Lithuanian coins with a rider and double cross on the shield

The rarest type of Lithuanian coinage is discussed by the author. The most important facts regarding the 40 coins in the group are presented, concerning their appearance. The author attempts to identify the subgroups within this class of coins. Recent research leads the author to ascribe this type of coin to Jogaila and date it to the years 1387-1392 (1401)

High-Speed Quadratic Electrooptic Nonlinearity in dc-Biased InP

We present experimental data on degenerate four-wave mixing as well as simulation results of fast optical nonlinearities in highly-excited semiinsulating InP under applied dc-field. Hot-electron transport governed enhancement of optical nonlinearity is obtained by applying a dc-field of 10-14 kV/cm at full-modulation depth of a light-interference pattern. The hydrodynamic model, which incorporates both free-carrier and photorefractive nonlinearities is used to explain the experimentally observed features. We show that the enhancement of optical nonlinearity is due to the quadratic electrooptic effect

Time-Resolved Transient Grating Spectroscopy for Studies of Nonequilibrium Carrier Dynamics in Wide Band-Gap Semiconductors

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

High-Speed Quadratic Electrooptic Nonlinearity in dc-Biased InP

We present experimental data on degenerate four-wave mixing as well as simulation results of fast optical nonlinearities in highly-excited semi-insulating InP under applied dc-field. Hot-electron transport governed enhancement of optical nonlinearity is obtained by applying a dc-field of 10-14 kV/cm at full-modulation depth of a light-interference pattern. The hydrodynamic model, which incorporates both free-carrier and photorefractive nonlinearities is used to explain the experimentally observed features. We show that the enhancement of optical nonlinearity is due to the quadratic electrooptic effect

High-Speed Quadratic Electrooptic Nonlinearity in dc-Biased InP

We present experimental data on degenerate four-wave mixing as well as simulation results of fast optical nonlinearities in highly-excited semi-insulating InP under applied dc-field. Hot-electron transport governed enhancement of optical nonlinearity is obtained by applying a dc-field of 10-14 kV/cm at full-modulation depth of a light-interference pattern. The hydrodynamic model, which incorporates both free-carrier and photorefractive nonlinearities is used to explain the experimentally observed features. We show that the enhancement of optical nonlinearity is due to the quadratic electrooptic effect

The detrimental effect of AlGaN barrier quality on carrier dynamics in AlGaN/GaN interface

Carrier recombination and scattering at the semiconductor boundaries can substantially limit the device efficiency. However, surface and interface recombination is generally neglected in the nitride-based devices. Here, we study carrier recombination and diffusivity in AlGaN/GaN/sapphire heterointerfaces with AlGaN barriers of different quality. We employ the light induced transient grating and time-resolved photoluminescence spectroscopy techniques to extract carrier lifetime in different depths of the GaN buffer as well as in the AlGaN barrier, and to evaluate the carrier diffusion coefficient in the buffer. Moreover, we assess interface recombination velocity, Shockley-Read-Hall and radiative recombination rates. We reveal the adverse barrier influence on carrier dynamics in the underlying buffer: AlGaN barrier accelerates the nonradiative carrier recombination in the GaN buffer. The interface recombination velocity in the GaN buffer increases with decreasing AlGaN barrier quality, and the dominating recombination mechanism switches from Shockley-Read-Hall to interface recombination. These phenomena are governed by a cumulative effect of various interface-deteriorating barrier defects. Meanwhile, the carrier diffusivity in the GaN buffer is not affected by the AlGaN barrier. We conclude that barrier-accelerated interface recombination can become a major carrier loss mechanism in AlGaN/GaN interface, and may substantially limit the efficiency in nitride-based UV LEDs