Sunghwan Kim

Optical cavities provide a powerful means to engineer light–matter hybrid states by coupling confined electromagnetic fields with matter excitations. Achieving in situ control of the coupling strength is essential for investigating how such hybridization evolves with the coupling strength. In this work, we use a symmetry-changing structural phase transition in lead halide perovskites to reversibly tune the phonon–photon coupling strength, leveraging the fact that their phonon frequencies and oscillator strengths are dictated by lattice symmetry. Terahertz time-domain spectroscopy of MAPbI3 embedded in nanoslot cavities reveals three polariton branches above the critical temperature Tc ≃ 162.5 K and the emergence of an additional branch below Tc, activated by a new phonon mode in the low-temperature phase. The full dispersion is accurately reproduced using a multimode Hopfield model, confirming that all normalized coupling strengths remain in the ultrastrong coupling regime. These results demonstrate symmetry-controlled tuning of ultrastrong light–matter coupling in optical cavities via temperature.

Plasmonic metasurfaces supporting multiple resonances are highly desirable for enhancing optical fields at distinct wavelengths. Here, we demonstrate doubly stacked nanogap arrays that exhibit dual Fabry–Pérot resonances of the gap plasmons. Numerical simulations reveal that both resonances appear at different wavelengths due to the lateral length difference between the upper and lower nanogaps. Furthermore, we fabricated stacked nanogap arrays by alternating metal-insulator deposition, electron beam (e-beam) lithography, and ion milling. Due to the dual plasmon resonance, the photoluminescence spectra of spin-coated dyes (R6G and IR-820) show distinct modifications near the two reflection dips. These results highlight stacked nanogaps as a promising platform for co-localized multicolor dye excitation and multispectral photoluminescence engineering, with potential applications in multi-wavelength light sources and multicolor displays.

The multimode ultrastrong coupling (USC) regime has emerged as a novel platform for accessing previously inaccessible phenomena in cavity quantum electrodynamics. Of particular interest are cavity‐mediated correlations between local and nonlocal excitations, or equivalently, between modes at zero and finite in‐plane momentum, which offer new opportunities for controlling light–matter interactions across space. However, direct experimental evidence of such interactions has remained elusive. Here, we demonstrate nonlocal multimode coupling in a Landau polariton system, where cavity photons simultaneously interact with the zero‐momentum cyclotron resonance and finite‐momentum magnetoplasmons of GaAs two‐dimensional electron gas. Our slot cavities, with their subwavelength mode volumes, supply in‐plane momentum components that enable the excitation of finite‐momentum matter modes. Terahertz time‐domain magnetospectroscopy measurements reveal a clear splitting of the upper‐polariton branch, arising from hybridization between magnetoplasmon modes and the cavity–cyclotron‐resonance hybrids. Extracted coupling strengths confirm USC of the cyclotron resonance and strong coupling of the magnetoplasmon modes to the cavity field, respectively. The experimental results are well captured by the multimode Hopfield model and finite‐element simulations. These findings establish a pathway for engineering multimode light–matter interactions involving zero‐ and finite‐momentum matter modes in the USC regime.

This article is dedicated to the cherished memory of Prof. Dai‐Sik Kim, a visionary leader and an inspiring mentor who profoundly shaped international as well as domestic landscape of optical science, and addresses topics that were central to his scientific passion in nanophotonics. Prof. Kim’s research spanned surface plasmonics, near‐field optics, and terahertz plasmonics and spectroscopy, each reflecting his enduring commitment to exploring the interaction between light and matter beyond conventional limits.

Phonons play a central role in fundamental solid-state phenomena, including superconductivity, Raman scattering, and symmetry-breaking phases. Harnessing phonons to control these effects and enable quantum technologies is therefore of great interest. However, most existing phonon control strategies rely on external driving fields or anharmonic interactions, limiting their applicability. Here, we realize multimode ultrastrong light–matter coupling and theoretically show the modulation of phonon emission. This regime is realized by coupling two optical phonon modes in lead halide perovskites to a nanoslot array functioning as a single-mode cavity. The small mode volume of the nanoslots enables high coupling strengths in the phonon-polariton system. We show theoretically that the nanoslot resonator mediates an effective interaction between phonon modes, leading to superthermal phonon bunching in thermal equilibrium between distinct modes. Our findings are well described by a multimodal Hopfield model. This work establishes a pathway for engineering phononic properties for light-harvesting and light-emitting technologies.

