arXiv subjects

H. C. Ong

Publications and source records attributed to H. C. Ong.

10 recordsLinked to original sources

arXiv2026

Study of polarization-dependent band inversions and edge states from two-dimensional square lattice plasmonic crystals

When two topologically trivial and nontrivial systems are brought together, a localized energy state is formed at the interface. For crystalline quantum and classical systems, their topology can be determined by studying the eigenmode symmetries at high symmetry points (HSPs) in the Brillouin zone. As electromagnetic systems are usually leaky, their radiations retain the eigenmode information, thus providing a means for probing the band topology. Here, we formulate the far-field characteristics of the eigenmodes at HSPs in 2D square lattice plasmonic systems and reveals, unlike the conventional tight-binding model, several polarization-dependent band inversions occur at theΓand X points, rendering subtle changes in their band topology. In particular, by carefully tuning the system geometry to facilitate trivial and non-trivial phases, polarization selective 0D and 1D edge states that possess distinct field symmetries are realized. The evolution of 0D or 1D edge state depends on the degree of bulk-edge overlapping. We perform angle- and polarization-resolved diffraction spectroscopy on 2D Au plasmonic nanohole arrays to verify the theory and observe such states. Our study demonstrates the applicability of simple far-field technique for diagnosing the band topology of various crystalline classical systems in electromagnetics and acoustics.

physics.optics↗

arXiv2023

Determination of the Zak phase of one-dimensional photonic systems via far-field diffraction

Bloch waves in 1D periodic systems carry Zak phase, which plays a key role in determining the band topology. In general, for systems that possess inversion symmetry, the Zak phase of an isolated band is quantized as 0 or Pi and is associated with the spatial field symmetries at the Brillouin zone center and boundary. The phase is Pi if the field symmetries are different but is 0 when they are the same. Since the radiation losses from leaky systems are strongly associated with the Bloch waves, one may probe the far-field continuum to determine the Zak phases. Here, we formulate the diffractions from photonic systems at the zone center and boundary and find their spectral profiles reveal the Bloch wave symmetries and thereby the corresponding Zak phase. The field symmetries also generalize the occurrence of bound states in the continuum at high symmetry points. For verification, we have studied the Zak phases of one-dimensional TM plasmonic and TE photonic crystals by electrodynamic simulations and measuring the optical properties of plasmonic crystals using Fourier space diffraction spectroscopy and common path interferometry. In addition, a topological protected interface state is demonstrated when two 0 and Pi systems are joined together. The results prove our method provides a simple way for characterizing the band topology of non-Hermitian systems via far-fields.

physics.optics↗

arXiv2023

Study of the surface lattice resonance on basis orientation for achieving ultrahigh quality factor > 12000

Periodic nanoparticle arrays can support surface lattice resonances (SLRs), which arise from the hybridization between localized surface plasmons (LSPs) and diffractive Rayleigh anomalies (RAs). In contrast to LSPs, SLRs enjoy a much higher quality (Q) factor. As the Q factor depends on many system parameters, a good understanding of them is essential for optimization. Here, we study the dependence of the Q factor of SLR from 2D Au nanorod arrays on nanorod orientation. It is found the Q factor is 112 when the nanorod is lying perpendicular to the incident plane but gradually increases by more than 13 times to 1460 upon rotating it azimuthally. The increase of the Q factor is due to the interplay between the coupling strength and the frequency detuning between the LSP and RA as well as the decay rate of the LSP. By optimizing these parameters, we can achieve a Q factor reaching over 12000, which is 6 times higher than the best reported so far.

physics.optics↗

arXiv2022

Small mode volume topological photonic states in one-dimensional lattices with dipole--quadrupole interactions

