arXiv subjects

Tadaaki Nagao

Publications and source records attributed to Tadaaki Nagao.

8 recordsLinked to original sources

arXiv2026

Single-nanoparticle detection using quasi-bound states in the continuum supported by silicon metasurfaces

The detection of single particles or molecules represents a critical milestone in the development of biosensing technologies. Recently developed optical sensors based on quasi-bound states in the continuum (qBICs) have primarily focused on detecting global refractive index changes, aiming to simultaneously enhance both refractive index sensitivity and quality ($Q$) factors. However, sensors capable of resolving local refractive index perturbations, such as the binding of a nanometer-sized molecule on a surface, remain elusive and have not yet been demonstrated in BIC geometries due to the limited $Q$ factors and relatively large mode volumes. Here, we demonstrate low-contrast BIC metasurfaces that can perform sensing with a virus-sized single-nanoparticle resolution. The qBIC resonance operating at the critical coupling condition exhibits an experimental $Q$ factor of 4.5 x 10$^4$ in heavy water. The strong interaction between the localized electric field and polystyrene nanoparticles with a diameter of 100 nm enable the experimental observation of step-like resonance wavelength shifts, serving as signatures of individual particle binding events. Furthermore, binding-induced modifications to the qBIC resonance alter the optical confinement and asymmetry factor, inducing changes not only in the resonance wavelength but also in the linewidth and amplitude with single-particle sensitivity. Combined with position-insensitive response and free-space accessible features, low-contrast BIC metasurfaces provide a user-friendly platform for next-generation single-molecule sensing integrated with microfluidic systems.

physics.optics↗

arXiv2024

Low-Contrast BIC Metasurfaces with Quality Factors Exceeding 100,000

Dielectric metasurfaces operating at quasi-bound states in the continuum (qBICs) can achieve exceptionally high radiative quality ($\textit{Q}$) factors by introducing small asymmetries into their unit cells. However, fabrication imperfections often impose major limitations on the experimentally observed $\textit{Q}$ factors. In this study, we experimentally demonstrate BIC metasurfaces with a record-high $\textit{Q}$ factor of 101,486 under normal excitation of light in the telecom wavelength range achieved by employing low-contrast silicon pairs. Our findings show that such ultrahigh-$\textit{Q}$ factors can be attained by leveraging both the high radiative $\textit{Q}$ factors of higher-order qBIC modes and reduced scattering losses in shallow-etched designs. Additionally, we demonstrate stable sub-picometer-level wavelength fluctuations in water, with a limit of detection of $10^{-5}$ for environmental refractive index changes. The proposed approach can be extended to BIC metasurfaces with many other configurations and operating wavelengths for ultrahigh-$\textit{Q}$ applications in both fundamental physics and advanced devices.

physics.optics↗

arXiv2023

Vibrational coupling to quasi-bound states in the continuum under tailored coupling conditions

Photonic resonance modes can be spectrally coupled to the vibrational modes of molecules in the mid-infrared regime through interactions between localized electric fields and nearby molecules. According to recent studies, radiative loss engineering of coupled systems is a promising approach for tailoring coupling conditions and enhancing the molecular signals. However, this strategy has only been realized using the localized surface plasmon resonances of metal nanostructures, which suffer from increased ohmic loss in the mid-infrared region and face serious limitations in achieving high quality (Q) factors. In this study, we adopt silicon-based metasurfaces formed on silicon-on-insulator wafers to achieve high Q factors and tune the coupling conditions between the quasi-bound states in the continuum (qBICs) and molecular vibrations. The coupling between the resonance mode and polymethyl methacrylate molecules is tailored from weak to strong coupling regimes by simply changing the structural asymmetry parameter and utilizing the intrinsically high Q factors of the qBIC modes. In addition, we identify the optimal asymmetry parameter that maximizes the enhanced molecular signal, opening a route toward realizing highly sensitive surface-enhanced infrared spectroscopy using complementary metal-oxide semiconductor compatible all-dielectric materials.

physics.optics↗

arXiv2019

An On-Chip Quad-Wavelength Pyroelectric Sensor for Spectroscopic Infrared Sensing

Merging photonic structures and optoelectronic sensors into a single chip may yield a sensor-on-chip spectroscopic device that can measure the spectrum of matters. In this work, we propose and realize an on-chip concurrent multi-wavelength infrared (IR) sensor. The fabricated quad-wavelength IR sensors exhibit four different narrowband spectral responses at normal incidence following the pre-designed resonances in the mid-wavelength infrared region that corresponds to the atmospheric window. The device can be applied for practical spectroscopic applications such as non-dispersive IR sensors, IR chemical imaging devices, pyrometers, and spectroscopic thermography imaging.

