Surface-dominated photocarrier dynamics in non-vdW 2D semiconductor

Surface-dominated photocarrier dynamics of Bi2O2Se nanosheets, an emerging non-van der Waals 2D semiconductor, are investigated. Thickness-dependent ultrafast optical spectroscopy reveals a long bulk lifetime and a low surface recombination velocity (SRV). The exceptionally low SRV highlights Bi2O2Se as a promising candidate for efficient optoelectronic and photonic applications, maintaining low photocarrier loss even at the ultrathin limit.    

"Low Surface Recombination Velocity and Enhanced Photocarrier Dynamics in Bi2O2Se Nanosheets"

TGPark et al., Advanced Optical Materials (2025).

Many-body effects in exciton resonance of highly n-doped regime

Strong many-body interactions of excitons and charge carriers are a hallmark of 2D semiconductors, arising from quantum confinement and reduced screening in these atomically thin materials. In this work, the influence of background doping on exciton many-body interactions is investigated. Transient absorption microscopy reveals a photoinduced exciton resonance redshift, attributed to band-gap renormalization (BGR) at a low background-doping density, transitioning to a blueshift at a high background-doping density due to dominant Pauli blocking (PB) and vertical excitation shifts (VE). A transient, large energy splitting between excitons and trions is also observed at the highly doped regime, explained by exciton-electron interactions. 

"Ultrafast exciton and trion dynamics in highly 𝑛-doped MoS2 monolayers: Many-body effects"

TGPark et al., Physical Review B (2025).

Ultrafast manipulation of topological invariants by light-driven strain

We observed bandgap modulation in the bulk states of a topological insulator driven by out-of-plane interlayer vibrations that alter the van der Waals gap. 

Specifically, the expansion of the van der Waals gap reduces the bulk bandgap in the topological insulator, enabling hybridization of topological surface states by allowing backscattering. 

Using ultrafast NIR and THz spectroscopy, we captured spectroscopic evidence of ultrafast topology switching induced by light-driven interlayer vibrations. 

"Ultrafast acousto-optic modulation at the near-infrared spectral range by interlayer vibrations"

TGPark et al., Nanophotonics (2024).

"Spectroscopic evidence of ultrafast topological phase transition by light-driven strain"

TGPark et al., ACS Nano (2024).

Interlayer vibrations launching by ultrafast laser actions

An ultrafast spectroscopic study revealed the role of optical properties in generating and detecting photoinduced strain waves, whose confinement initiates coherent interlayer vibrations.

The frequency and lifetime of these interlayer vibrations can be quantitatively described using a linear chain model and an acoustic mismatch model. 

This approach enables us to quantitatively assess the elastic coupling between the bottom layer and the substrate, with values ranging from 0 (free-standing) to 2 (tightly bound).

"Coherent control of interlayer vibrations in Bi2Se3 van der Waals thin-films"

TGPark et al., Nanoscale (2021).

"Ultrafast formation of quantized interlayer vibrations in Bi2Se3 by photoinduced strain waves"

TGPark et al., Optics Express (2022).

Interfacial charge transfer between metallic-TMDC and Dirac materials

Transient reflection showed the interlayer electron transfer across the vdW interface. Interlayer coupling can enhance optical properties of Dirac materials in ultrabroadband spectrum within ultrafast timescale. Coherent phonon spectroscopy revealed the mechanical properties (sound velocity, acoustic impedance) as well as elastic coupling with substrate.

"Ultrafast interfacial carrier dynamics and persistent topological surface states of Bi2Se3 in heterojunctions with VSe2" 

TGPark et al., Communications Physics (2022).

"Interlayer coupling and ultrafast hot electron transfer dynamics in metallic VSe2/graphene van der waals heterostructures"

TGPark et al., ACS Nano (2021).

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Characterizing charge transport and ultrafast carrier dynamics in perovskite solar cells.

THz spectroscopy can capture the electrical properties of charge carriers, including carrier density, mobility, and diffusion lengths. Optical pump and THz probe measurement is powerful tool to characterize the photocarrier transport and their ultrafast dynamics. THz spectroscopy revealed that a composition engineering lead to the enhancement of photocarrier transport in active material and high solar-cell efficiency.

"Efficient perovskite solar cells via improved carrier management"

J. J. Yoo et al., Nature (2021). (journal cover)

"Cross-correlation between crystallinity and optoelectronic properties of mixed-perovskite thin films through multiple time-resolved spectroscopy"

J. Park al., Advanced Energy Materials (2024).