The objective of this research is to experimentally verify that nonlinear optical processes are achievable in strained quantum cascade laser structures for 3-5 m photon emission. The approach is to (i) synthesize (via molecular beam epitaxy) arsenic-based structures with strained active and unstrained injector regions, (ii) compare experimental data with simulation data for 3-5 m laser structures, and (iii) explore orientation-patterned substrates for improved phase matching.
The intellectual merit includes expanded utilization of free space optics communications, which has critical applications for security as well as last-mile access in remote locations. If nonlinear optical processes are demonstrated in strained quantum cascade laser structures, then the potential for temporary high-bandwidth communication links between moving platforms can be achieved with greater performance than current technologies. Currently, free space optics suffers from significant performance issues that are attributed to water absorption of the laser beam in atmosphere and short distance limitations. Appropriately, this research effort serves to experimentally verify the feasibility of nonlinear, high-power 3-5 m devices that will operate in the water transparency window of the electromagnetic spectrum, which is suitable for free-space optics communications.
The broader impact of this research effort includes strong educational components as experiments aim to confirm existing theory about nonlinear optical processes and explore phase matching strategies. It emphasizes quality research experiences for the next generation of scientists, including traditionally underrepresented students by creating unique opportunities for recruitment, hands-on experiences, and support.
This work performed under NSF award # 0840357 explores tunable oscillator strength and nonlinear susceptibility in strained quantum cascade lasers (QCLs) that lead to frequency mixing within a GaAs matrix. This research effort involves AlGaAs/InGaAs QCL cavity design with a projected photon emission wavelength near the 3.8 micron range and lower. This is achieved by reducing the conduction band minima in the quantum wells in a QCL structure and incorporating a bounded fourth energy level within the active region. For the initial part of this study, a self-consistent Schrodinger-Poisson solver is employed to examine the QCL design parameters: composition, thickness, and strain. Then, we analyzed the effects of strain within the AlGaAs/InGaAs active region between AlGaAs/GaAs injectors on a [111] GaAs matrix for the purpose of enhancing nonlinear susceptibility. We particularly focused on the influence of indium composition in the active region including the impact on oscillator strength. Our results highlight critical regions within the AlGaAs/InGaAs QCL design space where the oscillator strength values are maximized. Initial results also demonstrate the feasibility of strained AlGaAs/InGaAs devices on GaAs for producing higher order harmonics that lay below the 4 micron spectral limit. AlGaAs/InGaAs QCL device structures been produced by molecular beam epitaxy on (100) and (111) substrates to explore the differences in non-linear susceptibility. We have performed more than twenty experiments that have shown epitaxy on (111) substrates is more challenging than growth on (100) substrates. Specifically, the growth window (i.e. growth temperature, beam flux ratio, growth rate) for producing strained AlGaAs/InGaAs structures on (111) with optical quality is more narrow than that for (100) samples. We have successfully produced several QCL structures on (111) are in the process of characterizing these structures. The intellectual merit of this effort is verification of the idea that nonlinear optical processes are achievable in strained, high-power quantum cascade laser structures for mid-infrared (MIR) applications. The motivation for this research involves the ability to produce higher power sources that operate within the 3-5 mm atmospheric window of the electromagnetic spectrum. The 3-5 micron range reveals an important water transparency window, which is promising for the advancement of detection of spectral gas lines [such as ethane (at 3.25 micron)], remote sensing, IR countermeasures, secure communications, IR imaging, and medical therapies. The broader impact of this work is that it confirms existing theory about intracavity frequency mixing. While the focus of this study involves the development of higher power QCL structures, the funds from this effort also provided support for under-represented minorities who are enrolled in our graduate program.
