It is shown that when a light pulse is incident upon an underdense relativistic ionization front, the light pulse can be simultaneously upshifted in frequency and compressed in duration. The reflection and transmission coefficients are examined. Examples for both overdense and underdense fronts are given to show that a relativistic ionization front can generate tunable radiation from millimeter to ultraviolet wavelengths.

• Received 27 June 1991

DOI:https://doi.org/10.1103/PhysRevA.44.5118