Enhancement of high harmonics in plasma waveguide and high-repetition-rate HHG with few-cycle OPCPA

Abstract

High-order harmonic generation (HHG) is a well-known method to get coherent light source from UV to extreme ultraviolet radiation (XUV) region, meanwhile with short pulse. Major efforts on HHG are currently devoted to increase high harmonics (HH) efficiency and photon flux for the request of wide applications. High repetition rates HH source has great experimental benefits, which significantly increases the signal-to-noise ratio and reduce measurement durations in many fields such as condensed matter physics. This thesis studies two aspects of enhancement of HH and high-repetition-rate HH source generation. Chapter 1 introduces the background of HHG. The HH development with driving laser parameters is reviewed and listed in details. The experiment related to HH enhancement and high-repetition-rate HHG is surveyed. Chapter 2 displays the laser system and experimental apparatus for HH enhancement with channeling configuration. The entire laser system is shown with schematic layout and specifications for each stage. The device for cluster formation is introduced. The design and calibration of extreme ultraviolet radiation (XUV) spectrometer for characterization of HH spectrum is presented. Chapter 3 shows the formation of plasma waveguide by interaction between intense laser pulse and clustered Argon gas. The time for the waveguide formation is investigated by probing beam with different delay time and Michelson Interferometer (MI) to encode the waveguide information into interferogram. The electron density distribution is obtained by extracting phase information with Abel inversion algorithm from the interferogram. It demonstrates that the plasma waveguide is formed around 20 ns after the interaction. The second pulse is guided through the created waveguide with different delay time and its beam profile is measured to confirm the formation of plasma waveguide. Chapter 4 presents the enhancement of high order harmonics in a plasma waveguide configuration formed in clustered Ar gas. The highest enhancement of HH compared without a waveguide is 9 times at the 27th order harmonic from Ar. The blue shift of the high-order harmonics is also observed. The numerical simulation by solving the time-dependent Schrödinger equation in strong field approximation (SFA) for enhancement of HH by plasma waveguide is performed. It agrees well with the experimental results. Further simulation shows that the blue shift of HH is mainly due to self-phase modulation (SPM) in a plasma channel. The plasma waveguide promises a way to enhance HH. Chapter 5 demonstrates the high-repetition-rate HHG with a 300 kHz few-cycle OPCPA laser and Argon gas. The OPCPA laser system is introduced in detail with performance parameters. The experimental apparatus for HHG is shown which including beam-lock system, beam image transfer system. The pulse in the interaction chamber is characterized with Spectral phase interferometry for direct electric-field reconstruction (SPIDER) apparatus and compressed by changing the bounce number on chirp mirrors and finely tuning the wedge pairs in OPCPA to compensate the dispersion. The high-repetition-rate HH is generated with Ar gas and the OPCPA laser. The beam profile of HH is measured. The highest photon energy reaches 32 eV for 21st order harmonics.