Resonant optical nonlinearities in cascade and coupled quantum well structures

Resonant or near resonant optical nonlinearities in semiconductor coupled quantum-well systems are discussed. Quantum engineered coupled or cascade quantumwell structures can provide giant nonlinear susceptibilities for various optical nonlinear processes. Nonlinearities integrated within quantum cascade lasers (QCL) showed great potential in various applications in the infrared range. Several schemes of nonlinearities are proposed and discussed in this work. Integrating difference frequency generation (DFG) with QCL can yield long wavelength radiation, such as terahertz light. The DFG process does not require population inversion at a transition associated with low photon energy; however, this requirement is necessary to lasers, such as QCL, and is hard to meet, because of the thermal backfilling and inefficient injection or pumping at room temperature. Therefore terahertz radiation due to DFG QCL for room temperature is proposed. On the other hand, the second harmonic generation can double laser frequency, and then push radiation frequency of AlInAs/GaInAs/InP based QCL to short wavelengths such as 3 μm and shorter. Optical nonlinearities can extend working frequencies of light sources, and also can help to improve light detection. For example, a sum frequency generation can upconvert mid/far-IR signal into near-IR signal with strong near-IR pump light, namely high efficient near-IR photon detector could be employed to detect mid/far-IR light. A specific designed quantum well structure of this frequency up-conversion scheme is discussed. A scheme of monolithic in-plane integration of the optical nonlinearities with QCL is also proposed. In this scheme, an optical nonlinear section is made from the same quantum well structure of a QCL, and is under an independent applied bias. Due to the independence of the applied bias, the nonlinearities can be tuned flexibly. In particular, a widely tunable Raman laser based on this scheme could be achieved. A frequency up-conversion based on sum frequency generation process in coupled quantum-well structure is also proposed for mid-infrared detection. By converting mid-IR signal to near-IR, superior near-IR detector such as silicon avalanche photo diode (APD) can be employed. The scheme can provide lower noise equivalent power (NEP) or higher detectivity compared with regular semiconductor photo detectors. A scheme of lasing without inversion (LWI) based on QCL for THz radiation is proposed. A ladder type three-level system for LWI process is integrated into a boundto- continue high power QCL at 10 μm. The proposed LWI generates THz signal at 69 μm. An optical gain about 80 cm-1 is achieved, against a waveguide loss about 30 cm-1 in a semi insulator (SI) surface plasmon waveguide