  A method for the controlled generation of intense high frequency
electromagnetic fields by a breaking Langmuir wave (relativistic flying
mirrors) in a gradually inhomogeneous plasma is proposed. The wave breaking
threshold depends on the local plasma density gradient. Compression, chirping
and frequency multiplication of an electromagnetic wave reflected from
relativistic mirrors is demonstrated using Particle-In-Cell simulations.
Adjusting the shape of the density profile enables control of the reflected
light properties.
