We investigate sub-threshold photoexcited VO 2 , isolating photodoping from electron–lattice thermalization by pumping around the bandgap. Band-edge excitation yields the shortest relaxation dynamics, revealing intrinsically driven photodoping effects.

Recent studies in nonlinear optics have seen heightened interest in metallic heterostructures, which provide a promising avenue for exploring the enhancement of high harmonic generation due to their localized surface plasmon resonances. In this paper, a theory of third harmonic generation (THG) and sum frequency generation (SFG) is developed for metallic nanohybrids made of an ensemble of Au, Al and CuS metallic nanoparticles. The coupled-mode formalism based on Maxwell's equations is used to obtain the intensity of the THG and SFG light. We found that the intensity depends on the third-order susceptibilities which are evaluated by the density matrix method. Analytical expressions of the intensities of the THG and SFG light were calculated in the presence of the surface plasmon polaritons (SPPs) and the dipole-dipole interaction (DDI). These expressions can aid experimentalists in conducting new experiments. We found that there are four types of processes contributing to THG and SFG in the Al/Au/CuS nanohybrid. The first contribution is due to the probe photons, the second is due to the SPPs, the third is due to the DDI polaritons, and the fourth is the combination of photons and polaritons. We compared our theory with our experimental data of Al/Au/CuS nanohybrid and found a good agreement between our theory and experiments. Our theoretical findings can be applied to fabricate optical nano-amplifiers, and nanosensors by measuring the intensity of the output wave.

The nature of the insulator-to-metal phase transition in vanadium dioxide (VO 2 ) is one of the longest-standing problems in condensed-matter physics. Ultrafast spectroscopy has long promised to determine whether the transition is primarily driven by the electronic or structural degree of freedom, but measurements to date have been stymied by their sensitivity to only one of these components and/or their limited temporal resolution. Here we use ultra-broadband few-femtosecond pump-probe spectroscopy to resolve the electronic and structural phase transitions in VO 2 at their fundamental time scales. Our experiments show that the system transforms into a bad-metallic phase within 10 fs after photoexcitation, but requires another 100 fs to complete the transition, during which we observe electronic oscillations and a partial re-opening of the bandgap, signalling a transient semi-metallic state. Comparisons with tensor-network simulations and density-functional theory calculations show these features result from an unexpectedly fast structural transition, in which the vanadium dimers separate and untwist with two different timescales. Our results resolve the structural and electronic nature of the light-induced phase transition in VO 2 and establish ultra-broadband few-femtosecond spectroscopy as a powerful tool for studying quantum materials out of equilibrium.

Doped vanadium dioxide (VO2) nanoparticles (NPs) have significant potential for applications requiring temperature-dependent emissivity, reflectivity, or transmission. Thermochromic coatings in particular enable energy-saving smart windows and passive thermal radiators but are subject to tight performance constraints. A major challenge is preparing uniform layers of NPs, over large areas, with controllable size distributions and transition temperatures (T c). We describe the growth and transition characteristics of randomly distributed undoped and W-doped VO2 NPs formed by solid-state dewetting. Sizes and size distributions are controlled by anneal time, as particles grow via Smoluchowski aggregation before oxidizing into V2O5; shapes are determined by the interfacial energies between VO2 (V2O5) and the silicon substrate. Tungsten dopants concentrate at the NP surface, increasing the energy barrier for and slowing the rate of dewetting, aggregation, and oxidization. Surprisingly, the doped NPs exhibit lower T c and sharper hysteresis than comparably doped thin films. These results advance our capacity to engineer doped VO2 NPs, yield valuable insights into VO2-substrate interactions, and highlight the distribution of W-dopants in VO2 NPs.

