Optical Probing of Ultrafast Laser-Induced Solid-to-Overdense-Plasma Transitions

Understanding the target dynamics during its interaction with a relativistic ultrashort laser pulse is a challenging fundamental multi-physics problem involving at least atomic and solid-state physics, plasma physics, and laser physics. Already, the properties of the so-called pre-plasma formed as the laser pulse's rising edge ionizes the target are complicated to access in experiments and modeling, and many aspects of this laser-induced transition from solid to overdense plasma over picosecond time scales are still open questions. At the same time, applications like laser-driven ion acceleration require precise knowledge and control of the pre-plasma because the efficiency of the acceleration process itself crucially depends on the target properties at the arrival of the relativistic intensity peak of the pulse. By capturing the dynamics of the initial stage of the interaction, we report on a detailed visualization of the pre-plasma formation and evolution. Nanometer-thin diamond-like carbon foils are shown to transition from solid to plasma during the laser rising edge with intensities < 10^16 W/cm^2. Single-shot near-infrared probe transmission measurements evidence sub-picosecond dynamics of an expanding plasma with densities above 10^23 cm^-3 (about 100 times the critical plasma density). The complementarity of a solid-state interaction model and a kinetic plasma description provides deep insight into the interplay of ionization, collisions, and expansion

Charakterisierung expandierter ultradünner DLC-Folien für die Laser-Protonenbeschleunigung

Die in dieser Arbeit betrachteten Parameter mit ihren Abhängigkeiten von der Laserintensität können weiteren PIC-Simulationen als Inputparameter gegeben werden. Diese betrachten das Problem der Laser-Plasma-Wechselwirkung kinetisch und könnten so in Verbindung mit den Ergebnissen der MULTIfs Simulationen zu einem besseren Verständnis der gesamten Laser-Plasma-Wechselwirkung führen

Laser-driven ion acceleration from carbon nano-targets with Ti:Sa laser systems

Over the past few decades, the generation of high energetic ion beams by relativistic intense laser pulses has attracted great attentions. Starting from the pioneering endeavors around 2000, several groups have demonstrated muliti-MeV (up to 58 MeV for proton by then) ion beams along with low transverse emittance and ps-scale pulse duration emitted from solid targets. Owing to those superior characteristics, laser driven ion beam is ideally suitable for many applications. However, the laser driven ion beam typically exhibits a large angular spread as well as a broad energy spectrum which for many applications is disadvantageous. The utilization of nano-targets as ion source provides a number of advantages over micrometer thick foils. The presented PhD work was intended to investigate laser driven ion acceleration from carbon nano-targets and demonstrate the potential feasibility for biological studies. Two novel nano-targets are employed: nm thin diamond-like-carbon (DLC) foil and carbon nanotubes foam (CNF). Both are self-produced in the technological laboratory at Ludwig-Maximilians-Universität München. Well-collimated proton beams with extremely small divergence (half angle) of 2 degrees are observed from DLC foils, one order of magnitude lower as compared to micrometer thick targets. Two-dimensional particle-in-cellsimulations indicate a strong influence from the electron density distribution on the divergence of protons. This interpretation is supported by an analytical model. In the same studies, the highest maximum proton energy was observed with a moderate laser intensity as low as 5*10^18W/cm^2. Parallel measurements of laser transmission and reflection are used to determine laser absorption in the nano-plasma, showing a strong correlation to the maximum proton energy. This observation indicates significance of absorbed laser energy rather than incident laser intensity and is supported by an analytical model. The ion energy also depends on pulse duration, a reduced optimum pulse duration is found as compared to micrometer thick targets. This behavior is attributed to a reduction of transverse electron spread due to the reduction of thickness from micrometer to nanometer. These remarkable proton bunch characteristics enabled irradiating living cells with a single shot dose of up to 7 Gray in one nanosecond, utilizing the Advanced Titanium: sapphire LASer (ATLAS)system at Max-Planck-Institut of Quantum Optics (MPQ). The experiments represent the first feasibility demonstration of a very compact laser driven nanosecond proton source for radiobiological studies by using a table-top laser system and advanced nano-targets. For the purpose of providing better ion sources for practical application, particularly in terms of energy increase, subsequent experiments were performed with the Astra Gemini laser system in the UK. The experiments demonstrate for the first time that ion acceleration can be enhanced by exploiting relativistic nonlinearities enabled by micrometer-thick CNF targets. When the CNF is attached to a nm-thick DLC foil, a significant increase of maximum carbon energy (up to threefold) is observed with circularly polarized laser pulses. A preferable enhancement of the carbon energy is observed with non-exponential spectral shape, indicating a strong contribution of the radiation pressure to the overall acceleration. In contrast, the linear polarization give rise to a more prominent proton acceleration. Proton energies could be increased by a factor of 2.4, inline with a stronger accelerating potential due to higher electron temperatures. Three-dimensional (3D) particle-in-cell (PIC) simulations reveal that the improved performance of the double-layer targets (CNF+DLC) can be attributed to relativistic self-focusing in near-critical density plasma. Interestingly, the nature of relativistic non-linearities, that plays a major role in laserwakefield-acceleration of electrons, can also apply to the benefit of laser driven ion acceleration

Ultrafast and intense laser interaction with gases and solids

A dedicated laboratory has been built in DCU to study the interaction between ultra-short, intense laser radiation with both solid and gaseous materials. A commercial laser system with high intensity (3 mJ), and short temporal dura- tion ( 32 fs) has been installed and characterised as part of this project. A static, single shot Nomarski interferometer has been developed and by exploiting the non- linear response of an optical filter, the pulse width of the laser has been measured. The interaction of high intensity laser radiation with solid materials was also investigated. Experiments using a wire probe to characterise a silicon laser produced plasma were performed. The emission of ions and electrons within the laser plasma as well as the angular distribution of both species has been measured and the effects of laser intensity, laser wavelength and laser polarisation are investigated. In a complementary experiment, the expansion dynamics of the laser plasma are studied with space resolved Langmuir probe measurements. These results are analysed and presented in the context of double layer formations within the plasma plume. A Wiley-McLaren spectrometer has been built to study the interaction of high intensity laser light with gas jets. The design and construction of the spectrometer is detailed as well as its characterisation. The effects of polarisation are studied with molecular nitrogen as a gas target