Long wavelength quantum cascade lasers

Quantum cascade (QC) lasers operating on intersubband transitions between conduction band states in InGaAs/AlInAs heterostructures have proven so far to be extremely versatile, covering the range of wavelengths of the two atmospheric windows (3.4 /spl mu/m, 13 /spl mu/m). Their extension to even longer wavelengths is, however, problematic, due to the increasingly smaller radiative efficiency of intersubband transitions and to the optical losses from intraband free-carrier absorption, which are roughly proportional to /spl lambda//sup 2/. A possible solution consists in making use of interminiband transitions in semiconductor superlattices (SL), which present the advantage of large oscillator strengths and intrinsic population inversion and can be driven with very large current densities. Here we present the realization of a QC laser employing these so-called "chirped" superlattices and operating at /spl lambda//spl sim/17 /spl mu/m, which represents the first demonstration of semiconductor injection laser based on intra-band transitions at wavelengths beyond the atmospheric windows. The structure was grown by molecular beam epitaxy in the InGaAs/lnAlAs material system lattice matched to InP substrate.