Hyunyong Choi

van der Waals (vdW) heterostructures provide a powerful method to control the alignment of energy bands of atomically thin 2D materials. Under light illumination, the optical responses are dominated by Coulomb-bound electron-hole quasiparticles, for example, excitons, trions, and biexcitons, whose contributions accordingly depend on the types of heterostructures. For type-II heterostructures, it has been well established that light excitation results in electrons and holes that are separated in

Abstract Light–matter interactions illuminate the nature of solids and provide a second look on associated carrier dynamics. In particular, graphene and 3D topological insulators (3D TI) with broadband electromagnetic excitation have revealed to host exotic dynamic interactions. Much of Dirac‐point physics arises from discrete lattice symmetries and nontrivial Z 2 classification of Bloch states. In this review, ongoing spectroscopic works on graphene and 3D TI are presented, where special attent

The nature of spin transport in the bulk and side surface of three-dimensional topological insulator thin film geometry is a relatively unexplored subject, compared to the extensively studied top and bottom surface states. Here we employ time- and space-resolved helicity-dependent photocurrent measurements to investigate the effect of optically excited bulk carriers on the spin-polarized topological side surface conduction. Time-resolved femtosecond double-pulse excitation reveals that the spin

The fields of layered material research, such as transition-metal dichalcogenides (TMDs), have demonstrated that the optical, electrical and mechanical properties strongly depend on the layer number N. Thus, efficient and accurate determination of N is the most crucial step before the associated device fabrication. An existing experimental technique using an optical microscope is the most widely used one to identify N. However, a critical drawback of this approach is that it relies on extensive

Abstract Photoresponse on the silicon nanowire (SiNW) and organic semiconductor interfaces embedding an insulating barrier is understood by a photogating effect associated with charge separations. Still elusive one is when the thickness of organic semiconductor is decreased down to a few molecular layers, where the photoresponse can be strongly altered by the spatial confinement of photoinduced carriers. In this work, the photoresponse modulation of SiNW field‐effect transistors coated with an u

Twisted van der Waals heterostructures have led to emerging layer-dependent correlated physics in moiré potentials. While optoelectronic controls over interlayer electronic coupling have been reported, the concomitant interlayer vibration has not yet been controlled. Here, we report experimental evidence of ultrafast optical control over the amplitude and oscillation period of interlayer breathing phonons in WSe₂/WS₂ heterobilayers. Femtosecond optical excitation above the

We demonstrate infrared photodetectors based on graphene-Bi2Se3 heterostructures with high responsivity (≥1.9 A/W) at room temperature. Strong photogating effect across the tunneling barrier and built-in potential enables the internal quantum efficiency larger than 100 %.

We present an active buffer control based TCP agent scheme that can overcome TCP performance degradation caused by handovers in broadband wireless networks. The agent performs an active buffer control and adjusts the sending rate of the TCP server by forcing three duplicate acknowledgements while it splits an end-to-end TCP connection into two TCP connections. So, high utilization is achievable even with frequent handovers. We also propose an active buffer control algorithm deployed in the agent

We present ultrafast terahertz dynamics in topological insulator (Bi1-xInx)2Se3. We find that photogenerated electrons suppress the increase of scattering at high temperature. The surface-bulk interaction strongly depends on the dynamic condition of topological phase transition.

We report the first anisotropic two excitons dynamics in bulk ReS2. Both exciton states show anisotropic dynamics upon carrier injection with a stronger response in higher exciton state (X2) than the lower state (X1).

We present a relay path determination scheme that maximizes the network lifetime and guarantees the end-to-end reliability in the wireless sensor networks with mobile sink. The simple flooding algorithm may render an easy means for the determination but the resulting frequent collisions and profuse relay hops disable determining energy-efficient and reliable paths. To resolve this problem, we present a transmission backoff (TB)- based broadcasting algorithm that uses a transmission deferring app

We report the broadband optical conductivity and ultrafast carrier dynamics of few-layer graphene. Equilibrium spectra exhibit significant THz and near-IR absorption, consistent with a model of intra- and interband transitions in a dense Dirac electron plasma. Ultrafast THz transmission changes after photoexcitation are dominated by excess holes, with a 1.2-ps exponential decay that reflects minority-carrier recombination.

We demonstrate a new optoelectronic platform using WSe2-Bi2Se3 heterostructures to generate and detect the valley-coupled spin-polarized photocurrents at room temperature. The light polarization and the external electric field can manipulate the magnitude of the current.

Terahertz (THz) spectroscopy can characterize the collective oscillations of free particles in two-dimensional (2D) materials. The resonant response appearing in the transmitted THz spectra is relevant to the 2D plasmon mode. Here, we investigate the spectral extinction of the THz wave transmitted through the graphene-integrated Bi₂Se₃ microstructure, where the bias voltage applied to the gate electrode controls the device sheet conductance. Comparing the spectral response

We report a deterministic creation of color centers in diamond by employing single-shot laser writing. After thermal annealing treatment, we have confirmed that the optical emission and spin coherence consist with the conventional single NV.