Cyclic behavior of ultrafast self-modulation of the light-absorption spectrum under conditions of pump and stimulated emission in GaAs

Under picosecond photogeneration of charge carriers in GaAs, accompanied by intense stimulated emission of the semiconductor itself, ultrafast self-modulation of its light-absorption spectrum takes place, which consists in the appearance of regions of local absorption enhancement (bumps) in the spectrum. The ultrafast self-modulation is found to exhibit a cyclic behavior; i.e., the pattern of the self-modulation of the spectrum (the number and spectral position of the bumps) is repeated after a certain time T _c falling in the picosecond range. The cycle period T _c varies over the time span of the pump pulse and depends on the pulse energy, which means that T _c is a function of the pump intensity. Assuming that self-modulation of the absorption reflects self-modulation of the charge-carrier energy distribution in GaAs under pumping, experimental results can be formulated as follows: in the process of the ultrafast self-modulation, deviations of the occupancies of different energy levels from the Fermi distribution evolve with time in a mutually related way; the distribution of the occupancy depletion in the conduction band repeats cyclically in time; and the cycle period decreases as the intensity of the pump increases.     

Relation between the Relaxation of Intrinsic Stimulated Picosecond Emission from GaAs with a Characteristic Charge-Carrier Cooling Time

Semiconductors - During the powerful picosecond optical pumping of a thin (~1 μm) GaAs layer, a stimulated intense (up to 1 GW/cm2) picosecond emission arises. It is found that the...     

Anticorrelation between the Intensity of Stimulated Picosecond Emission in GaAs and the Characteristic Time of Charge-Carrier Cooling

Semiconductors - During the powerful picosecond optical pumping, intense stimulated picosecond emission arises in a thin GaAs layer. It is found that, first, the maximum emission intensity...     

Stimulated-emission spectrum arising from interband absorption of a picosecond optical pulse in a thin layer of GaAs

Pumping a thin layer of GaAs with a high-power picosecond optical pulse leads to nonstationary edge emission. Experimental data are obtained for the way the time-integrated power spectrum of this emission varies with beam diameter and energy of the optical pump pulse. These data are sufficient to confirm the stimulated nature of the emission, whose duration is in the picosecond time range. Fiz. Tekh. Poluprovodn. 32, 537–541 (May 1998)     

A protrusion in the absorption spectra of GaAs excited by high-power picosecond light pulses

Absorption spectra of GaAs excited by short high-power light pulses are calculated. The appearance of a “protrusion” in the spectra is caused by the deviation of the electron distribution function from the Fermi function. The distortion of the distribution function is related to LO phonon-assisted relaxation of electrons between the states involved in the formation of a “hole” in the gain region and a “protrusion” in the absorption region. It is shown that the temperature of optical phonons responsible for the relaxation of photoexcited electrons differs from the lattice temperature and the time of recovery of the perturbed Fermi distribution via electron-electron collisions is nearly equal to the characteristic time of interaction between electrons and optical phonons.     

Self-synchronization of the modulation of energy-levels population with electrons in GaAs induced by picosecond pulses of probe radiation and intrinsic stimulated emission

Picosecond optical pumping leads to the initiation of intrinsic picosecond stimulated emission in GaAs. As was established previously, due to the interaction of pulses of probe radiation with those of intrinsic emission, the dependence of the absorption α of the probe pulse on its delay τ with respect to the pump pulse is modulated with oscillations. It is found that the oscillatory dependences α(τ) have a similar shape only in the case of certain combinations of energies of the interacting pulses. As a result, it is assumed that the above interaction is, in fact, a synchronization of modulations (formed by pulses) of charge-carrier populations at energy levels; this synchronization occurs in the direction of the reconstruction of detailed equilibrium. The real-time picosecond self-modulation of the absorption α is measured for the first time. The characteristics of this self-modulation as well as absorption α and intrinsic emission self-modulation characteristics measured previously by correlation methods are now accounted for by the concept of synchronization.     

Participation of electron-phonon interaction in the ultrafast self-modulation of absorption of light in GaAs. Relation of modulation of absorption with the spectrum of stimulated radiation in GaAs

Ultrafast (varying for ～ 1 ps) self-modulation of the absorption spectrum of light takes place during the picosecond-scale photogeneration of charge carriers and intense intrinsic stimulated radiation in GaAs. With the modulation, formation of local amplifications of absorption in the spectrum (juts), which are attributed to local depletion of electron populations in the conduction band, is implied. It is found experimentally that the location of the juts in the spectrum is repeated over the interval determined by the energy of the longitudinal optical (LO) phonon and masses of the electron and heavy hole. This circumstance confirms the previous assumption about the substantial role of the electron-(LO phonon) interaction in ultrafast self-modulation of the absorption spectrum. The previously established notion of the relation of the shape of modulation of the absorption spectrum with the shape of the time-integrated spectrum of intrinsic picosecond radiation is also expanded to the case when ultrafast self-modulation of the absorption spectrum manifests itself.     

Effect of the diameter of the photoexcited region on the picosecond relaxation of bleaching in a thin layer of GaAs

It is found experimentally that the relaxation of bleaching in GaAs, which takes place when charge carriers are photogenerated by a high-power picosecond optical pulse, is slowed as the beam diameter F increases. Relaxation of the bleaching is caused by a decrease in the concentration of carriers due to recombination-induced superluminescence in GaAs. It is found that as the diameter F increases, the rate of superluminescence recombination slows, although the intensity of the superluminescent emission should increase. This apparent contradiction is explained with the help of theory. Fiz. Tekh. Poluprovodn. 32, 542–545 (May 1998)