Sejeong Kim

Circular dichroism (CD) sensing plays a pivotal role in probing molecular chirality in biomedical sciences. However, engineering superchiral electromagnetic fields that can reliably amplify the faint signatures of chiral analytes remains profoundly challenging. Central to this difficulty is the need to balance two competing demands: maximizing the enhancement of chiral fields while maintaining a sufficient interaction volume for effective molecular interrogation. Here, we introduce a figure of merit (FOM) that captures the enhancement and spatial coverage of the superchiral fields. We examine the effects of helix-geometry evolution on the FOM. The optimized triple-strand helix markedly enhanced the analyte CD signal, yielding a FOM of 2.43 × 10¹⁰ nm³, which surpassed prior configurations by over an order of magnitude. Our findings provide a systematic design framework for 3D chiral structures for assessing their chiroptical sensing performance, particularly in scenarios involving clusters of randomly oriented small molecules or a large chiral molecule.

Chiral nanophotonic structures have attracted significant attention due to their wide-ranging applications in chiral sensing, on-chip optical devices, and optical communication. Although most chiral structures to date have been generated using conventional design approaches—guided by human intuition and followed by parameter sweeps—these methods are inherently limited in efficiently exploring complex, high-dimensional design spaces. Recently, inverse design methods have emerged to overcome traditional limitations by systematically and efficiently navigating vast parameter spaces to discover non-intuitive chiral geometries with enhanced performance. In this work, we present a range of chiral nanophotonic structures along with their corresponding design approaches, as well as performance metrics. This perspective focuses on inverse design methodologies, such as metaheuristic optimization, topology optimization, and data-driven approaches, tailored to the needs of enhanced chiroptical response. We also introduce conventional design strategies and compare them with optimization- and learning-based design pipelines. Particular emphasis is placed on the formulation of objective functions for circular dichroism and helical dichroism sensing, structural symmetry breaking, and computational constraints in structure design. We further highlight representative high-performance chiral metasurface and metamaterial designs developed using these strategies and conclude by outlining open challenges and future research directions.

An optical vortex beam has attracted significant attention across diverse applications, including optical manipulation, phase-contrast microscopy, optical communication, and quantum photonics. To utilize vortex generators for integrated photonics, researchers have developed ultra-compact vortex generators using fork gratings, metasurfaces, and integrated microcombs. However, those devices depend on costly, time-consuming nanofabrication and are constrained by the low signal-to-noise ratio due to the fabrication error. As an alternative maneuver, spin-orbit coupling has emerged as a method to obtain the vortex beam by converting spin angular momentum (SAM) without nanostructures. Here, we demonstrate the creation of an optical vortex beam using van der Waals (vdW) materials. The significantly high birefringence of vdW materials allows the generation of optical vortex beams, even with materials of sub-wavelength thickness. In this work, we utilize an 8 µm-thick hexagonal boron nitride (hBN) crystal for the creation of optical vortices carrying topological charges of ±2. We also present the generation of an optical vortex beam in a 320 nm-thick MoS₂ crystal with a conversion efficiency of 0.09. This study paves the way for fabrication-less and ultra-compact optical vortex generators, which can be applied for integrated photonics and large-scale vortex generator arrays.

Deep learning has been a critical part of designing inverse design methods that are computationally efficient and accurate. An example of this is the design of photonic metasurfaces by using their photoluminescent spectrum as the input data to predict their topology. One fundamental challenge of these systems is their ability to represent nonlinear relationships between sets of data that have different dimensionalities. Existing design methods often implement a conditional generative adversarial network in order to solve this problem, but in many cases, the solution is unable to generate structures that provide multiple peaks when validated. It is demonstrated that in response to the target spectrum, the bidirectional adversarial autoencoder is able to generate structures that provide multiple peaks on several occasions. As a result, the proposed model represents an important advance toward the generation of nonlinear photonic metasurfaces that can be used in advanced metasurface design.

Investigating chiral light-matter interactions is essential for advancing applications in sensing, imaging, and pharmaceutical development.

