Search

Thin films of strontium barium niobate SrxBa1−xNb2O6 (SBN) were successfully fabricated on fused quartz substrates using pulsed laser deposition (PLD) technique. The optical and non-linear optical properties were characterized. High-resolution spectroscopic measurements in either time domain or frequency domain were conducted. For undoped SBN thin films, a broad-band emission at UV and extending to the visible was observed by excitation at 355 nm. It is attributed to the exciton luminescence of the SBN host in the film. In degenerate four-wave-mixing (DFWM) measurements, a considerable enhancement of the third order susceptibility χ(3) by 2 orders of magnitude, in transverse alignment was obtained. High-resolution spectroscopic measurements in frequency domain were conducted at the temperatures varying from 270 K down to 2.3 K. The very well resolved fine structure of 5D0–7F multiplet emission was observed in the annealed sample. The hole-burning experiment shows a hole of width 100 MHz with depth as high as 30% with the laser pumping at 5774 A˚. It is suggested that Eu3+ ions may substitute Nb, occupying 6-fold sites. [Copyright &y& Elsevier]

Abstract: Laser-induced ultrafast nonlinear optical (NLO) response was observed in transient absorption, reflection and NLO measurements in VO2 thin films. The films were prepared using pulsed laser deposition technique with metallic vanadium and V2O5 powder employed as target material. The obtained thin films exhibit a thermal, as well as a laser-induced phase transition (PT) from semiconductor state at room temperature to metallic state. A typical hysterisis was observed for electric resistivity and optical reflectance verses temperature T. For the laser-induced PT in femtosecond pump–probe experiment, the PT was found to be ultrafast, immediately upon laser excitation. Kinetics study suggests that the PT is realized via an intermediate state, presumably a Wannier–Mott exciton (WME) state, followed by a resonant transition to the metallic phase state. The presence of the intermediate state is responsible for an extremely large third-order susceptibility observed in a nonlinear holographic experiment. In a 80nm thick VO2 thin amorphous film, the value of χ (3) was measured to be 1.3×10−8 esu which is four orders of magnitude greater than a standard reference sample of CS2. In addition, the nonlinear response was further enhanced with diffraction signal increased by a factor of 4 after the laser excitation. A signal dip at ∼1.2ns was repeatedly observed, which is presumably due to the excitation process associated with the intermediate state. Finally, the resonant transition from the intermediate state to the metallic state, giving rise to a maximum diffraction signal intensity from the formed transient grating. [Copyright &y& Elsevier]

Abstract: As a kind of phase transition functional material, Vanadium dioxide (VO2) thin films deposited on fused quartz substrate were fabricated using pulsed laser deposition (PLD) technique. Europium was introduced for structure study. By laser excitation at 526nm, VO2 thin film undergoes a reversible and ultrafast phase transition from semiconductor to metallic state, which results in a change of optical properties. In fluorescence measurement, Eu emission was found severely quenched in all as-grown thin films. After annealing the sample in air, a red Eu-emission appeared. The emission spectrum is characterized by a pronounced twin peak, centered at 617nm (5D0–7F2), surrounded by a set of broad, but relatively weaker bands (emission from 5D0 to 7F j manifold). The emission lifetime increased when the sample annealed at higher temperature for longer time. Each spectral component is actually a doublet which is the spectral overlap of emissions from Eu3+ situated in two sites with different configurations. One is a linear h-Eu3+-h, where h stands for holes. Another is a right-angle configuration of h-Eu3+-h with Eu3+ in the corner. In as-grown VO2 film, Eu3+ ions can either substitute V4+, leaving a negative charge around (Eu3+–O)–, or substitute V5+, leaving two negative charges around (Eu3+–O)−−. Due to trapped electrons in a large radius state, it covers Eu3+V4+–V5+ complexes. It suggests that the screening by degenerate electronic gas may result in switching off the Eu-related optical response for a wide spectral region, causing emission quenching in VO2 films. [Copyright &y& Elsevier]

VO2 thin films deposited on fused-quartz substrates were prepared by a pulsed laser deposition technique. Vanadium dioxide shows an ultrafast, passive phase transition (PT) from a monoclinic semiconductor phase to a metallic tetragonal rutile structure when the sample temperature is above 68°C. The fast PT can also be optically induced by laser excitation. In this paper, we report the optical properties of prepared thin films by absorption and photoluminescence (PL) measurements. In addition, a degenerate-four-wave-mixing measurement was also conducted using a 30 ps YAG pulsed laser operated at 532 nm. It is the first time, to our knowledge, that a large polarizability and relatively slow nanosecond excited state dynamical processes have been identified in VO2. The polarizability is found to be ∼300 times greater than the χ(3) value of a CS2 reference. The mechanism for slow process is believed to be associated with the structural change. [Copyright &y& Elsevier]

Optical quality SrxBa1−xNb2O6 (SBN) thin films doped with Pr3+ ions were successfully grown on fused quartz substrates using a pulsed laser deposition technique. Both as-grown and annealed samples were studied at either room temperature or at 10 K. Luminescence measurements show that most 4f2→4f2 emissions from Pr3+ centers were efficiently quenched in the films. Strong red emission from 1D2 to 3H4 at ∼610 nm accompanied by weak emission to other 3Hj multiplets could be observed only by UV excitation into the SBN conduction band. At 10 K, the main emission peak splits into four components. In addition, a broadband SBN-host luminescence was identified at ∼560 nm in the annealed sample, associated with the charge-transfer vibronic exciton. As an interstitial ion, Pr3+ may occupy a 15-, 12-, 9- or 6-fold cage surrounded by oxygen and was found to occupy all four possible sites in SBN thin films. This behavior is different from Eu3+- SBN where Eu3+ only occupies a 6-fold site. [Copyright &y& Elsevier]