Optical Phase Change Materials Research Articles

Solution-based processing of Ge2Sb2Se4Te optical phase change materials

Phase change materials (PCMs) are important building blocks in solid-state memory and photonic devices. Solution-based processing promises large-area, cost-effective, conformal coating of optical PCMs (O-PCMs) for photonic applications. In this work, a solution processing route was developed for Ge2Sb2Se4Te1 (GSST), a target PCM of interest due to its large optical contrast, broadband transparency, and improved glass-forming capability. An alkahest solvent mixture of ethanedithiol and ethylenediamine was used as a solvent system to fabricate solution-derived GSST thin films and films from these solutions were prepared and characterized using SEM, XRD, and Raman spectroscopy.

Design of a switchable triple-waveguide-based TE-converted polarization beam splitter based on lithium niobate

The lithium-niobate-on-insulator (LNOI) platform has been the new force to drive the developments of integrated photonics, where efficient polarization management and mode conversion are the fundamental issues to be solved. In this work, we proposed a switchable TE-converted polarization beam splitter (PBS) based on optical phase change material (PCM) and a triple-waveguide in simulation. To optimize and analyze the proposed design, the 3D finite difference time domain (3D-FDTD) method is applied in this work. The simulated results indicate that when PCM is in the amorphous state, the device can effectively separate the TM and TE modes, achieving a low insertion loss (<0.5dB) for both modes at 1550 nm. While the PCM is in the crystalline state, the proposed device can realize the efficient conversion from the input TE0 mode to output TE1 mode, which obtains a mode conversion efficiency of 99.5%, crosstalk of −26dB, and insertion loss of 0.5 dB at 1550 nm. We hope such a device can make compact integration and capacity improvement for the integrated photonics in LNOI.

Angle-insensitive perfect light transmission and absorption in one-dimensional photonic crystal heterostructures

Abstract Prefect light transmission in metamaterials typically suffers a frequency shift as the incident angle of wave increase, limiting their applicability across wide angles. One-dimensional photonic crystal heterostructures, with elliptic-shaped equi-frequency contours, exhibit angle-insensitive properties. However, there remains an inevitable blue shift in the optical response as the incident angle increases. In this work, we present two photonic crystal heterostructures, one of which is composed of the optical phase-change material Sb2S3. By combining two heterostructures with distinct Zak phases, we achieve perfect light transmission. Specifically, as incident angle of wave increases, the transmission peak can be strictly maintained without a frequency shift by tuning the phase (refractive index) of Sb2S3. Moreover, by jointing the heterostructures with a thin silver film, we demonstrate a Tamm plasma polariton mode with angle-insensitive property. This mode can facilitate angle-tolerant light absorption. Our work provides an alternative way to designing high angle-tolerant and tunable devices.

Solution-derived Ge–Sb–Se–Te phase-change chalcogenide films

Ge–Sb–Se–Te chalcogenides, namely Se-substituted Ge–Sb–Te, have been developed as an alternative optical phase change material (PCM) with a high figure-of-merit. A need for the integration of such new PCMs onto a variety of photonic platforms has necessitated the development of fabrication processes compatible with diverse material compositions as well as substrates of varying material types, shapes, and sizes. This study explores the application of chemical solution deposition as a method capable of creating conformally coated layers and delves into the resulting modifications in the structural and optical properties of Ge–Sb–Se–Te PCMs. Specifically, we detail the solution-based deposition of Ge–Sb–Se–Te layers and present a comparative analysis with those deposited via thermal evaporation. We also discuss our ongoing endeavor to improve available choice of processing-material combinations and how to realize solution-derived high figure-of-merit optical PCM layers, which will enable a new era for the development of reconfigurable photonic devices.

