Within a year after the operation of the first laser, P. Franken demonstrated the generation of the  second harmonic of Ruby laser radiation in 1961 [4]. Soon after that Kaiser and co-workers [5] observed the nonlinear optical phenomenon of two-photon absorption which was theoretically predicted by Goppert Mayer [6] in 1931. With the  availability of  the more  powerful lasers   a multitude of nonlinear optical phenomena such as sum and difference frequency generation [7], parametric generation and amplification of light [8.], optical rectification [9], stimulated Raman scattering [10],  self focusing and filamentation [11-13], self and cross phase modulation and self steepening phenomena [14] to name a few were discovered in the 1960's. Fundamental theoretical contributions of Bloembergen [15], Akhmanov and Khokhlov [16]  established nonlinear optics as an independent branch of optical sciences by mid sixties.   In 1971, Stepanov demonstrated optical phase conjugation in degenerate four-wave-mixing scheme [17]. Z'eldovich used the stimulated Brillouin Scattering process in 1972 to achieve the same [18]. Time reversed wavefront generation using optical phase conjugation became work horse for the variety of applications including aberration correction and self targeting [19,20]. Observation  of optical bistability in 1976 by Gibbs and coworkers [21], which was predicted by Szoke et-al in 1969 [22], sparked intense research activity in nonlinear optics and its applications to all optical computing. Investigations on the effects of cubic nonlinearity on the propagation of the laser pulses in the optical media led to the realization of optical solitons in 1983 which opened new vistas for all optical communications [23].   A variety of very versatile and ultra high resolution - both time and frequency - spectroscopic techniques based on degenerate and nondegenerate four-wave-mixing processes such as coherent  raman scattering,  pump-probe, laser excited grating(LEGs), photon echo,  saturation and multiphoton spectroscopies, to name a few, were developed in the due course of time which generated wealth of new insights into the physics and chemistry of materials [24]. These discoveries sparked intense research activity in area of  nonlinear optics during the 1980s. Bloembergen was awarded Nobel prize for his outstanding contributions to the field in 1981.  With the introduction of chirped-pulse-amplification [25], intense laser sources took new leap to a regime where the electric field strengths started to exceed the intra-atomic coulomb field and added a new chapter of intense field-matter interaction [26]. Discovery of newer phenomena such as higher harmonic generation(HHG) [27-29], Tunneling ionization [30, 31] and the barrier suppression ionization [32], above threshold ionization(ATI) [33]  followed. lasers approaching the fundamental intensity limit-Schwinger's limit [34] - are being developed to study relativistic nonlinear optical phenomena.