arXiv subjects

Jeong Woo Han

Publications and source records attributed to Jeong Woo Han.

8 recordsLinked to original sources

arXiv2026

Terahertz two-dimensional coherent spectroscopy: unlocking coherent quantum intraband nonlinear dynamics

Quantum materials host a rich spectrum of low-energy collective excitations- phonons, magnons, plasmons, polaritons, and Higgs modes- that fundamentally govern their emergent properties, but remain inaccessible to conventional linear THz spectroscopy when nonlinear pathways overlap or non-perturbative dynamics emerge. Terahertz two-dimensional coherent spectroscopy (THz-2DCS), realized through phase-stable high-field THz pulse sequences, directly resolves this challenge by delivering phase-resolved multidimensional spectra that uniquely disentangle coherent intraband nonlinear quantum dynamics from incoherent backgrounds. Here, we establish a unified theoretical framework by using density-matrix formalism combined with the optical Bloch equations, from which nonlinear pathways are systematically derived and visualized on the Bloch sphere. This framework provides insight into low-energy nonlinear dynamics exhibiting homogeneous or inhomogeneous broadening depending on the scattering mechanism, enabling the interpretation of nonlinear signals in both the perturbative regime, described by a susceptibility expansion, and the non-perturbative regime. Building on this foundation, we comprehensively survey THz-2DCS investigations of nonlinear plasmon, phonon, magnon, and polariton. Finally, we outline future opportunities, including integration with optical-pump THz-probe platforms for nonequilibrium quantum control.

physics.optics↗

arXiv2026

Generalizing Thermal Transport in High-Contrast Metamaterials through Interfacial Fresnel Reflection

The rapid growth of generative AI has intensified the need for efficient heat dissipation in large-scale data centers. To control heat flow, thermal metamaterials with layered structures have been widely used, which impart the anisotropic properties of thermal conductivities. However, the conventional effective medium approximation (EMA) often fails to provide accurate predictions in systems with a high thermal conductivity contrast between adjacent layers embedded in a background medium. Here, we generalize the EMA by introducing two corrective coefficients that extend its validity to regimes where the conventional EMA was previously inapplicable, i.e., high-contrast thermal metamaterials with the background medium. Notably, one of these coefficients that we proposed has the same mathematical form as the Fresnel reflection coefficient in optics. This allows us to interpret the "reflection-like" behavior of heat flow as it penetrates adjacent layers with high thermal contrast. Our findings suggest that heat diffusion, traditionally viewed as a purely dissipative process, can be understood intuitively through the framework of ray optics.

physics.optics↗

arXiv2025

THz carrier dynamics in $SrTiO_{3}/LaTiO_{3}$ interface two-dimensional electron gases

A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like $SrTiO_{3}$ (STO) and $LaTiO_{3}$ (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect transistors or detectors. Of particular interest is the integration of these oxide systems in silicon technology. To this end we study the carrier dynamics in a STO/LTO/STO heterostructure epitaxially grown on Si(001) both experimentally and theoretically. Linear THz spectroscopy was performed to analyze the temperature dependent charge carrier density and mobility, which was found to be in the range of $10^{12}$ $cm^2$ and 1000 $cm^2V^{-1}s^{-1}$, respectively. Pump-probe measurements revealed a very minor optical nonlinearity caused by hot carriers with a relaxation time of several 10 ps, even at low temperature. Density functional theory calculations with a Hubbard U term on ultrathin STO-capped LTO films on STO(001) show an effective mass of 0.64-0.68 $m_{e}$.

cond-mat.mtrl-sci↗

arXiv2025

Extreme Terahertz Nonlinearity of AlGaN/GaN-based Grating-Gate Plasmonic Crystals