Study of the photothermal response of a multilayer structure doped with VO2@Au nanoshells

In this paper, we demonstrate a theoretical study of a multiphysics problem to solve for the photothermal response of a one-dimensional multilayer structure containing a layer doped with VO2@Au nanoshells. The VO2@Au nanoshell consists of a gold (Au) shell and a core of the phase change material vanadium dioxide (VO2) where the VO2 core transitions from a semiconductor state to a conductor state at the critical temperature of 68 °C. This behaviour results in thermal induced optical tunability through this reversible phase change of the VO2, due to the temperature dependent optical and thermal properties. The presence of the VO2 core, functioning as an ultra-fast and reversible optical phase-change material, leads to the emergence of photothermal induced bistability. The layer doped with the VO2@Au nanoshell is approximated as an effective medium using the Maxwell-Garnett Theory to enable an analytical solution. In this study, the optical response of the multilayer structure is obtained using the Transfer Matrix Method, while the thermal response for both stationary and transient states is solved using the Green’s Function Method and Kirchhoff’s Transformation. These equations are interconnected through the heat source term in the heat diffusion equations, representing the local heat generation induced by the continuous-wave laser applied to the structure. Our findings indicate that at the wavelengths of 658 nm and 747 nm, there are two distinct photothermal responses arising from the phase change of the VO2 core. At these wavelengths, the absorption of light increases and decreases, respectively, because of the VO2 phase change. This analytical method not only offers a thorough exploration of the fundamentals of induced photothermal responses in multilayer structures but also holds considerable potential for various applications, including solar cells, photothermal therapy, and nanothermal sensors.

Створення активних оптичних метаповерхонь на плівках халькогенідних напівпровідників зі зміною фазового стану

The development of meta-optics is due to human aspirations for the maximum miniaturization of optical elements, the design and control of light fluxes, as well as the improvement of visualization and image processing. Metasurfaces, i.e. arrays with subwavelength distances (smaller than the wavelength of light), and optically thin elements trigger new physical mechanism and phenomena that are very different from those observed in three-dimensional bulk materials. Thus, meta-devices perform complete control and management of the characteristics of the light flux (phase, amplitude, polarization) with the help of one flat layer. Compared to traditional bulky lenses, metasurface lenses have advantages such as flatness, light weight, and compatibility with semiconductor manufacturing technology. The use of active (reconfigurable) metasurfaces, the characteristics of which can be dynamically rearranged after manufacturing, makes it possible to significantly expand the capabilities of meta-optics. The paper presents the results of the analysis of the properties and technologies of creating optically active metasurfaces for optical image processing and transformation systems. Generalized methods of forming metasurfaces are described: self-organization, selective chemical etching, holographic and lithographic. To implement the work of active (reconfigured) metasurfaces based on materials with a change in phase state (amorphous/crystalline), heating technologies with electric current pulses of various amplitudes and durations and the action of direct optical radiation are used. The analysis of materials for the formation of optically active metasurfaces and devices based on them that simulate the front of a light wave and work on reflection and transmission is presented. Special attention is paid to the use of photosensitive chalcogenide semiconductors as metamaterials with a phase change. Examples of materials such as Ge2Sb2Te5 (GST) and AgxInSb2Te (AIST), which have been used for decades in optical data storage and electronic memory devices, are given. A series of novel compositions of optical phase change materials such as Ge2Sb2Se4Te (GSST), Sb2S3, Sb2Se3, Ge2Sb2Te3S2 and In3SbTe2 for optical and photonic applications are also proposed. Direct laser recording on photosensitive films of chalcogenide semiconductors with the use of technological equipment for laser recording of master disks is proposed as a promising method of forming arrays with submicron distances and realizing the work of active metasurfaces.

Design of continuously tunable varifocal metalenses

Active metasurfaces with tunable optical properties enable reconfigurable or programmable photonic systems that can swiftly adapt to different functional needs. Realizing continuous tuning of phase-gradient metasurfaces, however, remains elusive. This is because active modulation of individual or few meta-atoms is traditionally deemed essential to continuous tuning, which nonetheless introduces significant technical difficulties associated with addressing and control of large meta-atom arrays. In this paper, we propose a novel active metasurface architecture to circumvent this challenge. The key notion is to modulate the gradients of metasurface phase profiles over large regions instead of setting the phase at individual locations, thereby enabling continuous tuning with only a minimal number of independently controlled elements. The design principle can be generically applied to different active tuning mechanisms. As an example, we modeled a varifocal metalens with focal length continuously tunable between 4 and 10 mm based on electrically switched optical phase change materials.

Time-Resolved Temperature Mapping Leveraging the Strong Thermo-Optic Effect in Phase-Change Materials.