We study the topological photonic states in one-dimensional (1-D) lattices analogue to the Su-Schrieffer-Heeger (SSH) model beyond the dipole approximation. The electromagnetic resonances of the lattices supported by near-field interactions between the plasmonic nanoparticles are studied analytically with coupled dipole--quadrupole method. The topological phase transition in the bipartite lattices is determined by the change of Zak phase. Our results reveal the contribution of quadrupole moments to the near-field interactions and the band topology. It is found that the topological edge states in non-trivial lattices have both dipolar and quadrupolar nature. The quadrupolar edge states are not only orthogonal to the dipolar edge states, but also spatially localized at different sublattices. Furthermore, the quadrupolar topological edge states, which coexist at the same energy with the quadrupolar flat band have shorter localization length and hence smaller mode volume than the conventional dipolar edge states. The findings deepen our understanding in topological systems that involve higher-order multipoles, or in analogy to the wave functions in quantum systems with higher-orbital angular momentum, and may be useful in designing topological systems for confining light robustly and enhancing light-matter interactions.

cond-mat.mes-hall↗

arXiv2022

Realization of superchiral surface lattice resonances in three-dimensional bipartite nanoparticle arrays

Optical chirality (OC) is a fundamental property of electromagnetic waves that plays a key role in governing chiral light-matter interaction. Here, we demonstrate how to obtain superchiral surface lattice resonances (SLRs), which arise from the hybridization between localized surface plasmons (LSPs) and diffractive Rayleigh anomalies (RAs), in nanoparticle arrays. We first study the coupling constants between LSPs and RAs in 2D Au monopartite nanorod arrays by angle-resolved reflectivity spectroscopy and finite-difference time-domain (FDTD) simulations. The complex dispersion relations and the near-fields of SLRs are then analyzed by temporal coupled-mode theory (CMT) for formulating the dependence of the coupling constants on the dipole orientation of the nanorod. The TE and TM coupling constants are found to depend strongly on the orientation of the dipole lying in the plane perpendicular to the propagation direction of RA. By using two orthogonally oriented nanorods, TE- and TM-SLRs can be excited independently. We then extend the CMT approach to rationally design superchiral SLRs based on 3D bipartite nanorods. The OC is shown to depend on the relative displacement between two nanorods, the near-field strength, and the interplay between the coupling constants and the Q-factor of the SLR resonance. We have achieved an averaged OC of 27 times stronger than that of the circularly polarized plane wave over the entire surface, featuring large area chiral surface waves that are useful for chirality-based applications.

physics.optics↗

arXiv2022

Realization of topological superlattices and the associated interface states in one-dimensional plasmonic crystals

In analogous to the Su-Schrieffer-Heeger (SSH) model, one-dimensional (1D) electromagnetic (EM) crystals can exhibit nontrivial topological properties. In particular, when a nontrivial EM crystal is in contact with its trivial counterpart, a topologically protected interface state is formed. While much attention has been focused on single interface state, multiple interface states can interact collectively when under suitable conditions, giving rise to trivial or nontrivial band topologies resembling to the standard SSH model. Here, we study the topological properties of 1D metallic superlattices that support multiple interface states. We first demonstrate single interface state exists at the boundary between two topologically distinct metallic arrays that carry Bloch-like surface plasmon polaritons. Such interface state is then used as the building block for further constructing the superlattices. By exploiting the separation between two dimerized interface states and the distinct inter- and intra-interface configurations to facilitate different intracell and intercell interactions, we vary the band topology of the superlattices. More importantly, new superlattice interface state is formed when trivial and nontrivial superlattices are brought together. The superlattice interface state is found to have smaller angular divergence and longer localization length than its single counterpart, thus is desired for robust signal transmission and high finesse cavity.

