Abstract
We compare the performance of 10 and 5 μm thick metal–metal waveguide terahertz quantum-cascade laser ridges operating around 2.7 THz and based on a 4-well phonon depopulation active region design. Thanks to reduced heat dissipation and lower thermal resistance, the 5 μm thick material shows an 18 K increase in continuous wave operating temperature compared to the 10 μm material, despite a lower maximum pulsed-mode operating temperature and a larger input power density. A maximum continuous wave operating temperature of 129 K is achieved using the 5 μm thick material and a 15 μm wide ridge waveguide, which lased up to 155 K in the pulsed mode. The use of thin active regions is likely to become increasingly important to address the increasing input power density of emerging 2- and 3-well active region designs that show the highest pulsed operating temperatures.
Highlights
THz quantum-cascade lasers (QCLs) demonstrations, thanks to (a) the strong mode confinement, which maximizes modal gain and minimizes losses, and (b) the transverse waveguide dimensions being scaled far below the wavelength, which reduces total power consumption.15 Epitaxyside down mounting to sapphire and AlN substrates allows heat extraction from both the top and bottom sides of laser ridges and has demonstrated improved cw operation for surface plasmon THz QCLs, as well as mid-IR QCLs.16,17 Buried heterostructures are a key technology that has improved cw operating temperatures for narrow mid-IR QCL ridges via improved heat removal from the sidewalls.18–20 While the use of buried heterostructures has somewhat improved the cw operation of THz QCLs, the strategy has not been widely adopted as it requires an inconvenient ion implantation step
We demonstrate that the thermal benefit of reducing the active region thickness from 10 to 5 μm in an MM waveguide can outweigh the increase in threshold gain, which results in a net increase in cw operating temperature
The 10 μm thick wafer was grown at Sandia National Laboratory, while the 5 μm thick wafer was grown by a commercial vendor, IQE plc
Summary
THz QCL demonstrations, thanks to (a) the strong mode confinement, which maximizes modal gain and minimizes losses, and (b) the transverse waveguide dimensions being scaled far below the wavelength, which reduces total power consumption.15 Epitaxyside down mounting to sapphire and AlN substrates allows heat extraction from both the top and bottom sides of laser ridges and has demonstrated improved cw operation for surface plasmon THz QCLs, as well as mid-IR QCLs.16,17 Buried heterostructures are a key technology that has improved cw operating temperatures for narrow mid-IR QCL ridges via improved heat removal from the sidewalls.18–20 While the use of buried heterostructures has somewhat improved the cw operation of THz QCLs, the strategy has not been widely adopted as it requires an inconvenient ion implantation step.21,22. This increased input power dissipation density works against high temperature cw operation due to increased heating.
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