Koichi Okamoto

Red emission remains a major bottleneck for practical full-color micro-light-emitting diodes (microLEDs) because the quantum efficiencies of InGaN decrease in the long-wavelength region. To enhance red emission, this study developed a honeycomb-latticed InGaN/GaN nanocolumn array integrated with an Ag-based plasmonic LED structure. Finite-difference time-domain simulations were used to optimize the lattice parameters, identifying a = 220 nm and D = 190 nm as the optimal geometries. This design provided an electric-field enhancement of approximately 4.5 at λ = 623 nm, indicating strong surface plasmon polariton coupling in the red region. A device fabricated with the same parameters exhibited up to a 6.3-fold photoluminescence enhancement under surface excitation, outperforming the 5.9-fold enhancement previously achieved under backside excitation. These results demonstrate that honeycomb-latticed plasmonic engineering is effective for boosting red emission in InGaN-based structures and offers a promising route to high-efficiency red microLEDs.

Plasmonic metallic nanostructures enhance light emission in optoelectronic devices, and silver random nanograin structures are particularly promising owing to their simple fabrication and large-area compatibility. However, their disordered morphology has precluded quantitative linking of surface structure to emission enhancement. We introduce a structural metric, referred to as effective power, by applying two-dimensional fast Fourier transforms to atomic force microscopy images of silver nanograin surfaces and integrating spatial frequency components within a coupling-eligible band determined from the surface plasmon polariton dispersion relation. Validation using red, green, and blue quantum dots yields a combined correlation coefficient of R = 0.83, demonstrating that emission enhancement across the visible spectrum can be quantitatively described using morphology alone.

We propose a dual-band near-infrared absorber based on the simultaneous excitation of borophene localized surface plasmons (LSPs) and Tamm plasmon polaritons (TPPs). By leveraging the ultrahigh intrinsic carrier density of borophene, this hybrid architecture effectively overcomes the mid-infrared spectral limitations of conventional two-dimensional materials, achieving near-perfect peak absorptances of 98.1% and 97.2%. Theoretical and numerical investigations reveal that the in-plane LSP mode and the longitudinally distributed TPP mode exhibit a highly decoupled nature. The LSP resonance can be actively and independently modulated via the borophene ribbon geometry and electron density, whereas the TPP mode is strictly governed by the underlying one-dimensional photonic crystal and incident angle. Combined with a robust polarization-selective response, this structurally simple yet highly tunable platform offers a versatile physical paradigm for designing next-generation active optical modulators and polarization-sensitive nanophotonic devices.

InGaN-based light-emitting diodes (LEDs) exhibit excellent emission efficiency in the blue spectral region but experience severe degradation of external quantum efficiency in the red region, primarily due to lattice strain and increased defect density. To address this limitation, we introduced surface plasmon polaritons into InGaN/GaN nanocolumn (NC) arrays arranged in a honeycomb lattice to enhance red-light emission. Deposition of Ag films on the photonic crystal NCs generated plasmonic crystals that allowed precise control of photonic–plasmonic band interactions. Angle-resolved photoluminescence measurements combined with electromagnetic field analysis revealed that strong coupling occurs when photonic and plasmonic field modes are spectrally and spatially matched. Rabi splitting was observed due to the strong coupling between photonic and plasmonic band states, and a 9.6-fold enhancement in red emission was achieved through the combined effects of strong and weak coupling. This controllable strong coupling between photonic and plasmonic band states provides a promising approach for overcoming the intrinsic efficiency limitations of InGaN-based red LEDs while simultaneously enhancing their optical functionality.

We propose and theoretically investigate a thermally tunable, dual-channel near-infrared absorber based on an amorphous silicon (a-Si)-integrated hybrid grating-photonic crystal. Utilizing rigorous coupled-wave analysis and coupled-mode theory, we demonstrate that the device achieves polarization-selective, near-perfect absorption (>98%) governed by mode-selective critical coupling. Due to distinct spatial confinement, the structure exhibits a completely decoupled thermal response. One resonant mode achieves critical coupling with a high linear thermal sensitivity of 118.3 pm/°C, while the other acts as an ultra-high- Q reference ( Q ≈13147) that remains spectrally locked with a negligible sensitivity of 4.4 pm/°C. Furthermore, the absorber features high fabrication tolerance and angular mode anti-crossing. This integration of an actively tunable channel and a temperature-insensitive reference within a single monolithic platform provides a robust paradigm for self-calibrating optical sensors and dynamic narrowband filters.

