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 2 μW of SHG light at 3.75 μm wavelength at a fundamental peak power and wavelength of 1 W and 7.5 μm, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 μW/W². An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 and 4.55 μm, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 μW/W². 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 4.7×10^-5 esu (2×10⁴pm/V) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.