Broadband Light Generation in Raman-active Crystals Driven by Femtosecond Laser Fields

Short pulse generation requires a wide phase-locked spectrum. Earlier the short pulses were obtained by expanding the spectrum of a mode locked laser from self phase modulation (SPM) in an optical fiber and then compensating for group velocity dispersion (GVD) by diffraction grating and prism pairs. Pulses as short as 4.4 fs have been generated (Steinmeyer et al., 1999). For ultrafast measurements on the time scale of electronic motion, generation of subfemtosecond pulses is needed. Generation of subfemtosecond pulses with a spectrum centered around the visible region is even more desirable, due to the fact that the pulse duration will be shorter than the optical period and will allow sub-cycle field shaping. As a result, a direct and precise control of electron trajectories in photoionization and highorder harmonic generation will become possible. But to break the few-fs barrier new approaches are needed. In recent past, broadband collinear Raman generation in molecular gases has been used to produce mutually coherent equidistant frequency sidebands spanning several octaves of optical bandwidths (Sokolov & Harris, 2003). It has been argued that these sidebands can be used to synthesize optical pulses as short as a fraction of a fs (Sokolov et al., 2005). The Raman technique relied on adiabatic preparation of near-maximal molecular coherence by driving the molecular transition slightly off resonance so that a single molecular superposition state is excited. Molecular motion, in turn, modulates the driving laser frequencies and a very broad spectrum is generated, hence the term for this process “molecular modulation”. By phase locking, a pulse train with a time interval of the inverse of the Raman shift frequency is generated. While at present isolated attosecond X-ray pulses are obtained by high harmonic generation (HHG) (Kienberger et al., 2004), the pulses are difficult to control because of intrinsic problems of X-ray optics. Besides, the conversion efficiency into these pulses is very low (typically 10−5). On the other hand, the Raman technique shows promise for highly efficient production of such ultrashort pulses in the near-visible spectral region, where such pulses inevitably express single-cycle nature and may allow non-sinusoidal field synthesis (Sokolov et al., 2005). In the Raman technique ns pulses are applied for preparing maximal coherence when gas is used as a Raman medium. When the pulse duration is shorter than the dephasing time T2 Source: Advances in Lasers and Electro Optics, Book edited by: Nelson Costa and Adolfo Cartaxo, ISBN 978-953-307-088-9, pp. 838, April 2010, INTECH, Croatia, downloaded from SCIYO.COM