDs represent the excellent switch capability of the GST based metasurfaces.3.3 Mie multipole resonances
To analyze the mechanism of the giant and large-range switchable polarization selectivity, the distributions of electric field magnitudes under the excitations of LCP and RCP light are given out in Fig. 5. Obviously, at a-GST state, the excited electric fields by LCP and RCP light are concentrated on metasurface layer. Differently, under the excitation of LCP light, the electric fields below metasurface are stronger than the ones above (Figs. 5(a) and (b)), which demonstrates that LCP light could transmit the a-GST metasurface. However, the electric fields below metasurface excited by RCP light are much stronger than the ones below in Figs. 5(c) and (d), which are consistent with the high reflection of RCP waves at a-GST state.
Fig. 5. The distributions of electric field magnitude at different planes at 1547 nm, which are driven by (a, b, e, f) LCP and (c, d, g, h) RCP light for (a-d) a-GST and (e-h) c-GST based metasurfaces, respectively. The GST strips are marked by the dash-line boxes.
With the transition from a-GST to c-GST, electric field of the c-GST metasurfaces are much weaker in Figs. 5(e)-(h), which means no obvious resonances are excited. In addition, the fields above and below the metasurface are extremely weak. Actually, the incident CPL are directly absorbed by the c-GST based metasurface due to the high imaginary part of refraction index for c-GST. Furtherly, the field distributions excited by LCP and RCP waves in the layer of metasurfaces are totally different, predicting distinct resonance modes, which lead to the giant polarization selectivity.
As the inset shown in Figs. 5(a) and (b), toroidal dipole arises from a head-to-tail configuration of magnetic dipoles, which in turn can be produced by currents flowing on the surface of a torus along its meridians. The thickness of GST strip in z direction offers the excitation condition of toroidal dipole. The electric field properties of the top-layer GST strip in x-z and y-z planes coincide to the scattered field characteristic of toroidal dipole resonance. Thus, the contribution of toroidal dipole is dominant for the high transmission of LCP waves. On the other hand, the electric fields excited by RCP waves offer the common characteristic of electric quadrupole, magnetic quadrupole, toroidal dipole and magnetic dipole resonances. The electric fields are focused outside the four corners of top-layer GST strip in Fig. 5(d), which is the characteristic of electric quadrupole. The electric fields distributed inside the top-layer GST strip represents the excitation of magnetic quadrupole. Moreover, the strong electric fields inside the bottom-layer GST strip in Figs. 5(c) and (d) denotes magnetic dipole resonance. Generally, a hybridization of Mie multipole resonances is excited, which leads to the high reflection of RCP waves at 1547 nm.
To verify the analysis of resonance modes above quantitatively, the radiated powers of multipole resonances were calculated according to the general multipole scattering theory [39]: electric dipole moment,
magnetic dipole moment, toroidal dipole moment,The individual scattered intensity of multipoles is calculated in Fig. 6(a). Under the excitation of LCP light, the scattered power of toroidal dipole is the highest, and other multipole resonances are relatively weak, which is consistent with the mode analysis in Figs. 5(a) and (b). Namely, the high transmission of LCP light near 1550 nm is mainly caused by toroidal dipole resonance.
Fig. 6. The scattered intensities of electric dipole (${I_P}$), magnetic dipole (${I_M}$), toroidal dipole (${I_T}$), electric quadrupole ($I_Q^e$), and magnetic quadrupole ($I_Q^m$) resonances under the excitations of (a), (c) LCP and (b), (d) RCP light in (a), (b) a-GST and (c), (d) c-GST based metasurfaces.
In contrast, the scattered intensity of toroidal dipole resonance excited by RCP light near 1547 nm is lower than the one excited by LCP light, whereas the scattered powers of other multipole resonance modes, i.e., electric quadrupole, magnetic quadrupole, toroidal dipole and magnetic dipole resonances, become a little higher near 1547 nm in Fig. 6(b). It means that the high reflection of RCP waves arises from the common excitation of multipole resonances.
By switching a-GST to c-GST sate, the multipoles resonances are much weaker than the a-GST ones. As shown in Figs. 6(c) and (d), the scattered intensities of multipoles resonances are at 10−25 order of magnitudes, which is one order of magnitude less than the a-GST ones (10−24). The weak scattering means no obvious Mie resonances is excited, which is agree with the electric field distributions in Figs. 5(e)–(h).
4. Conclusion
In this paper, we realized the multi-functions of switchable polarization selectivity in transmission and reflection modes and the large-range switch of absorption by controlling the phase change of GST in rotationally twisted GST nanostrips. At a-GST state, the high-efficiency polarization selectivity in transmission and reflection modes result from the selective excitation of Mie multipole resonances. With the GST phase change from amorphous to crystalline state, the AT(AR) undergoes a large-range switch from 0.92(−0.82) to −0.01(0.02), and the absorptions change from < 0.1 to > 0.9 at 1547 nm. Therefore, the function of the GST array is switched from giant polarization selectivity to high-efficiency and polarization less-dependent absorption by changing the phase of GST. Furthermore, before and after the phase change of GST, the MD reaches 94%, representing excellent switchable capability. The large-range dynamic switch for AT, AR and absorption in the multifunctional GST matasurfaces will accelerate the applications of metamaterial devices for integrated photonics.
Funding
National Natural Science Foundation of China (12004080, 61705046); Special Fund for Application, Science and Technology Planning Projects of Guangdong Province of China (2017B010127002).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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