Reconfigurable near-IR metasurface based on Ge 2 Sb 2 Te 5 phase-change material

A reconfigurable metasurface made of Ge2Sb2Te5 phase-change material was experimentally demonstrated in the 1.55 μm wavelength range. A nanostructured Ge2Sb2Te5 film on fused silica substrate was optimized to switch from highly transmissive (80%) to highly absorptive (76%) modes with a 7:1 contrast ratio in transmission independent of polarization, when thermally transformed from the amorphous to crystalline state. The metasurface was designed using a genetic algorithm optimizer linked with an efficient fullwave electromagnetic solver. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Initial work on metamaterials focused on the development of structures based on metallic resonators [1] that has resulted in the fabrication of left-handed materials with negative effective permittivity and permeability.By now, the study of metamaterials and their twodimensional counterparts, known as metasurfaces, has grown to encompass various macroscopic composites of periodic or non-periodic subwavelength structures whose function arises from a combination of the metamaterial structure and constituent materials composition and properties [2][3][4].Recently, nanofabrication techniques have enabled the creation of metamaterials operating in the optical (near infrared (IR)) frequency range [5].Unique properties of the optical metamaterials have been investigated with the aim to create perfect lenses [6,7], flat collimating lenses [8], invisibility cloaks [9,10], near perfect absorbers [11,12], and other devices for light propagation control [5].
Expanding upon the variety of metamaterials with static responses, there is a growing interest in developing reconfigurable metamaterials that would provide an alterable electromagnetic response as a result of changing the properties of a component material (see.e.g., reviews [13,14]).The availability of such reconfigurable metamaterials would greatly enhance the possibility of their employment in practical applications.As an intermediate step on the way from metamaterials with fixed frequency response to fully reconfigurable artificially structured materials, tunable metamaterials have been created with a limited range of the adjustable frequency response.In the mid-or near IR, the frequency response of the metamaterials was controlled using liquid crystals [15][16][17][18][19] or phase change materials (PCM) such as vanadium oxide [20][21][22][23] or chalcogenide alloys (e.g., refs [24][25][26][27][28][29][30].).Some limitations of these previous designs stem from the choice of the tuning constituents used in fabrication.For instance, liquid crystal devices have relatively slow switching speeds and demonstrate strong anisotropic behavior.VO 2 exhibits a very fast insulator-to-metal phase transition but has a low (68 °C) phase switching temperature.In comparison with other PCMs, chalcogenide alloys offer a number of benefits.Many chalcogenide alloys can undergo thermally, electrically, or optically controlled reversible phase transitions between amorphous and crystalline states with an associated large change in the optical and electrical characteristics.Chalcogenide alloys also offer advantages for fabrication due to their thermal and environmental stability, a broad operating temperature range, and the adjustability of their properties and operational wavelengths through composition modification, as dictated by a specific application [31][32][33].
A distinctive feature shared by most of the previously engineered tunable structures utilizing PCMs based on chalcogenide alloys, is that their tuning components (continuous layers of PCM) were hybridized with the metamaterials, i.e., they only changed the resonance conditions of the framework metamaterials (e.g., refs [24][25][26][27][28].).Here, we adopt another approach to making a reconfigurable photonic metasurface, in which the resonating medium is the nanostructured Ge 2 Sb 2 Te 5 (GST) chalcogenide alloy layer itself and another medium is not required to change the resonance.A similar approach was utilized in [29] where a onedimensional metasurface was etched in the GST film.In [30], the simulations of a twodimensional metasurface based on the GST film were presented.A unique optical response of the structure in the near IR is achieved in the present work [34], which demonstrates two disparate filter functions as a result of changing the structural phase of the chalcogenide film.Employment of the amorphous (dielectric) GST material where displacement currents act similarly to conduction currents in metallic metamaterial structures allows avoidance of losses inherent to metals at optical frequencies [35].

