Expansion Of Electron-hole Plasma Research Articles

Ultrafast Long-Distance Electron-Hole Plasma Expansion in GaAs Mediated by Stimulated Emission and Reabsorption of Photons.

Electron-hole plasma expansion with velocities exceeding c/50 and lasting over 10ps at 300K was evidenced by time-resolved terahertz spectroscopy. This regime, in which the carriers are driven over >30 μm is governed by stimulated emission due to low-energy electron-hole pair recombination and reabsorption of the emitted photons outside the plasma volume. At low temperatures a speed of c/10 was observed in the regime where the excitation pulse spectrally overlaps with emitted photons, leading to strong coherent light-matter interaction and optical soliton propagation effects.

Electron-hole Fermi liquid in nanosized semiconductor structures

The experimental and theoretical results on the quantum-sized electron-hole liquid plasma (EHLP) in semiconductors and analysis of the difference of it in comparison to the bulk one have been presented. The non-equilibrium Fermi EHLP can be created in the bulk and layered structures (insulator-semiconductor interfaces, thin films, quantum superlattices, etc.) at low temperatures and powerful laser radiation. In the quantum-sized structures, however, these phenomena appear at much higher temperatures, up to the room ones. The peculiarities of EHLP phenomena are: (1) appearance the very broad luminescence line in the low-energy side of its spectrum, which have constant width and energy position under variation of the light intensity as well as narrowing peak when increasing the temperature; (2) appearance of stimulated radiation with a relatively low excitation threshold (the so-called surface laser effect); (3) planar ballistic expansion of electron-hole plasma over long distances; (4) predicted effect of transformation of non-equilibrium 2D plasmons into radiative modes.

Carrier diffusion length measured by optical method in GaN epilayers grown by MOCVD on sapphire substrates

The carrier ambipolar diffusion length L of optically excited carriers in GaN epitaxial layers grown on sapphire substrate was estimated by an optical method using fitting of the experimental photoluminescence spectra recorded from the front and back sides of the samples by the theoretical equation describing light reflection, light absorption and carrier profile in the medium. The estimations were carried out in the range of excitation intensities from 5 W/cm 2 CW up to 1 MW/cm 2 (pulsed), using excitation at the wavelengths of 325, and 337.1 nm in order to vary the excited layer depth. It has been found that in the samples under study the value of L is about 120–130 nm and does not depend significantly on the excitation intensity up to 200 kW/cm 2. Further increase of excitation level leads to higher values of L about 150–170 nm, probably because of the electron-hole plasma expansion.

Electron-Hole Plasma Expansion in GaAs: Submicron Optical Time of Flight Investigations

Pump and probe differential reflection (ΔR) and transmission (ΔT) measurement of subpicosecond light pulses for either co- and counterpropagating pump and probe geometries are performed on thin GaAs samples. With this time of flight (TOF) method we study density dependent electron–hole plasma (EHP) expansion in GaAs perpendicular to the sample surface in the temperature range of 300 K ≥ TL ≥ 4 K. At a fluence of F = 800 μJcm—2 the expansion velocities increase with decreasing lattice temperature TL. The expansion velocities v at TL = 300 K and 4 K show a dependence on the delay time τ like v ∝ τ—2/3. At low lattice temperatures we find a much stronger dependence of the expansion velocities on the fluence of the pump pulses.

Plasma-expansion induced absorption and refraction changes in ZnSe epilayers

A novel transient-grating technique with a well-defined and moveable optical probe is used to investigate the spatial extent of any optical changes arising from optically-driven electron-hole plasma expansion in molecular-beam-epitaxially grown ZnSe. We present experimental data demonstrating that optically-driven plasma expansion does not extend significantly beyond the originally excited area when pumped to carrier densities appropriate to lasing action in these materials (1018cm−3).

Direct observation of Fermi-pressure-driven electron-hole plasma expansion in GaAs on a picosecond time scale

