Optimized second-harmonic generation in quantum cascade lasers

Optimized second-harmonic generation (SHG) in quantum cascade (QC) lasers with specially designed active regions is reported. Nonlinear optical cascades of resonantly coupled intersubband transitions with giant second-order nonlinearities were integrated with each QC-laser active region. QC lasers with three-coupled quantum-well (QW) active regions showed up to 12 muW of SHG light at 3.75 mum wavelength at a fundamental peak power and wavelength of 1 W and 7.5 mum, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 muW/W-2. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 And 4.55 mum, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 muW/W-2. Full theoretical treatment of nonlinear light generation in QC lasers is given, and excellent agreement with the experimental results is obtained. For the best structure, a second-order nonlinear susceptibility of 4.7 x 10(-5) esu (2 x 10(-4) pmN) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.