D. Stehr

Long lifetimes of quantum-dot intersublevel transitions in the terahertz range

Carrier relaxation is a key issue in determining the efficiency of semiconductor optoelectronic device operation. Devices incorporating semiconductor quantum dots have the potential to overcome many of the limitations of quantum-well-based devices because of the predicted long quantum-dot excited-state lifetimes. For example, the population inversion required for terahertz laser operation in quantum-well-based devices (quantum-cascade lasers) is fundamentally limited by efficient scattering between the laser levels, which form a continuum in the plane of the quantum well. In this context, semiconductor quantum dots are a highly attractive alternative for terahertz devices, because of their intrinsic discrete energy levels. Here, we present the first measurements, and theoretical description, of the intersublevel carrier relaxation in quantum dots for transition energies in the few terahertz range. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to approximately 2 ps at 30 meV (7 THz), in very good agreement with our microscopic theory of the carrier relaxation process. Our studies pave the way for quantum-dot terahertz device development, providing the fundamental knowledge of carrier relaxation times required for optimum device design.

High-performance fiber-laser-based terahertz spectrometer.

We have developed a rapid scanning terahertz (THz) spectrometer based on a synchronized two-fiber-laser system. When the system is set to the asynchronous optical sampling mode, THz spectra extending to 3 THz can be acquired within 1 μs at a signal-to-noise ratio of the electric field of better than 20. Signal averaging results in a dynamic range of more than 60 dB, and frequency components of more than 4 THz can be detected. When the lasers are set to the same repetition rate, electronically controlled optical sampling at a rate of 2.5 kHz is demonstrated, making the system versatile for different spectroscopic applications. Finally, we compare the THz emission spectra of a photoconductive switch that is pumped at 780 nm and a nonlinear DAST crystal excited at 1550 nm. We find that the spectral range of the spectrometer is significantly enhanced at higher frequencies, while the dynamic range remains constant.

Simultaneous time and wavelength resolved spectroscopy under two-colour near infrared and terahertz excitation

Time and wavelength resolved spectroscopy requires optical sources emitting very short pulses and a fast detection mechanism capable of measuring the evolution of the output spectrum as a function of time. We use table-top Ti:sapphire lasers and a free-electron laser (FEL) emitting ps pulses as excitation sources and a streak camera coupled to a spectrometer for detection. One of the major aspects of this setup is the synchronization of pulses from the two lasers which we describe in detail. Optical properties of the FEL pulses are studied by autocorrelation and electro-optic sampling measurements. We discuss the advantages of using this setup to perform photoluminescence quenching in semiconductor quantum wells and quantum dots. Carrier redistribution due to pulsed excitation in these heterostructures can be investigated directly. Sideband generation in quantum wells is also studied where the intense FEL pulses facilitate the detection of the otherwise weak nonlinear effect.

Tunable narrowband THz pulse generation in scalable large area photoconductive antennas

The generation and characterization of narrowband THz pulses by means of chirped pulse difference frequency generation in Auston-switch type photoconductive antennas is reported. Using optical pulses with energies in the range from 1 nJ to 1 µJ, we generate THz pulses with up to 50 pJ in energy and electric field strengths on the order of 1 kV/cm. Two emitter concepts are investigated and circumvention of the fast saturation for small area excitation by scaling of the THz emitter is demonstrated.

THz-driven quantum wells: Coulomb interactions and Stark shifts in the ultrastrong coupling regime

We investigate the near infrared interband absorption of semiconductor quantum wells driven by intense terahertz (THz) radiation in the regime of ultrastrong coupling, where the Rabi frequency is a significant fraction of the frequency of the strongly driven transition. With the driving frequency tuned just below the lowest frequency transition between valence subbands, a particularly interesting phenomenon is observed. As the THz power increases, a new peak emerges above the frequency of the undriven exciton peak, which grows and eventually becomes the larger of the two. This reversal of relative peak intensity is inconsistent with the Autler–Townes effect in a three-state system while within the rotating wave approximation (RWA). In the samples investigated, the Bloch–Siegert shift (associated with abandoning the RWA), exciton binding energy, the Rabi energy, and non-resonant ac Stark effects are all of comparable magnitude. Solution of a semiconductor Bloch model with one conduction and multiple valence subbands indicates that the ac Stark effect is predominantly responsible for the observed phenomenon.

Coherent control of a THz intersubband polarization in a voltage controlled single quantum well

Ultrashort terahertz pulses in the far-infrared spectral region centered around 2 THz are used to coherently control an intersubband polarization in a GaAs/AlGaAs quantum well structure at low temperature. While the first pulse excites a macroscopic polarization, a second temporally delayed pulse switches the polarization off or refreshes it depending on the relative time delay. The switching is directly demonstrated in the time-domain for the few picosecond long free-induction decay of the induced polarization. Model calculations based on the optical Bloch equations agree well with the experimental data.

Pump-probe spectroscopy of interminiband relaxation and electron cooling in doped superlattices

The picosecond dynamics of electrons in a doped GaAs∕AlGaAs superlattice have been investigated by pump-probe experiments using an infrared free-electron laser. We observe a fast bleaching of the interminiband absorption followed by thermalization and a slower cooling component. The latter can lead to a positive or negative transmission change, resulting from the temperature dependence of the linear absorption spectrum at the respective wavelength. We show that the superlattice in contrast to quantum wells provides a unique picosecond thermometer for the electron temperature based on the dependence of the absorption on the electron distribution function.

Long intersubband relaxation times in n-type germanium quantum wells

We measured the non-radiative intersubband relaxation time in n-type modulation-doped Ge/SiGe multi-quantum wells of different thickness by means of degenerate pump-probe experiments. The photon energy was tuned to be resonant with the lowest conduction band intersubband transition energy (14-29 meV), as measured by terahertz absorption spectroscopy and in agreement with band structure calculations. Temperature-independent lifetimes in excess of 30 ps were observed.

Singlet and triplet polaron relaxation in doubly charged self-assembled quantum dots

Polaron relaxation in self-assembled InAs/GaAs quantum dot samples containing 2 electrons per dot is studied using far-infrared, time-resolved pump–probe measurements for transitions between the s-like ground and p-like first excited conduction band states. Spin–flip transitions between singlet and triplet states are observed experimentally in the decay of the absorption bleaching, which shows a clear biexponential dependence. The initial fast decay (~30 ps) is associated with the singlet polaron decay, while the decay component with the longer time constant (~5 ns) corresponds to the excited state triplet lifetime. The results are explained by considering the intrinsic Dresselhaus spin–orbit interaction, which induces spin–flip transitions by acoustic phonon emission or phonon anharmonicity. We have calculated the spin–flip decay times, and good agreement is obtained between the experiment and the simulation of the pump–probe signal. Our results demonstrate the importance of spin-mixing effects for intraband energy relaxation in InAs/GaAs quantum dots.

Two-color pump-probe studies of intraminiband relaxation in doped GaAs∕AlGaAs superlattices

The miniband relaxation dynamics of electrons in doped GaAs∕AlGaAs superlattices are investigated by two-color infrared pump-probe experiments. By this technique, we are able to separate the different contributions from inter- and intraminiband relaxations to the transient behavior after an ultrafast excitation. In particular, the intraminiband relaxation is studied for different miniband widths below and above the optical phonon energy of GaAs. For minibands wider than this critical value, we find fast relaxation, nearly constant for different excitation intensities, whereas for narrow minibands, a strong temperature and intensity dependence of the relaxation is found. The results are in good agreement with previously published Monte Carlo simulations.