Applying pump and probe differential reflection $(\ensuremath{\Delta}R)$ and transmission $(\ensuremath{\Delta}T)$ of femtosecond light pulses for either co- or counterpropagating pump and probe geometries, a direct time of flight method with submicrometer resolution is presented. With this technique we study the density-dependent transport of photogenerated carrier plasmas perpendicular to the surface of GaAs samples for delay times 20 ps\ensuremath{\leqslant}\ensuremath{\tau}\ensuremath{\leqslant}1 ns. At a pump fluence of 800 \ensuremath{\mu}J ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ a relatively sharp charge-carrier front was observed, with high velocities of $14\ifmmode\times\else\texttimes\fi{}{10}^{5}$ cm/s at a delay time $\ensuremath{\tau}\ensuremath{\approx}20$ ps, decreasing as $v\ensuremath{\propto}{\ensuremath{\tau}}^{\ensuremath{-}2/3}$ to $2\ifmmode\times\else\texttimes\fi{}{10}^{5}$ cm/s at $\ensuremath{\tau}\ensuremath{\approx}350$ ps. The arrival times $\ensuremath{\tau}$ of the carriers at a fixed sample thickness depend on the fluence of the pump pulses F such as $\ensuremath{\tau}\ensuremath{\propto}{1/F}^{0.45}.$ The results are discussed in the framework of diffusive transport with a strongly density-dependent diffusivity D. The data can be described consistently with the assumption of Fermi pressure as the dominating driving force for plasma expansion.

Photoacoustic study of the electron–hole transport in a piezoelectric semiconductor

In a piezoelectric semiconductor acoustic waves can be excited via the inverse piezoelectric effect. This excitation process is the result of a screening of the applied electric field by the expanding space distribution of electrons and holes which are generated by pulsed laser radiation. The time and space characteristics of the electron–hole transport in this process are contained in the corresponding acoustic signals. The piezoelectric direct-gap semiconductors cadmium sulfur selenide and gallium arsenide are investigated experimentally at room temperature. A pulsed nanosecond UV excimer laser is used to excite electrons and holes on these samples. The generated longitudinal acoustic waves are detected by a laser interferometer. The obtained acoustic pulses of cadmium sulfur selenide show that both the expansion of the screened region in space and the electron–hole plasma expansion are supersonic at the time scale of laser action. The observations in this optoacoustic experiment indicate that the fast carrier transport is governed by photon recycling, i.e., the reabsorption of photons released by radiative recombination of electron–hole pairs. Experiments for gallium arsenide are in progress and will also be reported.

Supersonic radiative transport of electron-hole plasma in semiconductors at room temperature studied by laser ultrasonics

A piezoelectric semiconductor CdS 1− x Se x crystal under external electric loading was excited by pulsed nanosecond ultraviolet laser radiation. Acoustic waves were excited via the inverse piezoelectric effect due to the screening of the external electric field by expanding the space distribution of photogenerated electrons and holes. The duration of the interferometrically detected longitudinal acoustic pulses indicated that both the expansion of the screened region in space and the electron-hole plasma expansion are supersonic at the time scale of laser action. The value of 2 × 10 3 cm 2/s obtained for the electron-hole plasma diffusivity leads to the conclusion that the mechanism of this fast carrier transport is photon recycling, i.e. reabsorption of recombination radiation. This conclusion is also supported by the acoustic signals duration independence on magnitude and polarity of the external electric field.

The effects of carrier transport on the photoluminescence of degenerate electron-hole plasma in GaAs epilayers

Time resolved photoluminescence (TRPL) is employed to study photogenerated electron-hole plasma expansion under strong illumination conditions in thin GaAs epilayer. We have observed spectral dependencies similar to ones which are usually observed on carrier's cooling. However, in our case they are caused by the diffusion of the degenerate electron-hole plasma perpendicular to the crystal surface. We have studied the influence of the degenerated carrier transport on the luminescence characteristics of bulk samples, both experimentally and by simulations. We suggest using this method to measure the transport coefficients of both degenerate and hot excess carriers in semiconductor structures.

Fast-Drifting Electron-Hole Plasma in CdSe Studied by Ultrafast Pump-and-Probe Experiments

The ultrafast pump-and-probe technique was applied to study the electron-hole plasma (EHP) expansion in a thin platelet CdSe. Under the high-density surface excitation, the band absorption initially pushes out to low energy until 15 ps together with the decrease in the exciton absorption. The result is explained by assuming the constant-velocity EHP expansion to the depth of the sample. The expansion velocity of the boundary between the EHP phase and the exciton phase is 1.9×10 7 cm/s, which is much faster than the velocity of sound.

Spatial expansion of electron-hole plasma in Si.

