arXiv subjects

Hong-Gyu Park

Publications and source records attributed to Hong-Gyu Park.

12 recordsLinked to original sources

arXiv2026

Magneto-Optical Detection of Anisotropic Spin Currents in Altermagnetic RuO2

Altermagnets are a recently identified class of collinear antiferromagnets that host large spin-split electronic bands, offering a promising platform for efficient spin-current generation. Among proposed candidates, the metallic oxide RuO2 is predicted to exhibit strong altermagnetic spin splitting; however, whether it sustains robust magnetic order beyond the ultrathin thickness limit remains unresolved. Here, we employ optical probes to investigate charge-to-spin conversion in a 12-nm-thick (101)-oriented RuO2 film grown on sapphire. Polarization-resolved second-harmonic generation reveals nonlinear optical responses consistent with the surface symmetry and Néel order of RuO2. Under an applied current, both second-harmonic generation and polar magneto-optical Kerr effect measurements detect a pronounced, directionally anisotropic spin polarization, exhibiting enhanced signals for current along [010] and strongly suppressed responses for current along [-101], in agreement with the symmetry of the altermagnetic spin-splitter effect. Non-magnetic or Rashba-type mechanisms cannot explain this symmetry-selective response. Scanning transmission electron microscopy further reveals that substantial strain persists even in relatively thick films, providing a possible explanation for the observed behavior. Therefore, these results establish RuO2 as an efficient spin source and demonstrate the potential of altermagnets for field-free spintronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Lattice-mismatch Moire laser with strong flatband coupling

Inter-cell and/or interlayer coupling in Moire superlattices can generate flatbands and collective eigenmodes that enable emergent physical phenomena, motivating extensive exploration of Moire-inspired photonic devices. However, the experimental validation of robust inter-cell interactions in Moire photonic structures and the modulation of flatbands for specific photonic applications remain challenging. Here, we propose a lattice-mismatch Moire cavity and demonstrate nanolasers enabled by strong flatband coupling. In contrast to a twist-angle Moire cavity, a lattice-mismatch Moire cavity provides a stable flatband frequency and a substantial enhancement in Q factor compared to an isolated single-cell cavity, as the unit-cell size decreases. The photonic band-structure measurement of the small-unit-cell Moire cavity by photoluminescence reveals pronounced flatbands. Cell-resolved spectroscopy further confirms the presence of flatbands by identifying resonant peaks that consistently emerge across unit cells in a Moire cavity with a lattice mismatch of 102 nm, but not in a larger-unit-cell Moire cavity with a mismatch of 60 nm. Furthermore, mode selection is achieved by reducing the center-hole size, thus isolating the hexapole mode from the degenerate dipole modes while maintaining strong inter-cell coupling. Consequently, we demonstrate a low-threshold hexapole flatband laser in a single mode. Therefore, the systematic modification of the relative lattice parameters of the two constituent lattices offers a promising strategy for developing Moire nanolasers and flatband nanophotonic devices.

physics.optics↗

arXiv2025

Nonlinear chiral response from linearly achiral membrane metasurfaces

Chiral photonics aims to control and engineer light handedness for many applications in optical communications, biological and chemical sensing, and quantum technologies. While traditional approaches focus on engineering strong linear chiroptical response, nonlinear chiral phenomena remain largely unexplored. Here, we demonstrate experimentally a pronounced nonlinear chiral response in free-standing silicon membrane metasurfaces that are effectively achiral in the linear regime. By employing patterned membranes with both $C_4$-symmetry and intentionally broken in-plane symmetry, we reveal that strong nonlinear circular dichroism can be observed in third-harmonic generation. An unperturbed metasurface exhibits strong cross-polarized third-harmonic signal with nonlinear circular dichroism of the value $-0.83$, whereas in-plane symmetry breaking enables a co-polarized channel, and it reverses the sign of nonlinear circular dichroism that may be as large as the value $0.41$. Our findings suggest a novel approach for engineering nonlinear chiral responses in metasurfaces, complementing traditional approaches and paving the way towards advanced chiral metadevices.