Semiconductor quantum dots (QDs) are attractive materials for light-emitting devices, and the photoluminescence (PL) from QDs can be enhanced near a metal surface due to surface plasmon (SP) resonance. To integrate QDs into metal structures, QD/poly(methyl methacrylate) (PMMA) composite thin films are generally used. However, it has been reported that QDs tend to aggregate in the PMMA matrix. In this study, we fabricated two types of QD/polymer composite thin films with different degrees of QD aggregation by additionally using poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-MA), which is known to prevent QD aggregation. Furthermore, these two types of films were fabricated on Ag films, with the distance between the Ag films and the QDs controlled by Al₂O₃ spacer layers, and the PL enhancement was compared between the two film types. Finally, we reveal that QD aggregation in the polymer matrix significantly affects the PL enhancement. Although the aggregation trends differed between PMMA and PMMA-co-MA, the results suggest a possible increase in the internal quantum efficiency (IQE) in both film types.

Nonreciprocal radiation is essential for advanced thermal photonics but is constrained by static performance and large incident angle requirements in current Weyl semimetal (WSM)-based devices. Here, we propose a WSM–Ge2Sb2Te5 (GST) hybrid resonant structure to achieve thermally tunable dual functionality. When GST is amorphous, the structure exhibits strong small-angle (5°) nonreciprocal radiation at 14.09 μm, with a nonreciprocity coefficient of up to 0.95, driven by coupled guided-mode and Fabry–Pérot resonances. Upon annealing, GST transforms to the crystalline state, enabling broadband high absorption across 12–15 μm with reciprocal behavior. Structural parameters modulate the resonant wavelength by adjusting the effective refractive index or optical path, while GST crystallinity significantly regulates nonreciprocal performance. This non-volatile design overcomes key limitations of static nonreciprocal devices, holding promise for adaptive energy harvesting and intelligent electromagnetic protection.

Ultrafast luminescence represents a frontier in optical materials science, bridging fundamental investigations of light–matter interaction and the development of next‐generation photonic devices. Enhancing the speed of luminescence has long been a central objective, with continued relevance in both fundamental and applied contexts. In advanced photonic systems, rapid light emission enables the realization of extremely bright and temporally precise optical signals. This capability is crucial for quantum information technologies, where single‐photon sources with short radiative lifetimes are essential, as well as for classical high‐speed optical communication and sensing platforms. This review highlights recent progress in accelerating the emission process by leveraging cooperative emission effects, resonator‐enhanced coupling, and nonlocal or multipolar interactions, incorporating the authors’ original contributions and perspectives. These advances have led to the realization of novel coherent and incoherent bright light sources, high‐efficiency single‐photon emitters, radiative processes robust against thermal dephasing, and photon generation from materials typically regarded as nonluminescent due to thermal relaxation or self‐trapping. These mechanisms from both theoretical and experimental perspectives are explored, and insights into their integration for future photonic architectures are offered.

The electric field enhancement effect induced by localized surface plasmon resonance (LSPR) plays a critical role in imaging and sensing applications. In particular, nanocube structures with narrow gaps provide large hotspot areas, making them highly promising for high-sensitivity applications. This study predicts the electric field enhancement effect of structures combining silver nanocubes and a 10 nm thick silver thin film using the finite-difference time-domain (FDTD) method. We demonstrate that the interaction between the silver nanocubes and silver thin film allows control over sharp LSPR peaks in the visible wavelength range. Specifically, the structure with a spacer layer between the silver nanocubes and the silver thin film is suitable for multimodal imaging, while the direct contact structure of the silver nanocubes and the silver thin film shows potential as a highly sensitive refractive index sensor. The 10 nm thick silver thin film enables backside illumination due to its transparency in the visible wavelength region, making it compatible with inverted microscopes and allowing for versatile applications, such as living cell imaging and observations in liquid media. These structures are particularly expected to contribute to advancements in bioimaging and biosensing.