Semiconductor devices have become indispensable for generating electromagnetic radiation in everyday applications. Visible and infrared diode lasers are at the core of information technology, and at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications. But the terahertz region (1-10 THz; 1 THz = 10(12) Hz) between these ranges has remained largely underdeveloped, despite the identification of various possible applications--for example, chemical detection, astronomy and medical imaging. Progress in this area has been hampered by the lack of compact, low-consumption, solid-state terahertz sources. Here we report a monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure. The prototype demonstrated emits a single mode at 4.4 THz, and already shows high output powers of more than 2 mW with low threshold current densities of about a few hundred A cm(-2) up to 50 K. These results are very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems.

An all optical implementation of quantum information processing with semiconductor macroatoms is proposed. Our quantum hardware consists of an array of quantum dots and the computational degrees of freedom are energy-selected interband optical transitions. The quantum-computing strategy exploits exciton-exciton interactions driven by ultrafast multicolor laser pulses. Contrary to existing proposals based on charge excitations, our approach does not require time-dependent electric fields, thus allowing for a subpicosecond, decoherence-free, operation time scale in realistic semiconductor nanostructures.

The first global quantum simulation of semiconductor-based quantum-cascade lasers is presented. Our three-dimensional approach allows to study in a purely microscopic way the current-voltage characteristics of state-of-the-art unipolar nanostructures, and therefore to answer the long-standing controversial question: is charge transport in quantum-cascade lasers mainly coherent or incoherent? Our analysis shows that: (i) Quantum corrections to the semiclassical scenario are minor; (ii) Inclusion of carrier-phonon and carrier-carrier scattering gives excellent agreement with experimental results. 72.20.Ht, Since the seminal paper of Esaki and Tsu [1], semiconductor-based nanometric heterostructures have been the subject of an impressive theoretical and experimental activity, due to their high potential impact in both fundamental and applied research [2,3]. One of the main fields of research focuses on exploiting "band-gap engineering", namely the splitting of the bulk conduction band into several subbands, to generate and detect electromagnetic radiation in the infrared spectral region, as originally envisioned by Kazarinov and Suris [4].Unipolar coherent-light sources like quantum-cascade lasers (QCLs) [5] are complex devices, whose core is a multi-quantum-well (MQW) structure made up of repeated stages of active regions sandwiched between electron-injecting and collecting regions. When a proper bias is applied, an "electron cascade" along the subsequent quantized-level energy staircase takes place. QCLs are usually modelled in terms of n-level systems [6]. As pointed out in [7], such a macroscopic modeling can only operate as an a posteriori fitting procedure. In contrast, for a detailed understanding of the basic physical processes involved, a fully three-dimensional (3D) description is needed. More specifically, two main issues need to be addressed: (i) the nature of the hotcarrier relaxation within the device active region; (ii) the nature -coherent versus incoherent-of the physical mechanisms governing charge transport through injector/active-region/collector interfaces.Point (i) has been recently addressed in [7], where the usual macroscopic treatment of the device active region has been compared to a fully kinetic description, based on a Monte Carlo (MC) solution [8] of the following set of equations:(Here, the first two terms describe -still on a partially phenomenological level-injection/loss (i/l) of carriers with parallel or in-plane wavevector k in subband ν, while the last ones describe intra-as well as inter-subband inand out-scattering processes (kν → k ′ ν ′ ). As reported in [7], the quantum-cascade within the active region is mainly governed by LO-phonon emission. However, such a microscopic analysis, being limited to the device active region only, does not allow to answer point (ii); This issue is intimately related to the long-standing controversial question [9]: is charge transport in quantum-cascade lasers mainly coherent or incoherent?To provide a definite answer to this fundamental questi...