arXiv subjects

Wook-Jae Lee

Publications and source records attributed to Wook-Jae Lee.

5 recordsLinked to original sources

arXiv2026

Bound States in the Continuum and Unidirectional Guided Resonances Enabled by Competing Fourier Harmonics near the Fourth Stop Band

Bound states in the continuum (BICs) and unidirectional guided resonances (UGRs) are singular radiative eigenstates in planar photonic lattices characterized by complete or directional suppression of out-of-plane radiation, respectively. Here, we show that, near the fourth stop band, the formation of BICs and UGRs can be understood within a unified two-pathway radiation model, in which the Bloch eigenmode radiates through indirect and direct pathways mediated by the first and second Fourier harmonics of the dielectric modulation, respectively. In vertically symmetric photonic lattices, when the two radiation components cancel each other out in both the upward and downward directions, BICs emerge. The model yields an analytical BIC condition that can be satisfied by tuning a single lattice parameter, the duty cycle $\rho$, and is confirmed by finite-element simulations. By contrast, in vertically asymmetric photonic lattices, the upward and downward radiation-cancellation conditions no longer coincide, allowing UGRs to arise when the cancellation condition is satisfied in only one direction. We introduce a zero-contrast grating (ZCG), in which the upper protruding grating region produces pronounced up-down asymmetry, and show that two off-$\Gamma$ UGRs emerge and can be merged at the $\Gamma$ point by tuning the lattice parameters.

physics.optics↗

arXiv2024

Merging bound states in the continuum at third-order $Γ$ point enabled by controlling Fourier harmonic components in lattice parameters

Recent studies have demonstrated that ultrahigh-$Q$ resonances, which are robust to fabrication imperfections, can be realized by merging multiple bound states in the continuum (BICs) in momentum space. The merging of multiple BICs holds significant promise for practical applications, providing a robust means to attain ultrahigh-$Q$ resonances that greatly enhance light-matter interactions. In this study, we introduce a novel approach to achieve the merging of BICs at the edges of the fourth stop band, which opens at the third-order $Γ$ point, in one-dimensional leaky-mode photonic lattices. Photonic band gaps and BICs arise from periodic modulations in lattice parameters. However, near the third-order $Γ$ point, out-of-plane radiation arises by the first and second Fourier harmonic components in the lattice parameters. Accidental BICs can emerge at specific $k$ points where an optimal balance exists between these two Fourier harmonic components. We demonstrate that these accidental BICs are topologically stable, and their positions in momentum space can be precisely controlled by adjusting a specific lattice parameter that influences the strength of the first and second Fourier harmonic components. Furthermore, we show that accidental BICs can be merged at the third-order $Γ$ point, with or without a symmetry-protected BIC, by finely adjusting this specific lattice parameter while keeping other parameters constant.

physics.optics↗

arXiv2023

Transition from quasi-unidirectional to unidirectional guided resonances in leaky-mode photonic lattices

Unidirectional light emission from planar photonic structures is highly advantageous for a wide range of optoelectronic applications. Recently, it has been demonstrated that unidirectional guided resonances (UGRs) can be realized by utilizing topological polarization singularities in momentum space. However, the practical application of these topological unidirectional emitters has been limited due to their intricate geometric configurations, requiring special efforts with high-cost fabrication processes. In this study, we show that unidirectional light emission can be achieved in conventional one-dimensional zero-contrast gratings (ZCGs), which can be easily fabricated using current nanofabrication technologies. In ZCGs, the interband coupling between even-like and odd-like waveguide modes leads to the formation of quasi-UGRs, characterized by significantly higher decay rates in either the upward or downward direction compared to the opposite direction. We demonstrate that these quasi-UGRs evolve into genuine UGRs with an gradual increase in grating thickness. Moreover, the emission direction of UGRs can be selectively steered either upward or downward by adjusting the lattice parameters. In addition to quasi-UGRs and UGRs, our study also reveals additional topological phenomena in ZCGs, including exceptional points and quasi-BICs.

physics.optics↗

arXiv2023

Merging and band transition of bound states in the continuum in leaky-mode photonic lattices

Bound states in the continuum (BICs) theoretically have the ability to confine electromagnetic waves in limited regions with infinite radiative quality ($Q$) factors. However, in practical experiments, resonances can only exhibit finite $Q$ factors due to unwanted scattering losses caused by fabrication imperfections. Recently, it has been shown that ultrahigh-$Q$ guided-mode resonances (GMRs), which are robust to fabrication imperfections, can be realized by merging multiple BICs in momentum space. In this study, we analytically and numerically investigate the merging and band transition of accidental BICs in planar photonic lattices. Accidental BICs can merge at the edges of the second stop band, either with or without a symmetry-protected BIC. We show that as the thickness of the photonic lattice gradually increases, the merged state of BICs transitions from the upper to the lower band edge. Using coupled-mode analysis, we present the analytical merging thickness at which multiple accidental BICs merge at the second-order $Γ$ point. Our coupled-mode analysis could be beneficial for achieving ultrahigh-$Q$ GMRs in various photonic lattices composed of materials with different dielectric constants.

physics.optics↗

arXiv2017

Monolithic InGaAs nanowire array lasers on silicon-on-insulator operating at room temperature

Chip-scale integrated light sources are a crucial component in a broad range of photonics applications. III-V semiconductor nanowire emitters have gained attention as a fascinating approach due to their superior material properties, extremely compact size, and the capability to grow directly on lattice-mismatched silicon substrates. Although there have been remarkable advances in nanowire-based emitters, their practical applications are still in the early stages due to the difficulties in integrating nanowire emitters with photonic integrated circuits (PICs). Here, we demonstrate for the first time optically pumped III-V nanowire array lasers monolithically integrated on silicon-on-insulator (SOI) platform. Selective-area growth of purely single-crystalline InGaAs/InGaP core/shell nanowires on an SOI substrate enables the nanowire array to form a photonic crystal nanobeam cavity with superior optical and structural properties, resulting in the laser to operate at room temperature. We also show that the nanowire array lasers are effectively coupled with SOI waveguides by employing nanoepitaxy on a pre-patterned SOI platform. These results represent a new platform for ultra-compact and energy-efficient optical links, and unambiguously point the way toward practical and functional nanowire lasers.

cond-mat.mes-hall↗