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Brief background
To capture the time evolution of a fast process, we need an even faster probe that allows us to "freeze" the motion into a series of snapshots. Recent advancements in intense, ultrashort laser pulses provide such a probe, enabling scientists to not only observe but also control electron and nuclear dynamics on their natural timescales - ranging from attoseconds (10-18 seconds) to femtoseconds (10-15 seconds). Understanding and controlling these ultrafast electron dynamics is of fundamental and technological importance, as electron motion underpins processes such as current flow in materials and the initiation of chemical reactions. See the website of the Nobel Prize in Physics 2023 for the press release, popular/advancement information on the historical background of attosecond science.
Our research sits at the intersection of ultrafast condensed matter physics, AMO physics, and computational physics. We focus on developing theoretical frameworks and numerical tools to predict, control, and investigate ultrafast, non-perturbative phenomena resulting from strong laser-matter interactions in both condensed-matter and gas-phase systems. Through close collaboration with experimentalists, we aim to initiate novel ultrafast spectroscopy capabilities that can probe dynamic processes and reveal intricate system properties. Some research topics are given below,
Over the past decade, attosecond science has expanded beyond gas-phase systems to include the study of condensed matter. This rapidly growing interdisciplinary field holds the promise of new light sources, faster electronics, and novel spectroscopic techniques for probing quantum materials. Outlined below are some of our research interests. For some research highlights, see link.
High-harmonic generation in solids
High-harmonic generation (HHG), an extreme form of laser frequency up-conversion, is a cornerstone of attosecond science. In crystalline solids, this process is intuitively understood in terms of a three-step model (see figure) - tunneling, propagation and recombination - repeating withinin each optical cycle of a laser pulse. Solid-state HHG has emerged as a powerful ultrafast spectroscopic technique, capable of probing intricate material properties such as band structure, Berry curvature, and topological features. It also provides valuable insights into sub-cycle dynamics like electron-hole recollisions and dynamical Bloch oscillations.
Attosecond transient absorption/reflection spectroscopy
Attosecond transient absorption and reflection spectroscopy (ATAS/ATRS) are powerful pump-probe techniques used to investigate ultrafast dynamics in condensed matter. These techniques involve measuring the absorption or reflection of a time-delayed probe pulse following an initial pump pulse that triggers the dynamics. ATAS/ATRS offer exceptional time resolution, allowing for the precise observation of the underlying processes like exciton decay on femtosecond to attosecond timescales.
When an intense light field irradiates an atom or molecule, it can trigger a range of strong-field phenomena, including ionization, molecular breakup, and high-harmonic generation. Characterizing and understanding these processes are of fundamental and technological significance, with potential applications in controlling chemical reactions and monitoring electron dynamics. Outlined below are some of our research interests. For some research highlights, see link.
Strong-field-induced molecular breakup
When a molecule is exposed to short, intense laser pulses, it can undergo ionization and fragmentation. By detecting the ionized electrons and molecular fragments, we gain insights into the underlying electron-electron and electron-nuclei correlations that occur during strong light-matter interactions.
Attosecond spectroscopy of molecular structure and dynamics
Attosecond spectroscopy techniques, such as attosecond transient absorption spectroscopy (ATAS) and light-induced electron diffraction (LIED), provide valuable insights into molecular structure and the coupled dynamics of electrons and nuclei, with the potential to create 'attosecond movies' that capture these ultrafast processes.
The fascinating phenomena and advanced spectroscopy techniques in strong-field and ultrafast physics demand precise numerical and theoretical modeling. Our goal is to develop codes and methods that not only elucidate experimental results but also possess strong predictive capabilities. Outlined below are some of our research interests. For some research highlights, see link.
First-principle methods and code development
Strong light-matter interactions require numerical solutions that extend beyond standard perturbative methods commonly used in nonlinear optics. The complex interplay of coupled degrees of freedom in condensed matter and AMO physics during strong light-matter interactions necessitates accurate and efficient computational codes. Our research focuses on both the microscopic physics described by the Schrödinger equation and the macroscopic effects governed by Maxwell's equations.
Model building
Although first-principle numerical solutions can achieve high accuracy, they are computationally expensive and often function like a 'black box,' with the internal processes remaining opaque. To gain a deeper understanding of the underlying physics, we develop models that provide insight and intuition into the fundamental mechanisms at play.
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