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Journal of the Optical Society of America B

Dephasing in the gain region of II–VI semiconductor nanocrystals

Harald W. Giessen, B. Fluegel, Georg Mohs, Yuan Z. Hu, Nasser N. Peyghambarian, U. Woggon, Claus F. Klingshirn, P. Thomas, S. W. Koch

Published May 1, 1996• 19 citations
Publisher

Abstract

We investigate the dephasing times in highly excited CdSe and CdS0.3Se0.7 nanocrystals by spectral hole burning throughout the gain region. The energy dependence of the dephasing time T2 is compared between quantum dots in the strong-confinement regime and bulklike microcrystals. T2 in strongly confined quantum dots remains rather constant, whereas the bulklike sample shows a continuous increase of T2 toward the transparency point. This observation is attributed to the different gain mechanisms in the strong and the weak quantum-confinement regimes.

DOI
10.1364/josab.13.001039

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We study the coherence times of CuBr nanocrystals (quantum dots) in a borosilicate matrix under resonant excitation. Measurements are performed in two different experiments: Degenerate four-wave mixing with pulses of approximately 100 fs duration and spectral hole burning using a nanosecond excitation. Our results show a dependence of the dephasing times on the excitation intensity, and, when extrapolated to low intensity, a longer coherence time under fs than ns excitation. This result is tentatively explained as being due to spectral diffusion in the case of ns excitation.
We report what we believe to be the first observation of coherent transient absorption changes in glasses doped with quantum dots of CdS0.12Se0.88. These observations were made at room temperature with on-resonant laser pumping. We observe a transient bleaching on the high energy side of the pump center wavelength, and a transient absorption increase on the low energy side. The transient effects last for a time about equal to the pump pulse duration, 80 fsec, after which real carrier generation masks the effects. The results can be qualitatively explained by a spectral shift in the direction predicted by the optical Stark effect. This was a pump-probe experiment where the 622-nm pump center wavelength was on the long wavelength side of the lowest energy transition in the quantum dots but still well within the absorption band. The radii of the microcrystallites ranged from 20 to 40 Å, and T2 in similar samples was estimated to be 60 fsec or less. Although the laser pulsewidth is longer than the dephasing time in the sample, we were still able to see coherent effects.
This paper explores quantum-coherence phenomena in a semiconductor quantum-dot structure. The calculations predict the occurrence of inversionless gain, electromagnetically induced transparency, and refractive-index enhancement in the transient regime for dephasing rates typical under room temperature and high excitation conditions. They also indicate deviations from atomic systems because of strong many-body effects. Specifically, Coulomb interaction involving states of the quantum dots and the continuum belonging to the surrounding quantum well leads to collision-induced population redistribution and many-body energy and field renormalizations that modify the magnitude, spectral shape, and time dependence of quantum-coherence effects.
Persistent spectral-hole-burning (PSHB) phenomena have been observed in many kinds of semiconductor quantum dots (QDs). Luminescence hole burning and charged excitons have been observed in CuCl QDs in NaCl crystals. The observed PSHB phenomena are explained by the ionization of QDs. The energy of photoionized QDs is changed from the original energy and their new energies depend on the spatial arrangement of trapped carriers. Quantum confinement of carriers and the resulting strong Coulomb interaction between confined carriers and trapped carriers cause the energy change. PSHB has been widely applied to site-selective spectroscopy of QDs. Size-dependent excited exciton quantum states in CuCl quantum cubes, size-dependent excited electron-hole quantum states in CdTe QDs and renormalization of longitudinal optical phonons confined in CuCl QDs have been revealed by studying sideband holes. This also allowed the measurement of accumulated photon echo, which clarified the very sharp homogeneous spectral linewidth of QDs under weak excitation conditions.