Emission of light at half‐integer harmonics of the laser frequency

Calculations have been made of the spectrum of light emitted at half‐integer harmonics of the laser light frequency ω0. The emitted light results from either the Raman scattering of laser light by Langmuir waves or by the direct conversion of Langmuir waves to light. The Langmuir wave amplitude is calculated from convective instability theory for two‐plasmon decay in an inhomogeneous, plane‐stratified medium. The Raman scattering process generates harmonics at (n/2) ω0 where n=1,3,5 ...; however, only for n=1 and 3 is the emission level significant. The conversion process produces (1/2) ω0 light with a comparable efficiency to the Raman scattering process near the two‐plasmon instability threshold. Results of the calculation are in agreement with the general features of measured emission spectra.