Ultrafast laser probe of interband absorption edges in 3-D and 2-D semiconductors

Abstract

The wavelength of ultrafast laser probes near the fundamental (E0) absorption edge falls in the infrared and visible wavelengths in most diamond and zincblende structure semiconductors. Consequently the time-resolved reflectivity and transmission in a photo-excited sample are influenced by numerous factors: Drude reflectivity and free carrier absorption, in addition to interband saturation and band gap renormalization. In this work we demonstrate the utility of probing the higher (E1 and E2) absorption edges using two-photon absorption spectroscopy or ultraviolet wavelength probes. 1) In silicon we probe the E2 absorption edge by direct two-photon absorption using visible femtosecond pulses above the indirect edge. Longer pulses melt the sample before reaching peak intensities at which two-photon absorption becomes dominant. We extract the direct two-photon absorption coefficient over a wide spectral range, and distinguish other nonlinear absorption channels, and relate these results to the band structure of silicon. 2) Using an ultraviolet probe with visible pump, Drude effects and interband saturation become negligible, leaving renormalization of the E1 and E2 edges as the dominant influence on the probe. Time-resolved experiments compare the renormalization induced by hot carriers, cold carriers and lattice heating in 3D and 2D semiconductors.