Using time- and angle-resolved hemispherical elastic light scattering technique we reveal complex pathways of photoinduced nonlinear optical dynamics in VO2, V2O3 and V3O5 thin films. The structural dynamics was monitored by using an ultrafast diffraction conoscopy technique. The evolution of phases in these correlated oxides is substantially different and significantly depends on optical excitation, temperature and size of grains and domains. Strong optical nonlinearity along with its complex transient dynamics makes vanadium oxides attractive for high-contrast all-optical switches, high-speed optical data storage and holographic devices. The characteristic time of optical nonlinearity can be tuned from several femtoseconds to picoseconds by altering the excitation fluence and size of grains and domains. Additional control of ultrafast phase transition dynamics can be achieved by photoacoustical generation of strain waves. Depending on material morphology and level of optical excitation, the optical signal shows coherent oscillations caused by photoacoustic wave at picosecond and nanosecond time scales. Complex nonlinear dynamics of correlated vanadium oxides can provide a way for precise tuning of transient optical and electronic properties in photonic devices.
A spectrograph is described which is made with dual Holographic Optical Elements (HOEs) which are identical and parallel to each other. Both optics are collimating transmission HOEs with focal points that are at equal and opposite distances from each other. The identical HOEs are formed by the interference of a plane wave parallel to the grating plane with an off-axis spherical wave originating in the near-field. In playback, a spectrum can be formed from a point source radiator placed at the position of the recording spherical wave. If played back at an arbitrary wavelength other than the recording wavelength, the image exhibits coma. This spectrograph is intended for an unusual configuration where many nearly monochromatic sources of known wavelengths are separately positioned relative to the first HOE. The special application is in a space telescope capable of resolving spectra from habitable planets within 10 pc. HOEs of this type could be fabricated on membrane substrates with a low areal mass and stowable on rolls for insertion into the second Lagrange point. The intended application is for a 50 x 10 meter class primary objective holographic space telescope with 50 x 10 m HOEs in the spectrograph. We present a computer model of the spectrograph.. Experimental results are compared with predictions from theory. A single HOE is shown to perform over a wider bandwidth and is demonstrated.
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.
The photocatalytic activity of different porous layers of anatase nanocomposites has been monitored by measuring changes of the optical response of Rhodamine 6G water solution under UV irradiation in the presence of anatase films. We have produced series of porous anatase films with different methods to investigate their photocatalytic activity and its dependence on the parameters of the layers. Integrated sphere transmission and reflection spectra (200 - 1000 nm) with 3D angular resolved light scattering data gave a possibility to estimate the correct intrinsic absorption coefficient, the dispersion of the refractive index, the porosity (32 - 39%) and the thickness (90 - 1340 nm) of the samples. We have estimated the energy of the optical band-gap for both direct (3.66 - 4 eV) and indirect (3.33 - 3.4 eV) transitions. It has been shown that the photocatalytic activity depends sensitively on the nanopores which is determined by the molecular weights of pore agents.
The polarization properties of the light leaving from the planar waveguide (WG) being a basis of a planar polarization interferometer (which now is the most sensitive optical instrument, utilized as biosensors) were investigated. The angular distribution of a light emerges SiO2-Si3N4-SiO2 silicon structure was studied. The measurement of a polarization degree of an outcoming light depending on angle of an propagation is carried out.
The peculiarities of interfacial structure of self-assembled layer of dodecanthiol on the gold surface determined by quasi-elastic light scattering were employed to probe the lateral distribution and preferred orientation of the thiol molecules within the monolayer. The analysis of Angle Resolved Scattering in the framework of theoretical description involving both scattering on surface roughness and nonuniformities of dielectric permittivity of the near- surface layer allowed to establish the presence of correlation in counterphase between distribution of the thickness of the thiol coating and the relief of the gold surface. The distribution of the scatters in the thiol film is similar to that in as-deposited film of gold that is evidence of the dominant role of the structure of the relief of gold film in formation of the thiol layers. The formation of thiol layer on surface of polycrystalline gold is a complex process including adsorption, local self-assembly and surface-induced distortion of organized structure primary in the region of surface cavities. This study confirmed the utility of the proposed approach based on the nondestructive analysis of organized interfacial layers for detection spatial distribution and chemical functionality of self-assembled monolayers in practical sensing devices.
Multifractal analysis is performed for description of the surface topography of thin polycrystalline gold film. The film was obtained by thermal evaporation in vacuum. Its structure was modified by annealing at different temperatures in the range 20 - 200 degree(s)C. All films were imaged by Atomic Force Microscopy. Image was analyzed as a collection of layers taken parallel to the mean surface. Fractal subsets with different scaling properties were described by multifractal divergence (e.g. the difference between maximal and minimal values of the f((alpha) ) spectrum). This allowed us to highlight the effect of the temperature of film annealing on the surface structure. We found that fractal diversity jumps down in the temperature range 130 divided by 140 degree(s)C. Therefore, phase transition occurs in the system. Below the temperature of the phase transition the surface topography is characterized by high roughness and existence of small-scale irregularities. The melting also results in a decrease of the surface roughness due to the flowing down of gold crystallites. We also illustrate that surface transformation under low-temperature annealing changes the power spectral density and probability distribution of height functions.
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