Probing the phase transition in ${\mathrm{VO}}_2$ using few-cycle 1.8 $\mu \mathrm{m}$ pulses

We observe a nearly instantaneous triggering of the phase transition in ${\mathrm{VO}}_2$ using transient, time-resolved absorption techniques from few-cycle, infrared (1.8 $\mu \mathrm{m}$) laser pulses. The results are in agreement with the Mott-Hubbard insulator model, characterized by electronic holon-doublon pair creation that initiates the insulator-to-metal phase transition within the material. The spectral resolution provided by this technique can be exploited to measure the chirp of the probe pulses. Effects from probing above and below the band gap of the material are also discussed.

Figure 1

XRD $\theta \text{−}2\theta$ pattern of the ${\mathrm{VO}}_2$ film deposited on an $r$-plane sapphire substrate. The inset shows the $\varphi$ scans of the off-axis (210) orientation tilted with respect to the (200) (blue curve) and $(-211)$ (red curve) orientations, respectively.

Figure 2

(a) A pump-probe setup is constructed using few-cycle laser pulses centered at 1.8 $\mu \mathrm{m}$. The probe is separated from the pump using the surface reflection from a thin wedge. The energy of the pump is controlled using a half-wave plate, polarizer energy throttle. The low-energy probe is transmitted through the sample at a variable delay from the pump and its transmitted spectra are recorded. The delay between the two pulses is controlled with a dc stepper motor actuator. (b) The SHG-FROG trace (left) and fitted pulse duration (right) of the compressed output of the fiber setup showing a $<15$ fs pulse duration. The black circles and arrows show the uncompressed frequencies that make up the temporal pedestal of the pulse. (c) The spectrogram from the Ocean Optics spectrometer pumped to the maximum change in transmission with a laser fluence of 80 mJ ${\mathrm{cm}}^{-2}$. (d) The recorded percent change in transmission with the transition time (TT) given by the fitting algorithm found to be 24 fs FWHM.

Figure 3

A 100 nm epitaxially grown ${\mathrm{VO}}_2$ sample pumped with 80 mJ ${\mathrm{cm}}^{-2}$. (a) This shows the time- and frequency-resolved change in transmission given by Eq. (1). The sum between the colored lines is fitted and plotted below. (b) Each color curve corresponds to the wavelength range between the same colored lines in the top graph. The $\times$'s show the raw processed data given by Eq. (1), and the solid lines are the fit given by Eq. (2).

Figure 4

Change in transmission for the epitaxial ${\mathrm{VO}}_2$ sample. (a) Wavelength-resolved change in transmission for the 100-nm-thick epitaxially grown ${\mathrm{VO}}_2$ film, with a saturation point of $\sim 40\%$ transmission. The dotted vertical line corresponds to the 0.683 eV (1815 nm) band gap of the sample. (b) This shows the given $\eta$ for above and below the band gap (denoted by the dashed black line) fitted from the log of the change in transmission as a function of the log of laser fluence.