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 (mu)W of SHG light at 3.75 (mu)m wavelength at a fundamental peak power and wavelength of 1 W and 7.5 (mu)m, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to I (mu)W/W^2. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 and 4.55 (mu)m, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 (mu)W/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 * 10^-5 esu (2 * 10^4 pm/V) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.