Heterostructures combining two or more metal and/or semiconductor nanoparticles exhibit enhanced upconversion arising from localized nanoparticle resonances. However, plasmon-exciton coupling in semiconductor-metal nanostructures exhibits nanosecond relaxation times, and multi-plasmon metallic heterostructures are not broadly tunable. Here, we develop a biplasmonic heterostructure in which CuS and Au nanoparticle layers, separated by an alumina spacer of variable thickness, exhibit enhanced second- and third-harmonic generation due to dipole-dipole coupling between Au and CuS plasmons, as seen in the characteristic inverse sixth-power dependence of their separation in the measured harmonic enhancement and confirmed by numerical simulations of near-field CuS-Au nanoparticle coupling. Transient-absorption spectroscopy shows faster relaxation in Au/CuS (690 femtoseconds) compared to CuS heterostructures (929 femtoseconds). Moreover, nonlinear absorption measurements provide evidence for harmonic-induced plasmonic resonant energy transfer between the narrow Au and broad, tunable CuS plasmon resonances. This prototype for ultrafast upconversion showcases a strategy for high-efficiency, tunable plasmonic nonlinear devices with promising applications in photocatalysis, parametric down-conversion, and biomedical imaging.

Dual heterostructures integrating noble-metal and copper chalcogenide nanoparticles have attracted a great deal of attention in nonlinear optics, because coupling of their localized surface plasmon resonances (LSPRs) substantially enhances light-matter interactions through local-field effects. Previously, enhanced cascaded third-harmonic generation was demonstrated in Au/CuS heterostructures mediated by harmonically coupled surface plasmon resonances. This suggests a promising approach for extending nonlinear enhancement to higher harmonics by adding an additional nanoparticulate material with higher-frequency harmonic resonances to the hybrid films. Here we report the first observation of enhanced cascaded fourth- and fifth-harmonic generation in Al/Au/CuS driven by coupled LSPRs at the fundamental (1050 nm), second harmonic (525 nm), and third harmonic (350 nm) of the pump frequency. An analytical model based on incoherent dipole-dipole interactions among plasmonic nanoparticles accounts for the observed enhancements. The results suggest a novel design for efficiently generating higher harmonics in resonant plasmonic structures by means of multiple sum-frequency cascades.

A growing class of nonlinear materials employ the localized surface plasmonic resonance (LSPR) of nanoparticles to enhance harmonic generation. Material systems containing harmonically coupled metallic and semiconductor plasmonic nanoparticles have been shown to further increase performance. Here, we explore the effect of dual plasmonic interactions in bilayer CuS and Au nanoparticle films on third harmonic generation (THG). Detuning the CuS LSPR away from the excitation frequency changes the dominant upconversion pathway from THG to multiple photon photoluminescence (MPPL). Changing the size of the Au nanoparticle red shifts the LSPR from the second harmonic of the pump frequency and also eliminates the enhancement effect. When both LSPRs satisfy the harmonic condition, simultaneous excitation of CuS-Au nanoparticle films at the resonant frequency of each nanoparticle species enhances the generation of third harmonic light by sum-frequency generation, suggesting that the enhancement of THG in dually plasmonic nanoparticle films is the result of a cascaded nonlinear mechanism. An analytic model of the interaction between the plasmonic nanoparticles due to incoherent dipolar interactions is also presented. Understanding these processes opens a pathway for developing ultrafast, high-efficiency upconversion thin-film devices by clarifying the conditions that efficiently produce third harmonic generation without background MPPL or additional harmonics.

Metal nanoparticles can strongly enhance such nonlinear optical processes as harmonic generation and multiphoton photoluminescence due to their high electronic polarizabilities, intense optical resonances and high surface-to-volume ratio. Heavily doped semiconductor nanoparticles, such as metal chalcogenides, can also exhibit plasmonic resonances in addition to their excitonic response. Here we describe frequency upconversion in bilayer nanoparticle thin-film structures comprising copper sulfide and gold nanoparticles separated by insulating ligands or thin films when excited by near-infrared femtosecond laser pulses. The surprisingly large third-harmonic signal is due to coherent excitation of the LSPR in both semiconducting and metallic nanoparticles – a mechanism validated by a dipole-dipole model calculation

Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light-induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.

