High Intensity, Low/High contrast, Femtosecond Laser matter interaction

Terahertz (THz) radiation has been a subject of interest for the scientific community for quite a period of time due to numerous applications in imaging and security, biological, pharmaceuticals, and detection of cancerous tumors, to name a few. Hence, I will discuss femtosecond laser-matter interaction that was carried out as a part of the present research in two different regimes. At first, processes which occur at intensities < 10¹⁵ W/cm² that lead to Terahertz (THz) generation using two-colour laser-matter interaction in air and single-colour excitation in liquids. Following it, processes which occur at intensities > 10¹⁵ W/cm² that lead to relativistic electrons, thereafter acceleration of particles. Coming to a regime where we employed liquids like acetone, methanol, water, and ethanol, we could generate THz generation with MV/cm electric field strengths. We have also shown how to shift THz peaks by varying input pulse parameters and maintaining constant bandwidth of the laser pulse. Following this THz generation with MV/cm field strengths, nonlinear absorption nature has been probed in the bulk semiconductors at moderately high intensities, namely, <10¹⁵ W/cm². Interestingly, bulk semiconductor displays enhanced absorption of THz in the case of lesser bandgap whereas if the bandgap is high, it exhibits enhanced transmission. As the first part of this talk, I will discuss in detail about these results. High intense, high contrast laser-matter interaction for shock propagation in plasmas created by metals and dielectrics has been studied well, to date. However, at the same time the progress of similar studies in semiconductors remained slow. Towards these studies, we at Uphill lab have undergone measurements at relativistic laser intensities in silicon (Si) using ultrafast pump-probe reflectivity and Doppler spectrometry. In these experiments, we had observed an anomalous rise in reflectivity at approximately 9 ps after the main pulse interaction with the target. We attribute this increase in reflectivity mainly to the transient steepening of the plasma-density gradient at the probe critical surface due to explosive behaviour. In the next part, I will discuss regarding the acceleration of particles generated during laser matter interaction. The relativistic electrons that has been generated from the front side of target during this interaction lead to a sheath electric field that eventually resulted in the acceleration of the protons and ions. The electric fields created by relativistic electrons in this process is of the order of TV/m. In addition to these results, I will discuss about the initial results and enhancement achieved by changing the target surface parameters.