Markas Sudzius

Three-dimensional recording by tightly focused femtosecond pulses in LiNbO3

One of the authors M.S. thanks the Matsumae International Foundation for the research fellowship. Another author E.G.G. acknowledges support of the Australian Research Council through its Center of Excellence.

Investigation of nonequilibrium carrier transport in vanadium-doped CdTe and CdZnTe crystals using the time-resolved four-wave mixing technique

We present a novel way to determine the type of dominant carrier photoexcited from deep traps in a photorefractive semiconductor. A numerical analysis of a picosecond free-carrier grating dynamics has revealed an excitation intensity dependent grating diffusive decay time τD as well as effective carrier diffusion coefficient D, when the intensity varied in the range below that required to create a bipolar carrier plasma. According to the numerical analysis, an increase or decrease of effective diffusion coefficient D with excitation can be used as a criterion to distinguish the type of photogenerated carrier. We have verified this method experimentally by measuring dependences of effective D versus excitation density in a number of vanadium-doped and shallow-impurity codoped CdTe and ZnCdTe crystals, using for excitation a picosecond YAG:Nd laser (hν = 1.17 eV). The results were found to be in good agreement with predictions, based on carrier transport peculiarities in photorefractive crystals, and correlated well with the secondary ion mass spectroscopy data for each crystal.

Picosecond dynamics of spin-related optical nonlinearities in InxGa1−xAs multiple quantum wells at 1064 nm

Light-induced absorption and diffraction measurements of resonantly excited 10-nm-width InGaAs multiple quantum wells have been carried out, using circularly polarized beams at 1064 nm. Spin relaxation time τs≃280±15 ps has been determined by monitoring dynamics of light absorption bleaching at 0.04 mJ/cm2, while a nonlinearly compressed spin component in diffraction varied from 220±20 ps to 115±15 ps with excitation. The kinetics of carrier grating with randomized spins allowed the determination of the bipolar diffusion coefficient D=11.5 cm2/s, hole mobility of 230 cm2/V s, and carrier lifetime τR=0.73–1 ns.

Effects of photo-quenching-buildup and saturation of the induced impurity grating in ZnSe:Cr crystals

A model is proposed and calculations are performed to elucidate the effects of anomalous kinetics and dynamics of transient light self-diffraction signals (“a dip and a buildup” upon the termination of the grating generation pulse and saturation effects) observed in experiments on four-wave mixing in crystals doped with deep impurities. These observations are explained by the effects of optical depletion of local sites by the same grating generation pulse with only the subsequent buildup of the impurity grating through the capture of band carriers by these sites to form the previously created light-dynamic grating.

Evaluation of photoelectric processes in photorefractive crystals via the exposure characteristics of light diffraction

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.

Spin and carrier relaxation in resonantly excited InGaAs MQWs

Time-resolved absorption bleaching and four-wave mixing techniques have been used for studies of spin relaxation and nonequilibrium carrier dynamics in resonantly excited InGaAs/GaAs multiple quantum well structures. Kinetics of absorption bleaching at 1064 nm provided spin relaxation times of 240–280 ps and carrier lifetimes of 750–1000 ps at low excitations. At higher excitations, nearly three-fold increase of spin-relaxation rate has been measured by the FWM technique, while the decrease of carrier lifetime was much less pronounced. The kinetics of nonequilibrium carrier grating with randomized spins allowed determination of the in-plane bipolar diffusion coefficient in the wells D = 11.5 cm2 s−1 and hole mobility of 230 cm2 V−1 s−1.

Role of growth defects on carrier dynamics: semi-insulating GaAs

Using transient light-induced grating experiments, we demonstrate important consequences of interaction between photoexcited electrons and EL2 centers in semiinsulating GaAs at room temperature. Carrier lifetime is found to depend on the local density and ionization ratio of the EL2 centers. A substantial slow down of diffusive grating decay due to the interaction between electrons and photoionized EL2 donors is observed.

Reversible photomodification of LiNbO 3 and LiTaO 3 by femtosecond laser pulses

We present experimental and theoretical study of refractive index modification induced by femtosecond laser pulses in photorefractive crystals. The single pulses with central wavelength of 800 nm, pulse duration of 150 fs, and energy in the range of 6-130 nJ, tightly focused into the bulk of Fe-doped LiNbO3 and stoichiometric LiTaO3 crystals induce refractive index change of up to about 10-3 within the volume of about (2.0 x 2.0 x 8.0) μm3. The photomodification is independent of the polarization orientation with respect to the crystalline c-axis. The recorded region can be erased optically by a defocused low-intensity single pulse of the same laser. Recording and erasure can be repeated at the same position many times without loss of quality. These findings demonstrate the basic functionality of the ultrafast three-dimensional all-optical rewritable memory. Theoretically they are interpreted by taking into account electron photogeneration and recombination as well as formation of a space-charge field. The presented analysis indicates dominant role of photovoltaic effect for our experimental conditions, and suggests methods for controlling various parameters of the photomodified regions.

Reversible photomodification of LiNbO3 and LiTaO3 by femtosecond laser pulses

We present experimental and theoretical study of refractive index modification induced by femtosecond laser pulses in photorefractive crystals. The single pulses with central wavelength of 800 nm, pulse duration of 150 fs, and energy in the range of 6-130 nJ, tightly focused into the bulk of Fe-doped LiNbO3 and stoichiometric LiTaO3 crystals induce refractive index change of up to about 10-3 within the volume of about (2.0 x 2.0 x 8.0) μm3. The photomodification is independent of the polarization orientation with respect to the crystalline c-axis. The recorded region can be erased optically by a defocused low-intensity single pulse of the same laser. Recording and erasure can be repeated at the same position many times without loss of quality. These findings demonstrate the basic functionality of the ultrafast three-dimensional all-optical rewritable memory. Theoretically they are interpreted by taking into account electron photogeneration and recombination as well as formation of a space-charge field. The presented analysis indicates dominant role of photovoltaic effect for our experimental conditions, and suggests methods for controlling various parameters of the photomodified regions.

Space-charge limited free carrier transport in semi-insulating III-V compounds

We demonstrate a novel way to analyse carrier recombination and transport processes in photorefractive semiconductors via kinetics and exposure characteristics of light induced diffraction. The results of a picosecond degenerate four-wave mixing on free carrier gratings in semi-insulating GaAs and InP bulk crystals 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 nonequilibrium carrier transport or directly through linear electrooptic effect.