Transient reflection spectroscopy and degenerate four wave mixing technique were employed to study the light-induced insulator-to-metal phase transition (PT) and ultrafast relaxation dynamics in VO2. Spectral reflectivity during light- and thermally-induced PT shows close proximity in the relative change. The relaxation dynamics is strongly dependent on the film morphology, laser pump energy and substrate material. After light-induced PT the recovery time demonstrates a near-exponential dependence on the pump power. The recovery owing to cooling is considerably faster for VO2 films deposited on single-crystal Al2O3 or MgO substrates compare to VO2 on amorphous glass. The noticeable transient nonlinear optical response of metallic VO2 was observed and interpreted in terms of electronic-polaron and hole-polaron clustering.
The nonequilibrium carrier dynamics in spherical silver nanoparticles embedded in aluminophosphate glass system was explored by femtosecond optical pump-probe technique. Photoluminescence and absorption spectroscopy were used for characterization of linear optical properties and particle size estimation. The two temperature model is employed to study the hot electron subsystem and evolution of electronic and lattice temperatures. The electron scattering dynamics on the 10-13-10-12 sec scale and two-photon absorption process are discussed. The laser-induced coherent vibrations of silver nanoparticles were observed in transient transmission experiments for relatively large particles with radii ~35 nm.
Optical quality porous silicon (PSi) and nanoparticle crystalline-Si (c-Si) enriched SiO2 materials were studied by absorption; luminescence; time-resolved emission measurements. Using tunable ultrafast laser as a light source, the excited state dynamics was also investigated. In a nonlinear optical (NLO) spectroscopic study the pulsed degenerate-four-wave-mixing (DFWM) technique was employed. In absorption measurement all samples show a broad and intense UV absorption with a cut-off edge at ~400 nm. In photoluminescence (PL) measurement, red emission can generally be observed in both Psi and c-Si samples. At 8 K the broadband PL of PSi is peaked at ~670 nm, with the PL lifetime in microseconds regime. The band maximum shifts toward low energy side while excitation wavelength increases. The particle size distribution was estimated using the optical transmission data according to the size- energy gap relationship. DFWM measurement reveals a long lived, slowly decaying signal which emerges from the coherent response. It indicates that the excitation was highly localized. For c-Si sample, the luminescence maximum was found at higher energy side around 590 nm, but the emission lifetime is much shortened to as ~10ns. In order to understand the nature of nanocomposite, Eu dopant was introduced into the sample. The investigation of luminescence and time-resolved emission of Eu3+ shows that the c-Si nanoparticles are distributed in SiO2 matrix with an average size of the nanoparticles being around 2.2 nm. The valence band (VB) to conduction band (CB) energy gap is about 2.7 ev. By ultrashort (femtoseconds and picoseconds) laser pulse excitation the charge carriers produced in CB of c-Si nanoparticles contributes to the observed optical responses. The excited state dynamical process associated with the movement of charge carriers is characterized by an instantaneous response signal, followed by buildup of the slowly decaying signal within 500ps. The analysis of the charge creation, trapping and reactivation is discussed.
Eu3+-doped crystalline Si-enriched SiO2 nanocomposite thin films were prepared using Ar sputtering deposition on quartz substrates. By conventional laser spectroscopy the material was characterized in either frequency or time domain. The results show that the doped europium ions are present in trivalent state, as Eu3+. They are distributed in SiO2 matrix and on the boundary surface of c-Si nanoparticles. With increased excitation intensity at 532nm, two-photon absorption (TPA) induced new emission was observed. It is characterized by an additional broadband emission with a peak at 560nm and lifetime of ~ 0.8 s. This feature has been identified as the emission from Eu2+ ions. Further measurements reveal that the observed phenomenon originates from charge transfer process giving rise to a photoinduced transient valence switching from Eu3+ to Eu2+ in this particular material. Free carriers were originally created in the conduction band (CB) of c-Si nanoparticles by TPA, then trapped at the surrounding Eu3+ center due to strong Coulomb interaction. Luminescence of the formed divalent Eu2+ is characterized by a broadband d → f transition with fast decay rate. Degenerate four-wave-mixing experiment further revealed that in undoped sample TPA-created charge carriers in CB were trapped in shallow centers of Si nanoparticles. The trapping has an average time period of 500ps, and the carriers were then released for recombination. In Eu3+ -doped sample, however, the average time period of 500ps was no longer observable. It therefore suggests that the strong Coulomb attraction results in immediately capturing of the created charges by positive Eu3+ center.
Strontium barium niobate thin films were grown by PLD and characterized as to their structure, composition, and both linear and nonlinear optical properties. Attempted composition of very nearly x equals 0.61 was achieved, as shown within experimental error of RBS and PIXE techniques. Films were deposited on MgO and fused silica substrates at a range of growth rates, while keeping other factors constant. Films with excellent texture, and oriented with the c-axis normal to the substrate surface where obtained on the MgO substrates. Films grown on fused silica showed a range of structures, from essentially amorphous to well oriented, again with c-axis normal to the substrate surface. The absorption edge of the films was determined to be substantially blue-shifted in comparison with bulk material. This effect appears to correlate with film microstructure, with more disordered films grown on glass showing the largest shifts. Degenerate four-wave-mixing techniques were used to study the nonlinear optical response of the amorphous films. A considerable enhancement, by 2 orders of magnitude, of the third order nonlinear susceptibility (Chi) (3) in transverse alignment was found to occur with respect to bulk values.