Optical phase-change materials are highly promising for emerging applications such as tunable metasurfaces, reconfigurable photonic circuits, and non-von Neumann computing. However, these materials typically require both high melting temperatures and fast quenching rates to reversibly switch between their crystalline and amorphous phases: a significant challenge for large-scale integration. In this work, we use temperature-dependent ellipsometry to study the thermo-optic effect in GST and use these results to demonstrate an experimental technique that leverages the thermo-optic effect in GST to enable both spatial and temporal thermal measurements of two common electro-thermal microheater designs currently used by the phase-change community. Our approach shows excellent agreement between experimental results and numerical simulations and provides a noninvasive method for rapid characterization of electrically programmable phase-change devices.

Reconfigurable TE-pass polarizer based on lithium niobate waveguide assisted by Ge2Sb2Te5 and silicon nitride.

On-chip polarization management components play a critical role in tackling polarization dependence in the lithium-niobate-on-insulator (LNOI) platform. In this work, we proposed a reconfigurable TE-pass polarizer based on optical phase change material (GST) and the LNOI wafer. The key region is formed by a hybrid GST-S i 3 N 4 layer symmetrically deposited atop the centerline of the LNOI waveguide along the propagation direction where the GST is sandwiched in the middle of the S i 3 N 4 layer. Whether the polarizer will take effect depends on the phase states of the GST layer and the graphene and aluminum oxide layers are coated atop the G S T-S i 3 N 4 layer as the microheater to control the conversion of phase states. The proposed device length is 7.5µm with an insertion loss (IL)=0.22dB and extinction ratio (ER)=32.8dB at the wavelength of 1550nm. Moreover, it also has a high ER (>25d B) and a low IL (<0.5d B) in the operating bandwidth of 200nm. Such a high-performance TE-pass polarizer paves a new way for applications of photonics integrated circuits.

Wavelength-tunable infrared chiral metasurfaces with phase-change materials.

Optical phase-change materials exhibit tunable permittivity and switching properties during phase transition, which offers the possibility of dynamic control of optical devices. Here, a wavelength-tunable infrared chiral metasurface integrated with phase-change material GST-225 is demonstrated with the designed unit cell of parallelogram-shaped resonator. By varying the baking time at a temperature above the phase transition temperature of GST-225, the resonance wavelength of the chiral metasurface is tuned in the wavelength range of 2.33 µm to 2.58 µm, while the circular dichroism in absorption is maintained around 0.44. The chiroptical response of the designed metasurface is revealed by analyzing the electromagnetic field and displacement current distributions under left- and right-handed circularly polarized (LCP and RCP) light illumination. Moreover, the photothermal effect is simulated to investigate the large temperature difference in the chiral metasurface under LCP and RCP illumination, which allows for the possibility of circular polarization-controlled phase transition. The presented chiral metasurfaces with phase-change materials offer the potential to facilitate promising applications in the infrared regime, such as chiral thermal switching, infrared imaging, and tunable chiral photonics.

Comparing the thermal performance and endurance of resistive and PIN silicon microheaters for phase-change photonic applications

Optical phase-change materials have enabled nonvolatile programmability in integrated photonic circuits by leveraging a reversible phase transition between amorphous and crystalline states. To control these materials in a scalable manner on-chip, heating the waveguide itself via electrical currents is an attractive option which has been recently explored using various approaches. Here, we compare the heating efficiency, fabrication variability, and endurance of two promising heater designs which can be easily integrated into silicon waveguides—a resistive microheater using n-doped silicon and one using a silicon p-type/intrinsic/n-type (PIN) junction. Raman thermometry is used to characterize the heating efficiencies of these microheaters, showing that both devices can achieve similar peak temperatures but revealing damage in the PIN devices. Subsequent endurance testing and characterization of both device types provide further insights into the reliability and potential damage mechanisms that can arise in electrically programmable phase-change photonic devices.

Tunable topological phase transition in the telecommunication wavelength

Recent progress in the Valley Hall insulator has demonstrated a nontrivial topology property due to the distinct valley index in 2D semiconductor systems. In this work, we propose a highly tunable topological phase transition based on valley photonic crystals. The topological phase transition is realized by the inversion symmetry broken due to the refractive index change of structures consisting of optical phase change material (OPCM) with thermal excitation of different sites in a honeycomb lattice structure. Besides, simulations of light propagation at sharp corners and pseudo-spin photon coupling are conducted to quantitatively examine the topological protection. Compared with other electro-optical materials based on reconfigurable topological photonics, a wider bandwidth and greater tunability of both central bandgap frequency and topological phase transition can happen in the proposed scheme. Our platform has great potential in practical applications in lasing, light sensing, and high-contrast tunable optical filters.