Sub-10 nm nanostructures with high precision and uniformity are of significant interest due to their unique quantum properties and critical role in next-generation devices. However, current fabrication techniques are often constrained by the slow, small-area processes of electron beam lithography or the high costs of extreme ultraviolet (EUV) lithography, limiting broader accessibility. To address these challenges, we have advanced the atomic layer lithography method into an efficient, scalable approach for fabricating sub-10 nm nanogaps with high uniformity across entire wafer areas. The key strategy of this method is the direct employment of photoresist patterns as both protective and sacrificial layers, streamlining the process. Comprehensive optical and electron microscopy analyses, supported by COMSOL simulations and terahertz transmission measurements, confirm the formation of highly uniform, high-quality nanogaps across the entire wafer. Compared to the original atomic layer lithography, our approach offers enhanced patterning flexibility, including a simplified process, improved compatibility with a wide range of metals, and the ability to perform additional patterning on initial nanogap structures. This scalable technique, compatible with standard lithographic tools, provides a promising pathway for sub-10 nm nanogap fabrication, with strong potential for both academic research and industrial applications.

Gap plasmon (GP) resonance in static surface-enhanced Raman spectroscopy (SERS) structures is generally too narrow and not tunable. Here, we present an adaptive gap-tunable SERS device to selectively enhance and modulate different vibrational modes via active flexible Au nanogaps, with adaptive optical control. The tunability of GP resonance is up to ∼1200 cm^–1 by engineering gap width, facilitated by mechanical bending of a polyethylene terephthalate substrate. We confirm that the tuned GP resonance selectively enhances different Raman spectral regions of the molecules. Additionally, we dynamically control the SERS intensity through the wavefront shaping of excitation beams. Furthermore, we demonstrate simulation results, exhibiting the mechanical and optical properties of a one-dimensional flexible nanogap and their advantage in high-speed biomedical sensing. Our work provides a unique approach for observing and controlling the enhanced chemical responses with dynamic tunability.

Surface-enhanced Raman spectroscopy (SERS) enables sensitive observations of the chemical properties of various materials through highly localized optical field and gap plasmon (GP) resonance attributed to metal nanogap. However, GP resonance excited by static gap-plasmonic structures is too narrow and generally not tunable. Thus, tunable or broad-spectral SERS platforms have been little studied. Here, we present an adaptive gap-tunable SERS device to selectively enhance and modulate different vibrational modes via active flexible Au nanogap, with adaptive optical control. The tunability of GP resonance is ∼1200 cm^-1 by engineering gap width, facilitated by mechanical bending of a polyethylene terephthalate substrate. We confirm that the tuned GP resonance selectively enhances different Raman spectral regions of brilliant cresyl blue molecules. Additionally, we dynamically control the SERS intensity by manipulating the coupling efficiency of excited photons and electrons in the nanogap, enabled by wavefront shaping. We demonstrate simulation results with theoretical models, exhibiting the quantitative mechanical and optical properties of one-dimensional flexible nanogap and their advantage in high-speed biomedical sensing. Our work provides a unique approach for observing and controlling the enhanced chemical responses of various materials with dynamic tunability.

A flexible zerogap metallic structure is periodically formed, healing metal cracks on a flexible substrate. Zerogap is continuously tunable from nearly zero to one hundred nanometers by applying compressive strains on the flexible substrate. However, there have been few studies on how the gap width is related to the strain and periodicity, nor the mechanism of tunability itself. Here, based on atomic force microscopy (AFM) measurements, we found that 200 nm-deep nano-trenches are periodically generated on the polymer substrate below the zerogap owing to the strain singularities extant between the first and the second metallic deposition layers. Terahertz and visible transmission properties are consistent with this picture whereby the outer-bending polyethylene terephthalate (PET) substrate controls the gap size linearly with the inverse of the radius of the curvature.

Cracks are formed along the photolithographically pre‐determined lines with extremely high yield and repeatability, when Cu clusters are introduced between planarized Au thin films sequentially deposited on a PET substrate. These clusters act as nanometer‐sized spacers preventing the formation of contiguous metallic bond between the adjacent Au layers which will render prepatterned‐cracking impossible. While the effective gap width is initially zero in the optical sense from microwaves all the way to the visible, outer‐bending the PET substrate allows the gap width tuning into the 100 nm range, with the stability and controllability in the ranges of 100 s and Angstrom‐scale, respectively. It is anticipated that our wafer‐scale prepatterned crack technology with an unprecedented mixture of macroscopic length and Angstrom‐scale controllability will open‐up many applications in optoelectronics, quantum photonics and photocatalysis.