We present a comprehensive study of photoexcited carrier dynamics in differently grown InGaN/InGaN multiple quantum well (MQW) structures, modified by insertion of a wide interlayer structure and subsequent growth of differently shaped quantum wells (rectangular, triangular, trapezoidal). This approach of strain management allowed the reduction of dislocation density due to gradually increasing In content in the interlayer and shaping the smooth quantum well/barrier interfaces. A set of c-oriented MQW structures emitting at 470 nm were grown at Vilnius University, Institute of Applied Research, using a closed coupled showerhead type MOCVD reactor. Photoluminescence (PL) spectra of MQW structures were analysed combining continuous wave and pulsed PL measurements. Reactive ion etching of the structures enabled discrimination of PL signals originated in the InGaN interlayer structure, underlying quantum wells, and quantum barriers, thus providing growth-related conditions for enhanced carrier localization in the wells. Time-resolved PL and differential transmission kinetics provided carrier lifetimes and their spectral distribution, being the longest in triangular-shape QWs which exhibited the highest PL intensity. The light-induced transient grating (LITG) technique was used to determine the spatially averaged carrier lifetime in the entire heterostructure, in this way unravelling the electronic quality of the LED internal structure at conditions similar to device performance. LITG decay rates at low and high excitation energy densities revealed increasing with photoexcitation nonradiative recombination rate in the triangular and trapezoidal wells.

We applied time-resolved nonlinear optical techniques for investigation of carrier recombination and diffusion processes in a 420m thick CVD diamond, relatively free from structural and point defects. Injection of 10(15)-10(17)cm(-3) carrier density by two-photon excitation and monitoring free carrier absorption decay in 80-800K range provided lifetime of 720ns at T300K. At low temperatures, a linear increase of the carrier recombination rate with the injection level was fitted by an effective bimolecular recombination coefficient B=10(-11)-10(-9)cm(3)s(-1) and attributed to the impact of excitons and biexcitons, contributing to phonon-assisted indirect Auger recombination. At high temperatures, increase of recombination rate was governed by Coulomb enhanced direct Auger process with a 520meV threshold. An ambipolar carrier mobility with its peak value of approximate to 1500cm(2)V(-1)s(-1) at room temperature was determined by transient grating technique at approximate to 2x10(15)cm(-3) excess carrier density. A strong decrease of mobility at higher injections revealed the role of carrier-carrier scattering, exciton formation and bandgap renormalization. The simultaneous measurements of carrier lifetime and diffusivity provided carrier diffusion length, which varied in a 1-50m range, depending on injection and temperature.

Aims:  The aim of this study was to evaluate the inactivation efficiency of Listeria monocytogenes ATCL3C 7644 and Salmonella enterica serovar Typhimurium strain DS88 by combined treatment of hypericin (Hyp)‐based photosensitization and high power pulsed light (HPPL).

We report investigation of carrier recombination and diffusivity in bulk diamonds of different crystalline structure microcrystalline (MC) CVD-grown and single crystalline HPHT diamonds. Presence of neutral and positively charged nitrogen and hydrogen defects was determined from NIR and UVIR absorption spectra. Carrier injection into 1-mm thick bulk layers was realized by two-photon absorption at 351?nm wavelength. Carrier lifetimes of 150-330?ns in IIa type HPHT crystals correlated with N density, while the lifetimes in CVD crystal exhibited very fast (80?ps), slower one (38?ns), and mu s-duration thermally-activated (similar to 1.5?eV) decay components. The initial two components correlated with the grain size at the front and backsides of the MC diamond. Linearly increasing with injection carrier recombination rates were observed in both CVD and HPHT samples at 800?K, and fitted with effective recombination coefficient B?=?34 x 10-9?cm3/s. Ambipolar mobility and thermal diffusivity parameters in CVD and HPHT bulk crystals were measured by light-induced free carrier and thermal grating techniques.

We demonstrate applicability of time‐resolved free‐carrier absorption and transient grating techniques for investigation of carrier recombination and diffusion features in a bulk diamond. Carrier injection into a 1 mm thick, IIa type high‐pressure high‐temperature grown layer was realized by two‐photon absorption of ∼5 ps laser pulse at 351 nm wavelength. Kinetics of differential transmission in 80–800 K range at various excess carrier densities provided carrier lifetimes of 360 ns at room temperature and their temperature dependences. A linear increase of carrier recombination rate with injection in 450–800 K range resulted in carrier lifetimes up to 1 ns and was fitted by effective coefficient B* = 2 × 10−11– 4 × 10−9 cm3/s. The latter process was attributed to a trap‐assisted Auger recombination (TAAR) with coefficient CTAAR = B*/NTrap and tentatively ascribed to nitrogen related traps. An ambipolar carrier mobility with its peak value of ∼1500 cm2/Vs at room temperature was measured by transient grating technique at ∼1.5 × 1015 cm−3 excess carrier density.

