Photoinduced insulator-to-metal phase transition in $\mathrm{V}{\mathrm{O}}_2$ crystalline films and model of dielectric susceptibility

A light-induced insulator-to-metal ultrafast phase transition (PT) was observed in $\mathrm{V}{\mathrm{O}}_2$ thin films deposited on single-crystal sapphire and amorphous glass substrates. The PT and optical properties of $\mathrm{V}{\mathrm{O}}_2$ were characterized by transient reflection, absorption, and transient grating nonlinear optical measurements. $\mathrm{V}{\mathrm{O}}_2$ films deposited on crystalline substrates are spatially ordered and show noticeable anisotropy in optical response upon PT. Structural and optical properties are dependent on the shear plane of the sapphire substrate. Significant twinning was found for $\mathrm{V}{\mathrm{O}}_2$ deposited on a $\left(001\right){\mathrm{Al}}_2{\mathrm{O}}_3$ substrate while a $\mathrm{V}{\mathrm{O}}_2$ film on $\left(012\right){\mathrm{Al}}_2{\mathrm{O}}_3$ is highly textured, without detectable twinning. $\mathrm{V}{\mathrm{O}}_2$ dielectric susceptibility is described by a proposed model. The V-O-V charge transfer was considered as a major process contributing to the linear and nonlinear dielectric susceptibility. The PT time and relaxation dynamics were found to be dependent on the film morphology and concentration of structural defects.

Figure 1

The unit cell of $\mathrm{V}{\mathrm{O}}_2$ crystal. Dashed lines represent monoclinic cell. The tetragonal cell is shown by solid lines.

Figure 2

Diagram shows the cross section of tetragonal $\mathrm{V}{\mathrm{O}}_2$ lattice by the $\left(\overline{2}01\right)$ plane. Hatched and open circles represent vanadium and oxygen atoms, respectively. Solid and dashed lines show projection of oxygen octahedrons shifted on half of the lattice constant along the $c_t$ axis. The V-O-V charge transfer is shown by heavy arrows.

Figure 3

Experimental setup. $M$, mirror; OD, optical delay; BS, beam splitter; PD, photodetector.

Figure 4

X-ray diffraction patterns of $\mathrm{V}{\mathrm{O}}_2$ films deposited on crystalline ${\mathrm{Al}}_2{\mathrm{O}}_3$ and amorphous $\mathrm{Si}{\mathrm{O}}_2$ substrates.

Figure 5

X-ray diffraction intensity vs azimuthal orientation of the sample for (a) $\left(006\right){\mathrm{Al}}_2{\mathrm{O}}_3\left(R\right)$ and $\left(002\right)\mathrm{V}{\mathrm{O}}_2$ plane of $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(R\right)$ and (b) $\left(012\right){\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$ and $\left(011\right)\mathrm{V}{\mathrm{O}}_2$ plane of $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$.

Figure 6

Atomic force microscope images of $\mathrm{V}{\mathrm{O}}_2$ topography ($2\times 2\mu {\mathrm{m}}^2$ scan): (a) $\mathrm{V}{\mathrm{O}}_2∕\mathrm{Si}{\mathrm{O}}_2$, (b) $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(R\right)$, and (c) $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$.

Figure 7

Autocorrelation function of (a) $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(R\right)$ ($3\times 3\mu {\mathrm{m}}^2$ area) and (b) $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$ ($2\times 2\mu {\mathrm{m}}^2$ area). Arrows 1, 2, and 3 correspond to the substrate crystallographic directions $\left[100\right]{\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$, $\left[010\right]{\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$, and $\left[\overline{1}\overline{1}0\right]{\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$, respectively.

Figure 8

Normalized change of reflection in optical spectral range after light-induced (temporal dependence) and thermally induced (● symbols, static values) $I\text{−}M$ PT in $\mathrm{V}{\mathrm{O}}_2$ film.

Figure 9

Transmission spectra for $30\text{−}\mathrm{nm}$-thick $\mathrm{V}{\mathrm{O}}_2$ films for the low-$T$ insulator phase at $T=295\mathrm{K}$ and for the high-$T$ metallic phase at $T=410\mathrm{K}$.

Figure 10

Transient grating signal versus probe-pulse delay upon a light-induced insulator-to-metal phase transition. The inset shows a zoomed-in signal taken for the $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$ film. Pump beam crossing angle $\theta =9°$.

Figure 11

Transient grating signal versus probe-pulse delay at pump beam crossing angles $\theta =9°$ and $\theta =22.5°$ and grating periods $\Lambda =2.5\mu \mathrm{m}$ and $\Lambda =1\mu \mathrm{m}$, respectively, for $\mathrm{V}{\mathrm{O}}_2$ films on different substrates.

Figure 12

Diffraction efficiency for $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$ and $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(R\right)$ films vs azimuthal direction of light polarization. (a) Total normalized diffraction signal $\eta \left(\phi \right)$; solid line is a fit to ${\eta }_R\left(\phi \right)={\eta }_{0R}+{\eta }_{\Delta R}{\cos }^2\left(\phi \right)$. Anisotropic part of diffraction signal: (b) $\Delta {\eta }_C\left(\phi \right)$ for $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(C\right)$; dotted lines denote orientation of the sample related to the maximal diffraction efficiency. (c) $\Delta {\eta }_R\left(\phi \right)$ for $\mathrm{V}{\mathrm{O}}_2∕{\mathrm{Al}}_2{\mathrm{O}}_3\left(R\right)$.

Figure 13

Azimuthal dependence of the absorption coefficient variations during the $I\text{−}M$ PT for the 30- and $60\text{−}\mathrm{nm}$-thick $\mathrm{V}{\mathrm{O}}_2$ films deposited on an ${\mathrm{Al}}_2{\mathrm{O}}_3\left(R\right)$ substrate.