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 3-coupled quantum well (QW) active regions showed up to 2 uW of SHG light at 3.75 um wavelength at a fundamental peak power and wavelength of 1 W and 7.5 um, respectively. These lasers resulted in an external linear to nonlinear conversion efficiency of up to 1 uW/W2. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 and 4.55 um, respectively, resulted in a 100-fold improved external linear to nonlinear power conversion efficiency; i.e. up to 100 uW/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 × 10^4 pm/V) is calculated, several orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.