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Surface plasmon polaritons (SPPs) carry transverse optical spin within the evanescent field, which has enabled the demonstration of various chiral light-matter interactions in classical and quantum systems. To achieve high spin selectivity in the interactions, the elliptical polarization of the evanescent field should be made circular, but the engineering principle of the degree of circular polarization (DOCP) of SPPs has been lacking. In this study, we theoretically and numerically investigate the scaling behavior of the DOCP of the SPP field with respect to the modal effective refractive index (neff). The DOCP of the SPP field exhibits power-law scalability to the effective refractive index in the 1D layered system, regardless of the material, structural geometry, and excitation wavelength. The power-law scalability is also confirmed in 2D waveguide structures for in-plane and out-of-plane SPP fields, but the scaling exponents vary depending on the distance from the waveguide boundaries by the reduced symmetry of the given system. Due to Lorentz reciprocity, the power-law scalability can be extended to the coupling directionality of chiral emitters towards the plasmonic waveguide. To this end, we propose a chiral photonic platform for enhanced light-valley interaction, which utilizes simultaneous enhancement of the DOCP and coupling directionality. An incident SPP can excite a chiral emitter with high spin selectivity that unidirectionally couples the emitted light into the plasmonic waveguide depending on the valley polarization of excitons in 2D material. Our work provides a ground rule for designing chiral nanophotonic systems and paves the way for the exploration of scale-free phenomena of electromagnetic waves.

Allowing a high quality factor (Q-factor) to a sub-wavelength dielectric resonator, quasi-bound states in the continuum (Q-BIC) has gained much interest. However, the Q-BIC resonance condition is too sensitive to the geometry of the resonator and its practical broadband generation on a single-wafer platform has been limited. Here, we present that employing the base angle as a structural degree of freedom, the truncated nano-cone resonator supports the Q-BIC resonance with a high Q-factor of >150 over a wide wavelength range of >100 nm. We expect our approach will boost the utilization of the Q-BIC resonance for various applications requiring broadband spectral tuning., Comment: 4 pages, 3 figures

Embodying bosonic and electrically interactive characteristics in two-dimensional space, excitons in transition-metal dichalcogenides (TMDCs) have garnered considerable attention. The realisation and application of strong-correlation effects, long-range transport, and valley-dependent optoelectronic properties require customising exciton decay dynamics. Strains, defects, and electrostatic doping effectively control the decay dynamics but significantly disturb the intrinsic properties of TMDCs, such as electron band structure and exciton binding energy. Meanwhile, vacuum-field manipulation provides an optical alternative for engineering radiative decay dynamics. Planar mirrors and cavities have been employed to manage the light-matter interactions of two-dimensional excitons. However, the conventional flat platforms cannot customise the radiative decay landscape in the horizontal TMDC plane or independently control vacuum field interference at different pumping and emission frequencies. Here, we present a meta-mirror resolving the issues with more optical freedom. For neutral excitons of the monolayer MoSe2, the meta-mirror manipulated the radiative decay rate by two orders of magnitude, depending on its geometry. Moreover, we experimentally identified the correlation between emission intensity and spectral linewidth. The anisotropic meta-mirror demonstrated polarisation-dependent radiative decay control. We expect that the meta-mirror platform will be promising to tailor the two-dimensional distributions of lifetime, density, and diffusion of TMDC excitons in advanced opto-excitonic applications.

Optical vortices host the orbital nature of photons, which offers an extra degree of freedom in photonic applications. Unlike vortices in other physical entities, optical vortices require structural singularities, which restrict their abilities in terms of dynamic and interactive characteristics. In this study, we present the spontaneous generation and external magnetic field-induced manipulation of an optical vortex and antivortex. A gradient-thickness optical cavity (GTOC) consisting of an Al/SiO2/Ni/SiO2 multilayer structure realised the distinct transition between the trivial and non-trivial topological phases, depending on the magneto-optic effects of the Ni layer. In the non-trivial topological phase, the mathematical singularities generating the optical vortex and antivortex pair in the reflected light existed in the generalised parameter space of the thicknesses of the top and bottom SiO2 layers, which is bijective to the real space of the GTOC. Coupled with the magnetisation, the optical vortex and antivortex in the GTOC experienced an effective spin-orbit interaction and showed topology-dependent dynamics under external magnetic fields. We expect that field-induced engineering of optical vortices will pave the way for the study of topological photonic interactions and their applications., Comment: 22 pages, 4 figures

Interlayer excitons in van der Waals heterostructures of two-dimensional transition metal dichalcogenides have recently emerged as a fascinating platform for quantum many-body effects, long-range interactions, and opto-valleytronic applications. The practical implementation of such phenomena and applications requires further development of the long-lived character of interlayer excitons. Whereas material developments have successfully enhanced the nonradiative lifetime, the out-of-plane polarization nature of the interlayer excitons has made it challenging to improve the radiative lifetime with conventional photonic mirrors. Here, we propose and systematically analyze a plasmonic photonic crystal (PPhC) mirror that can increase the radiative lifetime of interlayer excitons by two orders of magnitude. Based on the vacuum field transition, the PPhC mirror supports spatially uniform radiative decay suppression over its territory, which is crucial for engineering the interlayer excitons not localized at a specific position. The PPhC mirror platform will offer new possibilities for realizing long-lived interlayer exciton-based nanodevices., Comment: 26 pages, 5 figures

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that perfect optical absorption (POA) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20 degree and magnetic domain imaging visibility of 0.47. Especially, POA-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The POA technique is promising for optical investigations and applications of nanomagnetic systems., Comment: 14 pages, 5 figures

We present a method to measure the vector-field light scattering of individual microscopic objects. The polarization-dependent optical field images are measured with quantitative phase imaging at the sample plane, and then numerically propagated to the far-field plane. This approach allows the two-dimensional polarization-dependent angle-resolved light scattered patterns from individual object to be obtained with high precision and sensitivity. Using this method, we present the measurements of the polarization-dependent light scattering of a liquid crystal droplet and individual silver nanowires over scattering angles of 50{\deg}. In addition, the spectroscopic extension of the polarization-dependent angle-resolved light scattering is demonstrated using wavelength-scanning illumination.