physics.optics↗

arXiv2021

Quality factor of plasmonic monopartite and bipartite surface lattice resonances

Surface lattice resonance (SLR) is the collective excitation of nanoparticle resonances arising from the hybridization between localized surface plasmons (LSPs) and propagating Rayleigh anomalies (RAs). When comparing with the corresponding LSPs, SLRs exhibit much higher quality factor. In fact, as the quality factor depends on the constituting resonances and their hybridization, how one can parametrize it in an analytic form is an important issue. We have studied the SLRs arising from 2D Au monopartite nanoparticle arrays by angle- and polarization-resolved reflectivity spectroscopy, temporal coupled mode theory (CMT) and finite-difference time-domain (FDTD) simulation. The scattering matrix of the SLRs is formulated, revealing the importance of the spectral detuning and the interaction strengths between the LSP and the RAs in governing the quality factor. We then extend the CMT approach to study bipartite arrays where nanoparticle dimer is employed and find the coupling between two LSPs plays a major role in further boosting the quality factor. Specifically, the coupling takes part in controlling the detuning factor as well as determining whether the coupled bright or dark mode is hybridized with the RAs. The dark mode hybridization can strongly enhance the quality factor which is otherwise not possible in the monopartite counterparts.

physics.optics↗

arXiv2020

Generalization of the circular dichroism from metallic arrays that support Bloch-like surface plasmon polaritons

The broken mirror symmetry in subwavelength photonic systems has manifested many interesting chiroptical effects such as optical rotation and circular dichroism. When such systems are placed periodically in a lattice form, in addition to intrinsic chirality, extrinsic chirality also takes part, and the overall effect depends not only on the basis and lattice but also the excitation configuration. Here, we study planar chiral nanohole arrays in square lattice that support Bloch-like surface plasmon polaritons (SPPs) and clarify how the system geometry and the excitation contribute to circular dichroism. By using temporal coupled mode theory (CMT), the dissymmetry factor and the scattering matrix of the arrays are analytically formulated. Remarkably, we find the dissymmetry factor depends only on the coupling polarization angle and the in-coupling phase difference between the p- and s-polarizations. Besides, the upper limit of the dissymmetry factor at +/-2 can be reached simply by orienting the lattice of the arrays for properly exciting the Bloch-like SPPs and at the same time making the basis mimic two orthogonal and relatively displaced dipoles, demonstrating the interplay between extrinsic and intrinsic chirality. The models have been verified by numerical simulations and experiments, yielding the dissymmetry factors to be 1.82 and 1.55, respectively, from the proposed dual slot system.

physics.optics↗

arXiv2017

Polarization conversion of non-specular diffraction orders from metallic nanohole arrays studied by Fourier space polarimetry

We use Fourier space polarimetry to study the incident angle- and polarization-dependent rotation angle and ellipticity of different diffraction orders emerging from a two-dimensional periodic Au array. The array has square lattice and circular nanoholes and thus is achiral. We find no polarization conversion occurs if the diffraction orders lie in the incident plane. However, for the orders that are diffracted away from the incident plane, their rotation angle and ellipticity vary considerably with incident angle and polarization. In particular, dramatic changes in rotation angle and ellipticity are observed when Bloch-like surface plasmon polaritons (SPPs) are excited. The experimental results are consistent with the finite-difference time-domain simulations. The transverse spin carried by the SPPs and a discrete dipole model are used complementarily to elucidate such angular and polarization dependences.

physics.optics↗

arXiv2016

Understanding the role of surface plasmon polaritons in two-dimensional achiral nanohole arrays for polarization conversion

We have studied the dependence of the rotation angle and ellipticity on the sample orientation and incident polarization from metallic nanohole arrays. The arrays have four-fold symmetry and thus do not possess any intrinsic chirality. We elucidate the role of surface plasmon polaritons (SPPs) in determining the extrinsic chirality and we verify the results by using finite-difference time-domain simulation. Our results have indicated the outgoing reflection arises from the interference between the nonresonant background, which preserves the input polarization, and the SPP radiation damping, which is linearly polarized but carries a different polarization defined by the vectorial field of SPPs. More importantly, the interference manifests various polarization states ranging from linear to elliptical across the SPP resonance. We analytically formulate the outgoing waves based on temporal coupled mode theory (CMT) and the results agree well with the experiment and simulation. From CMT, we find the polarization conversion depends on the interplay between the absorption and radiative decay rates of SPPs and the sample orientation.

physics.optics↗