Tunneling breakdown of a strongly correlated insulating state in $\mathrm{V}{\mathrm{O}}_2$ induced by intense multiterahertz excitation

We directly trace the near- and midinfrared transmission change of a $\mathrm{V}{\mathrm{O}}_2$ thin film during an ultrafast insulator-to-metal transition triggered by high-field multiterahertz transients. Nonthermal switching into a metastable metallic state is governed solely by the amplitude of the applied terahertz field. In contrast to resonant excitation below the threshold fluence, no signatures of excitonic self-trapping are observed. Our findings are consistent with the generation of spatially separated charge pairs and a cooperative transition into a delocalized metallic state by THz field-induced tunneling. The tunneling process is a condensed-matter analog of the Schwinger effect in nonlinear quantum electrodynamics. We find good agreement with the pair production formula by replacing the Compton wavelength with an electronic correlation length of 2.1 Å.

Figure 1

(a) Temperature dependence of the normalized transmittance through a 200-nm $\mathrm{V}{\mathrm{O}}_2$ film upon heating (red) and cooling (blue). (b) Optical microscope image of the 200-nm-thick $\mathrm{V}{\mathrm{O}}_2$ layer deposited on a diamond substrate with a diameter of 5 mm.

Figure 2

(a) Time-domain profiles of few-cycle multi-THz pump transients generated by 230 μm (red) and 1050 μm (blue) GaSe emitter crystals, respectively. (b) Amplitude spectra of the THz transients from panel (a). Blue graphs are vertically shifted for better visibility. (c) Relative transmission change $\mathrm{\Delta }T/T$ of the $\mathrm{V}{\mathrm{O}}_2$ film (orange) induced by a multi-THz transient (gray) with a peak electric field of 6.1 MV/cm and probed by the NIR pulse. The black solid line represents a fit to the steplike nonoscillating part of the pump-probe signal. Inset: Unpolarized Raman spectrum of the sample (blue) and Fourier transform of the oscillating part of $\mathrm{\Delta }T/T$ (red circles). (d) Oscillating part of the signal (red) fitted by a damped harmonic oscillation (blue) with a frequency of 5.9 THz.

Figure 3

(a) and (b) Relative transmission change $\mathrm{\Delta }T/T$ induced by multi-THz excitation at various peak electric fields and probed with an 8-fs-short NIR pulse. (a) Broadband excitation with peak electric fields ranging from 2.9 MV/cm (red) to 14.6 MV/cm (blue) and (b) narrowband excitation between 3.1 MV/cm (red) and 14.0 MV/cm (blue). The black solid line shows the transient change of multi-THz conductivity in arbitrary units after excitation with a 12-fs pulse at a wavelength of 800 nm and fluence of 3 mJ/cm taken from Ref. [28]. (c) $\mathrm{\Delta }T/T$ immediately after excitation as a function of the peak driving field for broadband (red circles) and narrowband (blue squares) pump transients. Inset: same data as a function of incident excitation fluence $F$. (d) Absolute change in the real part of optical conductivity $\mathrm{\Delta }{\sigma }_1$ at 1-ps pump-probe delay time measured by two-dimensional multi-THz spectroscopy for various driving peak fields (green triangles). The solid line shows a best fit to the data based on Eq. (2).

Figure 4

(a) Absorption coefficient of the 200-nm-thick $\mathrm{V}{\mathrm{O}}_2$ film. Arrows mark the frequency of infrared-active phonons. The dashed lines are interpolations in the frequency ranges affected by an absorption in the diamond substrate and a detector response. (b) Typical spectra of the multi-THz excitation transients used in our experiments. The graphs are shifted vertically for clarity. (c) Pump-probe signals for two different central frequencies of the multi-THz excitation pulse with a peak electric field of approximately 7 MV/cm. The red graph corresponds to the broadband spectrum centered at 20 THz which is fully resonant with the highest vibrational mode of the oxygen cage. The blue graph results from narrowband excitation centered at 30 THz, which is completely off resonant with respect to any vibrational modes.

Figure 5

Scenarios for NIR and multi-THz excitation. Red arrows denote photon energies of the excitation: (a) NIR excitation triggers a resonant interband transition. Excitonic charges are generated locally with respect to each other, resulting in neutral V-V dimers. (b) The low photon energy of the multi-THz transients keeps electrons from directly crossing the band gap. Instead, the high electric field tilts the potential, enabling the generation of spatially separated doublon-holon pairs by interband tunneling (orange dashed arrow). This process leads to charged V-V dimers and a suppression of excitonic self-trapping.