Ultrafast insulator-metal phase transition in VO2studied by multiterahertz spectroscopy

Abstract

The ultrafast photoinduced insulator-metal transition in VO 2 is studied at different temperatures and excitation fluences using multi-THz probe pulses. The spectrally resolved midinfrared response allows us to trace separately the dynamics of lattice and electronic degrees of freedom with a time resolution of 40 fs. The critical fluence of the optical pump pulse, which drives the system into a long-lived metallic state, is found to increase with decreasing temperature. Under all measurement conditions, we observe a modulation of the eigenfrequencies of the optical phonon modes induced by their anharmonic coupling to the coherent wave-packet motion of V-V dimers at 6.1 THz. Furthermore, we find a weak quadratic coupling of the electronic response to the coherent dimer oscillation resulting in a modulation of the electronic conductivity at twice the frequency of the wave-packet motion. The findings are discussed in the framework of a qualitative model based on an approximation of local photoexcitation of the vanadium dimers from the insulating state.

Thermochromic VO2 nanorods made by sputter deposition: Growth conditions and optical modeling

“…16 The metal-insulator transition can be influenced by a variety of external parameters, and recent work have considered the effects of strain [17][18][19] and lattice misfit with the substrate, 20,21 electric field or current activation, [22][23][24][25][26][27][28][29][30][31] light irradiation, [32][33][34][35][36][37][38] ion bombardment, 39,40 and gaseous ambience. 41 Considering the wealth of possibilities to modulate the properties of VO 2 -based thin films, it is not surprising that they are discussed for a vast number of applications such as thermochromic "smart" windows for energy efficient buildings, [42][43][44][45][46][47][48][49][50] variable-thermal-emittance surfaces for thermal radiation control, 51 oscillators and switches in (opto)electronics, 25,26,34,52,53 transistors of different types, 30,54,55 microcantilevers, 56,...…”

Tracking the evolution of electronic and structural properties of VO2during the ultrafast photoinduced insulator-metal transition

“…In the linear absorption regime, this fluence corresponds to exciting approximately 1 in 10 vanadium ions, and the total deposited energy is similar to the thermal energy required to drive the phase transition in equilibrium conditions. It has also been shown that, for excitation above the optical band gap, the threshold is independent of the pump wavelength 16 and only depends on the absorbed energy density and, as expected, the threshold value is reduced when the initial temperature of the sample is increased 7,9 . In addition, it has been observed that there are three distinct regimes for the photoinduced transition.…”

“…In these time-resolved experiments, a strong pump pulse excites the system and a second pulse probes a specific property as a function of time delay. To date, the time-dependent change in the optical reflectivity and conductivity [7][8][9] as well as the electronic 10,11 and lattice [12][13][14][15] changes during the dynamic IM transition have been measured. To drive the insulator-metal transition, an incident threshold fluence, F TH , of approximately 7 mJ cm −2 has been reported when the sample is excited with 800 nm laser light at room temperature.…”

VO2: Orbital competition, magnetism, and phase stability

“…[12][13][14][15] Much previous effort has been focused on understanding the physics behind the MIT, especially the role electron-electron correlation and electron-lattice coupling play in driving the simultaneous structural and electronic phase transition. [14,[16][17][18][19][20] Whether the insulating M 1…”