Optical Frequency Plasma Resonance in Gases

Abstract

An intense optical beam is reflected backwards from a laser-induced spark in argon and nitrogen gas at high pressures. The reflected beam is about 10% as intense, diverges with a comparable angle, and has the same frequency (to within about 0.01 cm−1) as the incident beam. By simultaneously focusing a weak second harmonic beam onto the discharge, the reflectivity at the higher frequency was measured. One percent of the incident harmonic beam was reflected in a nearly collimated beam, and with no apparent frequency shift (less than 0.05 cm−1). The proposed model is that stationary region having very high electron density (ne≈2×1021 electrons/cc) is created in the focal volume and exhibits plasma resonance at the laser frequency. The resonance would appear quite damped due to the high collision rate. Experiments have been performed which support the hypothesis and discriminate against several other possible explanations such as reflection from the mismatch of index of refraction at the shock front, stimulated Brillouin scattering, and stimulated plasma-wave scattering.