arXiv subjects

Sangsik Kim

Publications and source records attributed to Sangsik Kim.

10 recordsLinked to original sources

arXiv2026

Strontium ${}^{1}S_{0}\!\rightarrow\!{}^{1}P_{1}$ transition frequency measurements assisted by a photonic grating chip

We measure the absolute frequency of the ${}^{1}S_{0}\!\rightarrow\!{}^{1}P_{1}$ transition in strontium using two methods: fluorescence spectroscopy of a thermal atomic beam source from a compact low-power oven and velocity measurements of a slow atomic beam from a two-dimensional grating magneto-optical trap (2D gMOT). The measurements for both methods are performed in the same ultra-high vacuum chamber containing a diffraction grating chip which is placed below the strontium atoms that are being interrogated. The first method uses a probe laser beam incident on the grating chip such that the grating acts as an end mirror, with the first-order diffracted beam providing a retro-reflected probe beam. The counter-propagating laser beams traverse an atomic beam emitted from an oven, enabling spatially resolved fluorescence spectroscopy through CCD imaging and hyperfine-constrained multi-isotope fitting. The second method relies on a large profile cooling laser beam normally incident onto the grating chip which laser cools strontium atoms for a slow atomic beam source. The velocity of the atoms exiting the 2D gMOT is measured as a function of the laser detuning and intensity from which the resonance frequency can be estimated. The two methods are consistent within their quoted uncertainties. Using three datasets based on retro-beam spectroscopy measurements, and one dataset using slow atom beam velocity measurements, we determine the ${}^{1}S_{0}\!\rightarrow\!{}^{1}P_{1}$ transition frequency to be $650.503\,815(5)~\mathrm{THz}$. Our result provides a re-evaluation of this $461$ nm transition demonstrated on a compact laser cooling apparatus based on a diffraction grating platform.

physics.atom-ph↗

arXiv2026

Universal zero-crosstalk photonic integration via slab-engineered mode hybridization

Photonic integrated circuits have emerged as a scalable platform for optical computing, communication, and quantum technologies, where high-fidelity optical processing is essential. However, as photonic systems scale in complexity, inter-channel crosstalk accumulates across cascaded components, fundamentally degrading signal fidelity, limiting system-level performance, and constraining integration density. Existing crosstalk-suppression strategies rely on specialized nanostructures or platform-specific designs, hindering their adoption in standard foundry processes and across diverse material systems. Here we establish a universal and foundry-compatible route to eliminating crosstalk based on slab-engineered mode hybridization in standard rib waveguides. By tailoring the slab thickness, mode hybridization induces anisotropic modal perturbations that enable complete cancellation of coupling between adjacent waveguides. We experimentally demonstrate zero-crosstalk across diverse material platforms, including silicon-on-insulator, silicon nitride, thin-film lithium niobate, and germanium-on-insulator, spanning wavelengths from the visible to the mid-infrared. Our approach provides a manufacturable route toward scalable, high-fidelity, and high-density photonic integration, overcoming the long-standing trade-off between signal fidelity and integration density in large-scale photonic systems.

physics.optics↗

arXiv2025

Dual-Bus Resonator for Multi-Port Spectral Engineering

Microresonators are essential in integrated photonics, enabling optical filters, modulators, sensors, and frequency converters. Their spectral response is governed by bus-to-resonator coupling, typically classified as under-, critical-, or over-coupling. Conventional single-bus designs inevitably link the conditions for critical coupling, a transmission zero, and maximum intra-cavity power, preventing independent control of these phenomena and restricting the ability to engineer coupling regimes and resonance lineshapes. Here we propose and experimentally demonstrate a dual-bus racetrack resonator that breaks this constraint. Our design demonstrates complementary channel-specific coupling regimes and enables wavelength-dependent Lorentzian-to-Fano lineshaping. We model the device using three-waveguide coupled-mode theory and pole-zero analysis, which reveals that transmission zeros are decoupled from cavity-defined critical coupling and maximum intra-cavity power. Furthermore, the dual-bus scheme operates broadband, spanning visible to mid-infrared across all four transmission channels, highlighting its spectral richness and platform independence. These results establish a general framework for multi-port spectral engineering in integrated photonics, with broad implications for tunable filters, modulators, sensors, and nonlinear optical systems.

