Enabling Active Nanotechnologies by Phase Transition: From Electronics, Photonics to Thermotics.

Phase transitions can occur in certain materials such as transition metal oxides (TMOs) and chalcogenides when there is a change in external conditions such as temperature and pressure. Along with phase transitions in these phase change materials (PCMs) come dramatic contrasts in various physical properties, which can be engineered to manipulate electrons, photons, polaritons, and phonons at the nanoscale, offering new opportunities for reconfigurable, active nanodevices. In this review, we particularly discuss phase-transition-enabled active nanotechnologies in nonvolatile electrical memory, tunable metamaterials, and metasurfaces for manipulation of both free-space photons and in-plane polaritons, and multifunctional emissivity control in the infrared (IR) spectrum. The fundamentals of PCMs are first introduced to explain the origins and principles of phase transitions. Thereafter, we discuss multiphysical nanodevices for electronic, photonic, and thermal management, attesting to the broad applications and exciting promises of PCMs. Emerging trends and valuable applications in all-optical neuromorphic devices, thermal data storage, and encryption are outlined in the end.

Time-domain separation of optical properties from structural transitions in resonantly bonded materials.

The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.

Classification, registration and segmentation of ear canal impressions using convolutional neural networks.

Today, fitting bespoke hearing aids involves injecting silicone into patients' ears to produce ear canal molds. These are subsequently 3D scanned to create digital ear canal impressions. However, before digital impressions can be used they require a substantial amount of effort in manual 3D editing. In this article, we present computational methods to pre-process ear canal impressions. The aim is to create automation tools to assist the hearing aid design, manufacturing and fitting processes as well as normalizing anatomical data to assist the study of the outer ear canal's morphology. The methods include classifying the handedness of the impression into left and right ear types, orienting the geometries onto the same coordinate system sense, and removing extraneous artifacts introduced by the silicone mold. We investigate the use of convolutional neural networks for performing these semantic tasks and evaluate their accuracy using a dataset of 3000 ear canal impressions. The neural networks proved highly effective at performing these tasks with 95.8% adjusted accuracy in classification, 92.3% within 20° angular error in registration and 93.4% intersection over union in segmentation.

A Novel Ear Impression-Taking Method Using Structured Light Imaging and Machine Learning: A Pilot Proof of Concept Study with Patients' Feedback on Prototype.

INTRODUCTION: Taking an ear impression is a minimally invasive procedure. A review of existing literature suggests that contactless methods of scanning the ear have not been developed. We proposed to establish a correlation between external ear features with the ear canal and with this proof of concept to develop a prototype and an algorithm for capturing and predicting ear canal information. METHODS: We developed a novel prototype using structured light imaging to capture external images of the ear. Using a large database of existing ear impression images obtained by traditional methods, correlation analyses were carried out and established. A deep neural network was devised to build a predictive algorithm. Patients undergoing hearing aid evaluation undertook both methods of ear impression-taking. We evaluated their subjective feedback and determined if there was a close enough objective match between the images obtained from the impression techniques. RESULTS: A prototype was developed and deployed for trial, and most participants were comfortable with this novel method of ear impression-taking. Partial matching of the ear canal could be obtained from the images taken, and the predictive algorithm applied for a few sample images was within good standard of error with proof of concept established. DISCUSSION: Further studies are warranted to strengthen the predictive capabilities of the algorithm and determine optimal prototype imaging positions so that sufficient ear canal information can be obtained for three-dimensional printing. Ear impression-taking may then have the potential to be automated, with the possibility of same-day three-dimensional printing of the earmold to provide timely access.

Strongly tunable circular dichroism in gammadion chiral phase-change metamaterials.

A metal/phase-change material/metal tri-layer planar chiral metamaterial in the shape of a gammadion is numerically modelled. The chiral metamaterial is integrated with Ge2Sb2Te5 phase-change material (PCM) to accomplish a wide tuning range of the circular dichroism (CD) in the mid-infrared wavelength regime. A photothermal model is used to study the temporal variation of the temperature of the Ge2Sb2Te5 layer and to show the potential for fast switching the phase of Ge2Sb2Te5 under a low incident light intensity of 0.016mW/µm2.

