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.

High-power and highly directional semiconductor cylinder-lasers based on a new 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 "bowtie"-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 new resonator design, although demonstrated here in mid-infrared quantum-cascade lasers, should be applicable to any laser based on semiconductors or other high-refractive index materials.Lasers consist of two basic components. First, the active material in which light of a certain wavelength range is generated from an external energy source, such as electric current; second, the laser resonator, which contains the active material, provides feedback for the stimulated emission of light. The resonator largely influences the special features of the emitted light: power, beam directionality, and spectral properties, as well as the laser's physical features such as size and shape. Semiconductor lasers are the most widely used and versatile class of lasers. Their most common resonators are FabryPerot cavities, in which two cleaved semiconductor crystal planes act as parallel mirrors, reflecting the light back and forth through the active material.There have been many attempts to improve resonator properties. In particular, an increase of the reflectivity of the resonator mirrors is highly desirable. This allows low thresholds for the onset of laser action and a smaller volume of active material with concomitant moderate energy requirements and the ability to pack the lasers in a small space. * To whom correspondence should be addressed; email:fc@lucent One excellent example is the development of microdisk semiconductor lasers (1). These lasers exploit total internal reflection of light to achieve a mirror reflectivity near unity. Micro-disk, -cylinder or -droplet lasers form a class of lasers based on circularly symmetric resonators, which lase on "whispering-gallery modes" of the electromagnetic field (2,3,4). In such a mode light circulates around the curved inner boundary of the resonator, reflecting from the walls of the resonator with an angle of incidence always greater than the critical angle for total internal reflection, thus remaining trapped inside the resonator. There are only minute losses of light caused by evanescent leakage (tunneling) and scattering from surface roughness. This principle allowed the fabrication of the world's smallest lasers (2). Besides possible future applications in optical computing and networking, micro-lasers are of strong interest for research problems of cavity quantum electrodynamics, such as resonatorenhanced spontaneous emission and threshold-less lasers (5). Small resonators may also serve as model systems for the study of wave phenomena in mesoscopic systems, parti...

Quantum cascade ('QC') lasers are reviewed. These are semiconductor injection lasers based on intersubband transitions in a multiple-quantum-well (QW) heterostructure, designed by means of band-structure engineering and grown by molecular beam epitaxy. The intersubband nature of the optical transition has several key advantages. First, the emission wavelength is primarily a function of the QW thickness. This characteristic allows choosing well-understood and reliable semiconductors for the generation of light in a wavelength range unrelated to the material's energy bandgap. Second, a cascade process in which multiple-often several tens of-photons are generated per electron becomes feasible, as the electron remains inside the conduction band throughout its traversal of the active region. This cascading process is behind the intrinsic high-power capabilities of the lasers. Finally, intersubband transitions are characterized through an ultrafast carrier dynamics and the absence of the linewidth enhancement factor, with both features being expected to have significant impact on laser performance.The first experimental demonstration by Faist et al in 1994 described a QC-laser emitting at 4.3 µm wavelength at cryogenic temperatures only. Since then, the lasers' performance has greatly improved, including operation spanning the mid-to far-infrared wavelength range from 3.5 to 24 µm, peak power levels in the Watt range and above-room-temperature (RT) pulsed operation for wavelengths from 4.5 to 16 µm. Three distinct designs of the active region, the so-called 'vertical' and 'diagonal' transition as well as the 'superlattice' active regions, respectively, have emerged, and are used either with conventional dielectric or surfaceplasmon waveguides. Fabricated as distributed feedback lasers they provide continuously tunable single-mode emission in the mid-infrared wavelength range. This feature together with the high optical peak power and RT operation makes QC-lasers a prime choice for narrow-band light sources in mid-infrared trace gas sensing applications. Finally, a manifestation of the high-speed capabilities can be seen in actively and passively mode-locked QC-lasers, where pulses as short as a few picoseconds with a repetition rate around 10 GHz have been measured.

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We combine photonic and electronic band structure engineering to create a surface-emitting quantum cascade microcavity laser. A high-index contrast two-dimensional photonic crystal is used to form a micro-resonator that simultaneously provides feedback for laser action and diffracts light vertically from the surface of the semiconductor surface. A top metallic contact allows electrical current injection and provides vertical optical confinement through a bound surface plasmon wave. The miniaturization and tailorable emission properties of this design are potentially important for sensing applications, while electrical pumping can allow new studies of photonic crystal and surface plasmon structures in nonlinear and near-field optics.