3D Bulk Metamaterials with Engineered Optical Dispersion at Terahertz Frequencies Utilizing Amorphous Multilayered Split-Ring Resonators.

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.

Pixelated gradient thickness optical filter for visible light spectroscopy.

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.

Terahertz stretchable metamaterials with deformable dolmen resonators for uniaxial strain measurement.

In this study, we propose a terahertz stretchable metamaterial that can measure uniaxial strain. Gold dolmen resonators formed on a sheet of polydimethylsiloxane (PDMS) is deformed by strain, and its resonance peak exhibits the gradual decrease in reflectance without a frequency shift, which is suitable for imaging applications at a single frequency. The metamaterial was designed by mechanical and electromagnetic simulations and fabricated by microfabrication including a transfer process of gold structures from a glass substrate to a PDMS sheet. By measuring the reflectance and observing the deformation under different strains, the reflectance decrease was obtained at 0.292 THz despite the appearance of wrinkles on gold structures. Linear response and repeatability up to 20% strain were also confirmed. Furthermore, the strain measurement through a sheet of paper was demonstrated, suggesting that our method can be applied even in situations where opaque obstacles in the visible region exist.

Tunable Fabry-Perot interferometer operated in the terahertz range based on an effective refractive index control using pitch-variable subwavelength gratings.

We constructed a tunable Fabry-Perot interferometer (FPI) by controlling the effective refractive index of pitch-variable subwavelength gratings (PV-SWGs) that were incorporated into an FP cavity. The period of the PV-SWG can be varied to change the effective refractive index and shift the optical resonant frequency of the FPI. Compared with conventional methods that tune the optical resonance by adding fillers or deforming the cavity, the proposed FPI obtained a higher transmission and quality factor (Q-factor) for the transmittance peak, and its resonant frequency can be shifted by simply stretching the PV-SWG. A peak transmittance of 0.87, a Q-factor of 34, and a frequency shift of 17 GHz were obtained by the PV-SWG-based FPI for THz incomes around the frequency of 0.303 THz. As the effective refractive index and the working frequency can be tailored by altering the geometry design of the PV-SWG, the FPI holds significance for the development of THz communications and for applications at different wave bands.

Fabrication of functional metamaterials for applications in heat-shielding windows and 6G communications.

Windows with passive multilayer coatings can allow less energy to be used when maintaining comfortable indoor temperatures. As a type of effective solar energy management, these coatings can prevent the generation of excessive heat inside buildings or vehicles by reflecting near-infrared solar radiation (750-2000 nm) while retaining visible light transmission (400-750 nm) over a large range of viewing angles. To prevent overheating, they must also reflect rather than absorb near-infrared radiation. A transparent heat-shielding window is numerically and experimentally demonstrated in this study. High visual transparency (77.2%), near-infrared reflectance (86.1%), and low infrared absorption (<20%) over a wide range of oblique incident angles were achieved using nanometer-scale cross-shaped metamaterials manufactured by electron beam lithography. Furthermore, high terahertz transmittance (up to 82%) was also achieved for 6G communication system applications.

Reconfigurable THz metamaterial based on microelectromechanical cantilever switches with a dimpled tip.

We numerically and experimentally proposed a reconfigurable THz metamaterial (MM) by employing microelectromechanical cantilevers into a ladder-shaped MM (LS-MM). A fixed-free cantilever array with a dimpled tip behaved as Ohmic switches to reshape the LS-MM so as to actively regular the transmission response of THz waves. The cantilever tip was designed to be a concave dimple to improve the operational life without sacrificing the mechanical resonant frequency (fmr), and a fmr of 635 kHz was demonstrated. The device actively achieved a 115-GHz change in transmittance resonant frequency and a 1.82-rad difference in transmission phase shift, which can practically benefit advancing THz applications such as fast THz imaging and 6 G communications.

Micro-fabricated Si subwavelength grating for frequency-domain THz beam steering covering the 0.3-0.5 THz frequency band.

We designed and fabricated beam steering subwavelength grating (BS-SWG) with high efficiency, wide angles, and broadband beam steering in the terahertz (THz) range. Beam steering technology in the THz range by a fixed structure and frequency sweep has to date lacked a device combining high efficiency and a wide beam steering angle. A subwavelength structure using float zone Si, a low-loss dielectric, could combine both of these aspects, but no experimental demonstration in the THz range has been performed to our knowledge. The BS-SWG was designed with an efficiency of 0.708 at 0.4 THz and beam steering angles of -72.1°--34.8° by sweeping the incident frequency from 0.3 THz to 0.5 THz including the Beyond 5 G/6 G communication bands. An efficiency of 0.354 at 0.400 THz and beam steering angles of -74°--34° were experimentally achieved, demonstrating the potential of high-efficiency, wide-angle beam steering for THz communications, imaging, and radar applications.

Surface-plasmon-coupled optical force sensors based on metal-insulator-metal metamaterials with movable air gap.

We proposed surface-plasmon-coupled optical force sensors based on metal-insulator-metal (MIM) metamaterials with a movable air gap as an insulator layer. The MIM metamaterial was composed of an air gap sandwiched by a metal nanodot array and a metal diaphragm, the resonant wavelength of which was red-shifted when the air gap was narrowed by applying a normal force. We designed and fabricated a prototype of the proposed sensor and confirmed that the MIM metamaterial could be used as a force sensor with larger sensitivity than a force sensor based on Fabry-Pérot interferometer (FPI).

A Tactile Sensor Using Piezoresistive Beams for Detection of the Coefficient of Static Friction.

This paper reports on a tactile sensor using piezoresistive beams for detection of the coefficient of static friction merely by pressing the sensor against an object. The sensor chip is composed of three pairs of piezoresistive beams arranged in parallel and embedded in an elastomer; this sensor is able to measure the vertical and lateral strains of the elastomer. The coefficient of static friction is estimated from the ratio of the fractional resistance changes corresponding to the sensing elements of vertical and lateral strains when the sensor is in contact with an object surface. We applied a normal force on the sensor surface through objects with coefficients of static friction ranging from 0.2 to 1.1. The fractional resistance changes corresponding to vertical and lateral strains were proportional to the applied force. Furthermore, the relationship between these responses changed according to the coefficients of static friction. The experimental result indicated the proposed sensor could determine the coefficient of static friction before a global slip occurs.