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.

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.

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.

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.

Improvement of imaging performance of silicon micropore X-ray optics by ultra long-term annealing.

We have been developing a light-weight X-ray telescope using micro electro mechanical systems technologies for future space missions. Micropores of 20 µm width are formed in a 4-inch Si wafer with deep reactive ion etching, and their sidewalls are used as X-ray reflection mirrors. The flatness of the sidewall is an important factor to determine the imaging performance, angular resolution. It is known that hydrogen annealing is effective to smooth a Si surface. We tested 150 hour annealing to achieve the ultimately smooth sidewalls. After 50 hour, 100 hour, and 150 hour annealing, the angular resolution improved 10.3, 4.0, and 2.6 arcmin in full width at half maximum (FWHM) and 17.0, 14.5, and 10.8 arcmin in half-power width (HPW). In spite of this improvement, the edge shapes of the sidewall were rounded. Therefore, both edges of the sidewall were ground and polished, and then the angular resolution was improved further to 3.2 arcmin (FWHM) and 5.4 arcmin (HPW).

Spectral and angular shaping of infrared radiation in a polymer resonator with molecular vibrational modes.

We present a comprehensive approach for tailoring the spectral and angular properties of infrared thermal radiation by using a polymer resonator with molecular vibrational modes, consisting of a polymer thin film on a back-reflective substrate. To precisely design the resonator, we derived the infrared dielectric function of a poly(methyl methacrylate) (PMMA) thin film from the measured reflectance spectrum by fitting it with a Gaussian-convoluted Drude-Lorentz model while accounting for the inhomogeneous broadening caused by the disordered structure of polymers. Our experimental and numerical characterization confirms that the polymer resonator exhibits spectral shaping from quasi-broadband to narrowband due to the intrinsic molecular vibrational absorption of the polymer. The frequency-isolated and strong molecular vibrational absorption of the carbonyl stretching mode at 1730 cm-1 enables the narrowband shaping of the PMMA resonator. In addition, we confirm that the angular-shaping characteristics of this polymer resonator can be tuned, from omnidirectional to strongly angular selective, by changing its polymer film thickness. Modal dispersion analysis reveals that the angle-selectivity of the polymer resonator at an angle of incidence of 80° comes from coupling between the molecular vibrational mode and leaky mode. The proposed infrared radiation management strategy based on molecular vibrational modes of polymers is cost-effective, scalable, and works well with terrestrial matter, including organic compounds and gas molecules, showing promise for applications such as optical gas sensing and radiative thermal management.

Demonstration of sharp multiple Fano resonances in optical metamaterials.

We experimentally demonstrated multiple Fano resonances in optical metamaterials. By combination of two different sized asymmetric-double-bar (ADB) structures, triple Fano resonance was observed in the near-infrared region. In addition to Fano resonance due to anti-phase modes in isolated ADB structures, an anti-phase mode due to coupling among different sized ADBs was observed. Dependence of characteristics of resonances on size difference was also investigated. At specific conditions of size difference, quality factors of three Fano resonances were improved compared with ADB metamaterials consisting of one kind of ADBs. The results will help to realize applications using metamaterial resonators with multiple functionalities and high performance.

Reflection color filters of the three primary colors with wide viewing angles using common-thickness silicon subwavelength gratings.

We fabricated reflection color filters of the three primary colors with wide viewing angles using silicon two-dimensional subwavelength gratings on the same quartz substrate. The grating periods were 400, 340, and 300 nm for red, green, and blue filters, respectively. All of the color filters had the same grating thickness of 100 nm, which enabled simple fabrication of a color filter array. Reflected colors from the red, green, and blue filters under s-polarized white-light irradiation appeared in the respective colors at incident angles from 0 to 50°. By rigorous coupled-wave analysis, the dimensions of each color filter were designed, and the calculated reflectivity was compared with the measured reflectivity.

Comparison of electromagnetically induced transparency between silver, gold, and aluminum metamaterials at visible wavelengths.

Electromagnetically induced transparency (EIT)-like effects in silver, gold, and aluminum metamaterials consisting of dipole resonators and quadrupole resonators were demonstrated at visible wavelengths. Optical characteristics of the metamaterials could be controlled by the gap distance between the two resonators. EIT-like effects were observed at wavelengths between 603 and 789 nm, 654 and 834 nm, and 462 and 693 nm for the silver, gold, and aluminum EIT metamaterials, respectively. At wavelengths longer than around 650 nm, the silver metamaterials had better EIT-like features. At wavelengths shorter than around 650 nm, on the other hand, the aluminum metamaterials showed promising EIT-like results.

Experimental demonstration of sharp Fano resonance in optical metamaterials composed of asymmetric double bars.

We experimentally demonstrated Fano resonance in metamaterials composed of asymmetric double bars (ADBs) in the optical region. ADB metamaterials were fabricated by a lift-off method, and the optical spectra were measured. Around a wavelength of 1100 nm, measured optical spectra clearly showed sharp Fano resonance due to weak asymmetry of the ADB structures. The highest-quality factor (Q-factor) of the Fano resonance was 7.34. Calculated spectra showed the same tendency as the experimental spectra. Moreover, in a Fano resonant condition, out of phase of induced current flowing along each bar was revealed by electromagnetic field calculations. These antiphase currents decreased radiative loss of the Fano mode, resulting in a high Q-factor of the Fano resonance in ADB metamaterials. As the degree of asymmetry became small, the Q-factor decreased, and the Fano resonance disappeared because the effect of Joule loss became significant.

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.

Effects of expanded adverse childhood experiences including school bullying, childhood poverty, and natural disasters on mental health in adulthood.

