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We demonstrate highly sensitive detection of viruses using terahertz split-ring resonators with various capacitive gap widths. Two types of viruses, with sizes ranging from 60 nm (PRD1) to 30 nm (MS2), were detected at low densities on the metamaterial surface. The dielectric constants of the virus layers in the THz frequency range were first measured using thick films, and the large values found identified them as efficient target substances for dielectric sensing. We observed the resonance-frequency shift of the THz metamaterial following deposition of the viruses on the surface at low-density. The resonance shift was higher for the MS2 virus, which has a relatively large dielectric constant. The frequency shift increases with surface density until saturation and the sensitivity is then obtained from the initial slope. Significantly, the sensitivity increases by about 13 times as the gap width in the metamaterials is decreased from 3 µm to 200 nm. This results from a combination of size-related factors, leading to field enhancement accompanying strong field localization.

In this study, we developed hybrid humidity sensing methods by incorporating silk fibroin protein onto metamaterials, operating in the terahertz (THz) frequencies; the resonant frequency shifted but saturated at a specific thickness due to the limited sensing volume of the metamaterial. From the saturated value, we extracted the dielectric constant for the silk films. We also observed additional resonance shifts when we applied humid air to silk-coated metamaterials, due to the increased water molecule numbers on the film. Frequency shifts depend linearly on relative humidity. Also, in situ THz spectroscopy measurements reveal that the time response is instantaneous within our detection limit, especially upon exposure to humid air, whereas the small slowly decaying component appeared when we applied dry air. The time taken by the slow component in the drying process was 10-50 s, depending on film thickness. This could optimize humidity sensors as a fast and efficient detection tool to measure air humidity.

We present terahertz (THz) wave modulation by using free-standing ZnO nanowire (NW) network films. The ZnO NW films were virtually transparent against THz waves without UV illumination. Conversely, the THz waves were attenuated under very low-intensity UV illumination, making the ZnO NW film a promising platform for low-loss, low-power and all-optical THz modulators. The complex dielectric constants measurements reveal that the UV laser induces an enhancement in the ac conductivity while leaving the real part of the dielectric constant unchanged. The relatively slow time response implies that the UV-induced modulation is closely linked to the surface trap states. The THz attenuations showed clear saturation behavior with respect to the UV intensity, from which we extracted the ZnO NW surface trap density. (C) 2016 Optical Society of America

The shape dependence of target materials on the sensitivity of terahertz metamaterial sensors was investigated. Polystyrene microbeads with a known dielectric constant and spherical, ovular, lens-shaped, and star-shaped structures were studied. The resonant frequency showed a clear red-shift after the deposition of low-density microbeads owing to the change in the dielectric environment in the gap area of the metamaterials. Results of simulations based on a finite-difference time-domain (FDTD) method, in which the known dielectric constant of a polystyrene sphere was used, showed excellent agreement with experimental results. The shift in the resonant frequency increased linearly with the surface density, saturating at 60-80 GHz when the gap area was full of microbeads. More importantly, the resonant frequency shift was higher for non-spherical microbeads, such as the star-shaped microbeads. Therefore, the shape of the individual target material was a crucial factor in determining metamaterial sensor sensitivity. (C) 2015 Optical Society of America.

We demonstrated sensitive detection of individual yeast cells and yeast films by using slot antenna arrays operating in the terahertz frequency range. Microorganisms located at the slot area cause a shift in the resonant frequency of the THz transmission. The shift was investigated as a function of the surface number density for a set of devices fabricated on different substrates. In particular, sensors fabricated on a substrate with relatively low permittivity demonstrate higher sensitivity. The frequency shift decreases with increasing slot antenna width for a fixed coverage of yeast film, indicating a field enhancement effect. Furthermore, the vertical range of the effective sensing volume has been studied by varying the thickness of the yeast film. The resonant frequency shift saturates at 3.5 mu m for a slot width of 2 mu m. In addition, the results of finite-difference time-domain simulations are in good agreement with our experimental data. (C) 2014 Optical Society of America

We report huge field accumulations in rectangular aperture arrays on thin metal film by using shape resonance in THz frequency region. A huge far-field transmission enhancement is observed in samples of various widths ranging from 10 mum to 1.8 mum which correspond to only an order of lambda/100. Theoretical calculations based on vector diffraction theory indicates 230 times near-field enhancement in case of the 1.8 mum wide rectangular aperture. Transmission measurement through the single rectangular aperture shows that the shape resonance, not the periodicity, is mainly responsible for the transmission enhancement and the corresponding field enhancement.

We have performed femtosecond and picosecond nondegenerate two-beam and three-beam (ω1, ω1;ω2) four-wave mixing experiments by using two partially synchronized, independently tunable lasers with a simple sample-and-hold switch. When the jitter is compensated for externally, heavy- and light-hole beating is observed with these two mutually incoherent lasers, and the synchronous phases of oscillations with the detuning show that these beats are true quantum beats. We also see the strong completely off-resonant signals, and the below-bandgap signals are stronger than the signals at the continuum for equivalent detunings. This off-resonance signal has a Lorentzian dependence on detuning in both the femtosecond and the picosecond experiments. When ω2 is completely above or below the excitonic resonance, the spectrum of the diffracted beam is determined mostly by the laser spectrum, which can be explained by semiconductor Bloch equations.