title: Formula for the phase velocity of electromagnetic waves.
abstract: A new formula for the phase velocity of electromagnetic waves presented by Chen [Appl. Phys. Lett. 88, 121125 (2006)] is investigated and discussed here. The difference between the result obtained with the new formula and that obtained directly using the phase term is small for a fundament-mode Gaussian laser beam. However, this difference is qualitative in some high-order Gaussian-mode laser beams. Using the new formula for such beams, discontinuities arise in the distribution of the phase velocity. This distribution is not rotationally symmetric with respect to the optical axis, and an imaginary phase velocity may appear near these discontinuities.
authors.affiliation: Department of Physics, Zhejiang Normal University, Zhejiang 321004, China. hsh@zjnu.cn
authors.forename: Shihua
authors.initials: S
authors.lastname: Huang
authors.forename: Fengmin
authors.initials: F
authors.lastname: Wu
authors.forename: Bo
authors.initials: B
authors.lastname: Hu