physics.app-ph↗

arXiv2019

Terahertz Faraday and Kerr rotation spectroscopy of Bi$_{1-x}$Sb$_x$ films in high magnetic fields up to 30 Tesla

We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films of $\text{Bi}_{1-x}\text{Sb}_{x}$, an alloy system that exhibits a semimetal-to-topological-insulator transition as the Sb composition $x$ increases. By using a single-shot time-domain terahertz spectroscopy setup combined with a table-top pulsed mini-coil magnet, we conducted measurements in magnetic fields up to 30~T, observing distinctly different behaviors between semimetallic ($x < 0.07$) and topological insulator ($x > 0.07$) samples. Faraday and Kerr rotation spectra for the semimetallic films showed a pronounced dip that blue-shifted with the magnetic field, whereas spectra for the topological insulator films were positive and featureless, increasing in amplitude with increasing magnetic field and eventually saturating at high fields ($>$20~T). Ellipticity spectra for the semimetallic films showed resonances, whereas the topological insulator films showed no detectable ellipticity. To explain these observations, we developed a theoretical model based on realistic band parameters and the Kubo formula for calculating the optical conductivity of Landau-quantized charge carriers. Our calculations quantitatively reproduced all experimental features, establishing that the Faraday and Kerr signals in the semimetallic films predominantly arise from bulk hole cyclotron resonances while the signals in the topological insulator films represent combined effects of surface carriers originating from multiple electron and hole pockets. These results demonstrate that the use of high magnetic fields in terahertz magnetopolarimetry, combined with detailed electronic structure and conductivity calculations, allows us to unambiguously identify and quantitatively determine unique contributions from different species of carriers of topological and nontopological nature in Bi$_{1-x}$Sb$_x$.

cond-mat.mes-hall↗

arXiv2019

Dual-band in situ molecular spectroscopy using single-sized Al-disk perfect absorbers

In this manuscript, we have demonstrated that the antenna-enhanced infrared vibrational spectroscopy can be a powerful platform for high-sensitivity detection of ultrathin layer of polymer molecules and in situ monitoring of their crosslinking reaction as well as their kinetics of network formation. Our precisely designed dual-band perfect absorber (DPA) made of singles-sized resonator array shows strongly-enhanced nearfield intensity and polarization independence, providing a powerful antenna platform for the bond-selective vibrational sensing.

physics.app-ph↗

arXiv2015

Examining the performance of refractory conductive ceramics as plasmonic materials: a theoretical approach

The main aim of this study is to scrutinize promising plasmonic materials by understanding their electronic structure and correlating them to the optical properties of selected refractory materials. For this purpose, the electronic and optical properties of the conductive ceramics TiC, ZrC, HfC, TaC, WC, TiN, ZrN, HfN, TaN, and WN are studied systematically by means of first-principles density functional theory. A full ab initio procedure to calculate plasma frequency from the electronic band structure is discussed. The dielectric functions are calculated by including electronic interband and intraband transitions. Our calculations confirm that transition metal nitrides, such as TiN, ZrN, and HfN, are the strongest candidates, exhibiting performance comparable to that of conventional noble metals in the visible to the near-infrared regions. On the other hand, carbides are not suitable for plasmonic applications because they show very large losses in the same regions. From our calculated dielectric functions, the scattering and absorption efficiencies of nanoparticles made of these refractory materials are evaluated. It is revealed that TiN and TaC are the best candidate materials for applications in photothermal energy conversion over a broad spectral region. Furthermore, quality factors for localized surface plasmon resonance and surface plasmon polaritons are calculated to compare quantitative performances, and ZrN and HfN are found to be comparable to conventional plasmonic metals such as silver and gold

physics.comp-ph↗

arXiv2015

Band engineering of ternary metal nitride system Ti1-x ZrxN for plasmonic applications

Chemical composition is the primary factor that determines the electronic band structure and thus also influences the optical properties of plasmonic ceramics including nitrides and oxides. In this work, the optical and plasmonic properties of TiN, ZrN and their hypothetical intermediate alloys Ti1-xZrxN (x= 0, 0.25, 0.50, 0.75, and 1), are studied by using first-principles density functional theory. We demonstrate the effects of electronic band structure tuning (band engineering) on the dielectric properties by varying the concentration of metallic constituents. Our calculations reveal that bulk plasma frequency, onset of interband transitions, width of bulk plasmon resonance and cross-over frequency, can be tuned flexibly in visible spectrum region by varying the amount of Zr concentration in Ti1-xZrxN alloy system. We found that low threshold interband energy onset (~1.95 eV) leads to high losses in Ti rich compounds than that of ZrN which points to lower losses.

physics.comp-ph↗