2D materials are one of the most exciting research frontiers of materials science and physics. Rational design and control of novel systems for photophysics, many-body interactions, nano science and technologies are the longstanding goals for these two essential research topics. Here, Li Group uses static and ultrafast spectroscopy as the main research methods and combines advances of 2D materials and photonic cavities to achieve these goals. We aim to discover and manipulate new states of matter and devices with emerging properties through interdisciplinary research involving 2D materials, ultrafast spectroscopy, and cavity engineering.

2D Materials. Physics can be significantly different in 2D compared with that in 3D. 2D materials poccess emerging properties and provide a rich platform for the discovery of new states of matter and new opportunities to solve long-standing fundamental problems. Here, Li Group explores photophysics, magnetisms, and light-matter interaction in 2D semiconducting and magnetic materials, where different quasi-particles like electrons, holes, excitons, spins, and photons interact with each other. This research tends to provide new understandings as well as new opportunities for on-chip micro/nano opto-electronic, opto-magnetic, magneto-optical, and optical devices.

Ultrafast Spectroscopy.Ultrafast spectroscopy has become a powerful tool to provide a time-domain view for many physical and chemical processes, as well as a sensitive probe for the behaviors and lifetimes of excited states in numerous materials.  The basic idea is to use two ultra-short laser pulses with the pulse duration within femto-second (10^-15 second) time scale, one for “pump” and one for “probe”. The “pump” pulse serves as a stimuli or excitation that tunes the materials away from their ground states or equilibrium states, while the “probe” pulse then detects or measures the new status of the material generated by the “pump”. By changing the nature of two pulses (e.g., broad-band white light, THz, X-ray) and the delay time between them, ultrafast spectroscopy shows how different aspects (e.g., electronic transitions, spin precessions, phonon oscillations, lattice distortions) of materials evolve with time. Here, Li Group focuses on using different types of ultrafast spectroscopy and microscopy (including but not limited to transient absorption/reflection, time-correlated single photon counting, and time-resolved Faraday/Kerr rotation) to explore and manipulate the quasi-particle dynamics, collective excitations, and ordered states in 2D materials.

Cavity Engineering. If condensed matter or solid-state material is a platform for electrons, then cavity is a platform for photons. A cavity can confine, control, and amplify the vacuum and laser fields, it can be as simple as a pair of mirrors or a rationally designed photonic crystal (PhC) with a complex pattern. Similar to the electrons in condensed matters, photons in cavities or photonic crystals also have their own band structure, polarization, symmetry, and topology. Moreover, these designed features can hybridize with the ones of electrons, excitons, or collective excitations (e.g., phonons and magnons) in materials through light-matter interaction, providing a new and powerful strategy to establish and manipulate the states and properties of 2D materials. Here, Li Group aims to use rationally designed photonic cavities to confine, control, and amplify the vacuum oscillation or laser field amplitude to establish, sense, and manipulate the photoexcited states, charge carrier dynamics, collective behaviors, and electronic and magnetic ground states of 2D materials.