physics.optics↗

arXiv2022

Anisotropic subwavelength grating perturbation enables zero crosstalk in a leaky mode

Electromagnetic coupling via either exponentially decaying evanescent field or radiative wave is a primary characteristic of light, allowing optical signal/power transfer but limiting integration density in a photonic circuit. A leaky mode combines both evanescent field and radiative wave, causing stronger crosstalk and thus not ideal for dense integration. Here we show that a leaky mode with anisotropic perturbation rather can achieve completely zero crosstalk realized by subwavelength grating (SWG) metamaterials. The oscillating fields in the SWGs enable coupling coefficients in each direction to counteract each other, resulting in completely zero crosstalk. We experimentally demonstrate such an extraordinarily low coupling between closely spaced identical leaky SWG waveguides, suppressing the crosstalk by $\approx$40 dB compared to conventional strip waveguides, corresponding to $\approx$100 times longer coupling length. This leaky-SWG suppresses the crosstalk of transverse-magnetic (TM) mode, which is challenging due to its low confinement, and marks a novel approach in electromagnetic coupling applicable to other spectral regimes and generic devices.

physics.optics↗

arXiv2021

Exceptional points in lossy media enable decay-free wave propagation

Waves entering a spatially uniform lossy medium typically undergo exponential decay, arising from either the energy loss of the Beer-Lambert-Bouguer transmission law or the evanescent penetration during reflection. Recently, exceptional point singularities in non-Hermitian systems have been linked to unconventional wave propagation, such as the predicted extremely spatially broad constant-intensity guided modes. Despite such promises, the possibility of decay-free wave propagation in a purely lossy medium has been neither theoretically suggested nor experimentally realized until now. Here we discover and experimentally demonstrate decay-free wave propagation accompanied by a striking uniformly distributed energy loss across arbitrary thicknesses of a homogeneous periodically nanostructured waveguiding medium with exceptional points. Predicted by coupled-mode theory and supported by fully vectorial electromagnetic simulations, hundreds-of-waves deep penetration manifesting spatially constant radiation losses are experimentally observed in photonic slab waveguides. The uniform, decay-free radiative energy loss is measured across the entire structured waveguide region, regardless of its length. While the demonstrated constant-intensity radiation finds an immediate application for generating large, uniform and surface-normal free-space plane waves directly from the photonic chip surface, the uncovered decay-free wave phenomenon is universal and holds true across all domains supporting physical waves, opening new horizons for dispersion-engineered materials empowered by exceptional point physics.

physics.optics↗

arXiv2020

Exceptional coupling in extreme skin-depth waveguides for extremely low waveguide crosstalk

Photonic chips can miniaturize complicate optical systems very tiny and portable, providing versatile functionalities for many optical applications. Increasing the photonic chip integration density is highly desired as it provides more functionalities, low cost, and lower power consumption. However, photonic chip integration density is limited by the waveguide crosstalk, which is caused by the evanescent waves in the cladding. Here we show that the waveguide crosstalk can be suppressed completely with the exceptional coupling in extreme skin-depth (eskid) waveguides. The anisotropic dielectric perturbations in the coupled eskid waveguides cause such an exceptional coupling, resulting in infinitely long coupling length. We demonstrate the extreme suppression of waveguide crosstalk via exceptional coupling on a silicon-on-insulator (SOI) platform, which is compatible with a complementary metal-oxide-semiconductor (CMOS) process. The idea of exceptional coupling in eskid waveguides can be applied to many other photonic devices as well, significantly reducing entire chip footprints.

physics.optics↗

arXiv2018

Phased-Locked Two-Color Single Soliton Microcombs in Dispersion-Engineered Si3N4 Resonators

