High-Order Harmonic Generation and Attosecond Science

Since the first observation of high-order harmonics about two decades ago, research on high-order harmonic generation (HHG) has progressed while changing its focus. In its infancy, a major concern of research was to understand the underlying physics of HHG, then interest shifted to the development of a coherent source in the soft X-ray region. Research is now focused on attosecond science. Because HHG is based on tunneling ionization followed by radiative recombination during a single optical cycle of the fundamental excitation pulse, it can manifest itself as a variety of interesting phenomena caused by the interaction of light waves with electrons on the attosecond time scale. Therefore, HHG is a unique phenomenon that provides us with a method of observing attosecond quantum dynamics in atoms and molecules as well as with a unique coherent source covering a spectrum spanning several tens of octaves from ultraviolet to the soft X-ray region. In this report, I review the recent progress in attosecond pulse generation by HHG and its application to observing attosecond dynamics in atoms and molecules while focusing on our recent works.