Radiative Auger Recombination Research Articles

Carrier dynamics and optical nonlinearity of alloyed CdSeTe quantum dots in glass matrix

Size and pump-fluence dependent ultrafast carrier dynamics of CdSeTe QDs are investigated using femtosecond pump-probe techniques operating at two different repetition rates: 1 kHz (low-repetition rate), and 76 MHz (high-repetition rate). With a low-repetition rate laser and 3.1 eV excitation photon energy, multiple exciton generation (MEG) is observed and the optical responses of alloyed QDs clearly show three components: a fast decay ascribed to carrier recombination, an intermediate component associated with MEG decay, and a slow decay associated with radiative Auger recombination. With a high-repetition rate laser and excitation photon energy resonant with band-edge energy, obvious coherent phonon oscillations are observed in 4 nm CdSeTe QDs due to impulsive stimulated Raman scattering. Open-aperture Z-scan measurement is used to clarify the size and pump-fluence dependence of optical nonlinearity under femtosecond laser excitation. With increasing laser power, an evolution from saturable absorption to reverse saturable absorption in CdSeTe QDs is observed. The transition process is analyzed using a phenomenological model based on nonlinear absorption coefficient and saturation intensity. These results indicate that CdSeTe QDs in a glass matrix are a class of materials for potential application in all-optical switching devices.

Physics and technology of mid–infrared light emitting diodes

There is increasing interest in mid–infrared (mid–IR) light emitting diodes which operate in the 2–5 μm spectral region. Efficient LEDs operating at the characteristic absorption wavelengths of target gases, such as CH 4 , CO 2 and CO, have great potential for the next generation of optical gas sensors. The fundamental difficulties associated with realizing suitable mid–IR LEDs at different wavelengths, with high continuous wave (CW) output power at room temperature, relate principally to quantum efficiency and optical extraction of the light. Each of these will be briefly considered. Some of the different device designs and techniques used for the suppression of non–radiative Auger recombination and the reduction of Shockley–Read–Hall centres are discussed. Liquid phase epitaxy (LPE) continues to hold a strong position in mid–IR LED technology and many of the best LEDs currently available have been fabricated using this technique. In this respect, the LPE growth of associated InAs(Sb)–based epitaxial structures and their purification is briefly reported. An overview of the ‘state of the art’ is also given with respect to the application of mid–IR LEDs in practical gas sensors.

Radiative transitions between exciton sub-bands in semiconductors

It is shown both theoretically and experimentally, that radiative Auger-recombination of excitons and impact excitation of excitons by hot excitons or electrons in direct-gap semiconductors at appropriate conditions lead to inverse population of exciton and bi-exciton subbands and, consequently, to stimulated submillimeter (far infrared) radiation. In CdS crystals, excited by nitrogen laser at temperatures from 1.4–4.2 K to 300 K, a stimulated submillimeter radiation on exciton transitions was observed. At T=1.4–4.2 K a few milliwatts of total output in 50–120 mkm spectral interval was obtained by Hg- lamp excitation. The strong correlation between phenomena taken place in visible and submillimeter regions was established. Our data permit to obtain the most fulfil picture of exciton and bi-exciton energy spectrum and to explain the structure of P-band in luminescence spectra.

Radiative recombination of excitons in semiconductors at high excitation intensities

The shape of the exciton luminescence band of the gaseous phase of free excitons in crystal Si is investigated under high level excitation. Only the exciton-exciton interaction is considered and the influence of collisions of excitons with phonons and electrons is not taken into account. The theoretical shape of the exciton luminescence band qualitatively agrees with experiment. The process of the radiative Auger-recombination of two indirect excitons without the participation of phonons is studied in Ge and Si. The expression for the frequency and temperature dependence of the probability of radiation is obtained. The band has the asymmetrical Gaussian form with the steep short-wave tail. The predicted luminescence band in Ge is shifted at the long-wavelength side from the well-known exciton luminescence bands with participation of the phonons. The selection rules for the probability amplitude of the process under consideration are obtained.

Luminescence due to exciton-exciton collision in GaAs

Abstract Emission spectra of high-purity GaAs have been studied at 4.2 K under N2 laser excitation. The slope of the low-energy tail of the main band has been found to fit well with the theoretical prediction for radiative Auger recombination of free excitons. Measurements under electric field support the dominant contribution of this process in the luminescence of highly excited GaAs.

Optical Gain in Lightly Doped GaAs

Measurements of both small-signal gain and stimulated emission spectra for optically pumped GaAs are described. Saturation of the amplification process is shown to play an important role in determining the spectral distribution of the stimulated emission. These measurements indicate that intrinsic gain in high-purity material does not result from band-to-band recombination and in its place an interpretation in terms of free-exciton radiative Auger recombination is suggested.