Two terahertz quantum cascade lasers based on GaAs∕Al0.1Ga0.9As heterostructures are reported. Pulsed mode operation up to 84K and continuous wave (cw) power of 0.36mW at 10K are demonstrated for the laser which emits from 1.34to1.58THz. The other laser shows emission from 1.2to1.32THz with pulsed mode operation up to 69K and cw power of 0.12mW at 10K.

In this paper we review recent progress in obtaining laser action from semiconductor quantum cascade structures covering the low THz region of the electromagnetic spectrum, from 2 THz (λ 155 μm) down to the sub-THz region (λ > 300 μm). Particularly, laser active region designs based on bound-to-continuum transition and magnetically assisted intra-well transition are presented. The wide scalability of active region designs is discussed and illustrated with experimental data. Latest results including the demonstration of laser action from quantum heterostructure at 950 GHz are presented.THz quantum cascade lasers are based on semiconductor heterostructures and cover a wide spectral window which now extends below 1 THz.

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We present the development of a broad gain quantum cascade active region. By appropriate cascade design and using a symmetric active region arrangement, we engineer a flat gain and increase the total modal gain in the desired spectral range. Grating-coupled external cavity quantum cascade lasers using this symmetric active region are tunable from 7.6 to 11.4 μm with a peak optical output power of 1 W and an average output power of 15 mW at room-temperature. With a tuning of over 432 cm−1, this single source covers an emission range of over 39% around the center frequency.

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Laser emission over a broad range of frequencies from 2.8 to 4.1 THz is reported for a two-quantum well, photon-phonon cascade structure. Maximum operating temperatures of 125 K are reported, with optical peak powers in eccess of 30 mW from a double-metal ridge waveguide. The broadband nature of the gain curve is identified as due to coherent coupling of the injector and upper lasing states. Internal quantum efficiencies reaching 43 % are evaluated at 10 K.The laser operates in both polarities, showing laser action in reverse bias up to a temperature of 90 K. Simulations based on a full treatment of the structure with density matrix formalism are also presented and discussed.

Esaki and Tsu's superlattice 1 , made by alternating two different semiconductor materials, was the first one-dimensional artificial crystal that demonstrated the ability to tailor semiconductor properties. One motivation of this work was the realization of the Bloch oscillator 2,3 and the use of its particular dispersive optical gain 4,5 to achieve a tuneable source of electromagnetic radiation. However, these superlattices were electrically unstable in the steady state 6 . Fortunately, because it is based on scatteringassisted transitions, this particular gain does not arise only in superlattices, but also more generally in semiconductor heterostructures 7,8 such as quantum cascade lasers 9 (QCLs), where the electrical stability can be controlled 10 . Here, we show the unambiguous spectral signature of Bloch gain in a special QCL designed to enhance the latter by exhibiting laser action in the condition of weak to vanishing population inversion.In solids, electrons have a fixed relation between momentum and energy: they move along energy bands, as known from condensed-matter theory. When an electric field is applied they are accelerated but the lattice forces a periodic motion at a definite Bloch frequency. This phenomenon is known as Bloch oscillations, and the idea was successfully used by Zener to explain the dielectric breakdown 3 . However, in usual solids the strong scattering due to impurities and carrier-carrier interaction prevents the observation of such oscillations, as the lattice constant is too short to allow the electrons to complete even one oscillation cycle. In superlattices, the lattice constant can be chosen and a subtle engineering may allow electrons to achieve a few oscillations before scattering. As this phenomenon is fascinating from a condensed-matter point of view, it also opens new perspectives for optics because charge oscillations naturally couple to radiation and offer a way to emit coherent radiation.Therefore, the important question is whether these oscillations can be self-sustained and provide optical gain. First, Ktitorov 4 and then Ignatov and Romanov 5 addressed the problem theoretically with Boltzmann equations and succeeded in providing a definitive signature for Bloch oscillations in superlattices in terms of a particular spectral response: the Bloch oscillations are found to amplify the electromagnetic field (optical gain) on the low-energy side of the oscillation frequency, whereas they absorb photons on the high-energy side. This particular shaped gain-Bloch gain-is the main feature of the Bloch oscillator. A series of experiments 11-13 using pulsed ultrafast techniques have successfully shown the existence of Bloch oscillations as electrons are pumped in a higher energy band and collectively oscillate over their dephasing time. However, the Bloch gain extends to zero frequencies and the structure becomes unstable in the steady state, so far preventing the observation of net gain in superlattices, although some evidence in photocurrent 14 and more recently in absorpt...

A measurement of the linewidth enhancement factor α of a distributed feedback quantum cascade laser is presented. The measurement is based on a heterodyning experiment, in which one of the lasers is modulated at radio frequency. A value of α=0.02±0.20 is obtained for a modulation frequency of 500MHz. As the frequency is decreased, α increases and is consistent with a thermal chirp effect.