The demonstration of a photo-induced insulator-to-metal transition in vanadium dioxide (VO2) on a picosecond time scale in the mid-1990s foreeshadowed application of this photo-induced phase transition in silicon photonics devices. The talk introduces our current understanding of the phase-transition physics, but focuses on our recent demonstrations of sub-picosecond switching of cw and ultrafast signal pulses in silicon and silicon nitride waveguides. Direct in-line modulation of signal pulses achieves only modest contrast ratios. However, similar switching strategies deploying VO2 on resonant structures promise substantially higher contrast with smaller switching energies in silicon photonic structures with micron-scale form factors.

Solid-state systems can host a variety of thermodynamic phases that can be controlled with magnetic fields, strain, or laser excitation. Many phases that are believed to exhibit exotic properties only exist on the nanoscale, coexisting with other phases that make them challenging to study, as measurements require both nanometer spatial resolution and spectroscopic information, which are not easily accessible with traditional x-ray spectromicroscopy techniques. Here, we use coherent diffractive imaging spectroscopy (CDIS) to acquire quantitative hyperspectral images of the prototypical quantum material vanadium oxide across the vanadium L 2,3 and oxygen K x-ray absorption edges with nanometer-scale resolution. We extract the full complex refractive indices of the monoclinic insulating and rutile conducting phases of VO2 from a single sample and find no evidence for correlation-driven phase transitions. CDIS will enable quantitative full-field x-ray spectromicroscopy for studying phase separation in time-resolved experiments and other extreme sample environments where other methods cannot operate.

Hybrid material systems are a promising approach for extending the capabilities of silicon photonics. Given the weak electro‐optic and thermo‐optic effects in silicon, there is intense interest in integrating an ultrafast‐switching phase‐change material with a large refractive index contrast into the waveguide, such as vanadium dioxide (VO2). It is well established that the phase transition in VO2 thin films can be triggered by ultrafast, 800 nm laser pulses, and that pump‐laser fluence is a critical determinant of the recovery time of thin films irradiated by femtosecond pulses. However, thin‐film experiments are not reliable guides to a VO2:Si system for all‐optical, on‐chip switching because of the differences in VO2 optical constants in the telecommunication band, and the complex sample geometry and alignment issues in a waveguide geometry. This paper reports the first demonstration that the reversible, ultrafast photoinduced phase transition in VO2 can achieve sub‐picosecond response when small VO2 volumes are integrated into a silicon waveguide as the active element. The result suggests that VO2 can be pursued as a strong candidate for waveguide switching with sub‐picosecond on‐off times.

We demonstrate the first femtosecond-temporal and nanometer-spatial resolution imaging of a light induced phase transition, the insulator-to-metal transition of vanadium dioxide, using time-resolved resonant X-ray holography [1] at the Pohang Accelerator Laboratory X-ray free electron laser. Thin films of VO 2 deposited onto silicon nitride membranes were masked for holography [2] and heated to 325 K, where domains of the rutile metallic phase began to nucleate. Femtosecond 800 nm pulses then drove the system through the phase transition, with the domain growth and nucleation dynamics monitored by coherent X-ray scattering at the vanadium L 2,3 and oxygen K edges ( Fig 1 ). Dynamics at a range of time-scales are observed in the real space videos of the process: we observe nanometer sized domains forming within 200 fs, followed by both sub-picosecond and picosecond dynamics. Real space imaging-spectroscopy was performed on the long-lived transient state at 20 ps time-delay [3] , allowing us to comment on the existence of a proposed nanoscale monoclinic metallic precursor in the ultrafast phase transition [4] , [5] .

