THz Quantum Cascade Lasers

Another important photonic source for THz radiation is the Quantum Cascade Laser (QCL). This laser is unipolar and uses intersubband transitions in a series of steps made of coupled quantum wells. Electrically injected charge carriers cascade down a potential staircase, sequentially emitting photons at each step, see figure 3. The wavelength of the light emitted in a QCL is independent of the band gap of the material used, but can be tuned by tailoring the layer thicknesses during crystal growth. The first THz QCL was developed in 2002 emitting at 4.4 THz, with a peak output power of 2 mW at 8 K [R. Khler et al, Nature 417,
156 (2002).]. However THz QCLs still rely on cryogenic cooling for their operation which limits their potential applications.

THz Quantum Cascade Laser Designs

Currently, successful active region designs of THz QCLs include the bound-to-continuum design and the resonant phonon design, see figure 4. The depopulation of the lower laser level in the bound-to-continuum designs, rely on electron– electron scattering in minibands created using a superlattice structure. In the resonant phonon design, however, the depopulation of the lower laser level is realised through sub-picosecond electron–LO– phonon scattering.

Although there is no fundamental limits to the operation of THz lasers at room temperature,
current designs suffer from thermal back filling and reduced upper state life time as the
temperature increases. It may be necessary to explore other designs, which address the above limitations, in order to achieve higher temperature operation.

Resonance Gamma-X THz QCL

We are currently exploring a new THz QCL design based on resonance Gamma-X electron transfer to achieve population inversion, see figure 5 for a schematic diagram for implementation of Gamma-X depopulation mechanisms in GaAs/AlGaAs based QCL. Further details can be found in S. Rihani et al, Physica E, 41, 1240 (2009).