Abstract

We recently demonstrated that intracavity nonlinear optical processes can be introduced into passively mode-locked femtosecond lasers without disrupting mode-locking stability.¹ As a result femtosecond source lasers have been extended² to the UV using intracavity frequency doubling and to the IR using intracavity parametric downconversion. We demonstrate the ability of combined visible (hν_probe = 2.0-eV) and UV (hν_probe= 4.0-eV) femtosecond probing to elucidate low density carrier dynamics in novel single crystal thin films of Ge prepared by molecular beam epitaxial growth on a CaF₂ substrate. Use of optically thin films (thickness t ~ optical absorption depth α⁻¹ ~ 500 Å in Ge) provides a clear advantage in the study of low density carrier dynamics, because reflectivity and transmission changes caused by small refractive index changes are amplified by optical interference effects, as demonstrated previously in silicon films.³ In addition, vertical carrier diffusion is suppressed. Previous ultrafast spectroscopy of Ge has been performed with bulk samples or optically thick polycrystalline films at much higher levels of excitation.⁴

© 1990 Optical Society of America