Vapor-phase transport is a rapid, inexpensive method of growing nanoand microscale single crystals of vanadium dioxide, a correlated-electron material with a metal−insulator transition at ∼70 °C. Many growth parametersincluding time, temperature, precursor, ambient conditions, and substratehave been explored, and a variety of crystal morphologies has been produced, with most emphasis given to oriented nanowires. However, a comprehensive strategy for predicting/controlling the crystal morphology is still evolving. Here, we investigate the role of the substrate in platelet growth, highlighting three important types of interactions: chemical reactions at the surface, lattice matching effects, and surface energy. We present results on four different cuts of sapphire (Al2O3) and three of yttria-stabilized zirconia (YSZ) to differentiate the roles of these mechanisms. Each has significant effects: chemical reactions leading to Al-doped VO2 on Al2O3 and the formation of YVO4 on YSZ, lattice match producing preferred orientations on both, and high surface energy promoting growth of larger microcrystals. We suggest a framework for relating crystal morphology, orientation, and doping to substrate properties, in order to use intentional choice of the substrate to engineer the size, shape, orientation, and strain state of VO2 single crystals, a crucial step toward realizing VO2 crystal-based devices. ■ INTRODUCTION Vanadium dioxide (VO2) remains among the most promising reconfigurable materials with tunable optical and electrical properties. Its first-order phase transition from an insulating monoclinic phase (M1) to a metallic rutile phase (R) is accompanied by a change in resistivity by multiple orders of magnitude, sharp change in optical constants, and ∼1% change in lattice strain (shrinking along the cR axis, expanding normal to it, see Supporting Information, Section S1, Figure S1, and Table S1). This phase transition occurs at the readily accessible temperature of ∼70 °C and can be initiated optically on a femtosecond time scale; moreover, its critical temperature (Tc) and hysteretic response can be tuned via local strain, particle size, and doping, increasing its versatility. VO2 has been employed in sundry devices, from passive thermal control coatings to ultrafast photonic modulators and the complex physics underlying the phase transitionthe role of the structural versus the electronic transition and the difference between thermal and optical excitationhave made it an ideal subject in the study of correlated-electron materials. Bulk VO2 single crystalswhile less robust and less viable for large-scale applications than thin filmsoffer high crystalline quality, smooth faceted faces, and, in the absence of substrate strain, sharp, single-domain switching. When subject to strain, VO2 single crystals exhibit ferroelasticity, with strain-sensitive twin domains that form additional nucleation sites for the phase transition and generate complex patterns of coexisting of metallic and insulating domains. A broad range of applications requiring single-crystal VO2 have been proposed or demonstrated. Nanowires have been used for nanoactuators, tiny optically readable thermometers, nanoscale gas sensors, thermal rectifiers, and more. Microplatelets have Received: January 25, 2021 Revised: May 18, 2021 Published: June 2, 2021 Article pubs.acs.org/crystal © 2021 American Chemical Society 3770 https://doi.org/10.1021/acs.cgd.1c00088 Cryst. Growth Des. 2021, 21, 3770−3778 D ow nl oa de d vi a V A N D E R B IL T U N IV o n Ja nu ar y 24 , 2 02 2 at 1 6: 50 :5 1 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.

A key challenge to widespread implementation of silicon photonics is achieving optical switching at ultrafast speed and ultralow power using on-chip silicon modulators. Here we report experiments demonstrating that the ultrafast photo-induced phase transition in VO2 can be harnessed for all-optical in the telecommunications band when small VO2 volumes are integrated within a silicon waveguide. "On"-to-"off" switching speeds in this in-line modulator are less than 1 ps, thus consistent with Tbps speeds, and switching energies near threshold are less than 500 fJ for modulation depths near 6 dB. Early results showing significant reductions in switching energies in hybrid VO2:Si ring resonators will also be presented.

A heterostructure comprising CuS and Au nanoparticle films separated by insulating ligands yields enhanced second harmonic generation from coupling of localized surface plasmon resonance modes of CuS and Au at 1050 nm and 525 nm, respectively.

We demonstrate all-optical, subpicosecond switching in silicon photonics devices, using the femtosecond insulator-to-metal transition in vanadium dioxide as the driving mechanism for an in-waveguide modulator, and very low switching thresholds in a hybrid ring resonator.