I. RUCKMA" (a), J. KORNACK (a), J. KOLENDA (b), and M. PETRAUSKAS (b)One-beam transmission experiments and two-beam-coupling experiments with 35 ps-pulses at 1.06 pm are performed in CdTe and CdTe: Ge to investigate (i) the nonlinear (two-photon (TPA), free-carrier and two-step) absorption processes, (ii) their kinetics, and (iii) the transient energy transfer (TET) from stronger pump beam to the weaker probe beam, caused by coherent coupling which is observed around (with its maximum just before) zero delay. The competition between TPA and TET is demonstrated by oscillating intensity dependences of the probe-beam transmittivity measured and calculated at small delays, which can be used to realize a continuous transition from optical beam amplification to attenuation at a fixed delay in dependence on the pump intensity only. The influence of the pump-to-probe intensity ratio on TET is discussed. Ein-Strahl-Transmissionsexperimente undZwei-Strahl-Kopplungsexperimente mit 35 ps-Impulsen bei 1,06 pm werden in CdTe und CdTe : Ge durchgefiihrt, um (i) die nichtlinearen Absorptionsprozesse (Zwei-Photon-Absorption (TPA), freie Tragerabsorption und Zwei-Stufen-Absorption), (ii) ihre Zeitabhangigkeiten und (iii) den transienten Energietransfer (TET) vom intensiven Pumpstrahl in den Probenstrahl, der durch koharente Kopplung hervorgerufen und dessen Maximum zeitlich kurz vor der Verzogerung At = 0 beobachtet wird, zu untersuchen. Die Konkurrenz zwischen TPA und TET zeigt sich in oszillierenden Intensitiitsabhangigkeiten der Probenstrahltransmission, die im Bereich kleiner Verzogerungen gemessen und berechnet werden und die benutzt werden konnen, um bei fester Verzogerung einen kontinuierlichen Obergang von optischer Verstarkung zur Diimpfung nur in Abhangigkeit von der Pumpintensitat zu realisieren. Der EinfluD des Intensitatsverhaltnisses von Pump-zu Teststrahl auf den TET wird diskutiert. ') Invalidenstr. 110, DDR-1040 Berlin, GDR. ' ) Saulitekio al. 9, 232 054 Vilnius, USSR.
The methods of light-induced absorption and diffraction have been used to investigate the uniformity of Si and GaAs wafers and ion-implanted Si. Metastable annealing of defects, lifetime modification by swirl and radiation defects and anomalous depth penetration of ion-implantation-created defects have been demonstrated in Si. The mapping of GaAs wafers has been performed with picosecond and nanosecond light pulses and light absorption mechanisms are discussed.
The noncontact transient grating technique under constant electron-hole pumping to the density 4×1019 cm−3 was used to characterize the ambipolar diffusion coefficient Da in Si:Al, Si:In, and Si:Sb molecular-beam-epitaxy-grown layers. Da was found to be almost constant at a value of ≊8 cm2/s up to an equilibrium carrier density of 2×1019 cm−3 in the layer and was independent of the doping type. At higher doping density, evidence for a sharp increase in Da was observed. For example, Da increased to a value of 20 cm2/s at a doping density of about 1020 cm−3. The Da behavior is in reasonable agreement with results of the high-density ambipolar diffusion theory of Young and van Driel and is incompatible with majority-carriers diffusion coefficients according to the formula Da = 2DnDp/(Dn + Dp). An explanation for this behavior is given.
BYThe nonlinear optical properties of 3D-confined semiconductor microcrystallites (MC) embedded in a glass matrix have been the subject of intense research in the last years. Having small MCs with nearly identical sizes a characteristic MC absorption line can be observed in the linear spectrum up to room temperature. Its halfwidth is essentially influenced by the MC size distribution because the MCenergy states depend sensitively on the MC size. In e.g. /1 to 31 the bleaching of the MC-absorption line at laser excitation was studied. For MC sizes in the strong confinement limit the achievable bleaching value is distinctly smaller than expected for a simple two-level model. Different explanations were taken into consideration : traps in the MC, in the glass, and at the interface / 4 , 5 1 , Auger recombination / 5 to 7 1 , two-photon or free-carrier absorption into continuum states / 5 , 81. All these processes result in a partly deoccupation of the excited MC level. On the other hand, at resonant laser excitation of the MC simultaneously a darkening is observed, too, that means that after laser exposure a smaller bleaching value as well as a faster recovery of the bleaching of the MC line are obtained in comparison with the behaviour found in "fresh" samples /e.g. 8, 2 / . This darkening is irreversible and increases with exposure time. In some cases after long (MC-resonant or UV) laser irradiation a darkened area on the glass is visible, too. The darkening can be made reversible only by heating the sample over 450 O C . From our point of view at resonant MC excitation an energy transfer of excited carriers into the glass matrix occurs which is advanced by the MC-glass interface states and, as a consequence, (i) the observable bleaching of the MC line is smaller than expected and (ii) a darkening effect is obtained related with the properties of the glass matrix. The latter may be due to the charge state changes of network modifier ions or by structural changes in the glass. As e.g. discussed in / 9 / to explain permanent light-induced gratings in Eu-doped silicate glasses the ) Invalidenstr. 110, DDR-1040 Berlin, GDR.
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