Arunas Kadys

Antibacterial and antifungal activity of photoactivated ZnO nanoparticles in suspension

Antibacterial activity of photoactivated zinc oxide nanoparticles (ZnO NPs) against human pathogens Escherichia coli O157:H7, Listeria monocytogenes ATCL3C 7644 and plant pathogen Botrytis cinerea was investigated. Data indicate that photoactivated (λ = 400 nm) ZnO NPs at concentration 1 × 10(-3)M and incubation time 60 min reduced population of both bacteria by 7 log (CFU/ml). Clear dependence of antimicrobial properties of ZnO NPs on used concentration and incubation time was found. Scanning electron microscopy (SEM) images of treated bacteria indicate that treatment induced cell wall disintegration and lysis. Results obtained on examination of antifungal activity of ZnO NPs reveal that significant photoinactivation (58%) of B. cinerea was observed at NPs concentration 5 × 10(-3)M and incubation time of 24h. SEM analysis confirmed that substantial morphological changes occur in the microfungus after treatment. The data suggest that ZnO NPs in the presence of visible light exhibit strong antibacterial and antifungal activity. Such ZnO NPs properties obviously could be used for the development of effective fungicides in agriculture or innovative physical antibacterial agents, so important in medicine and food microbial control.

Non-Equilibrium Carrier Diffusion and Recombination in Semi-Insulating PVT Grown Bulk 6H-SiC Crystals

We applied a non-degenerate four wave mixing (FWM) technique to investigate carrier generation, diffusion and recombination processes in PVT-grown semi-insulating wafers of 6H-SiC at 300 K. The resistivity of samples, cut from different places of a boule as well as from different boules, varied in range from a few ⋅cm up to 1010 ⋅cm. Interband excitation (at 355 nm) and below bandgap excitation (at 532 nm) allowed to study dynamics of the bipolar plasma and the contribution of deep levels to carrier generation and recombination. The nonequilibrium carrier lifetime was shorter in the samples of higher resistivity, in accordance with the increasing density of deep levels. The bipolar plasma diffusion in high-resistivity samples (~109 ⋅cm) provided the value of the diffusion coefficient D = 4.4 cm2/s and hole mobility μh = (88 ± 6) cm2/Vs.

On the Correlation of the Structural Perfection and Nonequilibrium Carrier Parameters in 3C SiC Heterostructures

Photoelectric properties of 3C sublimation-grown epitaxial layers with different structural quality were studied by using time-resolved picosecond transient grating and free carrier absorption techniques. The layer quality was described by a parameter LTW which gives the total length of twin boundaries in a layer. Optical measurements of diffusion coefficients and carrier lifetimes in wide excess carrier density (N >1018 cm-3) and temperature range (10 K to 300 K) revealed the twin defect density dependent ambipolar mobility value at RT as well as essentially different temperature dependences of mobility of the layers. The larger value of absorption cross section in more defective layer at 1064 nm wavelength pointed out to contribution of defect-assisted absorption, which gradually vanished after the filling defect states by free carriers.

All-optical analysis of carrier and spin relaxation in InGaAs∕GaAs saturable-absorber structures

Results of an all-optical analysis of basic semiconductor parameters such as carrier mobilities, lifetimes, and electron spin relaxation time of implanted In0.25Ga0.75As∕GaAs multiple quantum well saturable-absorber structures for the 1060nm spectral range are presented. These parameters are determined in a wide range of optical excitation, even at the practical operation point of such devices. This is accomplished by the application of polarization-resolved pump-probe and four-wave-mixing spectroscopies. The all-optical approach allows the determination of mobilities and spin relaxation time from the same experiments and points to the D’yakonov-Perel mechanism to govern the electron spin relaxation at room temperature.

Nonequilibrium Carrier Diffusion and Recombination in Heavily-Doped and Semi-Insulating Bulk HTCVD Grown 4H-SiC Crystals

We applied four-wave mixing (FWM) technique for investigation of high temperaturechemical vapour deposition (HTCVD) grown 4H-SiC samples with different doping levels. The determined minority electron and hole mobilities in heavily doped crystals at doping densities of 1019 cm-3 were found to be equal to 116 and 52 cm2/Vs. In semi-insulating (SI) crystals, the ambipolar diffusion coefficient Da = 2.6 − 3.3 cm2/s and carrier lifetimes of 1.5 – 2.5 ns have been measured. Irradiation of SI crystals by 6 MeV electrons resulted in essential decrease of carrier lifetime down to ~ 100 ps and clearly revealed the defect-assisted carrier generation with respect to two-photon interband transitions before irradiation.

Investigation of Thermal Properties of Heavily Doped 4H-SiC Crystals by a Picosecond Transient Grating Technique

We applied a picosecond dynamic grating technique for investigation of thermal diffusivity, sound velocity, thermo-optic and photoelastic coefficients in heavily doped 4H-SiC substrates. Spatial modulation of thermal properties was achieved by intraband carrier absorption at excitation by an IR picosecond laser pulse (1064 nm) and subsequent carrier thermalisation. Decay of dynamic grating at its various periods and sample temperatures was monitored at 532 nm wavelength. The thermal diffusivity DT = 0.94 cm2/s, thermo-optic coefficient dn/dT = 3.610–5 K-1 and effective photoelastic constant = – 0.10 were determined at 300 K. DT value increased at lower temperatures and saturated below 100 K. Temperature dependences of thermo-optic coefficient and thermal expansion were found in good correlation with heat capacity peculiarities.

Carrier transport and recombination in resonantly excited InGaAs/GaAs MQWs

We present nonlinear optical studies of the non‐equilibrium carrier dynamics in InGaAs/GaAs quantum well structures. We exploit the free‐carrier and electron spin‐governed nonlinearities in pump‐probe and four‐wave mixing experiments to measure the carrier lifetime, spin relaxation time, and in‐well mobilities of electrons and holes. A saturation of excitonic nonlinearity at 0.08 ± 0.02 mJ/cm2 was determined by four‐wave mixing.