As discussed in Chap. 3, the non-thermal, photoexcited electrons and holes interact among themselves and other carriers present in the semiconductor. They achieve a distribution function characterized by a temperature in times of the order of 100 fs for the typical densities present in most femtosecond experiments. Initially, the temperatures of the electrons and the holes may be different, but a common temperature Tc for the carrier system is typically achieved in times of the order of a picosecond. The thermalized carriers are hot, i.e., their distribution function is characterized by a temperature Tc higher than the lattice temperature T L . Although some of the energy of the electronic system is lost to the lattice via carrier-phonon interactions during this thermalization process, most of the energy typically remains within the electronic system so that the non-thermal regime provides information primarily about carriercarrier interactions. The next phase of the relaxation occurs as the thermalized, hot electron-hole distributions cool and approach the lattice temperature. Much of our information about the carrier energy loss processes to the lattice comes from a study of this cooling process.Carrier-phonon interaction is one of the most fundamental scattering processes in semiconductors and plays a role in many different aspects of semiconductor physics. High-field transport is one such area, and investigation of highfield transport has provided considerable information about carrier-phonon scattering processes and rates [4.1-6]. Similarly, cw photoexcitation has been shown to create a steady-state hot-carrier distribution [4.7 -11]. A study of carrier temperature as a function of excitation intensity provides some information about the important scattering processes but is unable to provide quantitative information on scattering rates. We discuss in this chapter how ultrafast spectroscopy on picosecond time scales has been used to explore this aspect of semiconductor physics. We note that, although the process by which carriers gain energy is different for applied electric fields and photoexcitation, the process by which they lose energy to the lattice (i.e., by the emission of phonons) is the same in both cases. Therefore, information gained by optical studies is useful in understanding high-field transport, and vice versa. The primary advantage of the optical techniques is that they not only provide a means of generating a nonequilibrium carrier distribution, they also provide the best technique for measuring these distribution functions. In contrast, the transport measurements provide information that is averaged over the distribution function.The basic concept behind studies involving cw photoexcitation is that the rate of transfer of energy from photons to the electronic system must be equal

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Solids consist of 10 22 ±10 23 particles per cubic centimetre, interacting through in®nite-range Coulomb interactions. The linear response of a solid to a weak external perturbation is well described by the concept of non-interacting`quasiparticles' ®rst introduced by Landau. But interactions between quasiparticles can be substantial in dense systems. For example, studies over the past decade have shown that Coulomb correlations between quasiparticles dominate the nonlinear optical response of semiconductors, in marked contrast to the behaviour of atomic systems. These Coulomb correlations and other many-body interactions are important not only for semiconductors, but also for all condensed-matter systems.O ne of the most fascinating properties of quantum mechanical systems is that they can be in entangled states, that is, a coherent superposition of eigenstates unknown in classical systems. Entangled states have been observed in atomic physics, but they remain elusive in condensed matter. The primary dif®culty is that they survive only as long as the coherence is maintained, and numerous processes conspire to produce decoherence, dephasing and relaxation on an extremely short timescale (,10 -12 s) in condensed matter, which is typically composed of 10 22 ±10 23 particles cmthat interact through the in®nite-range Coulomb force. The complexity of the problem makes any theoretical approach extremely dif®cult. About ®ve decades ago, Landau proposed that real particles, strongly interacting among themselves but evolving in the real vacuum, be mapped onto`quasiparticles' . The quasiparticles arè dressed' by a part of the interaction, and are relatively long-lived excitations of the many-body system evolving in a`new vacuum' that consists of the`rest' of the many-body system and the part of the Coulomb interaction not accounted for in the quasiparticles. The quasiparticles are complex objects (Cooper pairs in a superconductor, excitons in a semiconductor, and so forth), and the new vacuum itself can be structured (it can have an antiferromagnetic order in the copper oxides) and dynamical (it can sustain phonons and magnons). For many solids, the concept of quasiparticles has been very useful for describing the ground state and small departures from it, that is, the most basic excitation and the linear response to weak external perturbations. However, the part of the Coulomb forces not accounted for in the formation of quasiparticles leads to interactions between these quasiparticles, inducing nonlinearities and destroying their phase coherence. Scientists interested in the linear response of a many-body system like to call these interactions`residual' . They are, however, not small, as shown below. They are residual only in the sense that we do not know (yet) how to treat them correctly. Understanding the in¯u-ence of these many-body interactions and Coulomb correlations is one of the central challenges of condensed-matter physics. Semiconductors form an ideal laboratory for quantitatively investigating the role of Co...

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A detailed calculation of the amplitude and phase response of ultrathin ZnTe and GaP electro-optic sensors is presented. We demonstrate that the inclusion of the dispersion of the second-order nonlinearity is essential. Significant structures in experimental data can be explained by the theoretical response function. Correcting for the detector characteristics, we determine the precise shape of electromagnetic transients with a time resolution of 20 fs. In addition, we show that ultrafast transport of photocarriers in semiconductors can act as an efficient source for coherent electromagnetic radiation covering the entire far-to-mid-infrared regime.

