Amrutha Gopal

Energy deposition dynamics of femtosecond pulses in water

We exploit inverse Raman scattering and solvated electron absorption to perform a quantitative characterization of the energy loss and ionization dynamics in water with tightly focused near-infrared femtosecond pulses. A comparison between experimental data and numerical simulations suggests that the ionization energy of water is 8 eV, rather than the commonly used value of 6.5 eV. We also introduce an equation for the Raman gain valid for ultra-short pulses that validates our experimental procedure.

Characterization of 700 μJ T rays generated during high-power laser solid interaction

Laser-produced solid density plasmas are well-known as table-top sources of electromagnetic radiation. Recent studies have shown that energetic broadband terahertz pulses (T rays) can also be generated from laser-driven compact ion accelerators. Here we report the measurement of record-breaking T-Ray pulses with energies no less than 0.7 mJ. The terahertz spectrum has been characterized for frequencies ranging from 0.1-133 THz. The dependence of T-Ray yield on incident laser energy is linear and shows no tendencies of saturation. The noncollinear emission pattern and the high yield reveal that the T rays are generated by the transient field at the rear surface of the solid target.

Ultra-energetic THz pulses from a laser-driven particle accelerator

Summary form only given. High peak-power THz sources find many applications in material science, non-linear optics and next generation particle accelerators. We report the experimental realization of a gigawatt (GW) class T-rays from a laser-driven particle accelerator. Laser-driven particle accelerators are one of the well studied and promising methods for the generation of energetic particle beams and radiation extending the whole electromagnetic spectrum. Recent studies have shown that powerful T-rays can also be generated during such an interaction [1]. A conversion efficiency of higher than 10-3 and a peak power above a GW makes our source the most efficient and powerful THz source known today.

700μJ THz pulses from a laser-driven particle accelerator

Here we report a laser plasma-driven source of T-rays with the highest pulse energy ever recorded in a laboratory. T-rays are emitted from the rear surface of a solid target in the non-collinear direction at incident laser intensities ~ 1019 W/cm2. Pulse energy measurements reported T-ray pulses with peak energies no less than 700 μJ. Temporal measurements using a single-shot electro-optic method showed the presence of sub-picosecond T-ray pulses with 570 fs duration, thus rendering the peak-power of the source higher even than that of state-of-the-art synchrotrons. A conversion efficiency of higher than 10−3 and an average power of 7 mW makes it the most efficient compact and powerful THz source known today. Spectral analysis revealed the presences of frequencies ranging from 0.1 − 133 THz, while most of the energy is localised in the low frequency region. The dependence of T-ray yield on incident laser energy is linear and shows no signs of saturation. The spatial distribution of the recorded T-rays indicates that most of the T-rays are emitted in the non-collinear direction from the rear-surface of a solid target and the contribution in the forward direction is very small. 2D particle-in-cell simulations show the presence of transient current at the target rear surface.

Quantitative two dimensional shadowgraphy for low density plasma characterization

Summary form only given. In plasma physics, shadowgraphy has always been considered as an important but a qualitative diagnostic. For optically thin plasmas quantitative data can also be extracted from shadowgrams. However, in contrast to other fields of physics, quantitative shadowgraphy can rarely be found in plasma physics. Density profiles are usually retrieved by means of interferometric techniques, which can give accurate results over a wide range of plasma densities. However, they are not suitable for real-time diagnostics as the processing of the interferogram requires phase unwrapping algorithms which often demand manual intervention. The aim of our investigation is to show that quantitative shadowgraphy can be a simpler alternative to interferometric methods for two dimensional, automated, real-time characterization of low density plasmas generated by intense laser pulses in gases. We verified the accuracy of the shadowgraphic method by retrieving density profiles of several test objects (optical fibers, gas jets, plasma filaments in air) and comparing the results with interferograms. Agreement has been found for samples satisfying the Rayleigh-Gans condition. An algorithm has been developed for the retrieval of the optical path using a multigrid solver that processes in real-time a 256times256 pixel frame in about 1 second on an AMD Turion 64-bit processor. According to our numerical simulations, the technique will be able to monitor in green light plasma samples up to a thickness of 100 micrometers with an electron density of up to 1018 cm -3