Hyun-Yong Choi

Abstract The optical Stark effect is a coherent light–matter interaction describing the modification of quantum states by non-resonant light illumination in atoms, solids and nanostructures. Researchers have strived to utilize this effect to control exciton states, aiming to realize ultra-high-speed optical switches and modulators. However, most studies have focused on the optical Stark effect of only the lowest exciton state due to lack of energy selectivity, resulting in low degree-of-freedom

The 1s exciton--the ground state of a bound electron-hole pair--is central to understanding the photoresponse of monolayer transition metal dichalcogenides. Above the 1s exciton, recent visible and near-infrared investigations have revealed that the excited excitons are much richer, exhibiting a series of Rydberg-like states. A natural question is then how the internal excitonic transitions are interrelated on photoexcitation. Accessing these intraexcitonic transitions, however, demands a fundam

Broadband detection of mid-infrared (IR) photons extends to advanced optoelectronic applications such as imaging, sensing, and telecommunications. While graphene offers an attractive platform for broadband visible/IR photodetection, previous efforts to improve its responsivity, for example, by integrating light-absorbing colloids or waveguide or antenna fabrication, were achieved at the cost of reduced photon detection bandwidth. In this work, we demonstrate room-temperature operation of a novel

Modulating light via coherent charge oscillations in solids is the subject of intense research topics in opto-plasmonics. Although a variety of methods are proposed to increase such modulation efficiency, one central challenge is to achieve a high modulation depth (defined by a ratio of extinction with/without light) under small photon-flux injection, which becomes a fundamental trade-off issue both in metals and semiconductors. Here, by fabricating simple micro-ribbon arrays of topological insu

Abstract Quantum beats, periodic oscillations arising from coherent superposition states, have enabled exploration of novel coherent phenomena. Originating from strong Coulomb interactions and reduced dielectric screening, two-dimensional transition metal dichalcogenides exhibit strongly bound excitons either in a single structure or hetero-counterpart; however, quantum coherence between excitons is barely known to date. Here we observe exciton quantum beats in atomically thin ReS 2 and further

Understanding the mutual interaction between electronic excitations and lattice vibrations is key for understanding electronic transport and optoelectronic phenomena. Dynamic manipulation of such interaction is elusive because it requires varying the material composition on the atomic level. In turn, recent studies on topological insulators (TIs) have revealed the coexistence of a strong phonon resonance and topologically protected Dirac plasmon, both in the terahertz (THz) frequency range. Here

Abstract Higher-order topological insulators are recently discovered quantum materials exhibiting distinct topological phases with the generalized bulk-boundary correspondence. T d -WTe 2 is a promising candidate to reveal topological hinge excitation in an atomically thin regime. However, with initial theories and experiments focusing on localized one-dimensional conductance only, no experimental reports exist on how the spin orientations are distributed over the helical hinges—this is critical

Photoexcited carrier relaxation is a recurring topic in understanding the transient conductivity dynamics of graphene-based devices. For atomically thin graphene oxide (GO), a simple free-carrier Drude response is expected to govern the terahertz (THz) conductivity dynamics--same dynamics observed in conventional CVD-grown graphene. However, to date, no experimental testimony has been provided on the origin of photoinduced conductivity increase in GO. Here, using ultrafast THz spectroscopy, we s

Coherent light-matter interaction can transiently modulate the quantum states of matter under nonresonant laser excitation. This phenomenon, called the optical Stark effect, is one of the promising candidates for realizing ultrafast optical switches. However, the ultrafast modulations induced by the coherent light-matter interactions usually involve unwanted incoherent responses, significantly reducing the overall operation speed. Here, by using ultrafast pump-probe spectroscopy, we suppress the

Resolving the complex interplay between surface and bulk response is a long-standing issue in the topological insulators (TIs). Some studies have reported surface-dominated metallic responses, yet others show semiconducting-like bulk photoconductance. Using ultrafast terahertz spectroscopy with the advent of Fermi-level engineered TIs, we discovered that such difference arises from the time-dependent competing process of two parameters, namely, the Dirac-carrier surface scattering rate and the b

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

Abstract The valley Hall effect (VHE) in two-dimensional (2D) van der Waals (vdW) crystals is a promising approach to study the valley pseudospin. Most experiments so far have used bound electron-hole pairs (excitons) through local photoexcitation. However, the valley depolarization of such excitons is fast, so that several challenges remain to be resolved. We address this issue by exploiting a unipolar VHE using a heterobilayer made of monolayer MoS 2 /WTe 2 to exhibit a long valley-polarized l

The first experimental demonstration of nonlinear terahertz difference-frequency generation in a hybrid graphene metadevice is reported. Decades of research have revealed that terahertz-wave generation is impossible in single-layer graphene. This limitation is overcome and nonlinear terahertz generation by ultra-short optical pulse injection is demonstrated. This device is an essential step toward atomically thin, nonlinear terahertz optoelectronic components.