W. Parz

Ultrafast probing of light-matter interaction in a midinfrared quantum cascade laser

In this work, we study the interaction of ultrashort midinfrared pulses with the active medium of an InGaAs∕InAlAs∕InP quantum cascade laser emitting at an 11.7μm wavelength. Applying an electro-optic sampling technique allowed us to measure the complete phase resolved transmission spectra at operating conditions below and above lasing threshold in a spectral range much broader than the gain band width. Far below threshold, we locate broadband resonant absorption, which spectrally overlaps with the electrically induced gain, forming areas of net absorption and net gain. Above threshold, gain clamping is seen, and it is found that echoes delayed by the round trip time experience spectral pulse shaping converging toward the emission spectrum.

Pulse-induced quantum interference of intersubband transitions in coupled quantum wells

We report the measurement of the transient intersubband absorption in a coupled quantum well semiconductor structure. Pulsed interband excitation is used to prepare a coherent population in the two lowest states of a three level system of electronic subbands. The time-dependent absorption to the upper state is probed by midinfrared time-domain spectroscopy. For certain pump–probe delay times we observe in the intersubband absorption spectra typical signatures of quantum interference: regions with reduced or enhanced absorption.

Ultrafast phase-resolved pump-probe measurements on a quantum cascade laser

The dynamics of optical gain in an InGaAs/AlInAs quantum cascade laser is studied in midinfrared pump-probe measurements. Pump and probe pulses of identical wavelength, polarization, and propagation direction through the laser waveguide are detected separately by electro-optic sampling. For injection currents below the lasing threshold, we observe an absorption decrease in the laser transition with a ≈3 ps recovery time. Above threshold, the gain strongly saturates and shows a fast recovery time decreasing with current. Such kinetics is superimposed by oscillations with a frequency of 0.8 THz originating from coherent electron tunneling through the injection barrier.

Intersubband gain-induced dispersion

Time-resolved transmission spectroscopy of a mid-infrared quantum cascade laser emitting at 11.7 mum allows us to iteratively retrieve the effective refractive index and the extinction coefficient of the gain medium in a broad spectral range with an accuracy of +/-7x10(-3). Besides a 3% slowdown of the group velocity we find a large induced group-velocity dispersion with changing signs in the vicinity of the gain maximum, disclosing implications for self-pulse formation in quantum-cascade lasers. Additionally we measured the temperature in the active region by exploiting the thermo-optic effect. A linear behavior with respect to the current and the duty cycle was observed.

Intraband relaxation of photoexcited electrons in GaAs/AlGaAs quantum wells and InAs/GaAs self-assembled quantum dots

We present time-resolved measurements of photoinduced intraband absorption in undoped GaAs/AlGaAs quantum well and InAs/GaAs self-assembled quantum dot structures after interband excitation. By using ultrabroadband mid-infrared pulses we determine the temporal evolution of the intersubband absorption spectrum of a double quantum well sample (subband spacing < longitudinal optical phonon energy) after excitation. From these measurements we determine an intersubband relaxation time of 14 ps at T = 5 K. By tuning mid-infrared pulses into resonance with intraband transitions between confined quantum dot states and the wetting layer continuum, the electron population of the quantum dot ground and first excited states is determined as a function of time-delay after the pump. We find that the most efficient relaxation pathway into the quantum dot ground state is the step-wise relaxation through the excited states of the dot. The capture time at T = 5 K is 4.7 ps.

Few-cycle terahertz generation and spectroscopy of nanostructures.

We report on new schemes for terahertz (THz) generation. The THz efficiency of photoconducting antennas can be increased by using a cavity effect for the near–infrared pump beam. The cavity is formed by a molecular beam epitaxy grown semiconductor Bragg mirror below the photoconducting layer. The optical confinement is accompanied by an electrical confinement suppressing undesired leakage currents and providing a constant electric field in the active layers. The performance of this cavity–enhanced emitter is further improved by using a mobility optimized low–temperature GaAs layer. This emitter is successfully used in a femtosecond Ti:sapphire laser cavity for highly efficient intracavity THz generation, where the photoconductive layer serves also as a saturable absorber. The broadband THz pulses generated are used for time–resolved spectroscopy of nanostructures. We study the dynamics of intersubband transitions in semiconductor quantum wells. The relaxation of carriers excited by a near–infrared pump pulse is investigated by measuring the THz absorption between the different subbands with our THz pulses. For transition energies below the optical phonon energy we find relatively long relaxation times with a strong dependence on the excited carrier density.

Terahertz Quantum Cascade Devices: From Intersubband Transition to Microcavity Laser

In this review, we report on the study of terahertz (THz) intersubband (ISB) transitions and on the optical devices based on them. We use time-resolved THz spectroscopy to examine ISB optical transitions in semiconductor quantum wells and quantum cascade lasers (QCLs). From these measurements, we obtain important information on the carrier relaxation, scattering mechanisms, and the gain. The waveguide losses are studied directly on the QCL devices and we show the main loss mechanism in the double-metal waveguides. Finally, we demonstrate THz-QCLs with low-mode volume optical cavity.

Time-domain spectroscopy of mid-infrared quantum cascade lasers

In this work we present phase-resolved measurements of the stimulated emission process in a quantum cascade laser emitting at 11.7 µm. In particular, we are focusing on the temperature performance of the intersubband gain in a spectrally broad region. We find that the gain turns into strong absorption at higher temperatures. We show that the phase information is mandatory to describe the correct relation between stimulated emission and absorption. Due to the importance of the correct phase estimation, we present an extended noise analysis of our optimized electro-optic sampling detection scheme. The dynamic range is >80 dB with signal components between 6 and 67 THz.

Intraband InAs/InAlGaAs/InP Quantum Dot Detectors for the MIR

In this work, the authors presented the results of a detailed optical characterization of specially designed stacked self-assembled InAs/InGaAlAs/InGaAs/InP QDIP structures grown by metalorganic vapor phase epitaxy. The QDIP active region consists of an InGaAs quantum well grown on InGaAlAs followed by a thin InGaAlAs layer on top of which the dots are nucleated. The ternary and the quaternary material are lattice matched to InP. The dots are then covered by a thin InP barrier which helps reducing the dark current and is more convenient to be grown at the same temperature as the dots themselves.

Ultrafast Spectroscopy As A Probe Of Light‐Matter Interaction In A Midinfrared Quantum Cascade Laser

Using an electro‐optic sampling technique, we study the interaction of ultra‐short midinfrared pulses with the active medium of a quantum cascade laser emitting at an 11.7 μm wavelength below and above the laser threshold. Gain clamping is observed above the lasing threshold and additional broadband resonant absorption is found at a low bias current.