Research

I.   Strong-Field Physics of Nanoparticles

Strong-field physics investigates the effects of intense electromagnetic fields—typically exceeding 10¹³ W/cm²—on matter. Such fields distort potential energy landscapes, drive nonlinear processes, and give rise to multiphoton ionization, tunneling, high-harmonic generation (HHG), and electron recollision. While strong-field interactions have been extensively studied in atomic and molecular systems, recent advances have opened a new domain in solid-state nanostructures.

In my research, I have focused on the generation of photoelectrons (PEs) from metal nanoparticles (NPs) exposed to intense infrared laser pulses, and on analyzing how their spectra are influenced by competing direct and rescattered photoemission pathways. To describe these processes, I extended the semi-classical three-step (simple-man) model of atomic strong-field ionization to nanoparticles. This framework involves: (1) electron release via quantum tunneling, (2) propagation from the NP surface to the detector along classical trajectories, and (3) rescattering and recombination at the NP surface. Each of these steps becomes significantly more complex for nanoparticles due to their intricate electronic and morphological structures and the emission of a much larger number of electrons, where PE–PE correlations, residual charging, and nanoplasmonic-field interactions all play a critical role.

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II.   Ultrafast Physics in Molecules and Nanoparticles
Ultrafast physics in atomic, molecular, and nanoscale systems explores phenomena that occur on extremely short timescales, revealing the detailed motion of electrons, nuclei, and molecules. These timescales are typically measured in attoseconds (10⁻¹⁸ seconds), femtoseconds (10⁻¹⁵ seconds), and picoseconds (10⁻¹² seconds). Electronic motion, which occurs on attosecond to femtosecond scales, includes rapid dynamics such as tunneling, charge transfer, and electronic excitation driven by external fields. Vibrational motion, unfolding over tens to hundreds of femtoseconds, involves nuclei oscillating around equilibrium positions through processes such as bond stretching and bending. Rotational motion, the slowest, occurs over picoseconds to nanoseconds and describes the rotation of molecules around their center of mass. These motions are strongly coupled: electronic excitation often induces vibrational and rotational dynamics. Modern ultrafast techniques—such as attosecond, femtosecond, and terahertz pump-probe spectroscopy—enable the study of these processes with unprecedented resolution.

a.   Attosecond Streaking Spectroscopy of Nanoparticles

In this research, We developed two models to investigate attosecond streaking spectroscopy from metal nanoparticles. Attosecond streaking is a pump-probe technique that probes electron dynamics at the natural timescale of electronic motion. It can be applied both to individual electron dynamics in atoms and molecules, as well as to collective plasmonic effects in solids and nanostructures.

In the classical model, we simulated streaked photoelectron energy spectra as a function of the delay between ionizing attosecond extreme ultraviolet (XUV) pulses and assisting infrared (IR) laser pulses. This was achieved by sampling over classical electron trajectories. The model involves four stages: XUV excitation, electron transport inside the nanoparticle, electron escape from the surface, and propagation to the detector.

In the quantum model, we calculated the plasmonic fields induced by IR pulses using Mie theory, and the T-matrix elements for photoemission. Based on these models, we proposed a method for reconstructing induced plasmonic fields with nanometer spatial and sub-femtosecond temporal resolution from streaked photoemission spectra.

b.   Ultrafast Dynamics of the Rydberg States of Molecules

In this research, we have developed a comprehensive analytical–numerical model to describe the ultrafast dynamics of autoionizing molecular Rydberg states excited by extreme ultraviolet (XUV) pulses and perturbed by near-infrared (NIR) pulses. By numerically solving the time-dependent Schrödinger equation with Fano-parameterized Rydberg resonances, the model enables us to track population evolution, state coupling, and competing decay pathways via autoionization and NIR ionization. Our first application to CO₂ molecules, excited by a high-harmonic generation (HHG) XUV pulse and probed by a delayed NIR pulse, successfully reproduces the main features of experimentally measured photoelectron yields.