Maximizing Performance of Quantum Cascade Laser-Pumped Molecular Lasers

Quantum-cascade-laser- (QCL) pumped molecular lasers (QPMLs) have recently been introduced as a source of powerful ($g\phantom{\rule{-2pt}{0ex}}1$ mW) tunable ($g\phantom{\rule{-2pt}{0ex}}1$ THz) narrow-band ($l\phantom{\rule{-2pt}{0ex}}10$ kHz) continuous-wave terahertz radiation. The performance of these lasers depends critically on molecular collision physics, pump saturation, and on the design of the laser cavity. Using a validated three-level model that captures the essential collision and saturation behaviors of the QPML gas nitrous oxide (${\mathrm{N}}_{2}\mathrm{O}$), we explore how the threshold pump power and output terahertz power depend on the pump power and gas pressure, as well as on the diameter, length, and output-coupler transmissivity of a cylindrical cavity. The analysis indicates that maximum power occurs as pump saturation is minimized in a manner that depends much more sensitively on pressure than on cell diameter, length, or transmissivity. A near-optimal compact laser cavity can produce tens of milliwatts of power tunable over frequencies above 1 THz when pumped by a multiwatt QCL.