Search

This paper reports an actuation method of a MEMS spiral metamaterial designed for THz (terahertz) electromagnetic frequency. Two-dimensional micro spirals with a diameter of $150\ \mu\mathrm{m}$ transform into three-dimensional micro spirals by the center of each micro spiral being pulled to the out-of-plane direction individually (Fig. 1). At the center, each micro spiral, arrayed on a silicon substrate, have an SU-8 bonding to another Parylene coated silicon substrate, and the micro spirals were stretched by separating the two substrates. It was confirmed that micro spirals were stably stretched to $536\ \mu\mathrm{m}$ at the maximum. The transmission characteristics obtained with THz-TDS (time domain spectroscopy) presented a large increase of polarization effect by the increase of micro spiral height.

We study the angular and frequency-angulardistributions of the low-frequency part (0.3–3 THz) of the terahertz emission from two-color femtosecond plasma spark experimentally and in 3D + time numerical simulations. Our experiments and simulations show, that the weakening in the focusing conditions results in the squeezing of the THz angular distribution and the formation of the bright conical emission.

We propose a tunable metamaterial actuated by pneumatic force. The unit of the proposed metamaterial has a pair of sprit-ring-resonators (SRR), and the gap between them is controllable around sub-micron-order in size. One SRR is formed on a thin cantilever, which can be bent due to differential pressure between the upper and lower surfaces of the cantilever, whereas the other is fixed on a thin membrane. We confirmed that controlling the gap ranging from sub-micron-order to a few-micron-order was suitable for tuning a resonant frequency of a terahertz metamaterial.

Summary form only given. Terahertz (THz) imaging technology attracts a lot of attentions for security and biochemical applications because many biochemical molecules have characteristic absorption spectra in the THz frequency region. THz camera, which can take real-time THz images is one of the most important detection devices for the development of THz imaging technology. NEC has developed a THz camera called IRV-T0830 consisting of uncooled microbolometer THz focal plane arrays (THz-FPAs) [1]. Although the operation of real-time imaging system with this camera was demonstrated recently [2], improvement of performance is still required. In this study, we have improved structure of the THz-FPAs to increase the sensitivity in the THz frequency region, and evaluated frequency-dependent sensitivity of the improved THz camera systematically.IRV-T0830 detects THz waves by sensing resistance change of the microbolometers. The resistance change is originated from the temperature increase by the absorption of incident THz waves. In addition, reflection layer is formed at the bottom of air gap located under the microbolometer layer in order to increase the THz absorption efficiency. Optical cavity is formed between reflection layer and THz absorption layer and it was found that the thickness of air gap is closely related to sensitivity of the THz camera [1]. Here, we have developed a new THz camera which air-gap thickness is near to wave length of THz frequency region to increase the sensitivity. THz beam used in sensitivity-measurement experiment was generated by tilted-pulse-front excitation of lithium niobate (LN) crystal. Spectrum of generated THz beam was obtained by electro-optic (EO) sampling method. The spectrum extended to 2 THz. We have measured frequency dependence of sensitivity of THz camera by putting THz band-pass filters in front of THz camera, and measured incident polarization direction angle dependence by rotating THz camera. It was found that sensitivities of both the new THz camera and IRV-T0830 depend on polarization of incident THz beam. Therefore, we estimated frequency dependence of sensitivities by averaging count numbers of each THz camera throughout all polarization angles. Frequency dependence of sensitivity for each THz camera is shown in Fig.1. The sensitivity of the new THz camera is three or four times larger than IRV-T0830 in frequency range of 1-2 THz. THz beam-spot images obtained by two cameras are shown in Fig.2. A band-pass filter which has transmission peak at 1.0 THz and full width at half maximum of 0.24 THz was used, and the power of incident THz beam penetrating the band-pass filter is about 100 μW. The image obtained by the new THz camera is much clearer than IRV-T0830. These results indicate that the structural modification of FPA successfully improved the performance of the THz camera, which should become a critical tool for the development of THz imaging technology.

We propose an electrostatically actuated spiral structures as a metamaterial for circularly polarized light in Terahertz (THz) frequency. Actuation of the spiral structures changes their geometry, and produces a tunable chirality resulting in circular dichroism, which is the differential absorption between the left and right circularly polarized light. Results of the THz-spectroscopy confirmed that the ON/OFF of circular dichroism was achieved with the proposing structures.

We proposed and demonstrated active control of terahertz optical activity via chiral patterned photo-excitation in a semiconductor with a spatial light modulator. This technique provides a new route to terahertz polarization modulators.

We demonstrated a new method of Terahertz cylindrical vector beam generation using segmented nonlinear crystals with three-fold rotational symmetry. The polarization dependent in-plane images obtained with a Terahertz camera clearly showed axial-symmetric polarization distributions. The electro-optic sampling image clearly visualized the longitudinal electric field at the focal point.

Recently, ultrafast control of magnetization via instantaneous stimulated Raman-type nonlinear optical processes has gained considerable interest. Here, we discuss that the conservation of angular momentum rules the light induced magnetization [1]. In light-magnetization interactions in solids, the balance of angular momentum included contributions from the light, the magnetization, and the crystalline lattice. In this presentation, we high-light the role of the lattice, in that it provides angular momentum in discrete units in correspondence to its rotational symmetry, which is the rotational analogue of the Umklapp process [2].

We demonstrate the enhancement of optical activity in the terahertz frequency region with metal chiral gratings by photo-excitation of the semiconductor substrate. This result indicates the possibility of the active control of the terahertz polarization.