Excitation and De-Excitation Mechanisms of Er-Doped GaAs and AlGaAs

Electrical and optical characterization have been performed on GaAs and AlxGa1-xAs samples doped with Er either by ion implantation or during Molecular Beam Epitaxial (MBE) growth. Deep Level Transient Spectroscopy (DLTS) and Temperature-Dependent Hall Effect (TDH) measurements indicated the presence of two hole traps in Er-doped GaAs, at 35 and 360 meV above the valence band maximum. The former (shallower) center was thought to be due to Er substituting for a Ga atom (ErGa) and giving rise to an isoelectronic impurity potential. The second center was attributed to an Er atom occupying an interstitial position (Eri). Annealing studies performed on Er-implanted GaAs indicated that the ErGa center preferentially formed at higher annealing temperatures ( \u3e 850°C), with the Eri reaching a maximum concentration at an annealing temperature of around 750°C. Optical characterization performed by Photoluminescence (PL) measurements showed that the Eri center gave a much stronger Er-related intra-4f shell emission. Mechanisms for the excitation of the 4f shells of these two centers are discussed. Similar optically active Eri centers may be forming in AlGaAs

Ultrafast spatio-temporal dynamics of terahertz generation by ionizing two-color femtosecond pulses in gases

We present a combined theoretical and experimental study of spatio-temporal propagation effects in terahertz (THz) generation in gases using two-color ionizing laser pulses. The observed strong broadening of the THz spectra with increasing gas pressure reveals the prominent role of spatio-temporal reshaping and of a plasma-induced blue-shift of the pump pulses in the generation process. Results obtained from (3+1)-dimensional simulations are in good agreement with experimental findings and clarify the mechanisms responsible for THz emission

Time resolved X ray absorption spectroscopy of infrared laser induced temperature jumps in liquid water

A time resolved X ray absorption study of the structural dynamics of liquid water on a picosecond timescale is presented. We apply femtosecond midinfrared pulses to resonantly excite the intramolecular O H stretching band of liquid water and monitor the transient response in the oxygen K edge absorption spectrum with picosecond X ray pulses. In this way, structural changes in the hydrogen bond network of liquid water upon an ultrafast temperature jump of approximately 20 K are investigated. The changes of the X ray absorption as induced by such a temperature jump are about 3.2 . This demonstrates that our method serves as a sensitive probe of transient structural changes in liquid water and that combined infrared laser synchrotron experiments with substantially shorter X ray pulses, such as generated with a femtosecond slicing scheme, are possibl

Transition from ballistic to drift motion in high-field transport in GaAs

With strong THz pulses, we measure ultrafast transport of electrons, holes, and an electron-hole plasma in GaAs. The transition from ballistic to drift-like transport is strongly influenced by electron-hole scattering

Physical limits of semiconductor laser operation: A time-resolved analysis of catastrophic optical damage

The early stages of catastrophic optical damage (COD) in 808 nm emitting diode lasers are mapped by simultaneously monitoring the optical emission with a 1 ns time resolution and deriving the device temperature from thermal images. COD occurs in highly localized damage regions on a 30 to 400 ns time scale which is determined by the accumulation of excess energy absorbed from the optical output. We identify regimes in which COD is avoided by the proper choice of operation parameters

Coherent optical generation of nonequilibrium electrons studied via band-to-acceptor luminescence in GaAs

Nonequilibrium electrons generated by coherent optical excitation of GaAs are studied in a wide range of carrier density. The electron distribution is monitored via spectrally resolved band-to-acceptor luminescence after continuous-wave, picosecond, or femtosecond laser excitation. Our data demonstrate that the coherent coupling between the laser radiation and the interband polarization and its dephasing strongly influence the initial carrier distribution. The energetic width of carrier generation is broadened due to rapid phase-breaking scattering events during carrier generation. Theoretical results from a Monte Carlo solution of the semiconductor Bloch equations including on the same kinetic level coherent and incoherent phenomena show that the broadening of the electron distribution is introduced mainly in the generation process whereas the recombination of electrons with bound holes makes a minor contribution. The theoretical results are in quantitative agreement with the experimental data

Ultrafast Coherent Generation of Hot Electrons Studied via Band-to-Acceptor Luminescence in GaAs

The distribution of hot electrons excited with femtosecond laser pulses is studied via spectrally resolved band-to-acceptor luminescence. Our data demonstrate for the first time that the coherent coupling between the laser pulse and the interband polarization strongly influences the initial carrier distribution. The energetic width of carrier generation is broadened due to rapid phase-breaking scattering events. Theoretical results from a Monte Carlo solution of the semiconductor Bloch equations including on the same kinetic level coherent and incoherent phenomena, are in excellent agreement with the experimental data

The Role of Nonequilibrium Dynamical Screening in Carrier Thermalization

We investigate the role played by nonequilibrium dynamical screening in the thermalization of carriers in a simplified two-component two-band model of a semiconductor. The main feature of our approach is the theoretically sound treatment of collisions. We abandon Fermi's Golden rule in favor of a nonequilibrium field theoretic formalism as the former is applicable only in the long-time regime. We also introduce the concept of nonequilibrium dynamical screening. The dephasing of excitonic quantum beats as a result of carrier-carrier scattering is brought out. At low densities it is found that the dephasing times due to carrier-carrier scattering is in picoseconds and not femtoseconds, in agreement with experiments. The polarization dephasing rates are computed as a function of the excited carrier density and it is found that the dephasing rate for carrier-carrier scattering is proportional to the carrier density at ultralow densities. The scaling relation is sublinear at higher densities, which enables a comparison with experiment.Comment: Revised version with additional refs. 12 pages, figs. available upon request; Submitted to Phys. Rev.

Future space experiment platforms for astrobiology and astrochemistry research

Space experiments are a technically challenging but a scientifically important part of astrobiology and astrochemistry research. The International Space Station (ISS) is an excellent example of a highly successful and long-lasting research platform for experiments in space, that has provided a wealth of scientific data over the last two decades. However, future space platforms present new opportunities to conduct experiments with the potential to address key topics in astrobiology and astrochemistry. In this perspective, the European Space Agency (ESA) Topical Team Astrobiology and Astrochemistry (with feedback from the wider scientific community) identifies a number of key topics and summarizes the 2021 “ESA SciSpacE Science Community White Paper” for astrobiology and astrochemistry. We highlight recommendations for the development and implementation of future experiments, discuss types of in situ measurements, experimental parameters, exposure scenarios and orbits, and identify knowledge gaps and how to advance scientific utilization of future space-exposure platforms that are either currently under development or in an advanced planning stage. In addition to the ISS, these platforms include CubeSats and SmallSats, as well as larger platforms such as the Lunar Orbital Gateway. We also provide an outlook for in situ experiments on the Moon and Mars, and welcome new possibilities to support the search for exoplanets and potential biosignatures within and beyond our solar system