The microscopic electronic arrangement and charge densities defines the physical, optical and chemical properties of the material. The scientific advancement in understanding the arrangement of matter has led to better understanding of both chemical as well as physical aspects of materials. For the bulk periodic or crystalline materials X-ray or electron beams can be used to decipher the crystalline arrangement and thereby map the electron density. However, even with these techniques at hand, the direct visualization of the valence electron densities inside solids remains an insurmountable challenge. Currently the understanding of high harmonic generation(HHG) in solids is mostly limited to dynamic of electron hole pair on the conduction and valence bands of solids however to gain insight into valence charge densities this process needs to explained with real space motion of electrons. Recent efforts in 2D materials have made significant steps in this direction but a sufficiently complete real-space picture is still lacking. We present a real space approach to high harmonic generation in solids in strong field regime. We show that in strong field regime the motion of electron in presence of laser field is mostly governed by it and periodic potential acts as a perturbation to the potential. Solving for equation of motion in presence of these two fields we show the dependence of emitted high harmonic radiation on laser field and potential. All the experimental observations such as cut-off dependence, wavelength dependence, orientation dependence and dependence of emitted intensity on input electric field reconcile with the theoretical prediction, hence bolstering our model. We also predict that such model can be used to reconstruct the valence electron densities for solids.
We have observed and studied a nonlinear response of dispersive dielectric multilayer mirrors (DM). It was found that the structure of the mirror itself causes strong enhancement of the electric field inside the multilayer stack consequently triggering strong two-photon absorption (2PA). We have developed a mathematical model, that allows estimation of the coefficient of the 2PA, β, subsequent prediction and to some extent tuning of the strength of the nonlinear response of any multilayer coating.
Extreme Ultraviolet (XUV) sources based on femtosecond laser - high order harmonic generation (HHG) in atomic gases are presently in a period of rapid development. The HHG sources offer the additional advantage of delivering much shorter pulses compared to those of plasma generation or synchrotron sources. Furthermore, because they are coherent sources with high photon energy, they support attosecond pulse (10-18 s) generation, pushing the frontiers in both science and technology. These pulses open up research on unprecedented time scales, characteristic of bound electron motion. Issues related to the production and characterization of such pulses are presented.