The study aimed to examine the association of expanded adverse childhood experiences (ACEs) with psychological distress in adulthood. The data from nation-wide online cohort was used for analysis. Community dwelling adults in Japan were included. The ACEs was assessed by 15 items of ACE-J, including childhood poverty and school bullying. Severe psychological distress was determined as the score of Kessler 6 over 13. Multivariable logistic regression analysis was conducted, by using sample weighting. A total of 28,617 participants were analyzed. About 75% of Japanese people had one or more ACEs. The prevalence of those with ACEs over 4 was 14.7%. Those with ACEs over 4 showed adjusted odds ratio = 8.18 [95% CI 7.14-9.38] for severe psychological distress. The prevalence of childhood poverty was 29% for 50-64 year old participants and 40% of 65 or older participants. The impact of childhood poverty on psychological distress was less than other ACEs in these age cohorts. Bullying was experienced 21-27% in young generations, but 10% in 65 or older participants. However, the impact on psychological distress in adulthood was relatively high in all age groups. ACEs have impacted mental health for a long time. Future research and practice to reduce ACEs are encouraged.

Fabrication of antireflection subwavelength gratings at the tips of optical fibers using UV nanoimprint lithography.

Antireflection (AR) layers at the tips of optical fibers are indispensable in high efficiency and low noise applications. We realized the AR structures with two-dimensional binary subwavelength gratings (SWGs) at the tips of optical fibers by using a dedicated UV nanoimprint machine. Using this technique, ideal AR structures with desired refractive indices can be realized at low cost in principle. The SWG with the period of 700 nm was fabricated at the tip of a single-mode optical fiber for optical communications system. The reflectance was decreased to less than 0.27% at measured wavelengths between 1460 nm and 1580 nm.

Association between adverse childhood experience and unintended pregnancy among Japanese women: a large-scale cross-sectional study.

Unintended pregnancy (UP) can negatively impact the health of mothers, children, and families. While Adverse Childhood Experiences (ACEs) are increasingly known to affect sexual health, the influence on pregnancy intention is not fully understood. This study examines the relationship between ACEs and UP and explores other related factors, using 5049 pregnant and postpartum women data from the Japan COVID-19 and Society Internet Survey (JACSIS). We measured participants' pregnancy intentions, ACEs, family functioning, and social network size. Logistic regression analysis provided odds ratios and 95% confidence intervals (CI). The prevalence of UP was approximately 16.5% (n = 893). Cumulative ACEs were consistently associated with UP, even after adjusting for intermediate variables in adulthood. The odds ratio for UP with a single ACE was 1.00 (CI: 0.82-1.21) but rose significantly with multiple ACEs: 1.39 (CI: 1.10-1.76) with double, 1.38 (CI: 1.02-2.86) with triple, and 1.81 (CI: 1.37-2.39) with more. Additionally, bad family functioning and lack of social networks emerged as contributors to UP. In conclusion, this study showed that ACEs are potentially correlated with UP. A deeper understanding of the transition from childhood experiences to UP is important for health interventions, necessitating further investigation.

Ultra-small silicon waveguide coupler switch using gap-variable mechanism.

Submicron-wide silicon waveguide coupler with gap variable mechanism is proposed for a compact optical waveguide switch. Two freestanding silicon waveguides are placed parallel with a submicron gap. The gap is changed by electrostatic comb-drive micro-actuators to control the coupling coefficient of the coupler. The fabricated device consisted of the silicon waveguides of 400 nm in width and 260 nm in thickness. The total size of the switch was 100 µm wide and 150 µm long. Decreasing the gap between the waveguides to 110 nm, the output intensity at drop port became a maximum while the output intensity at through port became a minimum. The extension ratio of the switch output was 17 dB for the waveguide displacement of 300 nm.

Investigation of strain sensing effect in modified single-defect photonic crystal nanocavity.

This paper reports the theoretical and experimental investigations on the strain sensing effect of a two dimensions (2D) photonic crystal (PhC) nanocavity resonator. By using the finite element method (FEM) and finite difference time domain (FDTD) simulations, the strain sensitivity of a high quality factor PhC nanocavity was calculated. Linear relationships between the applied strain and the shift in the resonant wavelength of the cavity were obtained. A single-defect silicon (Si) PhC cavity was fabricated, and measurements of the strain sensitivity were performed. Good agreement between the experimental and simulation results was observed.

Phase-shifter using submicron silicon waveguide couplers with ultra-small electro-mechanical actuator.

Phase shifter is an important part of optical waveguide circuits as used in interferometer. However, it is not always easy to generate a large phase shift in a small region. Here, a variable phase-shifter operating as delay-line of silicon waveguide was designed and fabricated by silicon micromachining. The proposed phase-shifter consists of a freestanding submicron-wide silicon waveguide with two waveguide couplers and an ultrasmall silicon comb-drive actuator. The position of the freestanding waveguide is moved by the actuator to vary the total optical path. Phase-shift was measured in a Mach-Zehnder interferometer to be 3.0pi at the displacement of 1.0 mum at the voltage of 31 V. The dimension of the fabricated device is 50microm wide and 85microm long.

Large area, freestanding GaN nanocolumn membrane with bottom subwavelength nanostructure.

We propose, fabricate and characterize the freestanding GaN nanocolumn membrane with bottom subwavelength nanostructures. The GaN nanocolumns are epitaxially grown on freestanding nanostructured silicon substrate that is achieved by a combination of self-assemble technique and silicon-on-insulator (SOI) technology. Optical reflection is greatly suppressed in the visible range due to the graded refractive index effect of subwavelength nanostructures. The freestanding GaN nanocolumn membrane is realized by removing silicon substrate from the backside, eliminating the silicon absorption of the emitted light and leading to a strong blue emission from the bottom side. The obtained structures also demonstrate the potential application for anti-reflective (AR) coating and GaN-Si hybrid microelectromechanical system (MEMS).