Long-wavelength (16 µm) and lasing assisted depopulation Quantum Cascade lasers

Abstract

Through the continuous development and successful achievement of watt-level room temperature emission, Quantum Cascade (QC) Lasers have become important mid-infrared light source in a variety of applications. The engineering flexibility in the gain medium enables the realization of light emission across a wide wavelength range. Nevertheless, there’s still a lack of high-performance QC lasers in the long-wavelength (12 – 16 µm) region. In this wavelength range, the QC lasers would be especially valuable in the detection of organic molecules such as BTEX (benzene, toluene, ethylbenzene, and xylenes). The challenges lie in the difficulties to achieve population inversion due to the nature of electron transport across close energy levels and thermal excitations. Moreover, free-carrier absorption loss is approximately proportional to the square of the wavelength and hence become much larger at the long wave-length.

In this thesis, we address the challenges via new QC design concepts. To approach the issues fundamentally, we develop the scattering model considering interface roughness (IFR) scattering in addition to phonon scattering between all bound energy levels in the active core. IFR scattering is crucial to the performance of long-wavelength QC lasers but has long been neglected. Utilizing the improved model, we introduce new active core designs. Instead of having a single optical transition per stage as in traditional structures, we demonstrated a sequential double-optical transition active region, the so-called “lasing assisted depopulation” design that depopulates electrons from the first optical transition through the lasing action between the second lower optical transition at the same wavelength. Both optical transitions in our design lased successfully at 15.5 µm. We also observed stimulated interaction between the two optical transitions. Furthermore, to avoid thermal leakage current, we design a “super-short” injector structure with reduced stage size and large optical dipole moment. This design shows strong optical gain even before states alignment. This design lased between 15.5 µm and 16.3 µm. Linear current-voltage characteristic was observed, which shows that the lack of the injector buffer leads to hot electrons transporting between the excited states. Given the insights from the observed physical phenomena, this thesis also discusses a list of improvements for further actions.