Broadband tunable basic units for nonvolatile field programmable photonic gate array

Field programmable photonic gate arrays (FPPGAs) use 2×2 tunable basic units (TBUs) interconnected as a mesh to achieve various functionalities on the same chip. It is desirable to have TBUs that can retain the state without power and function over a broad bandwidth. Different TBU structures and materials are compared in this work to design a suitable nonvolatile broadband device. An optical phase change material (O-PCM) based Mach–Zehnder interferometer provided the required nonvolatile TBU with a high bandwidth, low insertion loss, and low crosstalk. Among the various O-PCM materials, Sb2S3 provided the lowest insertion loss of &lt;1dB and a low crosstalk of &lt;−25dB. The proposed TBU can enable a low-power FPPGA with a smaller footprint and broader bandwidth compared to their volatile thermo-optic or electro-optic counterparts.

Highly efficient and tunable polarization rotator based on lithium niobate on an insulator.

The lithium niobate on an insulator (LNOI) platform has greatly advanced the development of integrated photonics recently, where efficient polarization management components are indispensable. In this work, we propose a highly efficient and tunable polarization rotator based on the LNOI platform and the low-loss optical phase change material antimony triselenide (S b 2 S e 3). The key polarization rotation region is formed by a LNOI waveguide with a cross section of the double trapezoidal shape and a S b 2 S e 3 layer deposited atop the LNOI waveguide in an asymmetrical way, where an isolating layer of silicon dioxide is sandwiched between them to reduce the material absorption loss. Based on such a structure, we have achieved the efficient polarization rotation in a length of only 17.7µm, where the polarization conversion efficiency and insertion loss are 99.6% (99.2%) and 0.38dB (0.4dB) for the trans-electric (TE)-to-trans-magnetic (TM) rotation. If we further change the phase state of the S b 2 S e 3 layer, other polarization rotation angles besides 90° can also be obtained for the same device, revealing a tunable function. We believe that the proposed device and design scheme could offer an efficient method for realizing the polarization management on the LNOI platform.

ELight: Toward Efficient and Aging-Resilient Photonic In-Memory Neurocomputing

Optical phase change material (PCM) has emerged promising to enable photonic in-memory neurocomputing in optical neural network (ONN) designs. However, massive photonic tensor core (PTC) reuse is required to implement large matrix multiplication due to the limited single-core scale. The resultant large number of PCM writes during inference incurs serious dynamic energy costs and overwhelms the fragile PCM with limited write endurance, causing the severe aging issue. Moreover, the aged PCM would distort the stored value and significantly degrade the reliability of PTC. In this work, we propose a holistic solution, <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ELight</monospace> , to tackle both the aging issue and the post-aging reliability issue, where a proactive aging-aware optimization framework minimizes the overall PCM write cost and a post-aging tolerance scheme overcomes the effect of aged PCM. Specifically, in the aging-aware optimization part, we propose write-aware training to encourage the similarity among weight blocks and combine it with a post-training optimization technique to reduce programming efforts by eliminating redundant writes. Next, an efficient groupwise row-based weight-PTC remapping scheme is introduced to tolerate the reprogrammability degradation due to the aged PCM. Experiments show that <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ELight</monospace> can achieve over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20 \times $ </tex-math></inline-formula> reductions in the total number of write operations and dynamic energy cost with comparable accuracy. Moreover, <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ELight</monospace> can guarantee significant accuracy recovery under the aged PCM within photonic memories. With our <monospace xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ELight</monospace> , photonic in-memory neurocomputing will step forward toward practical applications in machine learning with order-of-magnitude longer lifetime, lower programming energy cost, and significant resilience against PCM aging effects.