Diamond is fairly recognized as a material of 21 century due to unchallenged combination of physical parameters, chemical and radiation resistance, biological inertness and so on. Unprecedented development of CVD – technology has increased essentially an interest to the material, especially in view of promising potentials of diamond application in microelectronics, optoelectronics, sensors, optics and laser physics . The problems of optical diagnostics and physical interpretation of thermal and electronic parameters of CVD-diamond as being a parameters of significance from point of view of practical application are solved in this project. Two teams of scientists from the Institute of Applied Research (Vilnius) and Institute of Physics, Academy of Sciences (Minsk) are involved in the project. New effective methods of measurement and special scientific equipment based on up-todate lasers and detectors of optical signals have been developed. The measuring techniques are of contactless nature, provide high temporal resolution and are currently realized in system OPTOPICOTEST (Minsk) and module HOLO-3 (Vilnius) . The current project is a consequence and further evaluation of early implemented joint investigations in the same field via two-sided scientific and technical cooperation in 20022005, as well as through the grant CBP.EAP.EV 982483 (2007 г.) in the framework of NATO program of scientific and technical cooperation. Investigations currently conducted are aimed to clarify physical factors that affect the free charge carrier (FCC) dynamics as well as the processes of heat transfer in CVD (polycrystals) and HPHT (synthetic monocrystals) diamonds of different structure and impurities content.. Two-photon absorption in diamond at 351 nm is proposed for FCC excitation. It enabled to carry out the volumetric excitation of samples and to measure the recombination time and diffusion coefficient within wide intervals of FCC density (10 310 cm) and sample temperature (80-800К).

We applied time-resolved free carrier absorption (FCA) to monitor non-equilibrium carrier dynamics in 4H epilayers and 3C SiC bulk crystals at excess carrier densities in the N = 1017 - 1019 cm-3 range. The numerical fitting of FCA decay kinetics provided the linear and nonlinear carrier recombination rates in the 40-390 K range and the absorption cross-sections eh at 1064 nm. In 4H, the decrease of the bulk lifetime (800 ns) with excitation provided the bimolecular and Auger coefficients B=(1.2±0.4)×10-12 cm3/s and C=(7±4)×10-31cm6/s, respectively, at room temperature. These values for 3C were 55-150 ns, (2.0±0.4)×10-12 cm3/s, and (2±1)×10-32 cm6/s, respectively. The rate of linear and nonlinear recombination increased at lower temperatures. A value of eh =4.4×10-18 cm2 for 3C SiC at 1.064 m was found 2.3 times smaller than that for 4H SiC.

We report on nonequilibrium carrier dynamics in a set of hydride vapor phase epitaxy (HVPE) GaN wafers of different thickness (11, 17, 41, 90, and 145 mu m) grown on the (0001) c-plane sapphire substrates Carrier lifetime tau(R) and diffusion length L(D) were determined by picosecond transient grating and free carrier absorption (FCA) techniques. The nonradiative recombination lifetime increased from 400 ps in the thinnest layer up to 25 ns in the thickest one L(D) varied from 0.24 to 1.9 mu m, respectively. The tau(R) and L(D) values in the 145 mu m-thick HVPE layer are the largest ones reported up to now in bulk GaN at room temperature. The data provided a relationship tau(R) proportional to d(3/2) between the carrier lifetime and wafer thickness in similar to 10-150 mu m thickness range. The latter dependence indicated the dramatic decrease of threading dislocation density in the probed subsurface area of the studied wafers from 4 x 10(9) to similar to 10(6) cm(-2). (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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 L-TW 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 >10(18) 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.

The aim of this study was to evaluate the optimal algorithm of high-power pulsed light technique constructed for decontamination of meat surface from pathogenic microorganisms. Our experimental data indicate that the high-power pulsed light is fast and effective chicken surface decontamination tool and can decrease food pathogens Salmonella and Listeria by 2.0 orders of magnitude in non-thermal conditions. The constructed equipment and obtained data may serve in the future for advanced development of high-power pulsed light technique which could be used for decontamination of food matrixes or food related surfaces in non-thermal conditions.

Nonequilibrium carrier recombination in highly excited epitaxial layers of 4H–SiC and free standing 3C–SiC was analyzed numerically and studied experimentally by the time-resolved free carrier absorption (FCA) technique. The measurement setup combined interband carrier excitation by a picosecond laser pulse and probing of carrier dynamics at excess carrier densities in the ΔN=1017–1020 cm−3 range by optically or electronically delayed probe pulses, thus providing temporal resolution of 10 ps and 10 ns, respectively. FCA decay kinetics at different excitation levels and subsequent numerical modeling were used to determine the bulk lifetime, surface recombination velocity, and bimolecular (B) and Auger recombination (C) coefficients at 300 K. Bulk lifetimes of ∼800 ns and ∼65 ns were determined in 4H and 3C epitaxial layers, respectively. The numerical fitting of FCA kinetics in the 4H layer provided values of B=(1.2±0.4)×10−12 cm3/s and C=(7±4)×10−31 cm6/s at lower excitations while the Auger coefficient decreased to C=(0.8±0.2)×10−31 cm6/s at ΔN∼1020 cm−3 due to screening of the Coulomb-enhanced Auger recombination. In 3C crystals, these values were measured to be B=(2.0±0.5)×10−12 cm3/s and C=(2.0±0.5)×10−32 cm6/s. The tendency for a strongly increased surface recombination rate in 3C at high excitation conditions was observed experimentally and associated with the screening of the surface potential by the high density carrier plasma.