In this paper we report measurements of the spatial and spectral evolution of the photoluminescence resulting from nanosecond pulsed-laser excitation of silicon at low temperatures. An imaging technique employing single-photon counting provides direct measurements of the expansion rates of the electron-hole plasma with 5-ns temporal resolution. We find that for 0.5-\ensuremath{\mu}J excitation energy the plasma expands from the excitation point at subsonic velocities\char22{}contrary to recent claims based on spectroscopic analysis. An alternative analysis of the recombination spectra indicates that local heating can account for the anomalously broad spectra at early times. For 50-\ensuremath{\mu}J excitation, time-resolved images show that the electron-hole plasma expands as a shell into the crystal, indicating that it is driven by an intense phonon wind. The expansion velocities are found to decrease smoothly as the temperature is increased through the critical temperatures (${T}_{c}$\ensuremath{\approxeq}23 K) of the electron-hole liquid (EHL). A theoretical calculation of the phonon-wind-driven expansion velocity yields a temperature dependence which is in reasonable agreement with the data. At low temperatures (l10 K) and near-annealing excitation levels, the EHL expansion velocity is observed to saturate at v=5.1\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cm/s. We attribute this nonlinear damping to a Cherenkov-like emission of TA phonons from the EHL. Finally, we present measurements of the free-exciton diffusion coefficient for temperatures between 15 and 40 K.

The parameters of quasi-two-dimensional electron-hole plasma stimulated by laser radiation

The effects caused by a non-equilibrium quasi-two-dimensional (quasi-2D) electron-hole plasma (EHP) created at the layer structure insulator-semiconductor interfaces with the help of a powerful laser radiation are described. These effects are: (1) a stimulated radiation with low excitation threshold (surface laser effect); (2) a planar ballistici expansion of electron-hole plasma over long distances; (3) an extensive non-equilibrium plasmon generation due to numerous recombination processes and their subsequent transformation into radiative modes.

Electron-hole plasma expansion in the direct-band-gap semiconductors CdS and CdSe.

The properties of the electron-hole plasma in direct-band-gap semiconductors are investigated by excite- and probe-beam techniques with use of CdS and CdSe as examples. By spatially resolved transmission and reflection spectroscopy and by using samples of different thicknesses, the diffusion length and other data of the plasma are determined. A ``slow drift'' is found, i.e., the drift velocity is smaller than the Fermi velocities of electrons and holes. The drift distances are of the order of 10 \ensuremath{\mu}m with some significant differences between CdS and CdSe. Various models used in the literature to fit the observed gain spectra are critically discussed.

Phonon-Wind-Driven Electron-Hole Plasma in Si

The spatial expansion of electron-hole plasma created by nanosecond pulsed-laser excitation of Si is examined by time-resolved luminescence imaging. On a several-nanosecond time scale, the plasma created by 0.5-\ensuremath{\mu}J excitation expands from the excitation point at subsonic velocities, contrary to a fast-diffusing plasma model recently proposed to explain spectroscopic anomalies. For 50-\ensuremath{\mu}J excitation, time-resolved images show a shell of plasma expanding at near-sonic velocities, indicating phonon-wind-driven transport with highly non-linear damping.

Experimental investigation of the electron-hole plasma expansion in CdS

Using a two beam method for gain and reflection spectroscopy, we introduce a new technique allowing for a spatial and temporal resolution of 5 μm and 2 nsec, respectively. With this method we are able to investigate the optical gain, the reflection and the spatial extension of an electron-hole plasma under stationary excitation conditions. The experiments are performed with the II–VI compound semiconductor CdS. Our results are compared with those from other authors who deduced the electron-hole plasma properties mainly from luminescence experiments.

Expansion of the Electron‐Hole Liquid Plasma at GaAs Surface

AbstractThe spectral and planar distribution of the luminescence intensity of the electron‐hole plasma (EHP) created by laser excitation in the subsurface region of a direct‐band semiconductor like GaAs in contact with a dielectric medium (Si3N4) is studied. High optical excitation levels (L ≈ ≈ 5 MW/cm2) make possible to detect: (1) the appearance of stimulated radiation mainly from the regions of the sample edges; (2) the expansion of EHP from the excited region predominantly along the GaAs‐Si3N4 interface up to a distance (l ≈ 50 to 100 μm), which is several times greater than the corresponding bulk value. The assumption is made that the electron‐hole liquid is created not only at the “tails” of the expanded plasma, but also within the excited regions (in the latter case after a partial cooling of EHP, when in the process of the relaxation the electron‐hole pairs concentration achieves the EHP‐condensation value n ≈ 1016 cm−3). The mechanism of a long‐wave range tail formation in EHL radiation, which is caused by the energy losses on non‐equilibrium plasmons, is discussed.

Electron–Hole Plasma Expansion in Direct‐Gap GaAs1−xPx

AbstractThe e‐h plasma in the ternary alloy GaAs1−xP2 below direct‐indirect crossover is studied at 2 K by means of very high excitation photoluminescence measurements. The most interesting feature observed is an extremely efficient expansion of the excited region when the particle density reaches the value n* needed to minimize the free energy. This behaviour and the corresponding spectral evolution with power density are accounted for by a model where pressure‐induced flow of particles away from the pumped region becomes competitive with recombination mechanisms.