INTRODUCTIONSince the concept of infrared (IR) photodetection by intersubband absorption in superlattices was first proposed and demonstrated by Smith et al.(1), the field of intersubband physics and applications has grown and attracted an increasing interest. Along with the demonstration and the analysis of intersubband absorption(2), came the possibility of studying a very basic quantum system in the laboratory. These efforts revealed the richness of intertweened basic physical effects that have to be understood or taken into account for the full mastering of intersubband physics and applications: Stark (3), depolarization (4, 5) and other energy shifts, exchange interactions in doped quantum wells(6), effects of real material systems, polarization(7) and parity selection rules -just to name a few. On the application side another important channel of interest was opened with the realization that very large optical nonlinearities could be achieved through intersubband transitions in superlattices (8-10) .In the same time the understanding of infrared photodetection by intersubband and bound-to-continuum transitions has been deepened and carried to new promising frontiers in a relatively short time; many additional features of these detectors including some relevant to users' needs, have been addressed and investigated: wide-band response(1l), grating-assisted frontal detection schemes (12, 13) (to overcome basic selection rules limitations), photoconductive gains(14), tunability of the wavelength of detection (15), are just a few of the issues that have been undertaken and partially resolved. Most notably detectors presenting very respectable performances in terms of responsivity, noise figures and, ultimately, detectivity, have been demonstrated mainly in the GaAs/AlGaAs system(16). Tipical wavelengths of detection in this material include the athmospheric window 8-12 ~m and the range of detection can be pushed up and down a few microns by different techniques(17,18),or by using different, yet compatible material systems. The time has quickly come when, considering such demonstrated performances and thriving on the advantages of the mature, reliable and integrable GaAs material technology, hopes have arisen for the possibility of fabricating focal staring
Intersubband Transitions in Quantum WellsEdited by E. Rosencher et oJ., Plenum Press, New York, 1992
15arrays with acceptable performances for many applications; and questions need be asked about the performances of (GaAs) multi quantum well detectors per se and with respect to other materials(19) and/or other schemes, including the Mercury-CadmiumTelluride (HgCdTe) system, which has been considered for years the natural choice for many applications in the wavelength range described above.Many are the issues involved when one tries to compare between different technologies and different schemes of detection: these include the preparation of materials, the maturity, reliability and integrability of the technology, the capability of fabricating large arrays at h...
