This work deals with the dynamics of the nonthermalized electron-hole plasma in the subpicosecond regime. We computed different collision rates (electron-electron, electron-hole, electron-phonon) and discussed the relative efficiency of the different processes for the cases of static and dynamic screening of the interaction. We also considered how the inclusion of the exchange affected the screening amplitude and the collision rates. Calculations were bases upon GaAs. We conclude that dynamic screening is crucial for electrons in the conduction band at high plasma density [say, at densities above (1-2)x10 16 /cm 3 ] whereas it generally plays a minor role at low density and when describing hole dynamics. The excitation of heavy holes into the light-hole band by collisions with the conduction electrons is discussed in heavily p-doped GaAs.

We report on femtosecond time-resolved transmission and reflectivity measurements on bulk GaAs. Spectral-hole burning is observed, to our knowledge, for the first time in GaAs at room temperature. Carrier thermalization occurs within 200 fs and shows no significant dependence on excitation density or excess energy in the range from 2 x 10 cm to 2 x 10 cm and 35 meV to 90 meV, respectively. Calculations of the carrier dynamics are performed and include full dynamic screening of the carrier-carrier and the carrier LO-phonon interaction. The calculated thermalization times agree well with the experimental results. Negative transmission changes above the spectral hole are mainly caused by a complementary increase of reflectivity, but not by an increase of absorption.

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The relevance of introducing optical interconnects (OI's) in monoprocessors and multiprocessors is studied from an architectural point of view. We show that perhaps the major explanation for why optical technologies have nearly been unable to penetrate into computers is that OI's generally do not shorten the memory-access time, which is the most critical issue for today's stored-program machines. In monoprocessors the memory-access time is dominated by the electronic latency of the memory itself. Thus implementing OI's inside the memory hierarchy without changing the memory architecture cannot dramatically improve the global performance. In strongly coupled multiprocessors the node-bypass latency dominates. Therefore the higher the connectivity (possibly with optics), the shorter the path to another node, but the more expensive the network and the more complex the structure of electronic nodes. This relation leaves the choice of the best network open in terms of simplicity and latency reduction. The bottlenecks resulting from and the benefits of implementing OI's are discussed with respect to symmetric multiprocessors, rings, and distributed shared-memory supercomputers.

The radiative recombination of free excitons in diamond has been previously reported by Dean et al. /l/. They showed that the valence band splitting due to spin-orbit interaction must be taken into account for a good understanding of their line shape. Their experiments were performed a t high temperature (T > 77 K) and with a relatively low resolution.In our experiments we used a cathodoluminescence apparatus, working in pulsed mode, described in preceding publications /2, 3/. The main characteristics are briefly recalled: 2 z < 1 n s for 0 < I < 2 A/cm , rise time: pulse length 8 : 10 n s < 8 < 1000 ns, acceleration voltage U: 20 kV < U < 100 kV.We worked generally with the following experimental conditions: 2 I * 1.6 A/cm , U = 50 kV, repetition rate f = 800 Hz.High injection r a t e s can be reached, so that we could observe the free exciton luminescence a t low temperature, down to about 4.2 K. The intensity of this line decreases very quickly below 77 K. We worked with a sample of "natural diamond", type I1 b.On Fig, 1 is reported the main emission line of free excitons a t 80 K. The maximum is situated a t 5.275 eV, as previously observed by Dean et al. /l/.The width a t half height is much l a r g e r than predicted when the valence band split off by the spin-orbit interaction is neglected.

Mental and physical health are both at risk under conditions of prolonged remoteness from home and removal from normal social support networks. This article examines relevant data from small groups isolated in (a) Antarctic summer stations, (b) a Transantarctic traverse expedition, and (c) a spacecraft simulator capsule.