Maximizing Performance of Quantum Cascade Laser-Pumped Molecular Lasers

Abstract

Quantum-cascade-laser-(QCL) pumped molecular lasers (QPMLs) have recently been introduced as a source of powerful (>1 mW) tunable (>1 THz) narrow-band (<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 (N 2 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.

Accurately Measuring Molecular Rotational Spectra in Excited Vibrational Modes

“…A rotational transition frequency in one of these modes may be measured if the QCL power is sufficient to make the gas lase on the pumped transition. In Chevalier et al 13 and Wang et al, 30 , an expression was derived for the threshold power P th required for this to happen. Under the assumption of low pressure operation (<20 mTorr) in which ballistic molecular wall collisions dominate all other relaxation mechanisms, lasing may occur and the rotational transition frequency may be measured if the QCL power exceedswhere ν IR is the frequency of the pumped transition, α cell , R cell , and L are the loss (0.3 m −1 ), radius (0.25 cm), and length (15 cm) of the QPML cavity, u is the absolute molecular velocity (379 m/s at 300 K for N 2 O), and |〈f|μ|i〉|2=μ02(JL+1)/(2JL+1) is the transition dipole matrix element for the N 2 O lasing transition (μ 0 = 0.161 D for N 2 O).…”

“…A rotational transition frequency in one of these modes may be measured if the QCL power is sufficient to make the gas lase on the pumped transition. In Chevalier et al 13 and Wang et al, 30 , an expression was derived for the threshold power P th required for this to happen. Under the assumption of low pressure operation (<20 mTorr) in which ballistic molecular wall collisions dominate all other relaxation mechanisms, lasing may occur and the rotational transition frequency may be measured if the QCL power exceeds…”

“…1 reveals that P th grows quadratically with pressure. 30 From this, a molecule-dependent pressure for minimizing P th may be found for each transition, which for N 2 O generally occurs below 20 mTorr. The higher pressure regime is less desirable for spectroscopy because pressure broadening increases the linewidth, laser tuning range, and thus the uncertainty of the rotational transition's center frequency.…”