High-order harmonic generation in laser-produced plasmas

High-order harmonic generation (HHG) allows for production of high energy photons in a broad spectral range reaching up to extreme ultraviolet (EUV) range with a compact tabletop setup. The coherent nature of the generated broadband radiation also facilitates the production of ultrashort attosecond bursts. While HHG is often driven in a noble gas medium, laser-produced plasmas (LPPs) can also serve as a HHG medium. An interesting finding for driving HHG in LPPs has been that the more complex atomic structures of some generation media has allowed for resonant enhancement up to two orders of magnitude of a single harmonic. Besides enabling generation of high-energy photons, HHG in LPPs also allows the investigation of the LPP itself. Chapter 2 of this thesis describes the two laser systems which are needed for the HHG in LPP experiments. The first laser system (producing the ’fundamental’ beam) is an optical parametric chirped-pulse amplifier (OPCPA) which drives the highly nonlinear HHG process within the LPPs. The second laser system (pump) is responsible for driving the plasma generation. It is a Nd:YAG laser system with sub nanosecond pulse shaping capabilities. In Chapter three, we report on our experimental setup for the HHG in LPP measurements. We form the plasma and drive the HHG in the experimental chamber of our vacuum setup and detect the emitted radiation with an EUV spectrometer. We also measure ion kinetic energy distributions from the generated plasma plumes with retarding field analyzer (RFA) Faraday cup detectors. We display initial HHG spectra obtained from our aluminium, nickel, silver, indium, and tin targets. Each target shows a distinct harmonic spectrum with clear variations in brightness and highest order harmonic generated. We investigate the effects of the pump laser parameters on the HHG spectra produced in tin plasmas in Chapter 4. To this extent, generate the LPP with square or Gaussian temporal shapes and scan the pulse duration, and we vary the pump to fundamental delay. We observe that HHG produced in LPPs pumped by the short, Gaussian-shaped pump pulses leads to a higher generation efficiency of the highest-order harmonics up to H25. Longer pump pulses result in more efficient generation of lower order harmonics. The HHG spectral distribution also shifts from shorter to longer wavelengths with increasing delay between pump and fundamental laser pulses. In the final chapter of this thesis, Chapter 5, we investigate the influence of a two-color drive field on the HHG spectra generated in aluminium and tin LPPs. We record lower-order harmonic spectra as a function of relative phase between the fundamental and its second harmonic, resulting in oscillating signal strengths of these harmonics. Individual harmonics oscillate out of phase with each other and show large phase jumps between neighboring harmonics, indicating that in our regime, the electron trajectories of individual harmonics are not solely governed by the electric fields of the driving fields. Instead, these experiments confirm that contributions from the atomic Coulomb potential cannot be neglected when driving HHG in conditions where the strong field approximation does not hold.