arXiv subjects

Munseong Bae

Publications and source records attributed to Munseong Bae.

7 recordsLinked to original sources

arXiv2026

Inverse-Designed Lithium Niobate Wavelength Demultiplexer via Birefringent Effective Index Approximation

Inverse design of thin-film lithium niobate (TFLN) photonic devices is computationally demanding because optical birefringence and fabrication-induced slanted sidewalls generally require three-dimensional electromagnetic models. We introduce a birefringent effective-index (BEI) method to reduce this problem to two dimensions while retaining polarization-dependent slab confinement and a representative cross section of the etched geometry. The method is integrated with adjoint topology optimization and fabrication constraints to design a 30 x 10 um demultiplexer that routes 1550 and 775 nm light to separate output ports. Quantitative comparisons with three-dimensional finite-difference time-domain simulations establish the accuracy and etch-depth dependence of the reduced model. The fabricated device provides mean signal-to-crosstalk ratios of 13.9 dB across 1540-1560 nm and 13.3 dB across 770-780 nm. A two-stage cascaded configuration increases the output extinction ratio to 26.8 dB in the telecom band and 20.3 dB in the near-visible band. This fabrication-aware reduced-dimensional approach enables optimizations of the multifunctional photonic devices for nonlinear optical and quantum applications on the TFLN platform.

physics.optics↗

arXiv2026

Near-unity efficiency optical vortex generation in van der Waals materials

Optical spin-orbit coupling provides a promising, fabrication-free route for developing ultra-compact optical vortex generators. However, the conversion efficiency has been theoretically limited to 0.5. Here, we demonstrate enhanced vortex generation efficiency by employing a Bessel beam as the input and propagating it through van der Waals (vdW) crystals. The large birefringence of vdW crystals and the single transverse wave vector of a Bessel beam allow a near unity spin-orbit conversion efficiency and a topological charge transition of $\ell \rightarrow \ell + 2$. Through combined analytical and experimental investigations, we demonstrate a conversion efficiency of up to 0.82 in hexagonal boron nitride (hBN) crystals with a thickness of $27.4\,\mu\mathrm{m}$. The higher efficiency of Bessel input beams over Gaussian beams is attributed to their distinct transverse wave vector distribution of constituent plane wave components. Furthermore, we demonstrate the dependence of conversion efficiency on the numerical aperture (NA) of the objective lens, which is in good alignment with theoretical predictions. These demonstrations provide a fabrication-free route to highly efficient optical vortex generation via microscale vdW materials platforms.

physics.optics↗

arXiv2025

Inverse-Designed Metasurfaces for Wavefront Restoration in Under-Display Camera Systems

Under-display camera (UDC) systems enable full-screen displays in smartphones by embedding the camera beneath the display panel, eliminating the need for notches or punch holes. However, the periodic pixel structures of display panels introduce significant optical diffraction effects, leading to imaging artifacts and degraded visual quality. Conventional approaches to mitigate these distortions, such as deep learning-based image reconstruction, are often computationally expensive and unsuitable for real-time applications in consumer electronics. This work introduces an inverse-designed metasurface for wavefront restoration, addressing diffraction-induced distortions without relying on external software processing. The proposed metasurface effectively suppresses higher-order diffraction modes caused by the metallic pixel structures, restores the optical wavefront, and enhances imaging quality across multiple wavelengths. By eliminating the need for software-based post-processing, our approach establishes a scalable, real-time optical solution for diffraction management in UDC systems. This advancement paves the way to achieve software-free real-time image restoration frameworks for many industrial applications.

physics.optics↗

arXiv2025

Inverse Design of Chiral Structures for Giant Helical Dichroism

Investigating chiral light-matter interactions is essential for advancing applications in sensing, imaging, and pharmaceutical development. However, the chiroptical response in natural chiral molecules and subwavelength chiral structures is inherently weak, with the characterization tool limited to optical methods that utilize the light with spin angular momentum (SAM). To overcome this, orbital angular momentum (OAM) beams, characterized by helical wavefronts, have emerged as a compelling research focus. Helical dichroism (HD) describes the differential absorbance of OAM beams with opposite signs of topological charges. By using inverse design with adjoint methods for topology optimization, we design the chiral structure optimized to increase HD response under OAM beam incidence, demonstrating a giant HD response of ~107% with topological charges $|\pm\ell|$ = 3 at the wavelength of 800 nm. This study reveals distinct helicity-dependent interactions between the structure and OAM beams, highlighting the potential for custom-tuned chiroptical responses.

physics.optics↗

arXiv2024

Spin-Orbit Coupling for Optical Vortex Generation in van der Waals Materials

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.

physics.optics↗

arXiv2024

Inverse Designed WS2 Planar Chiral Metasurface with Geometric Phase

Increasing attention is being paid to chiral metasurfaces due to their ability to selectively manipulate right-hand circularly polarized light or left-hand circularly polarized light. The thin nature of metasurfaces, however, poses a challenge in creating a device with effective phase modulation. Plasmonic chiral metasurfaces have attempted to address this issue by increasing light-matter interaction, but they suffer from metallic loss. Dielectric metasurfaces made from high index materials enable phase modulation while being thin. Very few materials, however, have high refractive index and low loss at visible wavelengths. Recently, some 2D materials have been shown to exhibit high refractive index and low loss in the visible wavelengths, positioning them as promising platform for meta-optics. This study introduces and details a planar chiral metasurface with geometric phase composed of WS2 meta-units. By employing adjoint optimization techniques, we achieved broadband circular dichroism.

physics.optics↗

arXiv2023

Inverse design and optical vortex manipulation for thin film absorption enhancement

Optical vortices (OVs) have rapidly varying spatial phase and optical energy that circulates around points or lines of zero optical intensity. Manipulation of OV offers innovative approaches for various fields, such as optical sensing, communication, and imaging. In this work, we demonstrate the correlation between OVs and absorption enhancement in two types of structures. First, we introduce a simple planar one-dimensional (1D) structure that manipulates OVs using two coherent light sources. The structure shows a maximum of 6.05-fold absorption gap depending on the presence of OVs. Even a slight difference in the incidence angle can influence the generation/annihilation of OVs, which implies the high sensitivity of angular light detection. Second, we apply inverse design to optimize two-dimensional (2D) perfect ultrathin absorbers. The optimized free-form structure achieves 99.90% absorptance, and the fabricable grating structure achieves 97.85% at 775 nm wavelength. To evaluate OV fields and their contribution to achieving absorption enhancement, we introduce a new parameter, OV circularity. The optimized structures generate numerous OVs with a maximum circularity of 95.37% (free-form) and 96.14% (grating), superior to our 1D structure. Our study reveals the role of high-circularity localized OVs in optimizing nano-structured absorbers and devices for optical sensing, optical communication, and many other applications.

physics.optics↗