We have obtained a coherent understanding of spin relaxation processes of electrons, holes, and excitons in quantum wells by investigating subpicosecond dynamics of luminescence polarization. We show that the spin behavior for electrons and holes in quasi-two-dimensional systems is distinct from that in bulk semiconductors and that many-body effects and formation processes play an important role in exciton spin relaxation. PACS numbers: 78.47.+p, 71.35.+Z, 7l.70.Ej, 71.70.Gm Investigations of polarization of interband optical transitions provide considerable information about the symmetry of electron, hole, and excitonic wave functions in semiconductors. Such studies have led to the identification of diflferent spin relaxation processes [1,2] in bulk semiconductors. In quantum wells, cw measurements of linear [3] and circular polarization [4,5] of near-bandgap luminescence have been reported and various theories have been proposed to explain these results [6-12]. Time-resolved spectroscopy of luminescence polarization has also been reported recently for very high (lO'^ cm ~^) carrier densities [13], for different samples at intermediate carrier densities with 150-fs time resolution [14,15], and for low densities [16,17].In spite of this intense activity, spin relaxation in quantum wells is poorly understood. The quality of samples is important since spin dynamics can be strongly influenced by localization or defects. On a more fundamental level, the differences in the behavior of electrons, holes, and excitons must be recognized and carefully investigated. The prediction of slower hole spin relaxation in quantum wells compared to bulk semiconductors [10] has not been investigated. Finally, the influence of the formation dynamics of excitons and of many-body effects must be considered.In this Letter, we discuss new insights into many of these unresolved issues obtained through our investigations of spin dynamics of electrons, holes, and excitons in quantum wells. Using «-modulation-doped quantum wells, we obtain the first measurement of hole spin relaxation time in a semiconductor. The measured ^=: 4 ps demonstrates that the usual assumption of instantaneous hole spin relaxation is incorrect. We also show that the electron Fermi sea can be spin polarized under certain excitation conditions. In /?-modulation-doped quantum wells, we measure electron spin relaxation time of ^^^ 150 ps, approximately a factor of 4 shorter than that in comparably /7-doped bulk GaAs. We will show that an electron-hole exchange is responsible for this reduction. For intrinsic quantum wells, we show that polarized and unpolarized spectra exhibit an unusual splitting that depends on excitation density and time delay. This splitting results from many-body exchange interactions between excitons and contributes to an increase in the exciton spin relaxation time at higher density. Finally, we explain why spin relaxation in nonresonantly created excitons is faster than in resonantly created excitons. We believe that our quantitative measurement...

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%'e show that excitons form with a time constant~~20 ps following the creation of electron-hole pairs by subpicosecond optical excitation. The excitons are initially formed in large-wave-vector states. At low temperatures, these nonthermal excitons relax in =400 ps to the K =0 states, which couple directly to light by interaction with other excitons and acoustic phonons. This leads to a slow rise of exciton luminescence and an unusual dependence of this rise time on temperature, excitation density, and excitation energy.The optical properties of excitons in quantum wells have been the subject of intense research in recent years for fundamental' and applied reasons. Many fundamental properties of excitons have been investigated by using ultrafast laser spectroscopy. For example, the dynamics of exciton ionization and the ac Stark effect of excitons induced by an intense optical field ' have been investigated using excite-and-probe spectroscopy. The homogeneous linewidth of excitons, and the influence of temperature and various collisions on this linewidth, have been investigated by four-wave-mixing experiments. The recombination dynamics of excitons has been investigated by time-resolved luminescence spectroscopy.In spite of this intense interest in excitons, one important aspect of excitons, the dynamics of formation of bound states of excitons following photoexcitation of electron-hole pairs, has remained essentially unexplored.The recent results of Kusano et al. are largely dominated by extrinsic effects. The complex process of formation of intrinsic excitons has been identified as an important problem since the introduction of the concept of excitons in solids, but has not been addressed either experimentally or theoretically. Photoexcited electron and hole (either from a geminate or a nongeminate pair) can form an exciton by interaction with acoustic and optical phonons, and also by carrier-carrier interactions. The relaxation of photoexcited pairs within the bands proceeds simultaneously with the exciton formation process. Also, the excitons can be initially formed in the ground as well as excited states, and in the singlet as well as triplet states (corresponding to the orthohydrogen and parahydrogen states). Furthermore, the excitons are very likely created with a large total momentum wave vector E, corresponding to the center-of-mass motion of excitons in the quantumwell planes. The relaxation of these nonthermal excitons into the singlet K =0 state (the only state that can directly couple to the photons) must also be considered. It is clear that an understanding of these aspects of excitons is a fundamental importance in the physics of elementary excitons in solids.We present in this article the first results on the dynamics of exciton formation and relaxation, which provide insight into many facets of exciton physics that have not been considered before. All results presented in this letter deal with intrinsic excitons. We probed exciton formation dynamics in GaAs quantum wells as a function of temperat...