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High-order harmonics from an ultraintense laser pulse propagating inside a fiber.

A strong effect of high harmonic radiation during the propagation of a high intensity short laser pulse in a thin wall hollow channel ("fiber") is found and studied via relativistic particle-in-cell simulations. The fiber has finite width walls comprised of an overdense plasma. Only the harmonic radiation with the harmonic number above critical value, for which the fiber walls are transparent, propagates outwards in the form of a coherent ultrashort pulse with very short wavelength.