Time-Resolved Optical Spectroscopies

Spectroscopy basically consists in retrieving information on any system through the use of optical information as a function of wavelength. The examples are countless, and the field is of major importance. For example, the structure of the hydrogen atom has been decoded through the sequence of absorption lines known as the Balmer series. Optical spectroscopy of solids is known to have provided invaluable information over the years, however, when it comes to dynamical properties, one faces right away the challenge that the typical timescales in solids can be incredibly short: 100 fs is a time that cannot be considered as a limit but rather as typical of a wide range of processes. Therefore, the development of ultrafast spectroscopic technique. Ultrafast optical spectroscopy is particularly useful when we try to obtain information on very short timescales, where other techniques cannot reach the necessary time resolution. The very high frequency of light, 5 × 1014 Hz in the visible range, indeed allows nowadays to reach time limits of the order of a few cycles of the light field only. It is even anticipated that the reduction of the laser wavelength toward X-rays, will allow breaking the barrier of the femtosecond, and therefore entering the attosecond regime. We will not describe here the different techniques which allow to produce ultrashort light pulses, the reader only needs to know that convenient means exist that can produce pulses as short as 5 fs. Once we have such pulses at our disposal, spectroscopic techniques will allow....