C.J.G.M. Langerak

Ultrafast detection and autocorrelation of picosecond THz radiation pulses with a GaAs/AlAs superlattice

We used a wide miniband GaAs/AlAs superlattice (at room temperature) for detection and autocorrelation of picosecond THz radiation pulses (frequency 4.3 THz) from a free-electron laser. The detection was based on a THz-field induced change in conductivity of the superlattice, and the correlation on the nonlinearity of the conductivity change at strong THz-pulse-power. The nonlinear conductivity change was due to two effects, which we attribute to dynamical localization of miniband electrons and to ionization of deep impurity centers.

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.

Experiments on semiconductor systems using a pulsed-field-magnet free-electron-laser combination

Abstract The FELIX free-electron laser is continuously tuneable over the wavelength range 5–100 μm and can provide great power and the possibility of time-resolved experiments in the ns to μs range. We describe a pulsed magnet specifically designed for semiconductor experiments using FELIX, which provides fields of up to 45 T, with a pulse-shape tailored to the macropulses of the FELIX source. The magnet consists of a conventional outer coil providing fields of up to 30 T with a pulse length of several ms, plus a single-layer inner coil with a rise-time of ∼10 μs, matched to duration of the FELIX macropulses. The magnet is to be extended to fields of up to 60 T.

Using far-infrared two-photon excitation to measure the resonant-polaron effect in the Reststrahlen band of GaAs:Si

Abstract We demonstrate that we can use far-infrared two-photon excitation with picosecond pulses to measure the resonant polaron effect of the 1s-3d +2 silicon impurity transition in the middle of the Reststrahlen band of GaAs:Si. Contrary to single-photon measurements, our two-photon measurements do not suffer from the problems of strong reflection and small penetration depth of light in the Reststrahlen band of the GaAs host lattice. As a result, we are able to measure a significant fraction of the magnetic-field dependent anticrossing of the 1s-3d +2 transition with the LO-phonon.

Intersubband lifetimes in InAs/GaSb superlattices using saturated absorption spectroscopy

Abstract We investigate intersubband relaxation rates above the optical phonon energy in a InAs/GaSb superlattice using saturation spectroscopy. A high-intensity free-electron laser tuned to the intersubband transition energy is used to saturate the absorption process revealing picosecond relaxation rates. The effects of the parallel magnetic field and laser energy on the relaxation processes are explored.

Resonant magnetopolaron coupling to both polar and neutral optical phonons in the layer compound InSe

Abstract Cyclotron resonance measurements in the highly polar ( α = 0.29) semiconductor InSe are reported in pulsed magnetic fields up to 37 T. For B > 18 T two cyclotron branches are found which are a consequence of the resonant polaron interaction. This is the first time that both branches have been observed simultaneously in 2D, and both are observed over a range of more than 10 T. The splitting is approximately 40% of the optical phonon energy which results in a change in effective mass of a factor of two between the upper and lower branches. A detailed comparison is made with a theory for the cyclotron resonance spectrum. A second, much weaker resonant interaction is found for B ≈ 18 T. This is due to a resonant interaction with homopolar phonons and a theory is constructed for this interaction. At resonance the splitting is a very sensitive function of the interaction strength, allowing accurate determination of the coupling constant g 2 = 0.001.

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

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 τd, 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 τd when the laser is resonant with the 1s-2p+ transition, for three values of the magnetic field.