Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO 2 quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO 2 . First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO 2 band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulatorto-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred. DOI: 10.1103/PhysRevLett.113.216401 PACS numbers: 71.27.+a, 71.20.Be, 71.30.+h, 79.60.-i Since its discovery in 1959 [1], studies of the VO 2 phase transition (PT) from a monoclinic (M 1 ) insulator (Fig. 1, top left) to a rutile (R) metal at T C ¼ 340 K (Fig. 1, top right) have revolved around the central question [2][3][4][5] of whether the crystallographic PT is the major cause for the electronic PT or if strong electron correlations are needed to explain the insulating low-T phase. While the M 1 structure is a necessary condition for the insulating state below T C , the existence of a monoclinic metal (mM) and its relevance to the thermally driven PT is under current investigation [6][7][8][9][10][11][12]. In particular, the role of carrier doping at temperatures close to T C by charge injection from the substrate or photoexcitation has been increasingly addressed [6,8,[13][14][15][16].One promising approach to disentangling the electronic and lattice contributions is to drive the PT nonthermally using ultrashort laser pulses in a pump-probe scheme. Time-resolved x-ray [17,18] and electron diffraction [16,19] showed that the lattice structure reaches the R phase quasithermally after picoseconds to nanoseconds. Transient optical spectroscopies have probed photoinduced changes of the dielectric function in the terahertz [20][21][22], near-IR [9,10,17,23], and visible range [23]. The nonequilibrium state reached by photoexcitation (hereinafter transient phase) differs from the two equilibrium phases, but eventually evolves to the R phase [17][18][19][20][21][22][23][24][25][26][27][28]. The observation of a minimum rise time of 80 fs in the optical response after strong excitation (50 mJ=cm 2 ), described as a structural bottleneck in VO 2 [24], challenged theory to describe the photoinduced crystallographic and electronic PT simultaneously [15,25].Time-resolved photoelectron spectroscopy (TR-PES) directly probes changes of the electronic structure. Previous photoelectron spectroscopy (PES) studies of VO 2 used high photon energies generating photoelectrons with large kinetic energies to study the dynamics of the electronic structure; however, with a low repetition rate (50 Hz [27]) and inadequate time resolution (> 150 fs) the ultrafast dynamics of t...

We demonstrate a hybrid silicon-vanadium dioxide (Si-VO 2 ) electro-optic modulator that enables direct probing of both the electrically triggered semiconductor-to-metal phase transition in VO 2 and the reverse transition from metal to semiconductor. By using a twoterminal in-plane VO 2 electrical switch atop a single-mode silicon waveguide, the phase change can be initiated electrically and probed optically, separating the excitation and measurement processes and simplifying the analysis of the metal-to-semiconductor dynamics. We demonstrate a record switch-on time for high-speed electrical semiconductor-to-metal transition, with switching times less than 2ns, and quantify the slower inverse transition, which is dominated by thermal dissipation and relaxation of the metallic rutile lattice to the monoclinic semiconducting

Coulomb correlations can manifest in exotic properties in solids, but how these properties can be accessed and ultimately manipulated in real time is not well understood. The insulator-to-metal phase transition in vanadium dioxide (VO 2 ) is a canonical example of such correlations. Here, few-femtosecond extreme UV transient absorption spectroscopy (FXTAS) at the vanadium M 2,3 edge is used to track the insulator-to-metal phase transition in VO 2 . This technique allows observation of the bulk material in real time, follows the photoexcitation process in both the insulating and metallic phases, probes the subsequent relaxation in the metallic phase, and measures the phasetransition dynamics in the insulating phase. An understanding of the VO 2 absorption spectrum in the extreme UV is developed using atomic cluster model calculations, revealing V 3+ /d 2 character of the vanadium center. We find that the insulator-to-metal phase transition occurs on a timescale of 26 ± 6 fs and leaves the system in a longlived excited state of the metallic phase, driven by a change in orbital occupation. Potential interpretations based on electronic screening effects and lattice dynamics are discussed. A Mott-Hubbard-type mechanism is favored, as the observed timescales and d 2 nature of the vanadium metal centers are inconsistent with a Peierls driving force. The findings provide a combined experimental and theoretical roadmap for using time-resolved extreme UV spectroscopy to investigate nonequilibrium dynamics in strongly correlated materials.he Coulomb interaction of charges in a solid depends sensitively on local screening, bonding structure, and orbital occupancy. These electronic correlation effects are known to manifest themselves in unusual properties, such as superconductivity, colossal magnetoresistance, and insulator-to-metal phase transitions (IMTs), which can be switched on or off via small perturbations. Understanding if and how these correlation-driven properties can be manipulated in real time will open the door to using these materials as ultrafast photonic switches and will establish the electronic speed limits for next-generation devices (1-3).Following the real-time dynamics of carrier interactions necessarily requires time-resolving the excitation and relaxation of electron correlations. Studies at longer timescales are complicated by simultaneous effects of structural distortion together with carrier screening and thermalization. In contrast, few-femtosecond extreme UV absorption spectroscopy (FXTAS) provides temporal resolution close to the Fourier limit for single-photon excitation with broadband visible light. It has the capability to separate electronic and structural effects on the basis of their intrinsic timescales and can isolate early-time electronic dynamics spectroscopically via atom-specific core-level electronic transitions (4, 5). Previous few-femtosecond and attosecond measurements of electron dynamics in solid-state systems have been restricted to simple band insulators or semiconductors and...

