High-Efficiency and High-Power Mid-Wave Infrared Cascade Lasers.

摘要

This report summarizes advanced quantum cascade laser (QCL) development during the Phase Ib performance period of DARPA's EMIL program. Examining tunneling in QCLs, we found that strong dephasing from interface roughness scattering poses a likely bottleneck for electron transport. We therefore proposed an 'ultra-strong coupling design' strategy and calculated the optimal coupling strength with a density matrix based model. By implementing the ultra-strong coupling design strategy in real QCLs, we demonstrated a major step forward in the overall device performance and achieved a record breaking wall plug efficiency (WPE) of approximately 50% (pulsed at cryogenic temperatures). Ultra-strong coupling also produced QCLs with exceedingly broad optical gain (e.g. > 400 cm-1 at peak emission wavelengths around 4.x m), as well as low threshold current density, large slope efficiency (approximately 5 W/A), and high WPE (approximately 23 %) in pulsed mode at 295 K. To further study transport in QCLs, electron transit times of different QCLs were measured and compared to calculated upper laser level lifetimes with and without interface roughness scattering. A better correlation is found when including interface roughness, suggesting that it plays a crucial role in determining intersubband lifetimes in mid-infrared QCLs and should routinely be included in design. Quantum Cascade (QC) lasers have seen and continue to see rapid performance improvements driven by applications such as chemical trace gas sensing or infrared countermeasures. The DARPA EMIL program has contributed immeasurable value to the development of high-performance QC lasers, especially in the short 4 5 m wavelength range. High wall-plug efficiency (WPE) was one of this program s main focus. This report summarizes our team s main activities and results during the Phase Ib performance period.