A mechanism of the condensation of e-h pairs in bulk GaAs at room temperature, which has been observed earlier, is proposed.The point is that the photon assisted pairing happens in a system of electrons and holes that occupy energy levels at the very bottoms of the bands. Due to a very high e-h density, the destruction of the pairs and loss of coherency does not occur because almost all energy levels inside a 30-60 meV band from the bottom of the conduction band prove to be occupied. As a result, a coherent ensemble of composite bosons (paired electrons and holes) with the minimum possible energy appears. The lifetime of this strongly non-equilibrium coherent e-h BCS-like state is as short as a few hundred of femtoseconds. In the high-density limit, collectively paired electrons and holes behave like Cooper pairs in a superconductor, and the BCS-like energy gap at the Fermi level is the order parameter of the macroscopic quantum state [1,6]. As noted before [7], the order parameter of the excitonic condensate is identical to its optical polarization and can be directly accessed by relatively simple optical measurements. Moreover, it has been theoretically predicted that the dephasing and relaxation kinetics of excitonic condensate depends on its density in some cases [7]. In particular, the polarization dephasing rate slows down with increasing density of the condensate. This implies that the destruction of coherency due to collisions does not happen despite of an enhanced collision rate with increasing density.In our recent experiments [8][9][10][11], we have studied the regime of the cooperative recombination in a highly non-equilibrium large density (> 3 . 10 18 cm -3 ) system of electrons and holes in bulk GaAs at room temperature.The main information about the properties of the electron-

Experimental results of the investigation of spectral characteristics of cooperative emission of high-density e-h plasmas in bulk GaAs have been presented. It has been shown that the properties of electrons and holes during the cooperative recombination are essentially different from these under lasing or normal spontaneous emission. The center of line of the cooperative emission ͑1.405-1.406 eV͒ is shifted inside the band gap. It lies by about 19 meV and by nearly 40 meV lower than the peak wavelength of lasing and amplified spontaneous emission, respectively. This corresponds to the e-h condensation at the bottoms of the bands. The features of the cooperative emission can be explained in terms of e-h pairing and build-up of a transient coherent e-h BCS-like state. The estimated value of the order parameter ⌬ is around 2-3 meV.

A review of an experimental study of superradiance in semiconductor inorganic structures is presented. It is demonstrated that unique properties of superradiant emission are determined by unusual properties of electrons and holes, namely, the formation of BCS-like state in a system of collectively paired electrons and holes. This can adequately explain all features of superradiance, including its femtosecond pulse duration, record peak power, optical spectrum, spatial and temporal coherency and macroscopically large fluctuations. The effect of non-equilibrium condensation of electrons and holes in the phase domain at room temperature is experimentally demonstrated. The critical temperature of condensation in a strongly degenerate system of electrons and holes is finally theoretically estimated. 4.8. Non-equilibrium condensation of the electrons and holes 12 5. Hypothesis: quasi-stable BCS-like electron-hole state 13 6. Mechanism of metastability of the quasi-equilibrium collective state 14 7. Optical spectra fitting. Parameters of the coherent state 16 8. Mechanism of the formation of the condensate 17 9. Low energy level of the collectively paired e-h pairs 17 10. Critical temperature of condensation in highly degenerate e-h system in the presence of resonant electromagnetic field 18 11. SR of excitonic condensates 20 12. Conclusions 20 Acknowledgment 21 References 22

We analyze theoretically the superradiant emission (SR) in semiconductor edge-emitting laser heterostructures using InGaN/GaN heterostructure quantum well (QW) as a model system. The generation of superradiant pulses as short as 500 fs at peak powers of over 200 W has been predicted for InGaN/GaN heterostructure QWs with the peak emission in the blue/violet wavelength range. Numerical simulations based on semiclassical traveling wave Maxwell-Bloch equations predict building up of macroscopic coherences in the ensemble of electrons and holes during SR pulse formation. We show that SR is covered by the Ginzburg-Landau equation for a phase transition to macroscopically coherent state of matter. The presented theory is applicable to other semiconductor materials.

Although diode lasers are almost ideal sources for ultrahigh-speed data communication systems, system performance remains critically dependent on the quality of the optical pulses that they generate. Uniquely among lasers, the output power can be modulated directly by modulating the diode current. However, this leads to relaxation oscillations and a roll-off at high frequencies that is superimposed on the frequency response of the drive circuit. This is limited by parasitics and maximum modulation frequencies range from 1 to 70 GHz, depending on the type of laser and its packaging. The higher values are obtained with short cavity lengths and tight optical confinement, the highest frequencies being achieved in vertical-cavity devices. Modulation bandwidth is usually limited by circuit parasitics, device heating and the maximum power-handling capability of the laser facets.The generation of picosecond and femtosecond pulses demands special techniques and three-gain switching, Q-switching and mode locking-are discussed in detail, with their relative advantages and disadvantages compared. Very short pulses inherently contain a significant spread of wavelengths and their generation requires the optical gain in the laser medium to extend across that wavelength range. While diode lasers satisfy this criterion better than many other types, the effect of gain non-linearities and carrier-transport effects prevent the Fourier-transform limit from being achieved in practice. As a result, external pulse-compression techniques, which exploit the detailed temporal and spectral properties of the laser pulses, such as frequency chirping, self-phase modulation and group velocity dispersion, are becoming more important, and diode lasers are increasingly challenged as primary sources by compact, efficient, diode-pumped solid-state lasers.The paper summarizes the levels of maximum pulse power and minimum duration that have been achieved using the various techniques.