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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.

In this work femtosecond spectral hole burning spectroscopy is used to resonantly excite ground state excitons in an ensemble of self-assembled InAs/GaAs quantum dots with a strong pump pulse. Two fundamental coherent nonlinear effects are observed with the aid of the intrinsic time- and frequency resolution of the setup: The low temperature Rabi oscillation of the two-level system associated with the excitonic ground state transition and the observation of two-photon absorption in the surrounding GaAs crystal matrix. The emergence of the latter effect also infers the existence of charged excitons in the nominally undoped QD sample, backed up by the observation of additional spectral holes next to the excitonic transitions. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

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.

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.

Using ultrafast terahertz spectroscopy, we measure the temporal evolution of the intersubband absorption spectrum of a $\mathrm{GaAs}∕{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ double-quantum-well structure with an energy spacing between the first two subbands smaller than the longitudinal optical phonon energy. We show that the interaction between the photoexcited carriers has a considerable influence on the time-dependent absorption. When varying the photoexcited sheet carrier density between $1\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ and more than $1\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, we find (i) a strong dependence of the intersubband scattering rate on the density of optically generated carriers, and (ii) a temporal shift of the intersubband resonance as the population in the second subband decays, i.e., as the photoexcited carriers relax into the quantum-well ground state.

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.

Presents time-resolved measurements of photoinduced intersubband absorption in an undoped GaAs/AlGaAs coupled quantum well excited by an ultrashort interband optical pulse. An interband pump pulse injects electrons into the first and second subband of an undoped asymmetric double quantum well (ADQW) with a level spacing smaller than the LO phonon energy. The time evolution of the electron population in these two subbands is monitored by probing the MIR intersubband transitions to a third (empty) subband.

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.

Femtosecond spectral hole burning experiments were performed on InAs/GaAs quantum dots to investigate the influence of high excitation intensities. The observation of additional spectral holes, as well as distinct absorption line broadening are both attributed to two-photon absorption processes. These spectral holes can be unambiguously assigned to degenerate two-photon biexciton creation, whereas the absorption line broadening is a direct result of Coulomb renormalization, initiated by carriers created through two-photon absorption in the surrounding GaAs crystal matrix and the InAs wetting layer.

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.

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.

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.

The spectral gain profile and modal gain coefficient can be measured by coupling a broadband mid infrared pulses generated by phase matched difference frequency mixing in a 30 mum thick GaSe crystal . This enables us to detect the transmitted light by coherent detection facilitating the electro optic effect in a crystal. The advantage of time resolved spectroscopy enables easy access to dynamic parameters of a InGaAs/AlInAs/InP QCL during operation even above threshold.

Spectroscopy done on the light emitted by quantum cascade lasers (QCLs) provides only limited information on the optical , thermal, and temporal properties in such a laser. We study such lasers by time domain electric field spectroscopy of sub-picosecond pulse trains propagating through the QCL cavity. This is done by generating broadband mid-infrared pulses by phase matched difference frequency mixing in a 30 µm thick GaSe crystal. The coherent detection works up to 45 THz, employing the electro optic effect in a 7 µm thick ZnTe crystal. This detection scheme is invariant to the strong co-emitted light fields of the QCL above threshold, and thus, allows us to measure the whole gain dynamics below and above threshold. Due to the recovery of the complete amplitude and phase information we can iteratively solve for the complex refractive index in a wide spectral range (18 – 33 THz). We determine the waveguide and the mirror losses at the emission frequency of 25.4 THz in good agreement with the values deduced from the threshold current densities of different resonator lengths [1].

We present measurements of the dynamics of photoexcited excitons inan ensemble of InAs/GaAs self-assembled quantum dots using a femtosec-ond spectral hole burning technique. We use this technique to examine theexciton spin relaxation and the line shape of optical transitions in the dots.PACS numbers: 73.21.La, 78.47.{p, 78.67.Hc

The optical gain dynamics in an InGaAs/AlInAs quantum cascade laser is studied by midinfrared pump-probe experiments and electro-optic sampling. We find an extremely fast gain recovery time of ≪ 1 ps.

Time and phase resolved spectroscopy of broadband mid infrared pulses transmitted through the active medium of a mid infrared quantum cascade laser reveal the full complex refractive index and its derivatives over a wide spectral range. Using electro-optic sampling, we can monitor the optical response even above threshold and study effects of gain depletion and decoherence.