First-generation diode lasers produced stimulated emission from thick active regions consisting of a single semiconductor material. However, not long after epitaxial growth techniques advanced to the point of allowing thin heterostructure layers to be deposited with a high degree of control and precision, the quantum well laser was born. It soon became apparent that besides providing a valuable vehicle for studying fundamental quantum physics and optics in a solid state environment, quantum well diode lasers were also capable of dramatic performance improvements over their bulk counterparts. The lowering of threshold current densities was especially impressive.

As a result of intensive research and development activities over the past decade, quantum well lasers have evolved far past their laboratory curiosity phase and into a commercially important growth technology. This has been accompanied by an improved understanding of the physical phenomena involved in the generation of coherent light in quantum wells. The vast potential for manipulating the electronic and optical properties using more complex quantum heterostructures and microcavity geometries (such as the vertical cavity surface emitting laser, or VCSEL) came to be appreciated. As the technology matured, it became necessary to construct more general device models capable of accurately predicting both static and dynamic aspects of the performance. Furthermore, whereas most of the early work focused on well-characterized GaAs-based quantum wells emitting in the near infrared, a recent emphasis has been to extend the wavelength range, both into the visible and near ultraviolet at one end of the spectrum and into the midwave and longwave infrared at the other. This has been accomplished through the consideration of a wider range of InP-based, GaSb-based, GaN-based, and II-VI material systems, as well as the adoption of novel approaches (such as the quantum cascade laser, which employs intersubband rather than interband lasing transitions).

The present Optics Express focus issue consists of eight invited articles, the authors of which were asked to survey several important recent developments in the theoretical modeling and design of advanced quantum well lasers. As a reflection of the current diversity and breadth of the field, several of the less mature material systems are represented, including nitride quantum wells for blue emission, antimonide and narrow-gap II-VI quantum wells for the mid-IR, as well as intersubband lasers for longer wavelengths. Among the novel configurations and phenomena treated are three-dimensional optical confinement in VCSELs with a dielectric aperture for spontaneous emission enhancement, one-dimensional microcavities in edge emitters to suppress spontaneous emission perpendicular to the lasing axis, the influence of Coulomb enhancement, carrier collision and heating phenomena on the optical gain, and the effects of engineered band structures on Auger recombination and internal losses. Advanced electronic and optical simulations for VCSELs and other structures are also discussed. Taken together, these far-ranging topics provide a representative cross section of new developments from researchers working at the forefronts of the field. We thank all of the authors for the time and effort invested in each contribution, and hope that the issue will prove profitable to readers.

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