A semiconductor injection laser that differs in a fundamental way from diode lasers has been demonstrated. It is built out of quantum semiconductor structures that were grown by molecular beam epitaxy and designed by band structure engineering. Electrons streaming down a potential staircase sequentially emit photons at the steps. The steps consist of coupled quantum wells in which population inversion between discrete conduction band excited states is achieved by control of tunneling. A strong narrowing of the emission spectrum, above threshold, provides direct evidence of laser action at a wavelength of 4.2 micrometers with peak powers in excess of 8 milliwatts in pulsed operation. In quantum cascade lasers, the wavelength, entirely determined by quantum confinement, can be tailored from the mid-infrared to the submillimeter wave region in the same heterostructure material.

Quantum Hall Meets Metamaterial: Controlling and tuning light-matter interaction is crucial for fundamental studies of cavity quantum electrodynamics and for applications in classical and quantum devices. Scalari et al. (p. 1323 ) describe a system comprising an array of metamaterial split-ring resonators and a series of two-dimensional electronic gases (2DEG) formed in GaAs quantum wells. In a magnetic field, the electrons in the 2DEG performed cyclotron orbits and formed Landau levels. Strong coupling was observed between photon and magnetic cyclotron modes, producing a tunable semiconductor system for studying the light-matter interaction of two-level systems.

High-power and highly directional semiconductor microcylinder lasers based on an optical resonator with deformed cross section are reported. In the favorable directions of the far-field, a power increase of up to three orders of magnitude over the conventional circularly symmetric lasers was obtained. A “bow-tie”–shaped resonance is responsible for the improved performance of the lasers in the higher range of deformations, in contrast to “whispering-gallery”–type modes of circular and weakly deformed lasers. This resonator design, although demonstrated here in midinfrared quantum-cascade lasers, should be applicable to any laser based on semiconductors or other high–refractive index materials.

Continuous wave operation of quantum cascade lasers is reported up to a temperature of 312 kelvin. The devices were fabricated as buried heterostructure lasers with high-reflection coatings on both laser facets, resulting in continuous wave operation with optical output power ranging from 17 milliwatts at 292 kelvin to 3 milliwatts at 312 kelvin, at an emission wavelength of 9.1 micrometers. The results demonstrate the potential of quantum cascade lasers as continuous wave mid-infrared light sources for high-resolution spectroscopy, chemical sensing applications, and free-space optical communication systems.