We directly trace the multi-THz conductivity of ${\mathrm{VO}}_2$ during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the electronic conductivity settles to a constant value already after one V-V oscillation cycle. Based on our findings, we propose a qualitative model for the nonthermal phase transition.

We investigate the dielectric properties of a thin $\mathrm{V}{\mathrm{O}}_2$ film in the terahertz frequency range in the vicinity of the semiconductor-metal phase transition. Phase-sensitive broadband spectroscopy in the frequency region below the phonon bands of $\mathrm{V}{\mathrm{O}}_2$ gives insight into the conductive properties of the film during the phase transition. We compare our experimental data with models proposed for the evolution of the phase transition. The experimental data show that the phase transition occurs via the gradual growth of metallic domains in the film, and that the dielectric properties of the film in the vicinity of the transition temperature must be described by effective-medium theory. The simultaneous measurement of both transmission and phase shift allows us to show that Maxwell-Garnett effective-medium theory, coupled with the Drude conductivity model, can account for the observed behavior, whereas the widely used Bruggeman effective-medium theory is not consistent with our findings. Our results show that even at temperatures significantly above the transition temperature the formation of a uniform metallic phase is not complete.

We have observed size effects in the structural phase transition of submicron vanadium dioxide precipitates in silica. The ${\mathrm{VO}}_2$ nanoprecipitates are produced by the stoichiometric coimplantation of vanadium and oxygen and subsequent thermal processing. The observed size dependence in the transition temperature and hysteresis loops of the semiconductor-to-metal phase transition in ${\mathrm{VO}}_2$ is described in terms of heterogeneous nucleation statistics with a phenomenological approach in which the density of nucleating defects is a power function of the driving force.

The optical and morphological characteristics of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases have been studied by correlating the temperature and sharpness of the transition with the processing parameters. Thermal annealing results in grain growth and improved crystallinity. Normally, larger crystallites show smaller hysteresis, as there is a greater probability of finding a nucleating defect in the larger volume. But at the same time, this improved crystal perfection, which accompanies the thermal annealing and grain growth, tends to a larger hysteresis, as there are fewer nucleating defects within the volume. We show that the width and shape of the hysteresis cycle are thus determined by the competing effects of crystallinity and grain size.

The ultrafast photoinduced insulator-metal transition in VO${}_2$ is studied at different temperatures and excitation fluences using multi-THz probe pulses. The spectrally resolved midinfrared response allows us to trace separately the dynamics of lattice and electronic degrees of freedom with a time resolution of 40 fs. The critical fluence of the optical pump pulse, which drives the system into a long-lived metallic state, is found to increase with decreasing temperature. Under all measurement conditions, we observe a modulation of the eigenfrequencies of the optical phonon modes induced by their anharmonic coupling to the coherent wave-packet motion of V-V dimers at 6.1 THz. Furthermore, we find a weak quadratic coupling of the electronic response to the coherent dimer oscillation resulting in a modulation of the electronic conductivity at twice the frequency of the wave-packet motion. The findings are discussed in the framework of a qualitative model based on an approximation of local photoexcitation of the vanadium dimers from the insulating state.