The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in Vo 2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

Thin-film materials with 'smart' properties that react to temperature variations, electric or magnetic fields, and/or pressure variations have recently attracted a great deal of attention. Vanadium dioxide (VO 2 ) belongs to this family of 'smart materials' because it exhibits a semiconductor-to-metal first-order phase transition near 340 K, accompanied by an abrupt change in its resistivity and near-infrared transmission. It is also of great interest in condensed-matter physics because it is a classic strongly correlated electron system. In order to integrate vanadium dioxide into microelectronic circuits, thin-film growth of VO 2 has been studied extensively, and studies of VO 2 nanoparticles have shown that the phase transition is size-dependent. This paper presents a broad overview of the growth techniques that have been used to produce thin films and nanoparticles of VO 2 , including chemical vapor deposition, sol-gel synthesis, sputter deposition and pulsed laser deposition. Representative deposition techniques are described, and typical thin-film characteristics are presented, with an emphasis on recent results obtained using pulsed laser deposition. The opportunities for growing epitaxial films of VO 2 , and for doping VO 2 films to alter their transition temperature and switching characteristics, are also discussed.

Ultrafast photoinduced phase transitions could revolutionize data-storage and telecommunications technologies by modulating signals in integrated nanocircuits at terahertz speeds. In quantum phase-changing materials (PCMs), microscopic charge, lattice, and orbital degrees of freedom interact cooperatively to modify macroscopic electrical and optical properties. Although these interactions are well documented for bulk single crystals and thin films, little is known about the ultrafast dynamics of nanostructured PCMs when interfaced to another class of materials as in this case to active plasmonic elements. Here, we demonstrate how a mesh of gold nanoparticles, acting as a plasmonic photocathode, induces an ultrafast phase transition in nanostructured vanadium dioxide (VO2) when illuminated by a spectrally resonant femtosecond laser pulse. Hot electrons created by optical excitation of the surface-plasmon resonance in the gold nanomesh are injected ballistically across the Au/VO2 interface to induce a subpicosecond phase transformation in VO2. Density functional calculations show that a critical density of injected electrons leads to a catastrophic collapse of the 6 THz phonon mode, which has been linked in different experiments to VO2 phase transition. The demonstration of subpicosecond phase transformations that are triggered by optically induced electron injection opens the possibility of designing hybrid nanostructures with unique nonequilibrium properties as a critical step for all-optical nanophotonic devices with optimizable switching thresholds.

We present a detailed study of the photoinduced insulator-metal transition in VO2 with broadband time-resolved reflection spectroscopy. This allows us to separate the response of the lattice vibrations from the electronic dynamics and observe their individual evolution. When exciting well above the photoinduced phase transition threshold, we find that the restoring forces that describe the ground state monoclinic structure are lost during the excitation process, suggesting that an ultrafast change in lattice potential drives the structural transition. However, by performing a series of pump-probe measurements during the non-equilibrium transition, we observe that the electronic properties of the material evolve on a different, slower, timescale. This separation of timescales suggests that the early state of VO2, immediately after photoexcitation, is a non-equilibrium state that is not well defined by either the insulating or metallic phase.

We demonstrate photothermally induced optical switching of ultra-compact hybrid Si-VO₂ ring resonators. The devices consist of a sub-micron length ~70 nm thick patch of phase-changing VO₂ integrated onto silicon ring resonators as small as 1.5 μm in radius. The semiconductor-to-metal transition (SMT) of VO₂ is triggered using a 532 nm pump laser, while optical transmission is probed using a tunable cw laser near 1550 nm. We observe optical modulation greater than 10dB from modest quality-factor (~10³) resonances, as well as a large -1.26 nm change in resonant wavelength Δλ, resulting from the large change in the dielectric function of VO₂ in the insulator-to-metal transition achieved by optical pumping.

Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in storage-class memories but also enables in-memory data processing to overcome the von Neumann bottleneck. In PCMs, data storage is driven by thermal excitation. However, there is limited research regarding PCM thermal properties at length scales close to the memory cell dimensions. Our work presents a new paradigm to manage thermal transport in memory cells by manipulating the interfacial thermal resistance between the phase change unit and the electrodes without incorporating additional insulating layers. Experimental measurements show a substantial change in interfacial thermal resistance as GST transitions from cubic to hexagonal crystal structure, resulting in a factor of 4 reduction in the effective thermal conductivity. Simulations reveal that interfacial resistance between PCM and its adjacent layer can reduce the reset current for 20 and 120 nm diameter devices by up to ~ 40% and ~ 50%, respectively. These thermal insights present a new opportunity to reduce power and operating currents in PCMs.

The multifunctional properties of vanadium dioxide (VO2) arise from coupled first-order phase transitions: an insulator-to-metal transition (IMT) and a structural phase transition (SPT) from monoclinic to tetragonal. The characteristic signatures of the IMT and SPT are the hysteresis loops that track the phase transition from nucleation to stabilization of a new phase and back. A long-standing question about the mechanism of the VO2 phase transition is whether and how the almost-simultaneous electronic and structural transitions are related. Here, we report independent measurements of the IMT and SPT hystereses in epitaxial VO2 films on c-sapphire with distinct morphologies. The measurements show that the IMT and the SPT are not congruent, in that the structural phase transition requires more energy to reach completion than the electronic, insulator-to-metal transition. This result is independent of nanoscale film morphology and grain orientation on the substrate, so that the non-congruence is an intrinsic property of the VO2 phase transition. Our conclusion is supported by effective-medium calculations of the dielectric function incorporating the measured volume fractions of the monoclinic and tetragonal states. The results are consistent with the existence of an intermediate metallic state in which the electron-electron correlations characteristic of the monoclinic state begin to disappear before the transition to the tetragonal structural state.

Metasurfaces offer unparalleled functionalities for controlling the propagation and properties of electromagnetic waves. But to transfer these functions to technological applications, it is critical to render them tunable and to enable fast control by external stimuli. In most cases, this has been realized by utilizing tunable materials combined with a top-down nanostructuring process, which is often complicated and time intensive. Here we present a novel strategy for fabricating a tunable metasurface comprising epitaxially grown nanobeams of a phase transition material, vanadium dioxide. Without the need for extensive nanolithographic fabrication, we prepared a large-area (>1 cm 2 ), deep-subwavelength (thickness of ∼λ/40) nanostructured thin film that can control light transmission with large modulation depth, exceeding 9 dB across all telecommunication wavelength bands. Furthermore, the transmission in the "on" state remains higher than 80% from near-to mid-infrared region. This renders our metasurface useful also as a phase-shifting element, which we demonstrate by carrying out cross-polarized transmission measurements. To provide insights about the relationship between metasurface morphology and its resulting optical properties, we perform full-field three-dimensional numerical simulations as a function of width, height, and edge-to-edge separation of the epitaxial VO 2 nanobeams.