Using degenerate four-wave-mixing technique the acoustic wave produced in transparent microstructured optical materials can be determined. The light source used in this paper was a mode-locked, Q-switched Nd:YAG laser operated at 532 nm with pulse width of approximately 20 ps. The laser beam was split into three pulses. By spatially overlapping inside the sample with an angle in a backward propagating scheme. A laser induced periodic interference pattern which serves as grating was formed in the sample. The third, `probe', beam incident on the region in the direction satisfied by the Bragg condition. The diffracted signal brings the information of the produced acoustic wave. The incident laser pulses coupled to the acoustic field of the material through effects of thermally induced acoustic strain and electrostrictive coupling. This results in a generation of two counterpropagating ultrasonic acoustic waves in the grating wave vector direction, which is so- called the Laser Induced Phonon Spectroscopy.
Fluorescence and time-resolved degenerate-four-wave-mixing (DFWM) techniques have been used to characterize the optical properties of KNbO3/KTaO3 superlattice grown by pulsed laser deposition on a KTaO3 substrate. The superlattice consisted of 8 layers with thicknesses of 40 nm (KTaO3) and 30 nm (KNbO3). In the fluorescence measurement a significant change of emission profile with the KNbO3 thin films deposited on the substrate has been observed. In DFWM measurements the time-resolved spectrum is characterized by a sharp coherent response signal peaked at the time of zero delay followed by a build-up of a longer- lived signal. The third-order susceptibility (chi) (3) was estimated to be dramatically increased by 2 orders of magnitude.
Time-resolved degenerate-four-wave-mixing (DFWM) techniques have been used to characterize the nonlinear optical response of a KNbO3/KTaO3 superlattice grown by pulsed laser deposition on a 1 mm thick, (001)-oriented KTaO3 substrate. With a 30 psec pulsed laser, the difference in the nonlinear optical response between bulk KTaO3 and the superlattice was measured. Results indicate that a significant contribution to the response signal is due to the KNbO3 superlattice. The (chi) (3) value was estimated to have increased by 2 orders of magnitude compared to the bulk crystal.
In this paper the results on high resolution spontaneous Raman spectroscopy of barium nitrate molecular ionic crystal at temperature from 8 to 600 K are presented. The temperature dependencies of the linewidth broadening and frequency shifts of internal Raman modes are investigated. The proposed mechanisms of vibronic relaxation of internal Raman modes exhibit low degree of phonon-phonon coupling in the medium. The phonon frequency shifts are found to be due to lattice expansion and are discussed in terms of Gruneisen parameter theory.
Using picosecond laser operated at 532 nm, the observed time-resolved degenerate-four-wave-mixing (DFWM) spectra of nonether polyphenylquinoxaline (PPQ) solution show significant change in optical nonlinearity with changing solvents. For PPQ in cresol the DFWM spectrum obtained is composed of an instantaneous coherent optical response signal associated with the third-order susceptibility and a rapidly damped, laser-induced acoustic phonon signal. The lifetime of the generated acoustic phonon was estimated to be 1.5 ns. For PPQ in chloroform, an anomalously enhanced phonon signal overwhelms the intense coherent component. The generated acoustic phonon in PPQ-chloroform is much longer- lived. The mechanism of the observed optical response is discussed and the (chi) 3 value is calculated through resolving the coherent peak from the intense phonon signal.
The nonlinear optical response of photorefractive oxide materials was investigated using four- wave-mixing (FWM) techniques with laser pulses having durations in the pico- and sub- picosecond range. The specific materials studied are KNbO3, KTaO3, KTa1-XNbXO3 (KTN), SrXBa1-XNb2O6, and Bi2TeO5. In each case there are several different physical processes that contribute to the nonlinear signals and their origins are analyzed in the context of time intervals within and after the cross-correlation time of the two write beams pulses. The role of the nonlinear absorption in the four-wave mixing (FWM) processes is compared for the different materials. Variation in the build-up of the electro-optic photorefractive effect is discussed.
Bismuth tellurite (Bi2TeO5) is a new photorefractive material that is now available as single crystals of high optical quality. The results of measuring the photorefractive properties of Bi2TeO5 crystals are reported and the properties of this material are compared to those of well known photorefractive crystals. The decay of the photorefractive signal in Bi2TeO5 has multiple components and several different contributions to this signal were identified. One of the components has a lifetime of months and therefore has potential for holographic memory applications. This long-lived photorefractive signal is attributed to ion displacements and it can be produced by both continuous wave and short pulse write beams. Several experimental conditions were investigated to enhance this photorefractive signal.