W. Th. Wenckebach

Design and performance of a THz emission and detection setup based on a semi-insulating GaAs emitter

We have built a relatively simple, highly efficient, THz emission and detection system centered around a 15 fs Ti:sapphire laser. In the system, 200 mW of laser power is focused to a 120 μm diam spot between two silverpaint electrodes on the surface of a semi-insulating GaAs crystal, kept at a temperature near 300 K, biased with a 50 kHz, ±400 V square wave. Using rapid delay scanning and lock-in detection at 50 kHz, we obtain probe laser quantum-noise limited signals using a standard electro-optic detection scheme with a 1-mm-thick (110) oriented ZnTe crystal or a (110) oriented 0.1-mm-thick GaP crystal. The maximum THz-induced differential signal that we observe is ΔI/I=7×10−3, corresponding to a THz peak amplitude of 95 V/cm. The THz average power was measured to be about 40 μW, to our knowledge, the highest power reported so far generated with Ti:sapphire oscillators as a pump source. The system uses off-the-shelf electronics and requires no microfabrication techniques.

Hole transport in strained Si

In this article Monte Carlo simulation of hole transport in uniaxially strained Si is reported. The results are obtained using an exact analytical diagonalization of a six band k⋅p model. Thus we include the spin-orbit, light hole and heavy hole subbands, anisotropy, non-parabolicity and strain. Optical phonon and acoustical phonon scattering are implemented. In weak electric fields the drift mobility is found to increase with strain. The increase depends on the strain being tensile or compressive. The difference found in the two cases can be explained by the larger scattering rate in the tensile case which is caused by the greater density of states in the heavy hole band. At higher fields the drift mobility drops rapidly in strained Si as a function of the electric field.

Active mode locking of a p-Ge hot hole laser

The generation of 200 picosecond pulses of far-infrared radiation from a p-Ge hot hole laser (50–140 cm−1) was achieved due to active mode locking by electrical intracavity modulation of the gain.

Gain of the mode locked p-Ge laser in the low field region

Following the earlier observation of active mode locking in the high field region of the Voigt configured p-Ge intervalence band laser, presently mode locking in the low field region is also reported. The experimental results on the effective small signal gain for active- as well as for self-mode locked operation are given.

A terahertz system using semi-large emitters: noise and performance characteristics

We have built a relatively simple, highly efficient, terahertz (THz) emission and detection system centred around a 15 fs Ti:sapphire laser. In the system, 200 mW of laser power is focused on a 120 microm diameter spot between two silverpaint electrodes on the surface of a semi-insulating GaAs crystal, kept at a temperature near 300 K, biased with a 50 kHz, +/- 400 V square wave. Using rapid delay scanning and lock-in detection at 50 kHz, we obtain probe laser quantum-noise limited signals using a standard electro-optic detection scheme with a 1 mm thick (110) oriented ZnTe crystal. The maximum THz-induced differential signal that we observe is deltaP/P = 7 x 10(-3), corresponding to a THz peak amplitude of 95 V cm(-1). The THz average power was measured to be about 40 microW, to our knowledge the highest power reported so far generated with Ti:sapphire oscillators as a pump source. The system uses off-the-shelf electronics and requires no microfabrication techniques.

Excite-Probe Determination of the Intersubband Lifetime in Wide Gaas/Algaas Quantum-Wells Using a Far-Infrared Free-Electron Laser

A direct excite-probe semiconductor lifetime determination in the picosecond regime has been made for the first time in the far infrared. We have used an RF-linac-pumped free-electron laser to determine the relaxation rate associated with intersubband absorption in GaAs/AlGaAs quantum wells having a subband separation smaller than the optical phonon energy. The measurement yields a relaxation lifetime of 40+or-5 ps. This is compared with a variety of other results obtained with less direct techniques.

Complete bleaching of the intersubband absorption in GaAs/AlGaAs quantum wells using a far‐infrared free‐electron laser

The intensity‐dependent intersubband absorption in GaAs/AlGaAs quantum wells with a subband separation smaller than the optical phonon energy has been measured with a pulsed far‐infrared free‐electron laser. Complete bleaching of the absorption is observed at I=200 kW/cm2. Fitting the data with a two‐level system yields a characteristic time constant of 1–2 ps. Possible interpretations, considering the finite pulse width of the laser, are discussed.

Calculation of the Hall scattering factor using a Monte Carlo technique

We present a method to calculate the Hall scattering factor using a Monte Carlo technique. The greater accuracy of the new method makes it possible to calculate rH in p-Si and Ge at room temperature much faster. The models employed in the calculation include all details of the valence band and acoustical and optical phonon scattering processes.

Generation of subnanosecond high power far infrared pulses using a FEL pumped passive resonator

Abstract By cavity-dumping a small passive, external resonator, subnanosecond far infrared pulses with enhanced peak powers were created. The resonator was pumped by radiation produced by the UC Santa Barbara free electron laser and subsequently cavity-dumped using a fast optical semiconductor switch.

Free electron laser induced two‐photon absorption in Hg1−xCdxTe

The powerful output of FELIX, the recently built Rijnhuizen free‐electron laser, is used for the first frequency dependent study of nonlinear optical excitation of Hg1−xCdxTe in the far‐infrared spectral region. Two‐photon interband absorption has been investigated as a function of power and wavelength from 20 to 40 μm. This nonlinear optical absorption is used to perform autocorrelation experiments yielding the length of the micropulses of the free‐electron laser on a femtosecond time scale. The predicted dependence of the micropulse length on the synchronization between optical and electron pulses in the laser cavity is observed.