Keisuke Watanabe has been a researcher at the Photonics Nano-Engineering Group in the Research Center for Materials Nanoarchitectonics (MANA) at NIMS since 2024. He received his PhD from Yokohama National University in 2019. He then moved to the University of Exeter in the UK as a Postdoctoral Research Fellow, before joining NIMS as an ICYS Research Fellow in 2021. His work focuses on nanophotonic devices, optical sensors, and bound states in the continuum.

Conventional thermal light sources exhibit broad spectral characteristics that depend on their Joule heating temperature, resulting in low energy efficiency. Therefore, technologies that selectively utilize the optical energy within the wavelength range of interest are highly desirable. We have previously developed wavelength-selective thermal infrared emitters and infrared sensors by controlling optical resonances through micro/nanofabrication of metals and thermally robust conductive ceramics [1, 2]. However, their wavelength selectivities have been limited to several tens of nanometers due to the unavoidable optical Ohmic losses associated with the Drude response of metals. To overcome this limitation, we propose a new approach that achieves further spectral narrowing by employing quasi-bound states in the continuum (qBICs) in dielectric nanostructures as a wavelength-selective mechanism. Figure 1(a) shows a schematic illustration of the proposed structure. A 1200-nm-thick SiO₂ spacer layer is placed above the NiAl absorbing layer, followed by a 1280-nm-thick polycrystalline silicon (poly-Si) layer on top, where shallow-etched paired-rod arrays [3] are arranged with a periodicity of 2500 nm. Figure 1(b) shows the absorption (emission) spectrum obtained by finite-difference time-domain simulation for a structural asymmetry parameter of α = 15%. An extremely sharp absorption peak with a linewidth of approximately 2 nm is obtained around the design wavelength of 4.5 μm while suppressing the broadband background radiation.

1. T. D. Dao et al., ACS Photonics 2(7), 964 (2015). DOI: 10.1021/acsphotonics.5b00195
2. T. D. Dao et al., Advanced Science 6(20), 1900579 (2019). DOI: 10.1002/advs.201900579
3. K. Watanabe et al., Nano. Lett. 25, 2777 (2025). DOI: 10.1021/acs.nanolett.4c05880