Benoit Deveaud

Capture of electrons and holes in quantum wells

The capture of electrons and holes by quantum wells in multiple quantum well samples of InGaAs/InP is investigated using subpicosecond luminescence spectroscopy. For samples with thin barriers, quantum capture or carrier thermalization dominates. For thicker barriers (>500 A), transport of carriers to the well dominates. We show that quantum capture time is <0.3 ps for holes and <1 ps for electrons. No significant dependence on well thickness is observed. Finally, Coulomb interaction between electrons and holes is shown to ‘‘trap’’ the electrons in unbound states in InGaAs before they are captured by the well.

Subpicosecond luminescence spectroscopy using sum frequency generation

We report an improved time‐resolved luminescence spectroscopy system using sum frequency generation. The system has the following attributes: high time resolution (<400 fs, currently limited by the laser), ability to determine absolute zero in time delay with high precision, wide spectral range, and large dynamic range which allows measurement of luminescence under weak photoexcitation. We illustrate these with examples of time‐resolved luminescence spectra from GaAs.

Optical studies of perpendicular transport in semiconductor superlattices

Optical studies of perpendicular transport of carriers in superlattices are reviewed. Steady-state optical measurements demonstrate the existence of perpendicular transport in superlattices. The nature of the transport has been recently investigated by using all-optical time-of-flight techniques. These studies have demonstrated that the transport for both electrons and holes indeed proceeds by extended Bloch states of the minibands in short-period superlattices. Furthermore, the mobility for such transport has been determined by these ultrafast luminescence techniques. The transition from Bloch transport to the localized transport with increasing period of the superlattice has been directly observed. >

Longitudinal spatial hole burning and associated nonlinear gain in gain-clamped semiconductor optical amplifiers

The longitudinal spatial hole burning (LSHB) in gain-clamped semiconductor optical amplifiers (GCSOAs) is investigated by means of a numerical model, which is based on position-dependent rate equations for the carrier density and the propagation equations for the optical power. The simulation results show that the carrier densities are nonuniformly distributed within the active layer of GCSOAs. The nonuniformity can be large, especially for high currents and optical signal powers near the saturation. It is found that the LSHB induces a gain nonlinearity, which causes interchannel cross talk when GCSOAs are used in wavelength division multiplexing (WDM) applications. In order to reduce this gain nonlinearity, two methods are analyzed: the use of low resistivity devices and the use of unbalanced Bragg mirror reflectivities.

Hot phonons and Auger related carrier heating in semiconductor optical amplifiers

We have directly measured the carrier temperature in semiconductor optical amplifiers (SOAs) via spontaneous emission and we demonstrate an unexpectedly high carrier temperature. The direct correlation of the temperature increase with the carrier density suggests Auger recombination as the main heating mechanism. We have developed a model based on rate equations for the total energy density of electrons, holes, and longitudinal-optical phonons. This model allows us to explain the thermal behavior of carrier and phonon populations. The strong heating observed is shown to be due to the combined effects of hot phonon and Auger recombination in the valence band. We also observe an evolution of the Auger process, as the density is increased, from cubic to square dependence with coefficients C/sub 3/ = 0.9 10/sup -28/ cm/sup 6/ s/sup -1/ and C/sub 2/ = 2.4 10/sup -10/ cm/sup 3/ s/sup -1/. This change is explained by the hole quasi-Fermi level entering the valence band.

INITIAL RELAXATION OF PHOTOEXCITED CARRIERS IN GaAs AND GaAs QUANTUM WELLS UNDER SUBPICOSECOND EXCITATION

Abstract After excitation by a subpicosecond pulse, we observe a very slow rise of the luminescence both in GaAs and in GaAs quantum wells. By comparing the results in GaAs and InP, we show that the slow rise in GaAs and GaAs quantum wells is due to the slow return of the electrons from the L to the Γ valley. Our results show the importance of electron-electron scattering and the inadequacy of a simple phonon cascade model at densities as low as 5×1016cm−3.

Extremely fast, high-gain and low-current semiconductor optical amplifier by optical speed-up at transparency

Summary form only given. High-speed semiconductor optical amplifiers (SOAs) are required in many devices for all-optical processing in WDM networks. They can be used as linear devices (optical gates for optical switching for example) or they can be required for their nonlinear properties (to realize complex functions like wavelength conversion). The extremely fast gain recovering in SOAs is required for high-bit-rate applications in future optical systems. The gain recovery time can be only moderately reduced by increasing the conventional SOA length but this also reduces the optical gain bandwidth. The SOA setup proposed in the present paper, called OSAT (optical speed-up at transparency), uses an assist light beam injected at the transparency point of the SOA, which gives rise to higher possible gain and higher speed at a much lower injection current while eliminating relaxation oscillations and dark holes. The principle of the OSAT is based on the carrier dependency of the material gain transparency. We have demonstrated that a number of features makes it very attractive for all-optical processing in WDM networks.

Two-photon absorption spectra in V-shaped quantum wires including excitonic effects and valence band mixing

We calculate the anisotropic two-photon absorption spectra of an AlGaAs-GaAs V-shaped quantum wire with realistic band structure, including excitonic effects. The absorption coefficient is obtained by integrating effective multiband Bloch equations accounting for the Coulomb interaction within the Hartree-Fock approximation. The various excitonic peaks are identified with respect to the involved energy subbands and the symmetry properties. Furthermore, A/sub 2/-excitons, which are dark for one-photon excitation, become bright for two-photon spectroscopy when the light is linearly polarized along a direction that is not a symmetry axis. We show that some of them may be observable.

150 fs Cross-Gain Modulation Experiments on Semiconductor Optical Amplifiers

get a bener understanding o f the fundamental physical processes involved in l h e interaction between a short optical pulse and a high density plasma. So far, the uluafast pain dynamics i n SOAs has been mainly studied by means of standard pump and probe (PBrP) measurements in which the wavelengths o f the pump and probe are identical. Here, we present a new experimental technique which is an ulwfast cross-gain modulation (XGM) behveen a I50 fa pump pulse and a weak cw probe signal. The advantages of the XGM o v a the P&P technique are two-fold. Firstly, the probe wavelength can be varied and therefore the gain recovery at different wavelengths around the pump wavelength can be obtained. Secondly, the probe sigoal i s a cw signal, thus having a very narrow linewidth and hence a high spectral resolution. Consequently, information is obtained on the spectral carrier population distributioo and ita

Direct probing of electron movement in superlattices by subpicosecond luminescence

Vertical transport in GaAs/AlGaAs superlattices is probed in structures with graded composition. Such structures allow both to impose a quasi electric field to the carriers and to evidence the carrier movement by the temporal changes in the luminescence line shape. The fit of this line shape by a drift‐diffusion model gives the transport properties of electrons. High mobility of the electrons is evidenced for the shortest period superlattices, in agreement with previous optical measurements. The importance of the transfer to the satellite valleys and of the finite capture time in the large well included as a marker are evidenced.