The III-nitride semiconductors have been proposed as candidate materials for new quantum cascade lasers in the nearinfrared (1.5-3 μm), and far-infrared (30-60 μm), due to the large conduction-band offset between GaN and Alcontaining alloys (>1 eV), and the large longitudinal optical (LO) phonon energy (90 meV), respectively. The challenges of III-nitride intersubband devices are twofold: material and design related. Due to large electron effective mass, the nitride intersubband materials require the ability to fine-tune the atomic structure at an unprecedented sub-nanometer level. Moreover, the III-N materials exhibit built-in polarization fields that complicate the design of intersubband lasers. This paper presents recent results on c-plane nitride resonant-tunneling diodes that are important for the prospects of farinfrared nitride lasers. We also report near-infrared absorption and photocurrent measurements in nonpolar (m-plane) AlGaN/GaN superlattices.
We demonstrate a dual wavelength mid-infrared Quantum Cascade Laser (QCL) utilizing a single active region to emit
at 5-μm and 9-μm. The novelty lies in the large energy difference between the two lasing energies, achieved through
simultaneous injection into the top 2 levels of a 4-level cascade employing InGaAs/InAlAs heterostructures latticematched
to InP. The gain and losses at both wavelengths were measured by two different methods, Hakki-Paoli and cutback
method, and were compared with theoretical predictions. The results for the gain of the 9-μm laser from the two
techniques are consistently lower than theoretical predictions. Moreover, the mid-infrared losses are larger than expected
at both wavelengths. We are investigating these devices for their potential application of quantum coherence to achieve
lasing without inversion. The intense fields generated by the 9-μm laser are expected to partially eliminate the resonant
absorption on the transition of interest at an energy corresponding to the difference between the energies of the two
lasers. Our results on the dual wavelength QCL provide insights in the detail charge transport and optical properties of
this design concept and open up the possibility for future optimization of inversionless lasers.
We discuss practical benefits of the nonlinear active quantum-cascade structures that support both laser action and, at the same time, nonlinear self-conversion of laser light into coherent radiation at different frequencies. We show that the proposed approach can greatly enhance the performance of quantum cascade lasers and provide new functionalities. Examples considered include extreme frequency up- or down-conversion, fast and wide-range electric tuning, and multi-frequency generation.
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 mm 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.