Low-Loss Regrowth-Free Long Wavelength Quantum Cascade Lasers

Optical power output is the most sought-after quantity in laser engineering. This is also true for quantum cascade lasers operating especially at long wavelengths. Buried heterostructure cascade lasers with epitaxial regrowth have typically shown the lowest loss due to high current confinement as well as superior lateral thermal conductivity at the expense of complexity and cost. Among the many factors affecting optical output are the widely used passivating materials such as Si
₃
N
₄
and SiO
₂
. These materials have substantial optical absorption in the long wavelength infrared, which results in optical loss reducing the output of the laser. In this letter, we report on quantum cascade lasers with various waveguide widths and cavity lengths using both PECVD grown Si
₃
N
₄
and e-beam evaporated HfO
₂
as passivating material on the same structure. Their slope efficiency was measured, and the cavity losses for the two lasers were calculated. We show that HfO
₂
passivated lasers have approximately 5.5 cm
^-1
lower cavity loss compared to Si
₃
N
₄
passivated lasers. We observe up to 38% reduction in lasing threshold current, for lasers with HfO
₂
passivation. We model the losses of the cavity due to both insulator and metal contacts of the lasers using Comsol Multiphysics for various widths. We find that the loss due to absorption in the dielectric is a significant effect for Si
₃
N
₄
passivated lasers and lasers in the 8-12-μm range may benefit from low loss passivation materials such as HfO
₂
. Our results suggest that low-loss long wavelength quantum cascade lasers can be realized without epitaxial overgrowth.