Superlattice quantum cascade lasers

Novel approaches to the design of inter-miniband quantum cascade lasers with undoped active regions are presented. The applied electric field is compensated across the superlattices, either by exploiting the field generated by extrinsic charges in special modulation-doped injection regions, or with an effective 'quasi-electric field' obtained by varying the superlattice period and duty-cycle so that a flat miniband condition is reached. Broadening mechanisms and optical losses introduced by the otherwise needed dopants in the superlattices are thus reduced, dramatically lowering the threshold current densities and improving the temperature performance of the lasers. At the same time the appealing properties of inter-miniband based devices, i.e. automatic population inversion and high current carrying capabilities, are preserved. Room temperature record-high peak (approx. 0.5 W) and average (14 mW) powers are observed for a λ = 7.6 μm laser. The lowest threshold current densities reported up to now in quantum cascade lasers at 300 K (J_th = 4.5 kA/cm²) are also obtained, which translates in very large maximum temperatures for continuous wave operation (160 K). Inter-miniband superlattice structures prove particularly useful in extending laser operation towards longer wavelengths, where the increased optical absorption losses and the reduced radiative efficiency require larger driving currents. This is shown by the realization of a laser with chirped superlattice active regions operating at λ = 17 μm, well beyond the longest wavelength (λ = 13 μm) so far demonstrated for conventional quantum cascade lasers based on inter-subband transitions.