R. Bratschitsch

Few-cycle THz spectroscopy of semiconductor quantum structures

Abstract Optically excited plasma oscillations in n-doped GaAs epilayers emit intense THz pulses. From THz emission experiments in doped superlattices the miniband properties can be revealed. Using a THz-pump and THz-probe technique we observe the response of the intersubband polarization in semiconductor quantum structures. THz cross-correlation measurements of modulation doped semiconductor quantum structures allow to determine the absorption, the dispersion, and the dephasing times of the quantized electrons.

Coherent THz emission from optically pumped parabolic quantum wells

Summary form only given. Several mechanisms for coherent THz emission from both semiconductor bulk and heterostructure devices have been presented. Recently, Sekine et al. reported on the optically pumped emission of THz radiation from grating-coupled intersubband plasmons in a doped single quantum well. We present experiments which show that modulation doped parabolic quantum wells (PQWs) emit coherent THz radiation corresponding to the intersubband plasmon when excited by near infrared femtosecond laser pulses.

Few-cycle THz spectroscopy of nanostructures

Abstract Optically excited plasma oscillations in n-doped GaAs epilayers emit intense THz pulses. Using a THz-pump and THz-probe technique we observe the response of the intersubband polarization in semiconductor quantum structures. THz Cross-correlation measurements of modulation doped semiconductor quantum structures allow to determine the absorption, the dispersion, and the dephasing times of the quantized electrons.

Coherent and incoherent intersubband dynamics

Presents time-resolved measurements of photoinduced intersubband absorption in an undoped GaAs/AlGaAs coupled quantum well excited by an ultrashort interband optical pulse. An interband pump pulse injects electrons into the first and second subband of an undoped asymmetric double quantum well (ADQW) with a level spacing smaller than the LO phonon energy. The time evolution of the electron population in these two subbands is monitored by probing the MIR intersubband transitions to a third (empty) subband.

Excitation of intersubband transitions by THz pulses

Summary form only given. When intersubband transitions are excited in semiconductor heterostructures, the charge carriers follow the driving field in an oscillatory motion. THz time-domain spectroscopy is an attractive tool to study the polarization following an exciting THz pulse. A time resolution which is shorter than the oscillation time of the driving field enables the observation of the coherent electron motion for the first time. The investigated GaAs/AlGaAs heterostructures are modulation doped parabolic quantum wells.

THz emission of coherent plasmons in semiconductor superlattices

Summary form only given. Ultrafast excitation of n-doped GaAs layers leads to THz emission from coherent plasma oscillations. The emission frequency of this THz source is given by the simple plasma frequency formula /spl omega//sub p/=(ne/sup 2//m*/spl epsiv/)/sup 1/2/ where n is the number of extrinsic electrons which are confined between the surface depletion field and the GaAs substrate. The desired frequency can be controlled by changing the number of electrons n via the doping concentration of the material. Another possibility to determine the emission frequency /spl omega//sub p/ is to change the effective mass m* of the electrons. This can be accomplished by placing the electrons in the periodic potential of a superlattice. We study the GaAs/AlGaAs heterostructures which are three superlattices.