Phase velocity of light modification by means of axially displaced Gaussian modes

It is shown that by superposition of axially displaced Gaussian modes, the phase of the complex amplitude of an optical field along its propagation axis can be determined. As a consequence, the phase velocity of light along the axis is affected. Increase and decrease of the phase velocity are demonstrated. The local decrease in phase velocity as shown, may have potential implications in the power exchange in vacuum between charged particles and electromagnetic fields. Phase velocity matching of a light wave is also of importance in a variety of non-linear interaction schemes.