ABSTRACT
The advent of femtosecond lasers in the early 1980s has been accompanied
by the development of diagnostic technique for the fast probing of swiftly
excited solid by X-rays, optical and electronic beams with time resolution
of around 10-100 fs. These achievements generated a broad variety of ex-
periments, which allowed observation for the first time new phenomena in
swiftly excited solid on the femtosecond time scale and on the space scale
of tens of nanometers. The most spectacular observations are the oscilla-
tions in the optical probe beam reflection from the laser-excited solid with
the frequency close to that of the cold phonons in the solid [Cheng et al.,
1991; Zeiger et al., 1992]. It was also found that the intensity of X-ray probe
beam diffracted from the laser-excited solid oscillates with the cold phonon
frequency [Sokolowski-Tinten et al., 2003].
