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# Accession Number:

## ADA295485

# Title:

## Theory and Group Velocity of Ultrashort, Tightly-Focused Laser Pulses.

# Descriptive Note:

## Interim rept.,

# Corporate Author:

## NAVAL RESEARCH LAB WASHINGTON DC

# Report Date:

## 1995-06-07

# Pagination or Media Count:

##
24.0

# Abstract:

## The wave equation describing an ultrashort, tightly focused laser pulse in vacuum is solved analytically. Plasma dispersive effects are also included. Based on exact short-pulse solutions, analytic expressions are obtained for pulse length evolution, pulse centroid motion and group velocity. Approximate short pulse solutions are obtained to arbitrary order in the parameter lambda2xL 1, where lamda is the pulse wavelength and L is the length of the pulse envelope. Comparisons are made to the solutions of the paraxial wave equation and to numerical solutions of the full wave equation.The exact analytic expression for the pulse group velocity vg, which is correctly determined from the motion of the pulse centroid, is in excellent agreement with the numerical solution. In vacuum, 1 - vgc approx. equals to lambda2piro2, where ro is the laser spotsize at focus. Estimates for the quantity 1 - vgc, based on the paraxial wave equation, are found to be in error by factor of two. AN

# Distribution Statement:

## APPROVED FOR PUBLIC RELEASE

#