We present a novel approach to enhance THz nonlinearity by the resonant excitation of two-dimensional plasmons in grating-gate plasmonic crystals. Using a high-electric-field THz pump-THz probe technique, we investigate the nonlinear interaction of spectrally narrow THz pulses with plasmon oscillations in a two-dimensional electron gas on an AlGaN/GaN interface integrated with metallic grating. Nonlinear effects are observed as ultrafast, pump-induced changes in THz transmission, with relative transparency strongly dependent on plasmonic mode excitation and saturating at pump fluences of about 200 nJ cm-2. The maximal relative transparency, reaching 45 % at 350 nJ cm -2, occurs under resonant excitation of a localized plasmon mode at the strong electrostatic modulation of 2DEG concentration. Transient dynamics reveal ultrafast relaxation times of 15-20 ps, while the effects can be observed at elevated temperatures of up to 150 K. A nonlinear model of plasmonic crystal, based on finite-difference time-domain electrodynamic simulations coupled with viscous hydrodynamic electron transport model, elucidates key nonlinear mechanisms, including near-field effects under metallic gratings, electron heating, plasmon resonance broadening, and redshift. These results demonstrate that even conventional semiconductors such as AlGaN/GaN can achieve nonlinear THz responses comparable to or exceeding those of graphene, showing strong potential for ultrafast THz modulation and nonlinear photonics applications.

physics.optics↗

arXiv2024

Influence on extracted complex refractive index from phase inaccuracy of reflection-type THz-time-domain spectroscopy

Reflection-type terahertz (THz) time-domain spectroscopy (THz-TDS) enables the measurement of optical properties of opaque samples in the THz frequency range, e.g., carrier density and mobility. In this study, we examine the influence of phase inaccuracy on the extracted complex refractive index from reflection-type THz-TDS. Phase inaccuracy often arises from the placement mismatch between the perfect reflector, serving as the reference, and the target samples in reflection-type THz-TDS. By considering two representative systems, where free and bound carriers dominate optical properties, and introducing arbitrarily shifted placement mismatch, we confirm that significant errors in the extracted complex refractive index occur when the mismatch position exceeds 3 μm.

physics.optics↗

arXiv2024

Accuracy of extracted optical conductivity by Kramer-Kronig analysis from reflectivity spectrum

Kramers-Kronig (KK) analysis has been widely used to extract the optical conductivity spectrum from a broad range of reflectance spectrum obtained from far-infrared to ultraviolet frequency ranges. In this study, we present how measurement uncertainty in the reflectivity spectrum affects the extracted optical conductivity spectrum obtained through KK analysis. We consider realistic uncertainties that can easily occur in reflectance measurement environments: (1) a rigid shift of the absolute reflectance in the whole measurement frequency window, and (2) a linear decrement of reflectance with increasing frequency. Our investigation reveals that the reliability of the extracted optical conductivity spectrum, especially in the lower-frequency range, should be carefully addressed, particularly when the reflectance is above approximately 95 %.

physics.optics↗

arXiv2023

Atomic scale understanding of initial Cu-Ni oxidation from machine-learning accelerated first-principles simulations and in situ TEM experiments

The development of accurate methods for determining how alloy surfaces spontaneously restructure under reactive and corrosive environments is a key, long-standing, grand challenge in materials science. Current oxidation models, such as Cabrera-Mott, are based on macroscopic empirical knowledge that lacks fundamental insight at the atomic level. Using machine learning-accelerated density functional theory with in situ environmental transmission electron microscopy (ETEM), we examine the interplay between surface reconstructions and preferential segregation tendencies of CuNi(100) surfaces under oxidation conditions. Our modeling approach based on molecular dynamics and grand canonical Monte Carlo simulations shows that oxygen-induced Ni segregation in CuNi alloy favors Cu(100)-O c(2x2) reconstruction and destabilizes the Cu(100)-O missing row reconstruction. The underpinnings of these stabilization tendencies are rationalized based on the similar atomic coordination and bond lengths in NiO rock salt and Cu(100)-O c(2x2) structures. In situ ETEM experiments show Ni segregation followed by NiO nucleation and growth in regions without MRR, with secondary nucleation and growth of Cu2O in MRR regions. This further corroborates the simulated surface oxidation and segregation modelling outcomes. Our findings are general and are expected to extend to other alloy systems.

cond-mat.mtrl-sci↗

arXiv2023

Strong transient magnetic fields induced by THz-driven plasmons in graphene disks

Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong (~1°) ultrafast Faraday rotation (~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm-2.

cond-mat.mes-hall↗