Enhanced far-field coherent thermal emission using mid-infrared bilayer metasurfaces.

A classical thermal source, such as an incandescent filament, radiates according to Planck's law. The feasibility of super-Planckian radiation has been investigated with sub-wavelength-sized sources in the last decade. In such sources, a crystal-dependent coupling of photons and optical phonons is possible at thermal energies corresponding to that at room temperature. This interaction can be used to tailor the far-field thermal emission in a coherent manner; however, understanding heat transfer during this process is still nascent. Here, we used a novel measurement platform to quantify thermal signals in a Ge2Sb2Te5/SiO2 nanoribbon structure. We were able to separate and quantify the radiated and conducted heat transfer mechanisms. The thermal emission from the Ge2Sb2Te5/SiO2 nanoribbons was enhanced by 3.5× compared to that of a bare SiO2 nanoribbon. Our model revealed that this enhancement was directly due to polaritonic heat transfer, which was possible due to the large and lossless dielectric permittivity of Ge2Sb2Te5 at mid-IR frequencies. This study directly probes the far-field emission with a thermal gradient stimulated by Joule heating in temperature ranges from 100 to 400 K, which bridges the gap between mid-IR optics and thermal engineering.

Programmable Wavefront Control in the Visible Spectrum Using Low-Loss Chalcogenide Phase-Change Metasurfaces.

All-dielectric metasurfaces provide unique solutions for advanced wavefront manipulation of light with complete control of amplitude and phase at sub-wavelength scales. One limitation, however, for most of these devices is the lack of any post-fabrication tunability of their response. To break this limit, a promising approach is employing phase-change materials (PCMs), which provide fast, low energy, and non-volatile means to endow metasurfaces with a switching mechanism. In this regard, great advancements have been done in the mid-infrared and near-infrared spectrum using different chalcogenides. In the visible spectral range, however, very few devices have demonstrated full phase manipulation, high efficiencies, and reversible optical modulation. In this work, a programmable all-dielectric Huygens' metasurface made of antimony sulfide (Sb2 S3 ) PCM is experimentally demonstrated, a low loss and high-index material in the visible spectral range with a large contrast (≈0.5) between its amorphous and crystalline states. ≈2π phase modulation is shown with high associated transmittance and it is used to create programmable beam-steering devices. These novel chalcogenide PCM metasurfaces have the potential to emerge as a platform for next-generation spatial light modulators and to impact application areas such as programmable and adaptive flat optics, light detection and ranging (LiDAR), and many more.

Lexicon for classifying ear-canal shapes.

The ear canal is usually described as an S-shaped funnel. In attempting to classify ear-canal shapes obtained from point clouds digitized from molds of 300 ears, the problem of designing criteria for distinguishing and organizing the canal shapes arose. In this work, we extracted features inspired by the S-shape characteristic (critical point, maximum, minimum, twist, writhe, translation, rotation) and, through them, introduced 14 types of ear-canal shapes. This classification allowed comparison of ears within a type and of ears between different types. It expanded our range of descriptors of canal shapes and unlocked perspectives for applications.

Low Energy Switching of Phase Change Materials Using a 2D Thermal Boundary Layer.