physics.optics↗

arXiv2023

High harmonic generation from a subwavelength dielectric resonator

Higher-order optical harmonics entered the realm of nanostructured solids being observed recently in optical gratings and metasurfaces with a subwavelength thickness. Structuring materials at the subwavelength scale allows for resonant enhancing of the efficiency of nonlinear processes and reducing the size of high-harmonic sources. Here we report the observation of up to a seventh harmonic generated from a single subwavelength resonator made of AlGaAs material. This process is enabled by careful engineering of the resonator geometry for supporting optical modes associated with a quasi-bound state in the continuum in the mid-infrared spectral range at around λ=3.7 μm pump wavelength. The resonator volume measures ~ 0.1 λ^3. The resonant modes are excited with an azimuthally polarized tightly focused beam. We evaluate the contributions of perturbative and non-perturbative nonlinearities to the harmonic generation process. Our work proves the possibility to miniaturize solid-state sources of high harmonics to the subwavelength volumes.

physics.optics↗

arXiv2023

Enhanced five-photon photoluminescence in subwavelength AlGaAs resonators

Multiphoton processes of absorption photoluminescence have enabled a wide range of applications including three-dimensional microfabrication, data storage, and biological imaging. While the applications of two-photon and three-photon absorption and luminescence have matured considerably, higher-order photoluminescence processes remain more challenging to study due to their lower efficiency, particularly in subwavelength systems. Here we report the observation of it five-photon luminescence from a single subwavelength nanoantenna at room temperature enabled by the Mie resonances. We excite an AlGaAs resonator at around 3.6 um and observe photoluminescence at around 740 nm. We show that the interplay of the Mie multipolar modes at the subwavelength scale can enhance the efficiency of the five-photon luminescence by at least four orders of magnitude, being limited only by sensitivity of our detector. Our work paves the way towards applications of higher-order multiphoton processes at the subwavelength scales enabled by the physics of Mie resonances.

physics.optics↗

arXiv2021

Electrically driven strain-induced deterministic single-photon emitters in a van der Waals heterostructure

Quantum confinement in atomically-thin TMDCs enables the realization of deterministic single-photon emitters. The position and polarization control of single photons have been achieved via local strain engineering using nanostructures. However, most existing TMDC-based emitters are operated by optical pumping, while the emission sites in electrically pumped emitters are uncontrolled. Here, we demonstrate electrically driven single-photon emitters located at the positions where strains are induced by atomic-force-microscope indentation on a van der Waals heterostructure consisting of graphene, hexagonal-boron nitride, and tungsten diselenide. The optical, electrical, and mechanical properties induced by the local strain gradient were systematically analyzed. In particular, single-photon emission was observed at the indentation sites at 4 K. The emission exhibits photon anti-bunching behavior with a g(2)(0) value of ~0.3, intensity saturation and a linearly cross-polarized doublet. This robust spatial control of electrically driven single-photon emitters will pave the way for the practical implementation of integrated quantum light sources.

cond-mat.mes-hall↗

arXiv2021

Nonlinear circular dichroism in Mie-resonant nanoparticle dimers

We study nonlinear response of a dimer composed of two identical Mie-resonant dielectric nanoparticles illuminated normally by a circularly polarized light. We develop a general theory describing hybridization of multipolar modes of the coupled nanoparticles, and reveal nonvanishing nonlinear circular dichroism (CD) in the second-harmonic generation (SHG) signal enhanced by the multipolar resonances in the dimer provided its axis is oriented under an angle to the crystalline lattice of the dielectric material. We present experimental results for this SHG-CD effect obtained for the AlGaAs dimers placed on an engineered substrate which confirm the basic prediction of our general multipolar hybridization theory.

physics.optics↗

arXiv2020

Lasing from multipole topological corner states

Topological photonics provides a fundamental framework for robust manipulation of light, including directional transport and localization with built-in immunity to disorder. Combined with an optical gain, active topological cavities hold special promise for a design of light-emitting devices. Most studies to date have focused on lasing at topological edges of finite systems or domain walls. Recently discovered higher-order topological phases enable strong high-quality confinement of light at the corners. Here we demonstrate lasing action of corner states in a nanophotonic topological cavity. We identify four multipole corner modes with distinct emission profiles via hyperspectral imaging and discern signatures of non-Hermitian radiative coupling of leaky topological states. In addition, depending on the pump position in a large-size cavity, we selectively generate lasing from either edge or corner states within the topological bandgap. Our findings introduce pathways to engineer collective resonances and tailor generation of light in active topological circuits.