A holographic beam splitter has been integrated into a picosecond four-wave mixing (FWM) scheme. This modification significantly simplified the procedure of dynamic grating recording, thus making the FWM technique an easy-to-use tool for the holographic characterization of wide band gap materials. The novel FWM scheme was applied for characterization of hydride vapor phase epitaxy-grown undoped GaN layers of different thickness. It allowed the determination of carrier lifetime, diffusion coefficient, and carrier diffusion length by optical means, as well as the study of carrier recombination peculiarities with respect to dislocation and excess carrier density.

Time-integrated and time-resolved FWM techniques were applied for characterization of the photoelectrical properties of undoped, S-doped, and Fe-doped InP wafers and for a mapping of their homogeneity. We performed measurements of spatial distribution of diffraction efficiency across the wafers by nanosecond FWM and investigated a physical origin of the observed variations by using time-resolved picosecond FWM. By analyzing the diffraction efficiency kinetics and its dependence on excitation energy, we evaluated the impurity-assisted carrier generation, recombination, diffusion processes, electrical activity of the defects, and their distribution across the wafers. Carrier lifetime variation from 2.5 ns to 7.5 ns across the undoped InP wafer was found, while the diffusion coefficient value of 8 +/- 0.5 cm(2)/s was almost constant. In S-doped InP wafer, wafer inhomogeneity was attributed to carrier generation peculiarities governed by spatial distribution of deep centers.

Aims: The aim of this study was to construct an advanced high-power pulsed light device for decontamination of food matrix and to evaluate its antibacterial efficiency. Key parameters of constructed device-emitted light spectrum, pulse duration, pulse power density, frequency of pulses, dependence of emitted spectrum on input voltage, irradiation homogenicity, possible thermal effects as well as antimicrobial efficiency were evaluated.Methods and Results: Antimicrobial efficiency of high-power pulsed light technique was demonstrated and evaluated by two independent methods - spread plate and Miles-Misra method. Viability of Salmonella typhimurium as function of a given light dose (number of pulses) and pulse frequency was examined.Conclusion: According to the data obtained, viability of Salmonella typhimurium reduced by 7 log order after 100 light pulses with power density 133 W cm(-2). In addition, data indicate, that the pulse frequency did not influence the outcome of pathogen inactivation in the region 1-5 Hz. Moreover, no hyperthermic effect was detected during irradiation even after 500 pulses on all shelves with different distance from light source and subsequently different pulse power density (0-252 W cm(-2)).Significance and Impact of the Study: Newly constructed high-power pulsed light technique is effective nonthermal tool for inactivation of Salmonella typhimurium even by 7 log order in vitro.Novel advanced high-power pulsed light device can be a useful tool for development of nonthermal food decontamination technologies.

Using interdisciplinary fields relevant to a highly excited semiconductor - nonequilibrium phenomena in high density plasma, light-induced changes of optical properties, and dynamic holography, we developed time-resolved four-wave mixing technique for monitoring the spatial and temporal carrier dynamics in wide band-gap semiconductors. This opened a new possibility to analyse fast electronic processes in a non-destructive "all-optical" way, i.e. without any electrical contacts. This technique allowed evaluation of recombination and transport processes and the determination of important carrier parameters which directly reveal the material quality: carrier lifetime, bipolar diffusion coefficients, surface recombination rate, nonlinear recombination rate, diffusion length, threshold of stimulated recombination. The recent experimental studies of differently grown group III-nitrides (heterostructures and free standing films) as well silicon carbide epilayers by nondegenerate picosecond four-wave mixing are presented.

We demonstrate a novel way to analyse carrier recombination and transport processes in photorefractive semiconductors via the exposure characteristics of light induced diffraction. The results of a picosecond four-wave mixing on free carrier gratings in semi-insulating GaAs crystals at various grating periods and modulation depths of a light interference pattern are discussed. The role of a deep-trap recharging in carrier diffusion and recombination is sensitively revealed through a feedback effect of a space-charge field to non-equilibrium carrier transport.

Degenerate four-wave mixing experiments with 25-ps pulses at a wavelength of 1.06 mu m have been performed in ZnTe crystals doped with deep vanadium impurity and codoped by aluminum or scandium. Complex analysis of the time resolved measurements together with exposure characteristics at various delay times of the probe beam revealed effective carrier generation from defect complexes and their subsequent recombination to Zn-vacancies in Al codoped samples. On the other hand, significantly faster carrier diffusion in Sc codoped crystal disclosed the build-up of a space charge field in deep traps through its feedback to carrier transport. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.