Long-wavelength „lÉ16 mm..., room-temperature, single-frequency quantum-cascade lasers based on a bound-to-continuum transition

The Quantum Cascade ~QC! laser is a unipolar semiconductor laser that has demonstrated high performance in pulsed operation in the mid-infrared wavelength range ~4–12 mm!. There has been a large effort to develop such devices for even longer wavelengths. These sources would be especially valuable for the detection of large organic hydrocarbon molecules like the BTX compounds in the 12–16 mm region or for radio-astronomy as local oscillators in heterodyne detectors. Devices designed using the so-called three quantum well active region were demonstrated with an operation wavelength up to l'13 mm. However, these devices exhibited fairly limited performances with a maximum operating temperature of Tmax5175 K. Devices based on chirped superlattice active regions demonstrated operation at much longer wavelengths, reaching 17, 19, 21 and finally up to l '24 mm. However, these structures were limited to operation temperatures below 200–240 K. When trying to design long-wavelength quantumcascade lasers, for photon energies larger than the reststrahlen band, population inversion is more difficult to achieve as the upper state lifetime decreases with emitted photon energy, due to the dependence of the optical phonon scattering rate on exchanged wave vector. The lower state lifetime, on the other hand, remains practically unchanged. For this reason, and especially if high temperature operation is seeked, long-wavelength QC lasers must be designed with extremely short lower state lifetimes. Transport in wide minibands, compared with the optical phonon energy, provides efficient lower state extraction mechanism. As the ‘‘boundto-continuum’’ design combines the fast miniband extraction from the lower state with the efficient resonant tunneling injection into the upper state, this approach is very well suited for long wavelength lasers. As shown schematically in Fig. 1, the active region of our structure spans the whole period and consists of a chirped superlattice presenting a tilted lower miniband whose width is maximum in the center and decreases on both sides close to the injection barriers. The upper state is created in the first minigap by a small well adjacent to the injection barrier. Its wave function has a maximum close to the injec-