We have measured the subpicosecond optical response of a solid-state, semiconductor-to-metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of VO2 thin films by making pump-probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties of the high-temperature metallic state being attained within 5 ps. The ultrafast change in complex refractive index enables ultrafast optical switching devices in VO2.
The photostability of dye molecules trapped in transparent solid matrices synthesized by the solgel technique was studied both experimentally and theoretically using a model with numerical and approximate analytical solutions. The model is based on a one-photon photodestruction process with the creation of an absorbing bleached molecule. We give the number of photons that different trapped dye molecules can absorb on average before they are bleached. Dyes such as Perylene Red, Perylene Orange, Pyrromethenes 567 and 597, Rhodamines 6G and B, DCM, a Xanthylium salt, and Neon Red were investigated; significant differences were observed. Some dye molecules in solvents were also studied; increased stability resulted when the molecules were trapped in solid matrices.
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