arXiv subjects

Kwang Jin Lee

Publications and source records attributed to Kwang Jin Lee.

6 recordsLinked to original sources

arXiv2026

Vortex-Beam Transient Absorption Microspectroscopy Resolves Ultrafast Free-Exciton and Polaron Diffusion in 2D Perovskites

Two-dimensional (2D) Ruddlesden Popper perovskites are promising optoelectronic materials with strongly confined excitonic properties; however, probing their ultrafast carrier transport dynamics, particularly the initial nonequilibrium diffusion regime, remains challenging because conventional transient absorption microscopy requires complex spatial imaging and lacks sufficient temporal sensitivity to resolve early time diffusion dynamics. Here, we demonstrate a vortex beam based transient absorption microspectroscopy platform (VTAM) enabling imaging free measurement of carrier transport by encoding spatial diffusion information into the mode dependent pump probe signal. By employing vortex probes with different topological charges, VTAM provides mode selective spatial sensitivity to excitonic dynamics with subpicosecond temporal resolution. Using VTAM, we resolved rapid free exciton (FE) diffusion followed by relaxation toward a slower steady state transport regime. A theoretically derived time dependent diffusion model separated transient and steady state transport contributions, yielding a transient diffusion enhancement (68.84 cm2 per s) and a steady state diffusion coefficient (1.85 cm2 per s), thus providing an initial diffusion coefficient (70.69 cm2 per s), and a cooling time of 0.35 ps. Measurements at the exciton-polaron (EP) resonance revealed strongly suppressed diffusion with nearly time independent signal ratios, indicating lattice-coupled EP transport. These parameters were extracted without spatial scanning or image reconstruction, establishing V-TAM as a powerful imaging free platform for investigating carrier transport in perovskites and other semiconductor systems.

physics.optics↗

arXiv2025

Ultrafast exciton polaron dynamics in 2D Ruddlesden Popper lead halide perovskites

Two dimensional Ruddlesden Popper (2D) RP hybrid perovskites exhibit substantially higher chemical and structural stability than their three dimensional (3D) counterparts, positioning them as promising candidates for next generation optoelectronics. While quasiparticle dynamics in 3D perovskites are well studied, their 2D analogues remain comparatively underexplored. Here we systematically investigate the branching, dynamics, and interactions of free excitons (FEs) and exciton polarons EPs in monolayer 2D RP perovskites using visible range femtosecond transient absorption TA spectroscopy. We prepared monolayer 2D RP perovskite thin films with varied organic spacers and distinct fabrication routes for comparative analysis. We find that the EP binding energy is 50 65 meV in (BA)2PbI4 and 37 39 meV in (PEA)2PbI4, consistent with spacer layer dependent coupling as corroborated by FTIR. We reveal a dynamic equilibrium between FEs and EPs that persists for tens of picoseconds. Notably, the TA signatures differ by fabrication route films from the newly developed process show weaker Auger annihilation and a reduced hot phonon bottleneck than those from the conventional route trends consistent with fewer traps and impurities in the former. Coupled rate equation modeling reproduces the transients and quantifies the processes of hot carrier relaxation, exciton exciton annihilation, exciton phonon coupling, and FE EP interconversion. These results demonstrate that the chemical synthetic process (fabrication route) and spacer choice significantly influence EP stability and population balance, offering practical levers for engineering ultrafast photophysics in 2D perovskites and guiding the design of advanced optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Optical-Field Driven Charge-Transfer Modulations near Composite Nanostructures

Optical activation of material properties illustrates the potentials held by tuning light-matter interactions with impacts ranging from basic science to technological applications. Here, we demonstrate for the first time that composite nanostructures providing nonlocal environments can be engineered to optically trigger photoinduced charge transfer dynamic (CTD) modulations in the solid state. The nanostructures herein explored lead to unprecedented out-of-phase behaviour between charge separation and recombination dynamics, along with linear CTD variations with the optical-field amplitude. Using transient absorption spectroscopy, up to 270 % increase in charge separation rate is obtained in organic semiconductor thin films. We provide evidences that composite nanostructures allow for surface photovoltages to be created, which kinetics vary with the composite architecture and last beyond optical pulse temporal characteristics. Furthermore, by generalizing Marcus theory framework, we explain why CTD modulations can only be unveiled when optic field effects are enhanced by nonlocal image dipole interactions. Demonstrating that composite nanostructures can be designed to use optical fields as CTD remote actuators opens the path for their use in practical and original applications ranging from photochemistry to optoelectronics.

cond-mat.mtrl-sci↗

arXiv2020

Annihilation mechanism of excitons in a MoS2 monolayer through direct Förster-type energy transfer and multistep diffusion

Atomically thin MoS2 layer is a direct bandgap semiconductor exhibiting strong electron-hole interaction due to the extreme quantum confinement and reduced screening of Coulomb interactions, which results in the formation of stable excitons at room temperature. Therefore, various excitonic properties of MoS2 monolayer are extremely important in determining the strength of light matter interactions including their radiative recombination lifetime and optoelectronic response. In this paper, we report a comprehensive study of the underlying annihilation mechanism of various types of exciton in MoS2 monolayer using the transient absorption spectroscopy. We rigorously demonstrate that the Förster-type resonance energy transfer is the main annihilation mechanism of A- and B-excitons while multistep diffusion process is responsible for C-exciton annihilation, which is supported by critical scientific evidence.

cond-mat.mes-hall↗

arXiv2019

Controlling exciton dynamics in two-dimensional MoS2 on hyperbolic metamaterial-based nanophotonic platform

The discovery of two-dimensional transition metal dichalcogenides (2D TMDs) has promised next-generation photonics and optoelectronics applications, particularly in the realm of nanophotonics. Arguably, the most crucial fundamental processes in these applications are the exciton migration and charge transfer in 2D TMDs. However, exciton dynamics in 2D TMDs have never been studied on a nanophotonic platform and more importantly, the control of exciton dynamics by means of nanophotonic structures has yet to be explored. Here, for the first time, we demonstrate the control of exciton dynamics in MoS2 monolayers by introducing a hyperbolic metamaterial (HMM) substrate. We reveal the migration mechanisms of various excitons in MoS2 monolayers. Furthermore, we demonstrate the Förster radius of the A-excitons can be increased by introducing HMMs through the nonlocal effects of HMMs due to the Purcell effect. On the other hand, the diffusion coefficient is unchanged for the C-excitons on HMMs. This study provides a revolutionary step forward in enabling 2D TMD nanophotonics hybrid devices.

cond-mat.mtrl-sci↗

arXiv2018

Hyperbolic Dispersion Dominant Regime Identified through Spontaneous Emission Variations near Metamaterial Interfaces

Surface plasmon polariton, hyberbolic dispersion of energy and momentum, and emission interference provide opportunities to control photoluminescence properties. However, the interplays between these regimes need to be understood to take advantage of them in optoelectronic applications. Here, we investigate broadband variations induced by hyperbolic metamaterial (HMM) multilayer nanostructures on the spontaneous emission of selected organic chromophores. Experimental and calculated spontaneous emission lifetimes are shown to vary non-monotonously near HMM interfaces. With the SPP and interference dominant regimes. With the HMM number of pairs used as the analysis parameter, the lifetime is shown to be independent of the number of pairs in the surface plasmon polaritons, and emission interference dominant regimes, while it decreases in the Hyperbolic Dispersion dominant regime. We also show that the spontaneous emission lifetime is similarly affected by transverse positive and transverse negative HMMs. This work has broad implications on the rational design of functional photonic surfaces to control the luminescence of semiconductor chromophores.

cond-mat.mtrl-sci↗