Joyeeta Nag

Ultrafast changes in lattice symmetry probed by coherent phonons.

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.

Ultrafast Phase Transition via Catastrophic Phonon Collapse Driven by Plasmonic Hot-Electron Injection

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.

Photothermal optical modulation of ultra-compact hybrid Si-VO 2 ring resonators

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.

Enhanced performance of room-temperature-grown epitaxial thin films of vanadium dioxide

Vanadium dioxide (VO2) in bulk, thin-film, and nanostructured forms exhibits an insulator-to-metal transition accompanied by structural reorganization, induced by temperature, light, electric fields, doping, or strain. We have grown epitaxial films of VO2 on c-cut (0001) sapphire following two different procedures: (1) room-temperature growth followed by annealing and (2) direct high-temperature growth. We find that variations in strain at the film-substrate interface in the two protocols leads to differences in morphologies and transition characteristics. Our results show that room-temperature-grown epitaxial films have smoother morphologies and better switching contrast, analogous to the enhanced performance of epitaxially grown compound semiconductors.

Non-congruence of thermally driven structural and electronic transitions in VO2

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 intrinsi...

Plasmonic Probe of the Semiconductor to Metal Phase Transition in Vanadium Dioxide

An array of 180 nm diameter gold nanoparticles (NPs) embedded in a thin vanadium dioxide film was used as a nanoscale probe of the thermochromic semiconductor-to-metal transition (SMT) in the VO2. The observed 30% reduction in plasmon dephasing time resulted from the interaction between the localized surface plasmon resonance of the NPs with the 1.4 eV electronic transitions in VO2. The NPs act as nanoantennas probing the SMT; homogeneous broadening of the gold plasmon resonance is observed at the temperatures where electron correlations are strongest in VO2.

Plasmon-enhanced low-intensity laser switching of gold::vanadium dioxide nanocomposites

Transient absorption of gold nanoparticle (NP) arrays covered by a 60 nm thick film of VO2 was measured using a mechanically shuttered 785 nm pump laser and a 1550 nm cw probe. Even though the Au NPs constitute only 4% by volume of the nanocomposite, they increase the effective absorption coefficient by a factor of 1.5 and reduce the threshold laser power required to induce the semiconductor-to-metal transition (SMT) by as much as 37%. It is argued that the NPs function as thermal initiators for the SMT and as “nanoradiators” to increase the scattering and absorption of light into interband transitions of the VO2.

Ultrafast compact silicon-based ring resonator modulators using metal-insulator switching of vanadium dioxide

We present an optical modulator based on a silicon ring resonator coated with vanadium-dioxide (VO2) motivated by the need for compact silicon-compatible optical switches operating at THz speeds. VO2 is a functional oxide undergoing metal-insulator transition (MIT) near 67°C, with huge changes in electrical resistivity and near-infrared transmission. The MIT can be induced thermally, optically (by ultra-fast laser excitation in less than 100 fs), and possibly with electric field. VO2 is easily deposited on silicon and its ultrafast switching properties in the near-infrared can be used to tune the effective index of ring resonators in the telecommunication frequencies instead of depending on the weak electro-optic properties of silicon. The VO2-silicon hybrid ring resonator is expected to operate at speeds up to 10 THz at low Q-factor and with shorter cavity lifetimes, thus enabling compact, faster, more robust devices. We have made ring resonator structures on SOI substrates with rings varying in diameter from 3-10 μm coupled to 5 mm-long nanotapered waveguides at separations of 200 nm. Rings were coated with 80 nm of VO2 by pulsed laser deposition. As proof-of-concept, by switching the VO2 top layer thermally, we were able to modulate the resonance frequency of the ring to match with the predictions from our FDTD simulations.

Heterogeneous nucleation and growth dynamics in the light-induced phase transition in vanadium dioxide.

We report on ultrafast optical investigations of the light-induced insulator-to-metal phase transition in vanadium dioxide with controlled disorder generated by substrate mismatch. These results reveal common dynamics of this optically-induced phase transition that are independent of this disorder. Above the fluence threshold for completing the transition to the rutile crystalline phase, we find a common time scale, independent of sample morphology, of 40.5 ± 2 ps that is consistent with nucleation and growth dynamics of the R phase from the parent M1 ground state.

Spectral modulation of single plasmonic nanostructures

We discuss in this paper the feasibility of dynamically modulating both resonance wavelength and spectral width of single nanostructures exhibiting plasmonic effects by cycling through a metal-insulator transition (MIT) in vanadium dioxide (VO2). Using full-field 3D finite-difference time domain (FDTD) simulation method with nonuniform mesh techniques, we study the effects of this modulation by varying the lateral dimensions of these nanostructures from 40 nm to 120 nm radially and changing its configuration as well, that is VO2 nanodisk on gold one and vice-versa. As an initial step towards fabricating those single composite nanostructures showing the greatest modulating effect, we start by making single NPs of VO2 and single gold NPs embedded between two 60 nm layers of VO2. The samples are fabricated on 130 μm thin glass substrates by electron-beam lithography, pulsed laser deposition of VO2 and electron-beam evaporation of gold. Using confocal extinction spectroscopy, we hereafter provide for the first time experimental observations of spectral tuning in these lithographically prepared single nanostructures. However, we discussed the variability in spectra obtained. Indeed, as the gold NP size decreases, it becomes comparable to the domain sizes of the embedding VO2 and this prevent the correct acquisition of the flat field. Hence the study of the tunability of gold particle plasmon resonance is imparted. However, we conclude that this study will be feasible for truly hybridized NP, that is gold nanodisk stacked on VO2 nanodisk and vice-versa. As hinted by our simulation studies and preliminary experimental results, these hybridized composite NPs could potentially be used in the dynamic spectral tuning of plasmonic waveguides.