Ultrafast dynamics and nonlinear optical response of VO₂ films with thickness- and laser fluence-induced phase transition

Abstract In this research, we studied the ultrafast dynamics and phase transition of VO 2 films with different thicknesses. Using transient absorption spectroscopy, the two VO 2 films with thicknesses of 59.5 and 148 nm showed broadband excited state absorption from 450–750 nm, and the maximum absorption peak was at ∼450 nm. Under low excitation fluences, the VO 2 films exhibit three sequential dynamic processes with different time scales from femtoseconds (fs) to nanoseconds (ns). Depending on the film thickness and excitation fluence, a phase transition is observed in the 148 nm VO 2 film at a time scale of ∼21 ps when the excitation fluence reaches 1.27 mJ cm −2 . However, the 59.5 nm VO 2 film did not undergo a phase transition in this fluence. The femtosecond nonlinear optical properties of the two VO 2 films were also investigated via a Z-scan technique. The nonlinear absorption coefficients and nonlinear refractive coefficients of the VO 2 films at a wavelength of 515 nm are <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>β</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>148</mml:mn> <mml:mi mathvariant="normal"> nm</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">eff</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> = 1220 ± 100 cm GW −1 , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>β</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>59.5</mml:mn> <mml:mi mathvariant="normal"> nm</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">eff</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> = 1020 ± 90 cm GW −1 , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>148</mml:mn> <mml:mi mathvariant="normal"> nm</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">eff</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> = −4.3 ± 0.7 × 10 −3 cm 2 GW −1 , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>59.5</mml:mn> <mml:mi mathvariant="normal"> nm</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">eff</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> = −2.1 ± 0.5 × 10 −3 cm 2 GW −1 , respectively. Compared with the reported values in the near-infrared wavelengths, the nonlinear absorption coefficients exhibit an order of magnitude enhancement.