Ultrafast Mid-Infrared Nanoscopy of Strained Vanadium Dioxide Nanobeams

Abstract

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...

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