The switchable optical and electrical properties of phase change materials (PCMs) are finding new applications beyond data storage in reconfigurable photonic devices. However, high power heat pulses are needed to melt-quench the material from crystalline to amorphous. This is especially true in silicon photonics, where the high thermal conductivity of the waveguide material makes heating the PCM energy inefficient. Here, we improve the energy efficiency of the laser-induced phase transitions by inserting a layer of two-dimensional (2D) material, either MoS2 or WS2, between the silica or silicon substrate and the PCM. The 2D material reduces the required laser power by at least 40% during the amorphization (RESET) process, depending on the substrate. Thermal simulations confirm that both MoS2 and WS2 2D layers act as a thermal barrier, which efficiently confines energy within the PCM layer. Remarkably, the thermal insulation effect of the 2D layer is equivalent to a ∼100 nm layer of SiO2. The high thermal boundary resistance induced by the van der Waals (vdW)-bonded layers limits the thermal diffusion through the layer interface. Hence, 2D materials with stable vdW interfaces can be used to improve the thermal efficiency of PCM-tuned Si photonic devices. Furthermore, our waveguide simulations show that the 2D layer does not affect the propagating mode in the Si waveguide; thus, this simple additional thin film produces a substantial energy efficiency improvement without degrading the optical performance of the waveguide. Our findings pave the way for energy-efficient laser-induced structural phase transitions in PCM-based reconfigurable photonic devices.

Reversible Tuning of Mie Resonances in the Visible Spectrum.

Dielectric optical nanoantennas are promising as fundamental building blocks in next generation color displays, metasurface holograms, and wavefront shaping optical devices. Due to the high refractive index of the nanoantenna material, they support geometry-dependent Mie resonances in the visible spectrum. Although phase change materials, such as the germanium-antimony-tellurium alloys, and post-transition metal oxides, such as ITO, have been used to tune antennas in the near-infrared spectrum, reversibly tuning the response of dielectric antennas in the visible spectrum remains challenging. In this paper, we designed and experimentally demonstrated dielectric nanodisc arrays exhibiting reversible tunability of Mie resonances in the visible spectrum. We achieved tunability by exploiting phase transitions in Sb2S3 nanodiscs. Mie resonances within the nanodisc give rise to structural colors in the reflection mode. Crystallization and laser-induced amorphization of these Sb2S3 resonators allow the colors to be switched back and forth. These tunable Sb2S3 nanoantenna arrays could enable the next generation of high-resolution color displays, holographic displays, and miniature LiDAR systems.

Rewritable color nanoprints in antimony trisulfide films.

Materials that exhibit large and rapid switching of their optical properties in the visible spectrum hold the key to color-changing devices. Antimony trisulfide (Sb2S3) is a chalcogenide material that exhibits large refractive index changes of ~1 between crystalline and amorphous states. However, little is known about its ability to endure multiple switching cycles, its capacity for recording high-resolution patterns, nor the optical properties of the crystallized state. Unexpectedly, we show that crystalline Sb2S3 films that are just 20 nm thick can produce substantial birefringent phase retardation. We also report a high-speed rewritable patterning approach at subdiffraction resolutions (>40,000 dpi) using 780-nm femtosecond laser pulses. Partial reamorphization is demonstrated and then used to write and erase multiple microscale color images with a wide range of colors over a ~120-nm band in the visible spectrum. These solid-state, rapid-switching, and ultrahigh-resolution color-changing devices could find applications in nonvolatile ultrathin displays.

Chalcogenide-gold dual-layers coupled to gold nanoparticles for reconfigurable perfect absorption.

Recently, tunable high absorptance from various nanophotonic structures has been demonstrated. However, most of these structures require nano-lithography, which is expensive and slow. Lithography-free tuneable absorbers are rarely explored for tuneable visible and near-infrared photonics. Herein, we demonstrate a gold (Au)/chalcogenide dual-layer that is resonantly coupled to Au nanoparticles (NPs). The structure exhibits angle and polarisation-independent high absorptance. At resonance, waveguide cavity-like modes are excited between the film and NPs whilst gap plasmon modes are excited between the NPs. Coalescence of the waveguide cavity-like modes, the gap plasmon modes, and the highly absorbing chalcogenide semiconductor not only leads to perfect absorptance but also a reconfigurable response via reversible structural phase transitions in the chalcogenide film. In the amorphous state, the design provides nearly perfect absorptance for both p- and s-polarisation states at an incident angle of 20°. However, after switching to the crystalline state, the peak absorptance spectrally broadens and redshifts from 980 to 1520 nm. This experimental observation was theoretically validated by the finite element method. Thermal-electric modeling was performed to show that the structural transition from crystalline to amorphous states is possible in just 5 ns, thus allowing high-speed reconfigurable perfect absorbers.

