Photoinduced Phase Transitions by Time-Resolved Far-Infrared Spectroscopy in ${\mathrm{V}}_2{\mathrm{O}}_3$

Using time-resolved far-infrared spectroscopy, we observe multiple routes for photoinduced phase transitions in ${\mathrm{V}}_2{\mathrm{O}}_3$. This includes (i) a photothermal antiferromagnetic to paramagnetic transition and (ii) an incipient strain-generated paramagnetic metal to paramagnetic insulator transition, which manifests as coherent oscillations in the far-infrared conductivity. The $\sim 100\mathrm{ps}$ conductivity oscillation results from coherent acoustic phonon modulation of the bandwidth $W$. Our results indicate that poor metals are particularly amenable to coherent strain control of their electronic properties.

Figure 1

(a) Temperature dependence of far-infrared conductivity of ${\mathrm{V}}_2{\mathrm{O}}_3$ thin film measured by THz time domain spectroscopy. (b) Real part of the optical conductivity as a function of photon energy for various temperatures. The arrows indicate the spectral range for the pump and probe pulses for the far-infrared time-resolved experiments.

Figure 2

(a) THz conductivity dynamics as a function of delay time ($\tau$) at various temperatures. (b) Rise-time dynamics at 120 and 300 K. The black lines are experimental data and the red dashed lines fits to a single exponential.

Figure 3

(a) $\Delta {\sigma }_1$ as a function of pump-probe delay. QS labels the quasishear wave and SR the surface Rayleigh wave. (b) $\Delta R/R$ with 1.55 eV probe showing a one-to-one correspondence with $\Delta \sigma$. (c) Fit to the $\Delta {\sigma }_{\mathrm{osc}}$ at 300 K using the known frequencies of the QS and SR [27]. The inset is a fit to the initial rise-time dynamics which is subtracted from the oscillatory component.

Figure 4

(a) Summary of the dynamics indicated on the phase diagram of ${\mathrm{V}}_2{\mathrm{O}}_3$. Red arrows indicate that rapid electron-phonon relaxation drives ${\mathrm{V}}_2{\mathrm{O}}_3$ into the metallic state; blue arrows illustrate the subsequent oscillations correspond to dynamically tuning the strain environment. (b) One example of an optical-pump terahertz probe at an initial temperature of 143 K with a pump fluence $4\mathrm{mJ}/{\mathrm{cm}}^2$; the blue arrow indicates the conductivity deduction induced by the strain wave only. (c) Peak conductivity reduction [$\Delta {\sigma }_{\mathrm{osc}}(t)$ at 50 ps] versus pump fluence and final temperature.