• Spatial Carrier Dynamics from Non-equilibrium to Equilibrium Regimes
  We study how surface passivation, additive treatment, defect modulation, and film morphology—including wrinkled and textured structures—affect spatial carrier dynamics on femtosecond to sub-nanosecond timescales. Using transient absorption microscopy (TAM), we can directly visualize these processes in real space. Such direct experimental observations of carrier behavior provide insights that conventional spectroscopy cannot capture. These insights provide strategies for the rational design of perovskite films aimed at enhancing photovoltaic performance. [Nature Physics 2020, 16 (2), 171-176, The Journal of Physical Chemistry Letters 2020, 11, (14), 5402-5406, Nature Nanotechnology  2022, 17, 190-196 and more to come.]

• Ultrafast Spin/Valley Dynamics
  Spin- and valley-dependent processes can be accessed via circularly polarization dependent measurements. This approach enables systematically correlate spin lifetimes and spatial spin dynamics across various materials with different symmetries, structures, chemical compositions, and surface/interface treatments. Our findings aim to deepen the fundamental understanding of perovskite-based spintronic and valleytronic device and guide future engineering. [Nature Materials 2023, 22, 977-984, and more to come.]

• Quasi-particle Dynamics
  Perovskites offer a versatile platform for the study of quasiparticles such as phonons, polarons, and exciton-polaritons. By observing quasiparticles behavior such as spatial propagation and excited state dynamics, we provide novel insights into the coupling mechanism between lattice, charge, and light, as well as the spatial dynamics involved in quasiparticle relaxation processes.

Ultimately, we aim to elucidate the fundamental physical mechanisms underlying energy transport and conversion in hybrid perovskites. We strive to offer new perspectives on ultrafast phenomena in perovskite thin films and crystals, thereby contributing to the advancement of next-generation optoelectronic and spintronic/vallytronic technologies.

2. Transition Metal Dichalcogenides 

Transition metal dichalcogenides (TMDs) are layered two-dimensional semiconductors that exhibit with strong light–matter interactions. The atomically thin nature of these materials leads to reduced dielectric screening, which enhances Coulomb interactions and facilitates the formation of tightly bound excitons. These excitons maintain stability even at room temperature with a wide range of optical phenomena, making TMDs attractive systems for studying exciton-related processes. Owing to broken inversion symmetry and pronounced spin–orbit coupling, TMDs also exhibit valley-specific selection rules, allowing selective access to spin and momentum states and providing unique distinctive opportunity to study spin and valley dynamics.

In addition to their intrinsic properties, the optical and electronic properties of TMDs are highly sensitive to environmental conditions. Variations in strain, defects, stacking order, and dielectric environment introduce spatial heterogeneity in both excitonic and carrier behavior. As a result, energy relaxation, recombination pathways, and carrier transport processes can vary significantly within a single TMD flake.

In our lab, we study these intriguing phenomena in TMDs utilizing femtosecond transient absorption spectroscopy and microscopy. These advanced spectroscopy techniques allow for direct visualization and quantitative analysis of exciton formation, transport, and recombination with femtosecond temporal and nanometer spatial resolution. This approach allows us to assess how ultrafast energy, charge carrier transport and relaxation processes are influenced by structural and environmental variations in TMD systems. Such insights significantly improve our understanding of excitonic behavior in two-dimensional semiconductors and contribute to the development of next-generation optoelectronic technologies based on TMDs.

[ACS nano 2020, 14, (11), 15374-15384, ACS Nano 2024, 18, 1, 264-271, and more to come ]

3. Quantum Dots  

Quantum dots (QDs) are nanoscale semiconductor particles that exhibit unique optical and electronic properties, such as size-dependent emission energy and discrete energy levels, arising from quantum confinement effects. These features distinguish QDs from bulk semiconductors and make them promising candidates for the next-generation optoelectronic materials.

Advances in time-resolved spectroscopy techniques have yielded valuable insights into carrier dynamics such as exciton generation, relaxation, and recombination in the QDs. Such conventional time-resolved measurements are typically limited to properties at the  level of individual quantum dot. However, for real world device applications, it is more important to understand the collective properties within the QD solids rather than the isolated individual properties. TAM measurement allows direct tracking of charge carrier transport dynamics by directly visualizing the spatio-temporal map of excited carriers in QD films. This capability not only reveals hidden carrier dynamics but also provides deeper insights into exciton transport mechanisms, which are critical for advancing applications in photovoltaics and quantum information technologies.

[Nature Materials 2022, 21, 533-539, Nano Letters 2021, 21, (21), 8945-8951, and more to come ]