R. Kersting

Time-domain measurement of intersubband oscillations in a quantum well

We report time-domain measurements of electron intersubband oscillations in quantum wells. We use an interferometric technique to measure the change in the profile of a few-cycle THz pulse due to propagation through a modulation-doped Al0.3Ga0.7As/GaAs multiple quantum well structure (10×510u2009A wells). From this data we obtain the absorption and the index of refraction due to electrons in the quantum well, and due to the GaAs substrate. Unlike existing studies of coherent charge oscillations of electrons and holes in heterostructures excited by ultrashort pulses of band-gap light, our all-THz measurements quantitatively determine the linear optical properties of the quantum well electrons.

Sampling a terahertz dipole transition with subcycle time resolution

We present a time-resolved technique to measure optical excitation processes with a time resolution shorter than the oscillation period of the exciting light. Our terahertz (THz) experiments fully resolve the polarization dynamics of electrons in semiconductor heterostructures when they are excited by a THz pulse. The time resolution of the polarization enables us to deduce the population dynamics of the excited state, which includes the dynamics of a virtual population in the case of off-resonant excitation.

Coherent terahertz emission from optically pumped intersubband plasmons in parabolic quantum wells

We report on few-cycle terahertz (THz) emission from modulation-doped parabolic quantum wells. The quantum wells are optically excited by near-infrared femtosecond laser pulses. The observed THz emission corresponds to the intersubband plasmon of the parabolic quantum well. The emission frequency is independent of the number of optically generated carriers. We identify the excitation mechanism of the intersubband plasmon and hence THz emission to be ultrafast field screening. This mechanism allows for an optically driven THz emission from a completely symmetric nanostructure, in contrast to quantum beats which require a broken symmetry for their excitation.

Coherent THz plasmons in GaAs/AlGaAs superlattices

Abstract We report on THz emission from coherent plasma oscillations in GaAs/AlGaAs superlattices after excitation by femtosecond laser pulses. The carriers which oscillate inside the superlattice are either dopant electrons or electrons that are optically generated by the laser pulses. The oscillations are observable from low temperatures up to room temperature. A nearly full or a full miniband prevents these oscillations as it is the case when the miniband is thermally saturated.

Few-cycle THz generation for imaging and tomography applications.

The methods for generating few-cycle THz radiation from semiconductors without external applied fields are reviewed. Their spectral characteristics, efficiency and prospects for imaging and tomography at terahertz frequencies are discussed.

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.

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.

THz time-domain spectroscopy of intersubband transitions

The ability to generate ultrashort pulses of THz radiation has made time-domain spectroscopy possible at THz frequencies. Until now, this technique has only been applied to spectroscopy of free photoexcited carriers. Here, we present a time-resolved study of intersubband transitions of extrinsic electrons in quantum wells (QWs).