Phase-locking has improved the beam quality and spectral purity of a 3.3THz (~91µm) quantum cascade laser (QCL), achieving single-mode operation at 2W output.

High radiative efficiency is one of the reasons that the output power is so high.

“Radiative efficiency is greater than 50% for the first time for any single-mode quantum cascade laser,” Lehigh University research engineer Sushil Kumar told Electronics Weekly. And “this is the first report of a multi-watt power output for a single-mode terahertz terahertz semiconductor laser.”

The laser consists of many synchronised sub-lasers – each purple block in the artist’s impression is an individual sub-laser.

The cavity of each sub-laser is a flat slab-like 10µm-thick GaAs/AlGaAs block, with emission upwards in the diagram, through the 10µm thickness. To initiate lasing, energy is confined in the cavity by metallic top and bottom coatings.

Left alone, such lasers have ill-defined emission characteristics, according to the university, and various ways have been tried to phase-lock many to form a single-mode narrow high-quality beam.

And the Lehigh team has invented just such a scheme, which is described in Optica paper ‘Phase-locked terahertz plasmonic laser array with 2W output power in a single spectral mode’.

“Our paper describes a phase-locking scheme for plasmonic lasers that is distinctly different from prior research on phase-locked lasers in the vast literature on semiconductor lasers,” said Lehigh researcher Yuan Jin. “The demonstrated method makes use of travelling surface waves of electromagnetic radiation as a tool for phase-locking of plasmonic optical cavities. The efficacy of the method is demonstrated by achieving record-high output power for terahertz lasers that has been increased by an order of magnitude compared to prior work.”

The coupling mechanism is not between cavities, but in the surrounding medium, on top of the cavity metal confinement layers, through ‘single-sided surface-plasmon-polariton’ waves.

This description and the diagram above are simplifications. For complete details, the full clearly-written paper is available free.

Lehigh University is in Pennsylvania. Kumar and Jin worked with John Reno of the US Sandia National Laboratories.