Ge 2 Sb 2 Te 5 film deposition and characterization
The reconfigurable near-IR metasurface involves a nanostructured GST PCM film on a double side polished fused silica wafer.The 150-nm-thick GST films were deposited by thermal evaporation of ground pieces of the GST ingot at room temperature at a rate of 0.15 ÷ 0.25 nm/s.The GST ingot for the evaporation was fabricated from high purity chemical elements (99.999% each, Alfa Aesar) batched into a fused silica ampoule inside a nitrogenpurged glove box.Prior to sealing, the material components and silica ampoule were kept under high vacuum (~10 −4 Torr) at 90 °C for 4 hours in order to remove any residual moisture.The sealed ampoule was heated to 950 °C and then kept at that temperature for 15 hours in a rocking furnace to provide homogenization of the melt.The ampoule was quenched in liquid nitrogen that produced a polycrystalline ingot.
The elemental composition of the films determined by energy-dispersive X-ray spectroscopy (EDS) was found to be the same as the composition of the bulk target alloy from which they were prepared.EDS was performed using a Hitachi S-3400-2 Variable Pressure SEM with a liquid nitrogen cooled EDS attachment using an accelerating voltage of 20 kV.
As-deposited GST thin films were amorphous as verified by X-ray diffraction (XRD) and electrical resistance measurements.The XRD patterns of the films were collected in the Bragg-Brentano configuration using a PANalyticalX'Pert Pro Materials Research Diffractometer with Cu-Kα radiation.To induce a phase transformation from amorphous to one of the crystalline states, the films were subjected to a heat treatment using Alwin 21 AG610 Rapid Thermal Annealing (RTA) System in N 2 flow for 10 sec. Figure 1(a) shows the XRD scans for the as-deposited amorphous GST film (lower spectrum) as well as for the films crystallized by RTA at 170 °C (middle spectrum) and at 370 °C (upper spectrum).The curves are shifted along the vertical axis for clarity.The reflections in the middle spectrum correspond to the face-centered cubic (fcc) structure (Fig. 1(b)) [36,37], whereas the reflections in the upper spectrum match the hexagonal structure of the GST material [38].Only (00l) reflections are present in the spectrum for the film with hexagonal lattice, indicating a preferred (c-axis) orientation of the crystalline structure.
The GST thin films in the amorphous and crystalline states were also studied using Raman scattering in a micro-Raman configuration.The studies were performed using a Bruker Senterra Ram integration tim substrates wa films.The R presented in F (125 and 153 ab initio mole octahedra for spectrum for [39] by a lar crystalline sta sub-lattice [40  on an optical model consisting of a semi-infinite fused silica substrate / GST film / surface roughness.The surface roughness was represented by a Bruggeman effective medium approximation of 0.5 void + 0.5 film material fractions.The parameterization of N included combinations of Sellmeier, Lorentz, Tauc-Lorentz, Gaussian, and Drude (for the films in the crystalline state) oscillators.Oscillator parameters as well as thicknesses of the GST film and the surface roughness layer were used as fitting parameters.Figure 1(d) shows the wavelength dependence of n and k for the amorphous and fcc crystalline states of the films that agree well with the GST material parameters reported in Ref [43].Here, the crystalline phase has higher values of n and k than the amorphous phase, which corresponds to differences in the chemical bonding in the films before and after the phase transition [32,[43][44][45].It should be noted that some change in the surface roughness (~7 nm increase) of the GST films after the phase transformation from the amorphous to crystalline state was determined using ellipsometry.This increase of the surface roughness is too small to influence the optical properties of the films at the device working wavelength.

Metasurface design and fabrication
A large difference in the complex index of refraction for the amorphous and crystalline phases of the GST PCM can be leveraged to create reconfigurable metamaterials or metasurfaces with specific properties that depend on the state of the constituent PCM [46].The specific goal in the present study was to design a metasurface whose response in the IR (1.55 μm wavelength range) could be reconfigured from high transmission when the PCM is in the amorphous state to high absorption when it is in the fcc crystalline state.To achieve this goal, a robust genetic algorithm (GA) optimizer [47] was employed to create a pixelized pattern in the GST layer deposited on the fused silica substrate.Within the periodic unit cell, the PCM layer was subdivided into an 8 × 8 grid of pixels and represented by 10 unique binary bits using 8-fold symmetry.Enforcing 8-fold symmetry in the design maintains polarization independence and contracts the parameter search space by reducing the number of bits required to encode the unit cell pattern.The encoding for a single triangle in the 8-fold symmetric unit cell shown in Fig. 2(a) is "0011,001,01,1," where "0" represents a "No GST" pixel and "1" represents a "GST" pixel.Furthermore, fabrication constraints that remove isolated pixels and diagonal connections in the geometry were enforced on the unit cell structure.The Cost function used in the GA to evaluate the overall performance of the metasurface structure is given by where the transmittance T and reflectance R for amorphous Am and fcc crystalline Cr phases, respectively, were calculated for the infinite planar array of unit cells by linking the optimizer with an efficient full-wave electromagnetic solver based on the periodic finite elementboundary integral (PFEBI) method [48].In addition, the Cost function was calculated for a range of wavelengths around 1.55µm, recording the best value in the range.The measured n and k values for the amorphous and fcc crystalline phases shown in Fig. 1(d) were used to model the PCM film in the Cost evaluation.
The GA evolved the unit cell size, GST layer thickness, and pattern, converging to a structure illustrated in Fig. 2(a,b) as the best candidate design for a reconfigurable transmission/absorption metasurface subject to the fabrication constraints and the Cost function defined in (1).The predicted scattering spectra from the metasurface shown in Fig. 2(c,d) reveal a high transmission peak of 82.1% in the amorphous state of the PCM and a high absorption maximum of 82.4% in the fcc crystalline state, indicating that the optimized structure meets the specified reconfigurable metasurface design criteria.
The 4 × 4 mm 2 samples of the GST-based metasurface with the pattern optimized by the GA as shown in Fig. 2(a,b), were fabricated using e-beam lithography and lift-off techniques.The patterns were exposed on a positive electron beam resist (Nippon ZEP 520A) spun onto a fused silica w with calibrati patterns.The µC/cm 2 using deposited by dissolving the array pattern t of the full-siz images of sma

