Intense laser-plasma interactions foster a plethora of highly non-linear regimes, the most vivid example possibly being the 22-photon ionization of helium even at modest intensities of (1014 - 1015) W/cm2 (A. L�Huillier et al., J. Phys. B: At. Mol. Phys. 16, 1363 (1983)), often touted as one of the most highly non-linear processes known in physics. At higher intensities, the dynamics of interaction becomes increasingly more convoluted and confounding, often rendering analytical modeling inadequate and resorting to more elaborate particle-in-cell simulations, which mimic the table-top experiment in an effort to understand the underlying physics. The very coupling of the laser energy to the plasma, often interpreted by a variety of mechanisms, each predominant in a given regime of interaction, the consequent formation of plasma waves, followed by �wave-breaking� and engendering �fast� electrons, the transport of these �fast electrons� laden with instabilities and turbulences � these are a few amongst the multitude of questions that still remain unanswered, despite an overwhelming interest and extensive research in this area over the last few decades.
The incidence of an intense laser on a solid excites �plasma waves� - waves of oscillating electrons in the neutralizing background of residual quasi-stationary ions, testified by their signature emission of the second-harmonic. An analogy with sea- or ocean-waves is not completely unwarranted, as the final predicament of these burgeoning waves is a similar �wave-breaking�, as the plasma waves come crashing down in a �tsunami�, liberating a host of �fast� electrons, resulting in bursts of x-ray bremsstrahlung and a simultaneous decrease in the second-harmonic emission. Thus, a time-resolved measurement of the x-ray bremsstrahlung and the second-harmonic emission illustrates the dynamics of the fundamental process of wave-breaking (A. S. Sandhu et al., Phys. Rev. Lett. 95, 025005 (2005)), which is responsible for the generation of fast electrons.
Figure - (a) Integrated yield of x-rays and (b) second-harmonic generation efficiency, both self-normalized, as a function of time-delay, clearly showing how the second harmonic generation dips when the x-ray yield maximizes. For further details, see A. S. Sandhu et al., Phys. Rev. Lett. 95, 025005 (2005).
Another pivotal characteristic of laser-plasma interactions is the plasma density profile and its temporal evolution dynamics, with particular emphasis on the critical density layer � the plasma density beyond which a laser cannot penetrate into the plasma. Nanosecond interactions differ inherently from picosecond and sub-picosecond interactions, which are typified by steep density gradients over exponential length scales of tens of nanometers due to the absence of the �coronal� plasma, typically observed in long-pulse nanosecond interactions with scale-lengths as large as hundreds of microns. The plasma density profile, along with the critical density, is of utmost importance in determining the ensuing physics of the laser-plasma interaction � for example, the role it plays in exciting parametric instabilities in the various density regions of the coronal plasma such as the stimulated Raman scattering occurring at the quarter-critical density. Our recent experiments (S. Mondal et al., Phys. Rev. Lett. 105, 105002 (2010)) present the first high-resolution temporal mapping of the ultrafast dynamics of the critical surface using femtosecond Doppler pump-probe spectroscopy. A probe laser pulse, time-delayed with respect to the main interaction laser pulse, suffers a Doppler shift as it gets reflected from the time-varying critical density layer. Monitoring the temporal evolution of the Doppler shift therefore mirrors the motion of the critical surface, which initially rides the compression wave launched by the interaction pulse and tracks it as it moves inward, resulting in a red Doppler shift. At later times, as the compression wave has propagated into a region of overdense plasma, the critical surface now sits in a region that is undergoing rarefaction as the plasma expands into the vacuum, thereby causing a blue Doppler shift.