ABSTRACT
Optoelectronics relies on emitters and detectors of electromagnetic radiation. The most widely used mechanism to generate light from a semiconductor is the recombination of electrons and holes across the bandgap of a semiconducting material. Soon after the invention of the laser, this photon-emitting process was used to demonstrate the first semiconductor lasers based on GaAs [1-3]. The bandgap of GaAs (1.4-1.5 eV) sets the emission wavelength in the near infrared (∼800 nm). Considerable effort has been undertaken to develop semiconductor lasers that operate at longer and at shorter wavelengths. For short wavelengths, cheap and efficient diode lasers based on GaAs and InP are available. A breakthrough has occurred by solving the technological problems of wide-gap semiconductors. Green, blue and ultraviolet current injection lasers based on II-VI compounds (ZnSe [4]) as well as on III-V compounds (GaN [5]) have been developed. For wavelengths in the mid-infrared (MIR, also called ‘long wavelengths’), narrow gap semiconductor lasers were demonstrated in the sixties [6]. The most prominent example is lead-salt compounds with an emission range from below 3 µm to about 30 µm. A gap exists in the far-infrared, while wavelengths beyond 1 mm can be readily generated by high-frequency oscillators. Figure B2.7.1 presents an overview of the emission ranges of various types of lasers. While the near-infrared range is well covered by commercially available lasers, quantum cascade lasers (QCLs) are very promising sources for the mid-and far-infrared range.