We propose and theoretically investigate a dispersion-engineered Si3N4 microring resonator, based on a cross-section containing a partially-etched trench, that supports phase-locked, two-color soliton microcomb states. These soliton states consist of a single circulating intracavity pulse with a modulated envelope that sits on a continuous wave background. Such temporal waveforms produce a frequency comb whose spectrum is spread over two widely-spaced spectral windows, each exhibiting a squared hyperbolic secant envelope, with the two windows phase-locked to each other via Cherenkov radiation. The first spectral window is centered around the 1550 nm pump, while the second spectral window is tailored based on straightforward geometric control, and can be centered as short as 750 nm and as long as 3000 nm. We numerically analyze the robustness of the design to parameter variation, and consider its implications to self-referencing and visible wavelength comb generation.

physics.optics↗

arXiv2018

Photonic waveguide mode to free-space Gaussian beam extreme mode converter

Integration of photonic chips with atomic, micromechanical, chemical and biological systems can advance science and open many possibilities in chip-scale devices and technology. Compact photonic structures for direct coupling of light between high-index single-mode waveguides and arbitrary free-space modes spanning hundreds of waves in cross-section would eliminate bulky optical components and enable integration of photonics into many new applications requiring wide beams, structured light and centimeter-scale propagation distances with low diffraction-limited losses. Conventional fiber-coupling approaches do not scale well for accurate, low-loss coupling across the extremely large mode scale mismatch ($\approx10^6$ times in modal area). Here we present an extreme mode converter that can transform the photonic waveguide mode to the diffraction-limited, free-space Gaussian beam, with a beam waist of about $160~\mu$m. Using two identical converters, we demonstrate a grating-to-grating coupling that couples the radiating beam back to the chip through a mirror reflection in free-space. Operating at 780~nm for integration with chip-scale atomic vapor cell cavities, our design can be adapted for visible, telecommunication or other wavelengths. Furthermore, other types of beams can be implemented by using the 2-stage expansion approach presented in this paper.

physics.app-ph↗

arXiv2017

Controlling evanescent waves using silicon photonic all-dielectric metamaterials for dense integration

Ultra-compact, densely integrated optical components manufactured on a CMOS-foundry platform are highly desirable for optical information processing and electronic-photonic co-integration. However, the large spatial extent of evanescent waves arising from nanoscale confinement, ubiquitous in silicon photonic devices, causes significant cross-talk and scattering loss. Here, we demonstrate that anisotropic all-dielectric metamaterials open a new degree of freedom in total internal reflection to shorten the decay length of evanescent waves. We experimentally show the reduction of cross-talk by greater than 30 times and the bending loss by greater than 3 times in densely integrated, ultra-compact photonic circuit blocks. Our prototype all-dielectric metamaterial-waveguide achieves a low propagation loss of approximately 3.7 dB/cm, comparable to those of silicon strip waveguides. Our approach marks a departure from interference-based confinement as in photonic crystals or slot waveguides, which utilize nanoscale field enhancement. Its ability to suppress evanescent waves without substantially increasing the propagation loss shall pave the way for all-dielectric metamaterial-based dense integration.

physics.optics↗

arXiv2016

Frequency Comb Generation in 300 nm Thick SiN Concentric-Racetrack-Resonators: Overcoming the Material Dispersion Limit

Kerr nonlinearity based frequency combs and solitons have been generated from on-chip optical microresonators with high quality factors and global or local anomalous dispersion. However, fabrication of such resonators usually requires materials and/or processes that are not standard in semiconductor manufacturing facilities. Moreover, in certain frequency regimes such as visible and ultra-violet, the large normal material dispersion makes it extremely difficult to achieve anomalous dispersion. Here we present a concentric racetrack-shaped resonator that achieves anomalous dispersion in a 300 nm thick silicon nitride film, suitable for semiconductor manufacturing but previously thought to result only in waveguides with high normal dispersion, a high intrinsic Q of 1.5 million, and a novel mode-selective coupling scheme that allows coherent combs to be generated. We also provide evidence suggestive of soliton-like pulse formation in the generated comb. Our method can achieve anomalous dispersion over moderately broad bandwidth for resonators at almost any wavelength while still maintaining material and process compatibility with high-volume semiconductor manufacturing.

physics.optics↗