Abstract
High harmonic generation (HHG) is a strong-field process that generates coherent extreme ultraviolet (XUV) pulses. Compared with the fundamental driving field, these XUV pulses have significantly shorter durations and higher photon energies, making HHG a powerful tool for studying transient electron dynamics with temporal resolution ranging from the attosecond to the few-femtosecond regime. In this presentation, I summarize my Ph.D. work, which is organized into two main sections.
First, I present the design and construction of a home-built attosecond-resolution spectroscopy apparatus. The apparatus consists of several key components. It begins with a gas-cell chamber for XUV generation via HHG, followed by an attosecond interaction chamber equipped with a sophisticated control system that facilitates user-friendly alignment for IR-XUV pump–probe experiments. To verify the spatial and temporal overlap of the two beams, an imaging beam path is designed for rapid switching between alignment and measurement modes. For spectral characterization, two home-built XUV spectrometers were constructed, and a commercial time-of-flight (TOF) spectrometer was integrated into the system.
Second, I present a high-precision transient absorption experiment on SiO₂ performed using this apparatus. The ultrafast response of wide-band-gap dielectrics to intense laser fields is central to attosecond science. Here, we combine attosecond transient absorption spectroscopy (ATAS) with a time-resolved theoretical framework to investigate the near-band-gap dynamics in SiO₂. An intense NIR pump pulse excites carriers across the band gap, and a delayed XUV pulse train generated by HHG probes the resulting absorption changes near the gap. We observe a transient absorption enhancement that blueshifts and broadens every optical cycle. To trace its origin, we solve the semiconductor Bloch equations (SBEs) including Coulomb interactions, explicitly accounting for electron–hole attraction and band-gap renormalization. The simulation faithfully reproduces the measured feature and reveals the modification of the band structure due to excitonic effects. Our work demonstrates that Coulomb correlations govern the near-band-gap transient absorption of SiO₂ on the femtosecond timescale, providing a sensitive test for theories of dielectric dynamics.
Anyone interested is welcome to attend.