Laser excitation dynamics of argon metastables generated in atmospheric pressure flows by microwave frequency microplasma arrays

Abstract

The optically pumped rare-gas metastable laser is a chemically inert analogue to diode-pumped alkali (DPAL) and alkali-exciplex (XPAL) laser systems. Scaling of these devices requires efficient generation of electronically excited metastable atoms in a continuous-wave electric discharge in flowing gas mixtures at atmospheric pressure. This paper describes initial investigations of the use of linear microwave micro-discharge arrays to generate metastable rare-gas atoms at atmospheric pressure in optical pump-and-probe experiments for laser development. Power requirements to ignite and sustain the plasma at 1 atm are low, <30 W. We report on the laser excitation dynamics of argon metastables, Ar (4s, 1s₅) (Paschen notation), generated in flowing mixtures of Ar and He at 1 atm. Tunable diode laser absorption measurements indicate Ar(1s₅) concentrations near 3 × 10¹² cm^-3 at 1 atm. The metastables are optically pumped by absorption of a focused beam from a continuous-wave Ti:S laser, and spectrally selected fluorescence is observed with an InGaAs camera and an InGaAs array spectrometer. We observe the optical excitation of the 1s₅→2p₉ transition at 811.5 nm and the corresponding laser-induced fluorescence on the 2p₁₀→1s₅ transition at 912.3 nm; the 2p₁₀ state is efficiently populated by collisional energy transfer from 2p₉. Using tunable diode laser absorption/gain spectroscopy, we observe small-signal gains of ~1 cm^-1 over a 1.9 cm path. We also observe stable, continuous-wave laser oscillation at 912.3 nm, with preliminary optical efficiency ~55%. These results are consistent with efficient collisional coupling within the Ar(4s) manifold.