Time-Resolved Spectroscopy in Semiconductors Using Intense Far-Infared (Sub)Picosecond Pulses

Abstract

We have performed far-infrared time-resolved experiments, in semiconductors, using (sub)picosecond pulses from the Free-Electron Laser for Infrared eXperiments FELIX at the FOM-institute ‘Rijnhuizen’, the Netherlands. As a first example we show time-resolved measurement of polarization beats in GaAs:Si, in which we combine the use of ultrashort pulses with photoconductivity measurements¹. Our sample consists of a 400 jam thick GaAs substrate with a 10 μm thick Si-doped GaAs layer on top. The sample is mounted in a magnet cryostat and kept at a temperature of 8 K. The energy of the Si-impurity transitions can be tuned by application of a magnetic field. We concentrate on the ls-2p⁺ donor transition. Two identical collinear far-infrared pulses of 5 ps duration from a Michelson interferometer setup, with a variable time separation τ_d, are weakly focused onto the sample. Electrons, resonantly photo-excited to the 2p⁺ state thermally decay to the conduction band and will give rise to an increase in the conductivity². In Fig. 1 we plot the measured photo-conductivity of the sample as a function of τ_d when the laser is resonant with the 1s-2p⁺ transition, for three values of the magnetic field.