Monolayer transition metal dichalcogenides are promising materials that not only are atomically thin but also have direct bandgaps, making them highly regarded in optics and optoelectronics. However, their photoluminescence exhibits almost random polarization at room temperature. The emission is also omnidirectional and weak due to the low quantum yield. These limitations hamper the development of practical optoelectronic devices and solid-state single-photon sources for quantum technologies. Here, we demonstrate the spatial control of photoluminescence polarization by coupling monolayer tungsten disulfide with photonic bands having bound states in the continuum. We design a dielectric photonic crystal slab with bound states in the continuum that spectrally overlap with the excitonic resonance of the monolayer of tungsten disulfide. Integration with the photonic crystal slab modulates the directionality and improves the intensity of photoluminescence through extraction and excitation enhancement. Our results will enable the development of compact on-chip optoelectronic and quantum photonic devices based on two-dimensional materials.

Chiral plasmonic nanomaterials have been widely utilized to study light‐matter interactions due to its capability to amplify chiroptic signals. Conventionally, chiro‐optic experiments have demonstrated interactions between circularly polarized light and materials. However, employing light with chiral phase, i.e., optical vortex, can generate a strong chiral response and holds the potential to unveil extensive material information owing to the infinite topological numbers. In this work, an array of 3D chiral nanoparticles is employed to demonstrate large helical dichroism (HD). Chiral gold nanoparticle arrays are illuminated by vortex beams of opposite helicity, which revealed the high HD value of 0.93. The chiral interaction is theoretically investigated, and enantioselective interaction can be explained by multipole analysis. It is determined that the strong HD is attributed to the interaction of higher‐order multipole moments such as electric quadrupole and magnetic quadrupole moments. This study provides deeper insight into understanding of the interaction between optical vortex and chiral plasmonic nanostructures and paves the way for next‐generation chiroptical applications ranging from ultrasensitive chiral spectroscopy to chiral quantum optics.

Van der Waals (vdW) layered materials have been shown to have excellent optoelectronic properties relevant to photovoltaics. Despite their promise, the demonstrated efficiencies of vdW material solar cells remain low and are seldom supported by statistics or spectral quantum efficiency analysis. In this study, we utilize a p-type WSe₂ absorber, forming a solar cell with a transparent front InO_x electron contact, and a rear Pd reflector/hole contact. We fabricate multiple devices providing statistics for 10 devices with an average 1 sun conversion efficiency above 5%, among which a champion efficiency of 6.37% is achieved. This is the highest AM 1.5G 1 sun efficiency reported for a vdW material solar cell, with a current density supported by external quantum efficiency analysis. This cell is also shown to have near unity quantum efficiency around λ = 600 nm. This work provides support to vdW materials being considered as serious candidates for future thin-film solar cells.

An optical vortex beam has attracted significant attention across diverse applications, including optical manipulation, phase-contrast microscopy, optical communication, and quantum photonics. To utilize vortex generators for integrated photonics, researchers have developed ultra-compact vortex generators using fork gratings, metasurfaces, and integrated microcombs. However, those devices depend on costly, time-consuming nanofabrication and are constrained by the low signal-to-noise ratio due to the fabrication error. As an alternative maneuver, spin-orbit coupling has emerged as a method to obtain the vortex beam by converting spin angular momentum (SAM) without nanostructures. Here, we demonstrate the creation of an optical vortex beam using van der Waals (vdW) materials. The significantly high birefringence of vdW materials allows generations of optical vortex beams with high efficiency in a sub-wavelength thickness. In this work, we utilize an 8-um-thick hexagonal boron nitride (hBN) crystal for the creation of optical vortices carrying topological charges of +2 and -2. We also present the generation of an optical vortex beam in a 320-nm-thick MoS2 crystal with a conversion efficiency of 0.09. This study paves the way for fabrication-less and ultra-compact optical vortex generators, which can be applied for integrated photonics and large-scale vortex generator arrays.