Intersubband semiconductor light sources: history, status, and future

After the first theoretical proposal in 1971, it took more than two decades until the first laser based on intersubband transitions in semiconductor quantum structures was demonstrated. The progress since then has indeed been spectacular with lasers now spanning the range from 3.5 to 140 μm. The physics of these devices can be elucidated and some crucial achievements was highlighted. Finally, the perspectives for closing the still existing wavelength gap (25-60 μm) and extending the covered range to even longer as well as shorter wavelength was discussed.