A. Halabica

Ultrafast insulator-metal phase transition in VO 2 studied by multiterahertz spectroscopy

The ultrafast photoinduced insulator-metal transition in VO2 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.

Transmission increase upon switching of VO2 thin films on microstructured surfaces

The authors compare transmission measurements of near-infrared light through VO2 thin films on smooth substrates and on ordered arrays of silica microspheres. When the samples are heated above the critical temperature for the semiconductor-metallic phase transition, smooth thin films show reduced transmission independent of thickness; however, the VO2 film deposited on the microspheres may show either reduced or enhanced transmission, depending on VO2 film thickness. They show that this at a first glance, unexpected behavior is directly related to the change of scattering efficiency upon the phase transition. This suggests that optical transmission through thin-film microstructures could be tuned by an appropriate combination of microsphere ordering and VO2 film thickness.

Pulsed infrared laser annealing of gold nanoparticles embedded in a silica matrix

Pulsed infrared laser irradiation was used to modify the optical and physical properties of gold nanoparticles in a SiO2 substrate. The primary laser wavelength used for annealing was 8 μm, where the primary absorption was in the substrate. The optical absorption spectra exhibit a typical surface plasmon peak at 520 nm, which increases with laser fluence. This implies growth of the gold nanoparticles during laser annealing but the size distribution remains narrow, unlike some thermal annealing procedures that produce bimodal size distributions. The results were confirmed by scanning transmission electron microscopy and were consistent with the calculations of expected extinction spectra.

Deposition of polymer barrier materials by resonant infrared pulsed laser ablation

We describe resonant infrared pulsed laser deposition (RIR-PLD) of cyclic olefin copolymer, a barrier and protective layer; for comparison, we describe RIR-PLD of polystyrene and poly(ethylene dioxythiophene) about which we already have significant knowledge. Film deposition based on resonant infrared laser ablation is a low-temperature process leading to evaporation and deposition of intact molecules. In this paper, we focus on deposition of this model barrier and protective material that is potentially useful in the fabrication of organic light emitting diodes. The films were characterized by scanning electron microscopy and Fourier-transform infrared spectroscopy. We also compared the properties of films deposited by a free electron laser and a picosecond optical parametric oscillator.

Ultrafast Conductivity and Lattice Dynamics of Insulator-Metal Phase Transition in VO2 Studied via Multi-Terahertz Spectroscopy

The mid-infrared ultrafast response of VO2 thin films following 10 fs interband excitation is investigated as a function of sample temperature and laser fluence. The dynamics of the terahertz conductivity provides an insight into the microscopic details of the phase transition.

THz Slow Motion of an Ultrafast Insulator-Metal Transition in VO2: Coherent Structural Dynamics and Electronic Correlations

The multi-THz conductivity of VO2 recorded during a photoinduced insulator-metal transition directly reveals the femtosecond dynamics of V-V stretching modes and electronic correlations. We suggest a novel qualitative model for the nonthermal phase transition.

Modification of noble metal nanoparticles in a silica matrix by pulsed tunable infrared laser irradiation

We have used pulsed tunable infrared laser irradiation to modify the optical and physical properties of metal nanoparticles in a SiO2 substrate. The nanoparticles were fabricated by implanting high-energy Au+ or low-energy Ag+ ions at a dose of 6.1016 ions/cm2. The substrate temperature was held at 400 oC during implantation. The depth of the nanoparticles was well within the 1/e absorption length of the SiO2 substrate at our primary laser wavelength of 8 &mgr;m. The infrared laser beam generated by a picosecond free electron laser (FEL) was scanned across the implanted surface at various fluences. The optical absorption spectra of the gold implanted sample show that the absorption maximum at 520 nm, which is related to the presence of gold colloids, increases with laser fluence. On the other hand, the absorption maximum at 415 nm in the spectra of the silver- implanted sample decreases with increasing laser fluence and shifts to slightly lower wavelengths. In both cases a visible change in the color of the sample is observed, a clear indication of changes in the size distribution of the nanoparticles. Previous experiments used nanosecond excimer lasers that directly interact with the nanoparticles to modify their size and size distribution in different matrices. Our successful modifcation of the nanoparticles by excitation of the matrix vibrational modes, rather then melting of the nanoparticles, shows another possible approach to the processing of nanocomposite optical materials.