Recent progress in nonlinear quantum cascade lasers
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Nonlinear light generation in quantum-cascade lasers (QCLs) has the potential to extend the operating wavelength of these devices outside the limits imposed by the fundamental properties of the materials of choice. The giant nonlinear susceptibilities of resonant intersubband transitions have been studied intensively both theoretically and experimentally over the past twenty years. However, the practical applications have been limited by the lack of efficient laser pumping and of convenient phase matching techniques. The first obstacle was overcome by monolithically integrating the nonlinear intersubband transitions within the active region of a quantum cascade (QC) laser. Sum-frequency and second-harmonic (SH) generation were the first nonlinear processes observed in QCLs. The optimization of the second-harmonic generation in InGaAs/InAlAs QCLs will be discussed in detail. The second challenge for achieving high efficiency nonlinear power conversion is the phase-matching of the fundamental and nonlinear light. We have developed a technique for modal phase-matching that takes advantage of the flexibility in the design of the QCL waveguide. An additional degree of freedom for tuning into exact phase-matching conditions is provided by the dependence of the refractive indices on the laser ridge width. Record nonlinear power of 2 mW at 4.45 m was achieved using an InP top-cladding waveguide and high-reflectance coating on the laser back facet. Reaching the milliwatt power range is significant as such power levels are sufficient for trace gas point sensors using mid-infrared light sources. The practical limitations of the phase-matching method will be evaluated and the experimental results will be compared with theoretical predictions.