Taiyu Okatani’s research while affiliated with Tohoku University and other places

Terahertz (THz) metamaterials have garnered attention for their unique electromagnetic properties and potential applications in biomaterial sensing, offering label-free, non-contact capabilities. However, their performance is limited by their diffraction limits, impeding the detection of small objects. This study presents an approach for fabricating three-dimensional (3D) THz metamaterials with vertically oriented, high-aspect-ratio multinanogaps. These 3D metamaterials comprise laminated cross-shaped metal layers, which induce electromagnetic resonance, and polymer layers that support the metal layers at the center of the cross shape. Air nanogaps formed between metal layers achieved aspect ratios of > 64. Conventional microfabrication techniques were employed, including spin coating, metal sputtering deposition, photolithography, ion milling etching, and oxygen plasma etching, without relying on electron beam lithography. Spectroscopic analyses of the fabricated metamaterials revealed that the multilayered structure exhibited a deeper dip than single-layered and double-layered configurations. To validate THz sensing, we used isopropyl alcohol (IPA) for THz spectroscopy applications; the spectra indicated significant peak frequency shifts in the transmission dip of the fabricated device with and without IPA. These findings highlight the application scope of the as-prepared 3D THz metamaterial in material sensing techniques, enhancing THz-metamaterial-based device performance.

Terahertz (THz) spectroscopy offers a non-contact analysis of biological samples; however, it faces challenges due to water absorption within the THz range. To address this issue, a THz metamaterial sensor was developed for high-sensitivity moisture measurements. The sensor is composed of a subwavelength metal layer, porous insulator, and metal layer, with its principle relying on resonant frequency shifts derived from changes in capacitance with and without the target gas within the porosity. The sensor experimentally demonstrated a significant frequency shift in response to humidity compared to the simulation results, indicating enhanced sensitivity because of the porous structure. This novel approach has the potential for practical analysis using THz waves.

We fabricate a device that configures the tunable air gap Fabry‒Pérot filter with a static gradient thickness filter to estimate air gap dimension in tunable filter. The chip-level microelectromechanical integration of gradient thickness filter can serve the purpose of device miniaturization while performing air gap sensing in visible wavelength range.

A 3D bulk metamaterial (MM) containing amorphous multilayered split‐ring resonators is proposed, fabricated, and evaluated. Experimentally, the effective refractive index is engineered via the 3D bulk MM, with a contrast of 0.118 across the frequency span from 0.315 to 0.366 THz and the index changing at a slope of 2.314 per THz within this frequency range. Additionally, the 3D bulk MM exhibits optical isotropy with respect to polarization. Moreover, the peak transmission and optical dispersion are tailored by adjusting the density of the split‐ring resonators. Compared to reported conventional approaches for constructing bulk MMs, this approach offers advantages in terms of the potential for large‐scale manufacturing, the ability to adopt any shape, optical isotropy, and rapid optical dispersion. These features hold promise for dispersive optical devices operating at THz frequencies, such as high‐dispersive prisms for high‐resolution spectroscopy.

We fabricate a microelectromechanical systems (MEMS)-based device configuring the tunable air gap Fabry–Pérot filter (FPF) with a static gradient thickness filter on the same platform. The proposed double filter configuration offers a wavelength calibration approach that accurately estimates the air gap dimension in the tunable air gap FPF. The wavelength calibration is performed by utilizing the spectrally-selective and spatially-resolved transmission characteristics of the tunable air gap FPF and the static gradient thickness filter, respectively. The MEMS-compatible chip-level integration of the static gradient thickness filter facilitates device miniaturization to enable its use in handheld devices.

We demonstrate spoof surface plasmon coupling of a terahertz wave propagating in free space into a planar silicon waveguide through a bull's eye structure with a subwavelength aperture. Spoof surface plasmon polaritons induced by the bull's eye structure on the backside of the substrate propagate to the frontside through the aperture and couple into the waveguide. Electromagnetic field simulations revealed that the spoof surface plasmon polaritons propagating to the frontside show directivity along the incident polarization direction, and that the phase can be controlled by placing a Y-branched silicon waveguide beside the aperture. A prototype device was fabricated by bonding a copper-plated substrate with a bull's eye aperture and a waveguide fabricated by silicon micromachining. A monochromatic wave of 0.42–0.49 THz from a backward terahertz-wave parametric oscillator was injected onto the bull's eye structure, and the intensity of the emitted wave from the end of the waveguide was measured. Directional coupling into the waveguide was confirmed from the intensity change depending on the incident polarization direction when using a straight waveguide. In addition, the phase difference between the two ends of the Y-branched waveguide was confirmed from the intensity change showing constructive or destructive interference depending on the polarization direction. These results indicate that it is possible to couple an incident wave into a planar waveguide perpendicular to it with controlling the phase via spoof surface plasmon coupling, suggesting its applicability to new experimental and practical systems in the terahertz band such as Beyond 5G/6G communications.

We numerically and experimentally developed a cantilever that provided both fast and analog actuation for THz metamaterials (MMs) by properly geometrizing a dimpled tip. Owing to its small size and light mass, the cantilever had a high mechanical resonance at 705 kHz. Cantilever arrays were fabricated with different tip gaps and integrated into a ladder-shaped MM (LS-MM). By changing the tip gap from 0.80 to 0.32 μm, the resonance of the transmittance spectrum changed from 1.235 to 0.795 THz, indicating that the reconfigurable LS-MM was capable of continuously tuning the resonance of the THz wave transmission with the tip gap. Additionally, the dimple served as an anti-stiction structure, providing the cantilever with a fabrication yield of 99.8%. This work shows a practical pathway to high-performance active metamaterials, which holds potential in advanced THz technologies such as 6G communications and fast imaging.

A miniature low-cost pixelated gradient thickness optical filter is proposed to achieve spectroscopy in the visible wavelength range. The optical filter consists of a two-dimensional array of metal-dielectric-metal thin films arranged in Fabry–Pérot filter configurations with discretely varying cavity thicknesses. The wavelength-selective characterization of each filter is performed by measuring the transmittance over the visible wavelength range. The pixelated gradient thickness filter is equipped with a CMOS image sensor, and its performance as a spectroscopic module is evaluated by illuminating different monochromatic wavelengths on it. The target spectra are successfully reconstructed from the output signals recorded in the sensor from the respective pixelated gradient thickness filters. The technological competence of the proposed filter will enable its use in handheld devices to widen its application range in day-to-day life.