Low-loss strong index modulated ultra-thermally stable optical phase-change material for broadband nonvolatile photonics

Optical phase change materials (O-PCMs) have attracted increasing attention and provide promising solutions for non-volatile reconfigurable photonics due to their large index contrast and non-volatile switching characteristics. In this work, we have explored the optical phase change properties of a dozen different O-PCMs and established the corresponding gene bank. Among these O-PCMs, Ge1Sb2Se3Te1 (G1S2S3T1) films exhibit unprecedented low-loss, strong light modulation capability, and ultra-thermal stability capability, which are prepared by pulsed laser deposition. Meanwhile, we investigate the phase transition kinetic mechanism of G1S2S3T1 films by transmission electron microscope, temperature dependent transmission spectroscopy, and temperature dependent Raman spectroscopy. It is found that the wide bandgap (1.09 eV) in the amorphous state, the large bandgap variation (0.51 eV) between amorphous and crystalline states, the needle-like configuration of its crystalline state, and the special phase change process contribute to strong refractive index modulation. Moreover, we find that their ultra-thermal stability is from a strong Sb–Se bond. Through the simulation, we found that G1S2S3T1 can be applied to prepare ultra-low loss waveguide devices. The present work represents a significant step in the development of broadband nonvolatile photonics derived from O-PCMs.

On-Chip Reconfigurable and Ultracompact Silicon Waveguide Mode Converters Based on Nonvolatile Optical Phase Change Materials.

Reconfigurable mode converters are essential components in efficient higher-order mode sources for on-chip multimode applications. We propose an on-chip reconfigurable silicon waveguide mode conversion scheme based on the nonvolatile and low-loss optical phase change material antimony triselenide (Sb2Se3). The key mode conversion region is formed by embedding a tapered Sb2Se3 layer into the silicon waveguide along the propagation direction and further cladding with graphene and aluminum oxide layers as the microheater. The proposed device can achieve the TE0-to-TE1 mode conversion and reconfigurable conversion (no mode conversion) depending on the phase state of embedded Sb2Se3 layer, whereas such function could not be realized according to previous reports. The proposed device length is only 2.3 μm with conversion efficiency (CE) = 97.5%, insertion loss (IL) = 0.2 dB, and mode crosstalk (CT) = -20.5 dB. Furthermore, the proposed device scheme can be extended to achieve other reconfigurable higher-order mode conversions. We believe the proposed reconfigurable mode conversion scheme and related devices could serve as the fundamental building blocks to provide higher-order mode sources for on-chip multimode photonics.

Photothermal and Photoacoustic Response of a VO2@Au Nanoshell Irradiated by a Nanosecond Laser Pulse

Plasmonic nanoparticles (PNPs) are considered as a proper mediator in photothermal therapy due to their capability to efficiently convert the absorbed light energy into localized heat. As the localized heat diffuses during therapy processes, it is crucial to control and detect the temperature in a non-invasive way. Synthesizing a new generation of PNPs that utilize optical phase-change materials leads to a tunable photothermal response without geometrical variation. This tunability originates from the prompt variation of optical and thermal properties during the phase-change transition. In this paper, we numerically study the photothermal response of a VO2@Au nanoshell in an aqueous medium when irradiated by a nanosecond pulsed laser (5 ns). For the temperature profiles, we simultaneously solve a self-consistent multiphysics problem consisting of electromagnetism and thermodynamics. We do our calculations for two wavelengths of 658 and 737 nm where the absorption spectrum of the nanoshell has two extrema during the VO2 core’s phase-change transition. Finally, for the two selected wavelengths, by coupling the structural dynamic physics to the problem, we calculate the acoustic pressure signals generated by the photothermal expansion of both the nanoshell and its surrounding medium. This photoacoustic signal could be considered as a non-invasive method to measure the local temperature in deep tissues accurately.

Characterizing optical phase-change materials with spectroscopic ellipsometry and polarimetry

This paper discusses the fundamentals, applications, potential and limitations of polarized light reflection techniques for the characterization of phase-change materials (PCMs). These techniques include spectroscopic ellipsometry, time-resolved ellipsometry and imaging ellipsometry as well as polarimetry. We explore the capabilities of spectroscopic ellipsometry in the determination of the extinction coefficient of PCMs and the capabilities of imaging ellipsometry to characterize PCMs. We show that ellipsometry is capable of more than the determination of thickness and optical properties, and it can be exploited to gain information about crystallization/amorphization kinetics and mapping anisotropies.

Phase Change Materials for Optics and Photonics: feature issue introduction

We introduce the Optical Materials Express feature issue on Phase Change Materials for Optics and Photonics. This issue comprises a collection of seventeen manuscripts on the development, characterization, control, and applications of optical Phase Change Materials.