Photoinduced semiconductor-metal phase transition in the surface layer of vanadium dioxide

The photoinduced semiconductor-metal phase transition occurring for a time Δt < 1 ps in the surface layer of vanadium dioxide is studied theoretically. A nonthermal mechanism of instability development is considered. An equation for the order parameter ξ of the photoinduced semiconductor-metal phase transition is derived. It is shown that the transition of the surface layer of VO₂ to the metallic state requires irradiation by a laser pulse whose energy density W exceeds a critical value W _c. The phase transition is initiated at the surface, after which the interface propagates deep into the sample. The critical energy density W _c, the velocity of propagation of the metal-semiconductor interface, the thickness z ₀, and the characteristic time Δt of formation of the metal layer are calculated. The theoretical results obtained are in agreement with the experimental data on irradiation of vanadium dioxide single crystals by high-intensity laser pulses.