Refractive-index dispersion of gases measured by optical harmonic phase matching

A new method for accurately measuring the dispersion of the refractive index of gases is presented. The technique makes use of the phase-matching condition which applies to a sample subjected to a spatially periodic electric field in an electric-field-induced second-harmonic generation experiment. The attainable accuracy is comparable to that of interferometric methods, and this method may be extended to measure the refractive index in the vacuum-ultraviolet range. Results are given for the refractive index at {λ}{≥}229 nm for He, Ar, H₂, D₂, N₂, and O₂.