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Optica Publishing Group
  • Quantum Electronics and Laser Science Conference
  • OSA Technical Digest (Optica Publishing Group, 1993),
  • paper QMG3

Dephasing at the direct gap of germanium

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Abstract

In contrast to many direct-gap semiconductors, little is known about the ultrafast dynamics at the direct band edge of indirect-gap semiconductors, e.g., germanium and silicon. Such information is of fundamental interest and delivers material parameters relevant for, e.g., Si/Ge heterostructures, Few experiments study the dynamics in Ge well (100 meV) above the direct optical transition1 (cooling of carriers). We here investigate the dephasing at the band edge of Ge in the low-excitation regime at. low temperatures (T = 1.8 K). The samples are platelets a few micrometers thick (optical density 0.6 at the exci- ton) of ultrapure, intrinsic Ge that are coated on both sides with an antireflection compound and are mounted free of strain. An infrared femtosecond (140-fs) continuum laser system based on a color-center laser and amplifier allows us to perform excitation from room temperature (Ex = 800 meV) all the way to T= 1.8 K (Ex = 887 meV). To get a closer insight into the dephasing dynamics we spectrally resolve the diffracted wave in degenerate femtosecond four-wave mixing (FWM). Figure 1 shows the spectra of the FWM signal for different time delays. The excitation is centered well within the continuum states (as depicted in the background). In this case a narrow peak is observed at the excitonic resonance, as is a significant contribution of the continuum states at the laser position. Hence we can distinguish between the dynamics of the exciton and the continuum states.

© 1993 Optical Society of America

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