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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)

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.

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.

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.

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

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.