Vanadium dioxide (VO(2)) is a promising reconfigurable optical material and has long been a focus of condensed matter research owing to its distinctive semiconductor-to-metal phase transition (SMT), a feature that has stimulated recent development of thermally reconfigurable photonic, plasmonic, and metamaterial structures. Here, we integrate VO(2) onto silicon photonic devices and demonstrate all-optical switching and reconfiguration of ultra-compact broadband Si-VO(2) absorption modulators (L < 1 μm) and ring-resonators (R ~ λ(0)). Optically inducing the SMT in a small, ~0.275 μm(2), active area of polycrystalline VO(2) enables Si-VO(2) structures to achieve record values of absorption modulation, ~4 dB μm(-1), and intracavity phase modulation, ~π/5 rad μm(-1). This in turn yields large, tunable changes to resonant wavelength, |Δλ(SMT)| ~ 3 nm, approximately 60 times larger than Si-only control devices, and enables reconfigurable filtering and optical modulation in excess of 7 dB from modest Q-factor (~10(3)), high-bandwidth ring resonators (>100 GHz). All-optical integrated Si-VO(2) devices thus constitute platforms for reconfigurable photonics, bringing new opportunities to realize dynamic on-chip networks and ultrafast optical shutters and modulators.

Long regarded as a model system for studying insulator-to-metal phase transitions, the correlated electron material vanadium dioxide (VO 2 ) is now finding novel uses in device applications. Two of its most appealing aspects are its accessible transition temperature (∼341 K) and its rich phase diagram. Strain can be used to selectively stabilize different VO 2 insulating phases by tuning the competition between electron and lattice degrees of freedom. It can even break the mesoscopic spatial symmetry of the transition, leading to a quasiperiodic ordering of insulating and metallic nanodomains. Nanostructuring of strained VO 2 could potentially yield unique components for future devices. However, the most spectacular property of VO 2  its ultrafast transitionhas not yet been studied on the length scale of its phase heterogeneity. Here, we use ultrafast near-field microscopy in the mid-infrared to study individual, strained VO 2 nanobeams on the 10 nm scale. We reveal a previously unseen correlation between the local steady-state switching susceptibility and the local ultrafast response to below-threshold photoexcitation. These results suggest that it may be possible to tailor the local photoresponse of VO 2 using strain and thereby realize new types of ultrafast nano-optical devices. KEYWORDS: Near-field, femtosecond dynamics, VO 2 , NSOM, phase transition, correlated electrons T he insulator-to-metal phase transition in vanadium dioxide (VO 2 ) has been the subject of extensive investigation since its discovery in 1959 (ref 1). Interest has stemmed in part from its relatively simple, nonmagnetic structure 2 and its accessible transition temperature (T c ∼ 341 K), which makes it relevant for technological applications.3−6 Nevertheless, the enduring appeal of VO 2 can be traced to the complex interplay between electron and lattice degrees of freedom that produce its intricate free-energy landscape. 7−12 This fine balance between competing interactions can be tuned by strain, leading to a rich phase diagram.13,14 Below T c , unstrained VO 2 is an insulator characterized by both strong electron−electron correlations and lattice distortion, where the vanadium ions form chains of dimerized pairs (monoclinic structure, M1). These dimers are dissociated in the rutile (R), metallic phase (T > T c ) in a process reminiscent of a Peierls transition. However, cluster dynamical mean-field theory calculations have shown that both lattice distortion and strong on-site electron− electron Coulomb repulsion are necessary to accurately model the band gap.9 Moreover, tensile strain applied to the insulating state can produce new, stable lattice structures that are intermediates between M1 and R and, surprisingly, these states are also insulators. High tensile strain along the rutile c R axis (>2%, along the direction of the dimerized chains in the insulating state) induces the monoclinic insulating phase M2 in which only every second row of vanadium ions is dimerized. 15,16 Meanwhile, moderate tensile strain results in...