ABSTRACT: Quantum-cascade lasers (QCLs) are lasers whose optical transitions are between different subbands of the conduction band of a semiconductor heterostructure. Because the laser transition takes place between different spatially quantized subbands of the conduction band, the emission wavelength is determined by the structure and is not by the band gap of the materials used. Today QCLs emit with a wide range of wavelengths, from 2.6 to 250 μm (1.2 THz) and have found application in a number of areas including gas sensing for security, environmental and medical uses, as well as for communication and defense-related IR countermeasures. Power of several watts with very good optical mode quality has been realized and corresponding efficiency approaching 30% at room temperature.
Driving these advances and successes is an increasing understanding of the critical design issues and physics of the QCL. This talk discusses two important such design issues: the use of composite quantum-well barriers and the use of reduced number of cascades.
The use of composite QW barriers both allows flexibility in designing structures that use extreme level of internal strain as well as engineering scattering. This design strategy appears to be universally applicable, across the entire range of QCL emission wavelengths. By using low barriers where the upper laser state has its maximum probability and high barriers where the lower laser state has its maximal probability in strain-compensated designs for short wavelength emission, the lifetime of the upper laser state can be increased, while decreasing the lifetime of the lower laser state.
QCLs designed for power typically contain 30-40 cascades, are less than two wavelengths in width, and laser ridge lengths are typically between 3 and 6 mm. Even with state-of-the-art efficiency and thermal management, room temperature operation of such lasers is fundamentally limited to several watts. Our lab has pioneered a path to power scaling that is not fundamentally limited through the use of fewer cascades combined with broad laser areas. We demonstrate the first room temperature continuous-wave emission of broad-area QCLs and discuss how this scaling concept can deliver MIR emission of 10's of watts at room temperature with beam quality required for high brilliance.
BIO: Ted Masselink Ph.D. is professor of physics at the Humboldt-University Berlin, Germany, with research foci in the areas of molecular-beam epitaxy, quantum dots in III-phosphide systems, low-frequency noise in semiconductors, integration of III-V on Si for electronic applications, and quantum-cascade lasers. He received the Ph.D. degree in 1986 from the University of Illinois, Urbana, and was Research Staff Member in the IBM Research Division at its T.J. Watson Research Center, Yorktown Heights, NY until 1994. Since then he has been full professor at the Humboldt University. He has authored or co-authored over 400 refereed publications, as well as a large number of conference presentations and is credited with over 9000 citations with an
“h-index" of 47. He has co-authored 15 distinct patents, patent applications, and disclosures including 9 distinct patents covering aspects of quantum-cascade lasers (QCLs) and is co-inventor of the pHEMT. His group has spun out 2 companies related to QCLs and he was on the board of directors of a start-up company developing and manufacturing solid-state lasers.