Spectral characteristics of high-order harmonics generated from noble gases (Ar, He, and Ne) driven by intense femtosecond laser pulses were investigated. High-order harmonic generation experiments were performed with a femtosecond Ti:sapphire laser operating at 10 Hz. The laser beam was focused into a gas jet with a spherical mirror of 60-cm focal length, and the gas density in the interaction region was measured by applying a focusing method, which detected the refraction of incident light due to nonuniform gas-density distribution. To estimate the focused laser intensity, the spatial profile of the focused beam was measured with a charge-coupled device (CCD) coupled with a microscope objective lens, which showed that the central Airy disk contained 73% of the energy.
We experimentally verified the generation of spectrally resolved high-order harmonics from different quantum paths by driving Ar and He atoms with intense laser pulses well above the saturation intensity for optical-field ionization. Moreover, by controlling the phase-matching condition high harmonics generated from the short quantum path were selected. It was shown theoretically that when only the short-path harmonics are used, an attosecond pulse train with one pulse per half-optical cycle can be generated with the pulse width. of about 150 attoseconds.
For the production of sharp and strong harmonics, high-order harmonics were coherently controlled using chirped femtosecond laser pulses. As the laser intensity was increased above the saturation intensity for optical-field ionization, the laser chirp needed to suppress harmonic chirp in the plateau region changed from positive to negative. We showed that the modification of a laser chirp condition in a rapidly ionizing medium should be included for the proper coherent control of high-order harmonics, necessitating the integral treatment of the interaction between atoms and a driving laser pulse.
In addition to the spectral analysis of high-order harmonics, this thesis presents experimental results on bright harmonic generation achieved through a guided propagation of intense femtosecond laser pulses in a long gas jet. A long uniform plasma column was produced when the gas jet was exposed to converging laser pulses, and good phase matching between the driving laser pulse and generated harmonics was obtained by optimizing laser parameters, medium density and position. The measured low divergence, 0.5 mrad (FWHM), of the 27th harmonic from an argon gas jet resulted from a large cross section of harmonic generation. In addition to low beam divergence and easiness of laser alignment, the harmonic generation in a long gas jet will provide a pathway to efficient harmonic generation with a large pumping energy, because there is no restriction on the transverse dimension of harmonic generation medium.