Large-Area Broadband Near-Perfect Absorption from a Thin Chalcogenide Film Coupled to Gold Nanoparticles.

Perfect absorbers that can efficiently absorb electromagnetic waves over a broad spectral range are crucial for energy harvesting, light detection, and optical camouflage. Recently, perfect absorbers based on a metasurface have attracted intensive attention. However, high-performance metasurface absorbers in the visible spectra require strict fabrication tolerances, and this is a formidable challenge. Moreover, fabricating subwavelength meta-atoms requires a top-down approach, thus limiting their scalability and spectral applicability. Here, we introduce a plasmonic nearly perfect absorber that exhibits a measured polarization-insensitive absorptance of ∼92% across the spectral region from 400 to 1000 nm. The absorber is realized via a one-step self-assembly deposition of 50 nm gold (Au) nanoparticle (NP) clusters onto a 35 nm-thick Ge2Sb2Te5 (GST225) chalcogenide film. An excellent agreement between the measured and theoretically simulated absorptance was found. The coalescence of the lossy GST225 dielectric layer and high density of localized surface plasmon resonance modes induced by the randomly distributed Au NPs play a vital role in obtaining the nearly perfect absorptance. The exceptionally high absorptance together with large-area high-throughput self-assembly fabrication demonstrates their potential for industrial-scale manufacturability and consequential widespread applications in thermophotovoltaics, photodetection, and sensing.

Sb2Te3 and Its Superlattices: Optimization by Statistical Design.

The objective of this work is to demonstrate the usefulness of fractional factorial design for optimizing the crystal quality of chalcogenide van der Waals (vdW) crystals. We statistically analyze the growth parameters of highly c axis oriented Sb2Te3 crystals and Sb2Te3-GeTe phase change vdW heterostructured superlattices. The statistical significance of the growth parameters of temperature, pressure, power, buffer materials, and buffer layer thickness was found by fractional factorial design and response surface analysis. Temperature, pressure, power, and their second-order interactions are the major factors that significantly influence the quality of the crystals. Additionally, using tungsten rather than molybdenum as a buffer layer significantly enhances the crystal quality. Fractional factorial design minimizes the number of experiments that are necessary to find the optimal growth conditions, resulting in an order of magnitude improvement in the crystal quality. We highlight that statistical design of experiment methods, which is more commonly used in product design, should be considered more broadly by those designing and optimizing materials.

Large-Area Silver-Stibnite Nanoporous Plasmonic Films for Label-Free Biosensing.

The development of various plasmonic nanoporous materials has attracted much interest in different areas of research including bioengineering and biosensing because of their large surface area and versatile porous structure. Here, we introduce a novel technique for fabricating silver-stibnite nanoporous plasmonic films. Unlike conventional techniques that are usually used to fabricate nanoporous plasmonic films, we use a room-temperature growth method that is wet-chemistry free, which enables wafer-scale fabrication of nanoporous films on flexible substrates. We show the existence of propagating surface plasmon polaritons in nanoporous films and demonstrate the extreme bulk refractive index sensitivity of the films using the Goos-Hänchen shift interrogation scheme. In the proof-of-concept biosensing experiments, we functionalize the nanoporous films with biotin-thiol using a modified functionalization technique, to capture streptavidin. The fractal nature of the films increases the overlap between the local field and the immobilized biomolecules. The extreme sensitivity of the Goos-Hänchen shift allows femtomolar concentrations of streptavidin to be detected in real time, which is unprecedented using surface plasmons excited via the Kretschmann configuration.

Evolutionary design of interfacial phase change van der Waals heterostructures.

