  Localized plasma waves can be generated by suddenly ionizing extrinsic
semiconductors with spatially periodic dopant densities. The built-in
electrostatic potentials at the metallurgical junctions, combined with electron
density ripples, offer the exact initial condition for exciting long-lasting
plasma waves upon ionization. This method can create plasma waves with a
frequency between a few terahertz to sub-petahertz without substantial damping.
The lingering plasma waves can seed backward Raman amplification in a wide
range of resonance frequencies up to the extreme ultraviolet regime. Chirped
wavevectors and curved wavefronts allow focusing the amplified beam in both
longitudinal and transverse dimensions. The main limitation to this method
appears to be obtaining sufficiently low plasma density from solid-state
materials to avoid collisional damping.
