Abstract

Lead chalcogenides represent an important class of narrow-gap semiconductors that have found application in photodetectors and laser diodes for the wavelength range between 3 and 30 µm.^[1] Nonlinear optical effects in these materials are relevant for switching devices and modulators to be operated in conjunction with laser diodes emitting in the mid-infrared. In this paper, we report the first picosecond investigation of optical nonlinearities in lead salt semiconductors on the picosecond time scale.^[2] PbSe and PbEuSe crystals grown by molecular beam epitaxy are studied in pump-probe experiments with tunable mid-infrared pulses of 8 ps duration. Generation of an electron-hole plasma by the pump pulse results in changes of the absorption close to the bandgap which are monitored in spectrally and temporally resolved measurements. In Fig. 1, results for PbSe are presented. The data of Fig. 1 a which were taken with the sample under Brewster angle give the band edge before (open circles) and 10 ps after excitation (solid circles). For an excitation density of 4×10¹⁷ cm^–3, we observe a distinct blue shift of the absorption spectrum due to band-filling by both electrons and holes. Fig. lb gives results for normal incidence where the stationary spectrum (open circles) exhibits strong Fabry-Perot resonances below the band gap. The spectrum observed after excitation (solid circles) shows - in addition to the blue shift of the band edge - a well pronounced change of transmission in the range of these resonances. The latter effect gives direct evidence of a strong change of the refractive index due to band-filling. Theoretical calculations of the transient absorption spectra demonstrate that band-filling by electrons and holes makes contributions of similar magnitude to the change of absorption. This behavior is due to the practically identical effective masses in the valence and the conduction band. The absorption edge calculated for an excitation density of 4×l0¹⁷ cm^–3 is plotted in Fig 1 a (dashed line) and is in agreement with the experimental results. The maximum change of the index of refraction occurs at photon energies slightly below the band gap and has a high value of Δn=-0.1, comparable to narrow-gap III-V compounds like InSb.

© 1992 IQEC