Generation and characterization of attosecond pulses obtained from high-order harmonics

Abstract

Generation and temporal characterization of attosecond pulses obtained from high-order harmonics have been investigated. From the time-frequency analysis of high-order harmonic generation it has been known that the attosecond pulses contain intrinsic chirp, corresponding to quadratic spectral phase variation. It was then suggested to compensate for the intrinsic attosecond chirp by exploiting the material dispersion of an x-ray filter. The analysis of the real and imaginary parts of the refractive index of metallic x-ray filter materials showed that a material has negative group delay dispersion in the spectral region of increasing transmittance. By compensating for the intrinsic chirp using an appropriately chosen x-ray filter, a method to generate single sub-50-attosecond pulses was proposed. The idea of the intrinsic attosecond chirp compensation by material dispersion has been extended to gaseous medium, which is generally used for the harmonic generation. Self-compression of attosecond pulses in the harmonic generation medium itself has also been demonstrated experimentally in a similar manner. Since the attosecond pulses were generated in an argon filled gas cell and compressed by exploiting the dispersion characteristics of argon itself, continuous chirp control was easily achieved by adjusting the gas pressure. The optimized attosecond pulse was also the most intense, and its duration of 206 as was very close to the transform-limited value of 200 as. For the complete characterization of the attosecond pulses, the frequency-resolved optical gating (FROG) technique was studied. The current progress of this method for the characterization of an attosecond pulse train is discussed along with the progress achieved in the experiments reported in this thesis. A Time-of-flight (TOF) spectrometer was constructed for these experiments. Optimization using the electron trajectory simulation was done and the resolution of the spectr...