Dear Colleagues,

The laser is 50 years old this year, and has been transformed from short-lived laboratory curiosity to indispensable part of a many branches of science, technology, manufacturing and medicine. While some of these dramatic advances in functionality and reliability have resulted from better design and new scientific understanding, most benefits have occurred due to improvements in laser materials, so it is appropriate in this special issue to review past developments and anticipate new ones.

Laser materials are those materials which are capable of amplification and generation (in the presence of a resonator) of coherent electromagnetic radiation under external pumping above a reasonable threshold. As a rule, these are gaseous or condensed media containing a sufficient density of active species which can generate gain by virtue of their energy level structure. The active species in laser materials may be neutral atoms, molecules, ions or charge carriers in semiconductors. Depending on the physical properties of the material, it is typically pumped by absorbing electromagnetic radiation at certain wavelengths – those which are absorbed efficiently by the material and for which the resultant excitation can be transferred rapidly and efficiently to the active species to produce population inversion. In other cases, the material may be pumped by electric discharge such as a plasma glow, and there are several other methods.

One especially interesting method is by forward biasing a PN junction in a semiconductor diode. This process generates electrons and holes in the vicinity of the junction, which then recombine to produce photons near the energy gap. With sufficient pumping (ie forward current) the probability of stimulated emission increases to the point where gain is produced and laser oscillation occurs. This is very important because it enables compact, rugged and efficient laser diodes to be mass produced which are highly suitable for optical communication and data storage, for example. At first these lasers required enormous current densities and lasted only minutes at cryogenic temperatures. Now they are so reliable and efficient that they are placed in the most inaccessible places, at the bottom of oceans, where they operate consistently for decades.

For this special issue, we invite papers dealing with recent results and novel trends across a broad range of laser materials, from gas and excimer lasers to solid state, fiber and semiconductor based systems. Means of producing, characterizing, forming and exciting novel laser materials are welcome topics, as are metamaterials and nanostructured media with highly novel properties. We welcome new semiconductors which extend the emission ranges of laser diodes to mid-visible (especially yellow-orange) and ultraviolet, and into the mid to far infrared. Finally, we are interested in highly speculative work on potential materials for emission at terahertz and X-ray wavelengths.

Prof. Dr. John Gerard McInerney
Prof. Dr. Alexander Biriukov
Guest Editors

• active laser component
• energy level
• inversion population
• optical pumping
• electric discharge pumping
• relaxation process
• radiative transition
• materials used in lasers or superluminescent systems (gain or cavity media, gases, dielectrics, semiconductors, nanostructures and clusters, microcavities, quantum confined media, active metamaterials, special considerations for pulsed or data modulated systems, THz UV and X-ray lasing)