We use an evolutionary algorithm to explore the design space of hexagonal Ge2Sb2Te5; a van der Waals layered two dimensional crystal heterostructure. The Ge2Sb2Te5 structure is more complicated than previously thought. Predominant features include layers of Ge3Sb2Te6 and Ge1Sb2Te4 two dimensional crystals that interact through Te-Te van der Waals bonds. Interestingly, (Ge/Sb)-Te-(Ge/Sb)-Te alternation is a common feature for the most stable structures of each generation's evolution. This emergent rule provides an important structural motif that must be included in the design of high performance Sb2Te3-GeTe van der Waals heterostructure superlattices with interfacial atomic switching capability. The structures predicted by the algorithm agree well with experimental measurements on highly oriented, and single crystal Ge2Sb2Te5 samples. By analysing the evolutionary algorithm optimised structures, we show that diffusive atomic switching is probable by Ge atoms undergoing a transition at the van der Waals interface from layers of Ge3Sb2Te6 to Ge1Sb2Te4 thus producing two blocks of Ge2Sb2Te5. Evolutionary methods present an efficient approach to explore the enormous multi-dimensional design parameter space of van der Waals bonded heterostructure superlattices.

Oxygen Tuned Local Structure and Phase-Change Performance of Germanium Telluride.

The effect of oxygen on the local structure of Ge atoms in GeTe-O materials has been investigated. Oxygen leads to a significant modification to the vibrational modes of Ge octahedra, which results from a decrease in its coordination. We find that a defective octahedral Ge network is the crucial fingerprint for rapid and reversible structural transitions in GeTe-based phase change materials. The appearance of oxide Raman modes confirms phase separation into GeO and TeO at high level O doping. Counterintuitively, despite the increase in crystallization temperature of oxygen doped GeTe-O phase change materials, when GeTe-O materials are used in electrical phase change memory cells, the electrical switching energy is lower than the pure GeTe material. This switching energy reduction is ascribed to the smaller change in volume, and therefore smaller enthalpy change, for the oxygen doped GeTe materials.

Phase-Change Memory Materials by Design: A Strain Engineering Approach.

Van der Waals heterostructure superlattices of Sb2 Te1 and GeTe are strain-engineered to promote switchable atomic disordering, which is confined to the GeTe layer. Careful control of the strain in the structures presents a new degree of freedom to design the properties of functional superlattice structures for data storage and photonics applications.

Strain-engineered diffusive atomic switching in two-dimensional crystals.

Strain engineering is an emerging route for tuning the bandgap, carrier mobility, chemical reactivity and diffusivity of materials. Here we show how strain can be used to control atomic diffusion in van der Waals heterostructures of two-dimensional (2D) crystals. We use strain to increase the diffusivity of Ge and Te atoms that are confined to 5 Å thick 2D planes within an Sb2Te3-GeTe van der Waals superlattice. The number of quintuple Sb2Te3 2D crystal layers dictates the strain in the GeTe layers and consequently its diffusive atomic disordering. By identifying four critical rules for the superlattice configuration we lay the foundation for a generalizable approach to the design of switchable van der Waals heterostructures. As Sb2Te3-GeTe is a topological insulator, we envision these rules enabling methods to control spin and topological properties of materials in reversible and energy efficient ways.

A zero density change phase change memory material: GeTe-O structural characteristics upon crystallisation.

Oxygen-doped germanium telluride phase change materials are proposed for high temperature applications. Up to 8 at.% oxygen is readily incorporated into GeTe, causing an increased crystallisation temperature and activation energy. The rhombohedral structure of the GeTe crystal is preserved in the oxygen doped films. For higher oxygen concentrations the material is found to phase separate into GeO2 and TeO2, which inhibits the technologically useful abrupt change in properties. Increasing the oxygen content in GeTe-O reduces the difference in film thickness and mass density between the amorphous and crystalline states. For oxygen concentrations between 5 and 6 at.%, the amorphous material and the crystalline material have the same density. Above 6 at.% O doping, crystallisation exhibits an anomalous density change, where the volume of the crystalline state is larger than that of the amorphous. The high thermal stability and zero-density change characteristic of Oxygen-incorporated GeTe, is recommended for efficient and low stress phase change memory devices that may operate at elevated temperatures.