Results a
The reflectan were measure Spectrometer films were tr equipped with reflectance at wafer using a L ing the exposu ese dose array g 10 µC/cm 2  The resonant increase of transmission in the structure (Fig. 3(a)) when the GST material is in the amorphous state is determined by a leaky guided mode resonance induced by the periodic pattern of air voids within the GST PCM layer [49,50].The average dielectric constant of the patterned PCM layer is higher than those of the cover (air) and the substrate, therefore the patterned layer acts simultaneously as a diffraction grating and a waveguide.Inside the patterned GST layer, the first diffracted orders ( ± 1) excite guided modes when proper phase matching conditions [51] are met (Fig. 3(d)).The grating formed by patterning the GST layer makes the guided modes leaky which brings about a radiation from the PCM layer extending into the air and substrate [52].As a result of the interaction between the incident wave, waveguide modes, and radiation fields, as well as the wavelength-dependent probability of the guided mode excitation, the structure provides a resonant response.The structure period, L, satisfies the conditions: λ R /n avg < L < λ R /n s , λ R /n c , where λ R is the resonance wavelength, n avg is the average index of patterned GST layer, and n s and n c are refractive indices of the substrate and cover (air).Thus, the patterned GST layer is a high spatial frequency grating and there are no diffraction orders (except for the zeroth order) in the transmitted light which improves the filter efficiency.The two-dimensionality of the pattern with 90° rotational symmetry makes the response polarization-independent.As noted above, the metasurface pattern design was optimized in such a way that the reflectance is low in both amorphous and crystalline states.This is achieved by shifting the reflection peak to the longer wavelengths upon transforming the patterned GST layer to the crystalline state (see Fig. 3(e,f) where the transmittance and reflectance spectra are shown in a broader wavelength interval compared to Fig. 3(a-c)).The transmission peak also becomes red-shifted and decreases due to higher absorption in the crystalline GST material.In the fcc crystalline phase the GST is conducting -a resistivity ρ = 2.4 mΩ-cm was found using the Drude term in the dielectric function determined by ellipsometry.The red-shift of the transmittance and reflectance maxima represents approximately a 1.3 times increase in the vacuum wavelength.This increase is associated with the change in the guided mode wavelength that was brought about by the increase in the refractive index induced by the phase transformation from amorphous to crystalline states.However, the change in the refractive index alone (~1.85 times) would produce a still higher red-shift.It is moderated by   the structure when the PCM is in the amorphous (dielectric) state.The distribution of electric and magnetic field maxima is commensurate with the period of the structure.At λ = 1.7 μm, the electric field maxima are at the ridges of the holes and the electric field penetrates through the holes (see electric field distributions in Fig. 6, panel c, and in the vertical slice under panel c) providing an increase in the transmission.At higher wavelengths (~1.85 μm) where transmission is low, the reflection from the structure is at a maximum (see Fig. 2(c)).Here, the absorption is relatively small (less than 20%) in the wavelength range of 1.4 -2 μm.In the crystalline state of the PCM, an increased intrinsic loss has the effect of reducing the field intensities in the structure as shown for all three wavelengths in Fig. 6(b,d,f), thereby damping the resonant transmission at the corresponding wavelengths.As a result, the absorption increases throughout the band as shown in Fig. 2(d).Thus, the simulations of the electric and magnetic field distributions using a full-wave electromagnetic computational model also attest that the structure provides switching from high transmission to high absorption upon inducing the amorphous to crystalline phase transition in the patterned GST layer.

Conclusion
We have presented the design and experimental realization of a reconfigurable metasurface based on a nanostructured Ge 2 Sb 2 Te 5 PCM that changes its response from being highly transmissive to highly absorptive in the near-IR (1.55 µm) wavelength range independent of polarization.Fabrication of the novel design generated using a GA in which the patterned GST layer itself acts as a metasurface demonstrates a considerable improvement in the transmission (80%) of the structure with the GST layer in the amorphous state as compared to previous works.After thermal switching of the nanostructured GST layer to the crystalline state, a high absorption (76%) is attained.A 7.5:1 contrast ratio in transmittance and a 5.4:1 contrast ratio in absorptance were observed while maintaining a low reflectance (5.7% and 13.5% in the amorphous and crystalline states, respectively).This reconfigurable metasurface demonstration using the nanostructured GST film shows great promise as a platform for developing other reconfigurable metasurface devices, as the metasurface response for each PCM state can be tailored by GA optimization according to the application requirements.
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Fig. 4 .
Fig. 4. Reflectance and transmittance spectra of the 300 × 300 µm 2 metasurface patterns formed in the amorphous Ge 2 Sb 2 Te 5 films using: (a), (b) different e-beam exposure doses (the same structure period) and (c), (d) different pixel sizes (variable structure period) as indicated in the insets.The spectra were taken at 20° incidence using the FT-IR microscope.