physics.optics↗

arXiv2019

Individual nanoantennas empowered by bound states in the continuum for nonlinear photonics

Bound states in the continuum (BICs) represent localized modes with energies embedded in the continuous spectrum of radiating waves. BICs were discovered initially as a mathematical curiosity in quantum mechanics, and more recently were employed in photonics. Pure mathematical bound states have infinitely-large quality factors (Q factors) and zero resonant linewidth. In optics, BICs are physically limited by a finite size, material absorption, structural disorder, and surface scattering, and they manifest themselves as the resonant states with large Q factors, also known as supercavity modes or quasi-BICs. Optical BIC resonances have been demonstrated only in extended 2D and 1D systems and have been employed for distinct applications including lasing and sensing. Optical quasi-BIC modes in individual nanoresonators have been discovered recently but they were never observed in experiment. Here, we demonstrate experimentally an isolated subwavelength nanoresonator hosting a quasi-BIC resonance. We fabricate the resonator from AlGaAs material on an engineered substrate, and couple to the quasi-BIC mode using structured light. We employ the resonator as a nonlinear nanoantenna and demonstrate record-high efficiency of second-harmonic generation. Our study brings a novel platform to resonant subwavelength photonics.

physics.optics↗

arXiv2018

Enhanced second-harmonic generation with structured light in AlGaAs nanoparticles governed by magnetic response

We employ structured light for the second-harmonic generation from subwavelength AlGaAs nanoparticles that support both electric and magnetic multipolar Mie resonances. The vectorial structure of the pump beam allows addressing selectively Mie-resonant modes and control the strength of the generated nonlinear fields. We observe experimentally the enhancement of the second-harmonic generation for the azimuthally polarized vector beams near magnetic dipole resonance, and match our observations with the numerical decomposion of the Mie-type multipoles for the fundamental and generated second-harmonic fields

physics.optics↗

arXiv2017

Focusing of light energy inside a scattering medium by controlling the time-gated multiple light scattering

The efficient delivery of light energy is a prerequisite for non-invasive imaging and stimulating of target objects embedded deep within a scattering medium. However, injected waves experience random diffusion by multiple light scattering, and only a small fraction reaches the target object. Here we present a method to counteract wave diffusion and to focus multiplescattered waves to the deeply embedded target. To realize this, we experimentally inject light to the reflection eigenchannels of a specific flight time where most of the multiple-scattered waves have interacted with the target object and maximize the intensity of the returning multiple-scattered waves at the selected time. For targets that are too deep to be visible by optical imaging, we demonstrated a more than 10-fold enhancement in light energy delivery in comparison with ordinary wave diffusion cases. This work will lay a foundation for enhancing the working depth of imaging, sensing, and light stimulation.

physics.optics↗

arXiv2017

Assembly of 3D plasmonic metamolecules by 2D AFM nanomanipulation of highly uniform and smooth gold nanospheres

Atomic force microscopy (AFM) nanomanipulation has been viewed as a deterministic method for the assembly of plasmonic metamolecules because it enables unprecedented engineering of clusters with exquisite control over particle number and geometry. Nevertheless, the dimensionality of plasmonic metamolecules via AFM nanomanipulation is limited to 2D, so as to restrict the design space of available artificial electromagnetisms. Here, we show that 2D nanomanipulation of the AFM tip can be used to assemble 3D plasmonic metamolecules in a versatile and deterministic way by dribbling highly spherical and smooth gold nanospheres (NSs) on a nanohole template rather than on a flat surface. Various 3D plasmonic clusters with controlled symmetry were successfully assembled with nanometer precision, the relevant 3D plasmonic modes (i.e., artificial magnetism and magnetic-based Fano resonance) were fully rationalized by both numerical calculation and dark-field spectroscopy. This templating strategy for advancing AFM nanomanipulation can be generalized to exploit the fundamental understanding of various electromagnetic 3D couplings and can serve as the basis for the design of metamolecules, metafluids, and metamaterials.

physics.optics↗