A photon rocket is a vehicle propelled by the reaction of light or other electromagnetic radiation. Photons have the highest specific impulse of any rocket propellant and will allow acceleration to 80% of the speed of light with a mass ratio of 3. For higher velocities then a 99.9% of light velocity requires a mass ratio of over 100.
Only engines that can convert matter into energy can be used.
Referring to my thermonuclear engine I suggest that a similar configuration would be able to contain a plasma at such a high pressure and temperature that 100% conversion of matter into energy would occur.
Using the electrical power generated directly then microwave energy could be directly generated and fed to a parabolic antenna to give a beam. This would result in thrust.
Calculations indicate that a one tonne vehicle would require 10E12 watt to give an acceleration of 1 g. However much lower power levels may be used to accelerate an orbiting spacecraft toward the stars.
Journeys to stars 100 light years away would only take a few weeks. See Relativity.
Perhaps "Flying Saucers" are an extraterrestial example of this type of spacecraft. The microwave horn that directs the propulsion photon beam forms the bell shape of the saucer.
Chris.