April 7, 2022
The China Institute of Semiconductors claims the first continuous wave (CW) operation of microcavity quantum cascade lasers (QCLs) down to a temperature of 5 °C at an emission wavelength of 8 μm [Qiangqiang Guo et al, ACS Photonics, published online 14 March 2022].
The researchers comment, “The CW operating temperature far exceeds freezing, enabling the manufacture of portable systems for use outside of a laboratory environment.” The team sees prospects for use in photonic integrated circuits, on-chip sensing, and microcavity frequency combs.
Researchers fabricated QCL material from solid-source molecular beam epitaxy on indium phosphide (InP) in notched elliptical Whispering Gallery Mode (WGM) high quality factor (Q) electromagnetic (EM) resonators (Figure 1).
Figure 1: CW microcavity QCL structure and design: (a) schematic; (b) calculated mode distribution; (c) simulated three-dimensional temperature profiles.
The 40-level QCL consisted of layers of indium gallium arsenide (InGaAs) and indium aluminum arsenide. InGaAs has also been used for optical confinement.
Since the excited EM modes are concentrated at the periphery of the structure, electrical insulation in the form of semi-insulating InP, prepared by doping with iron (Fe), was introduced in the center of the structure. The researchers explain: “This reduces the total injection current and thus the operating temperature in the active region. In theory, this design can eliminate most unnecessary heat dissipation, which is the primary limiting factor for CW operation of microcavity QCLs, while providing a horizontal channel for heat dissipation in the active area.”
The Fe-doped InP created by wet etching the top confinement layer and regrowing also had a similar refractive index as the cladding layers. The sidewalls of the device were optionally passivated with silicon dioxide to suppress current leakage through thermally activated surface states.
The aspect ratio of the ellipse was 1.2 with a semi-minor axis length of 80 μm. The notch on the wavelength scale – 2.5 μm wide, 1.6 μm deep – was located at the intersection of the minor axis and the boundary. The Fe-doped InP material had a semiminor axis of 65 µm.
The electrical isolation scheme was found to more than double the slope efficiency in pulse mode operation. Passivating the devices with silicon dioxide greatly reduced the peak output power and increased the threshold current. This was attributed to the high optical loss of the passivation at wavelengths above 7 μm. The output wave number was around 1250/cm, which corresponds to a wavelength of 8 μm.
In CW mode (Figure 2) the side mode rejection ratio (SMSR) was around 30 dB. Devices with galvanic isolation and surface passivation could be operated at temperatures up to +5 °C. The thermal conductivity of this device is 729.8/cm2-K, at 276.9/cm, was much higher than one with no electrical insulation2-K, consistent with the simulations performed to construct the lasers.
Figure 2: Electrical and optical properties of components with surface passivation in CW operation: (a, c) measured power-current-voltage characteristics as a function of the heatsink temperature (a) without and (c) with galvanic isolation; (b,d) CW spectra at -20 °C versus pump current according to panels (a) and (c), respectively.
The CW threshold current has been reduced by 40% when galvanic isolation measures are taken. “This surprising result is due to the reduction in unnecessary heat dissipation performance brought about by the electrical insulation, which effectively lowers the operating temperature of the active region,” comments the team.
The wavenumber spacing between modes was about 5.42/cm, again close to the simulation result of 5.53/cm. A mode hop was observed in the spectrum of the device with galvanic isolation. This results in corresponding kinks in the light output performance.
Microcavity quantum cascade laser InP InGaAs MBE
About the Author Mike Cooke is a freelance technology journalist covering the semiconductor and high technology industries since 1997.