Akihiro Isozaki

Enantiomeric switching of chiral metamaterial for terahertz polarization modulation employing vertically deformable MEMS spirals

Active modulation of the polarization states of terahertz light is indispensable for polarization-sensitive spectroscopy, having important applications such as non-contact Hall measurements, vibrational circular dichroism measurements and anisotropy imaging. In the terahertz region, the lack of a polarization modulator similar to a photoelastic modulator in the visible range hampers expansion of such spectroscopy. A terahertz chiral metamaterial has a huge optical activity unavailable in nature; nevertheless, its modulation is still challenging. Here we demonstrate a handedness-switchable chiral metamaterial for polarization modulation employing vertically deformable Micro Electro Mechanical Systems. Vertical deformation of a planar spiral by a pneumatic force creates a three-dimensional spiral. Enantiomeric switching is realized by selecting the deformation direction, where the polarity of the optical activity is altered while maintaining the spectral shape. A polarization rotation as high as 28° is experimentally observed, thus providing a practical and compact polarization modulator for the terahertz range.

Spiral metamaterial for active tuning of optical activity

We propose an electrostatically actuated spiral structure as a metamaterial for circularly polarized light in the terahertz (THz) frequency range. An array of planar spiral structures was fabricated with micro electro mechanical system technology, and the geometry of the structures can be changed by electrostatic actuation. The actuation transforms the planar spirals into three-dimensional helices, resulting in optical activity in which the differential polarization response of the material depends on whether the incident light is left- or right-circularly polarized. THz spectroscopy confirmed that the optical activity can be tuned with the proposed structures.

Out-of-plane actuation with a sub-micron initial gap for reconfigurable terahertz micro-electro-mechanical systems metamaterials

We propose a reconfigurable terahertz (THz) metamaterial that can control the transmittance by out-of-plane actuation with changing the sub-micron gap distance between electrically coupled metamaterial elements. By using the out-of-plane actuation, it was possible to avoid contact between the coupled metamaterial elements across the small initial gap during the adjustment of the gap size. THz spectroscopy was performed during actuation, and the transmission dip frequency was confirmed to be tunable from 0.82 to 0.92 THz for one linear polarization state and from 0.80 to 0.91 THz for the other linear polarization; the two polarizations were orthogonal. The proposed approach will contribute to the development of tunable metamaterials based on structural deformations.

Nanocrystalline porous silicon ultrasonic transmitter with patterned emission area

We propose a nanocrystalline porous silicon (nc-PS) ultrasonic transmitter with a variable directional pattern. The directional pattern is realized by using an interference effect. An ultrasonic wave is emitted from electrodes on the nc-PS layer according to the applied electrical current. The emission areas of the transmitter were patterned in order to use the interference between the emitted ultrasonic waves. To observe the interference effect, our device was designed to have two emission areas. The directional pattern induced by the interference effect depends on the distance between the emission areas, and the frequency and the phase difference of the currents applied to the emission areas. We confirmed that the sharpness and the lobe number of the directional pattern can be changed by the phase difference between two emission areas. We also confirmed that the direction of the main lobe can be changed by the phase difference. With the characteristics of the designable and controllable directional pattern, the proposed device is suitable for distance sensors, especially for proximity sensors.

Optofluidic time-stretch quantitative phase microscopy

Innovations in optical microscopy have opened new windows onto scientific research, industrial quality control, and medical practice over the last few decades. One of such innovations is optofluidic time-stretch quantitative phase microscopy - an emerging method for high-throughput quantitative phase imaging that builds on the interference between temporally stretched signal and reference pulses by using dispersive properties of light in both spatial and temporal domains in an interferometric configuration on a microfluidic platform. It achieves the continuous acquisition of both intensity and phase images with a high throughput of more than 10,000 particles or cells per second by overcoming speed limitations that exist in conventional quantitative phase imaging methods. Applications enabled by such capabilities are versatile and include characterization of cancer cells and microalgal cultures. In this paper, we review the principles and applications of optofluidic time-stretch quantitative phase microscopy and discuss its future perspective.

Parallel Helmholtz resonators for a planar acoustic notch filter

This paper reports on an acoustic planar notch filter with a sub-wavelength thickness at the notch frequency. The developed notch filter consists of a number of spherical Helmholtz resonators (HRs) connected to a hole created in a plate. The HRs were placed at the in-plane vertices of a regular polygon. A simulated pressure distribution revealed that this uniform arrangement of HRs improves the silencing effect because the uniform applied waves emitted from the HRs act as canceling waves to the cross-section of the short hole (in this case, the length of the hole is sub-wavelength). The total pressure emitted from the HRs is equal regardless of the number of HRs connected to the hole. Therefore, the arrangement of HRs is essential for realizing a planar notch filter. Simulated transmittance spectra showed that the depth of the dip in the transmittance increased with the number of uniformly arranged HRs. We confirmed that the experimental transmittance spectra of fabricated notch filters, which consisted of between one and six HRs, agreed with the simulated transmittance spectra. The design of the acoustic filter presented in this study and the corresponding analysis should motivate further development of thin acoustic filters.

Batch fabrication of a double-layer metamaterial resonator using scalloping structures

We propose a batch fabrication method for a double-layer microscale metamaterial resonator operating in the terahertz (THz) region. The proposed method takes advantage of scalloping structures formed by a general deep etching process. The scalloping structures function as a shadow mask. The shadow mask structure allows for the simultaneous fabrication of multiple metal layers with only a one-time evaporation step. The evaporated metal layers are electrically isolated from each other. In this paper, we fabricated a double-layer split-ring resonator (SRR) on a silicon substrate. The typical length of an arm of the SRRs was 40 µm, and the interlayer distance was 4 µm. Using energy-dispersive x-ray analysis, we confirmed that there was a nonevaporation area between the two SRR layers. We also confirmed that the fabricated sample functioned as a metamaterial in the THz region by transmittance measurements and simulation. These results prove that our method could lead to the realization of multilayer metamaterials.

Photodiode with micro texture for improving sensitivity at large angle of incidence for particle sensors

This paper reports on a photodiode (PD) with micro texture for miniaturizing particle sensors. We proposed an optical system for a small particle-sensor without focus lenses. The system requires a PD which is high sensitive at large angle of incidence. The micro texture was fabricated on a surface of the PD to achieve the required property. The micro texture was an array of columnar-like structures that had a part of inverse-tapered shape. The all surface of the structures was n-type doped layer. Experimental results show that the sensitivity of the PD with micro texture increased with an increase in the incident angle. Furthermore, we demonstrated that the proposed optical system using the PD detected light scattered from smoke of an incense stick. Hence, the PD is suitable for realizing small particle-sensors.

A smart, intermittent driven particle sensor with an airflow change trigger using a lead zirconate titanate (PZT) cantilever

This paper reports on a smart, intermittent driven particle sensor with an airflow trigger. A lead zirconate titanate cantilever functions as the trigger, which detects an airflow change without requiring a power supply to drive the sensing element. Because an airflow change indicates that the particle concentration has changed, the trigger switches the optical particle counter from sleep mode to active mode only when the particle concentration surrounding the sensor changes. The sensor power consumption in sleep mode is 100 times less than that in the active mode. Thus, this intermittent driven method significantly reduces the total power consumption of the particle sensor. In this paper, we fabricate a prototype of the particle sensor and demonstrate that the optical particle counter can be switched on by the fabricated trigger and thus that the particle concentration can be measured.

Measurement method for light transmittance of layered metamaterials

We propose a method to measure light transmittance of layered metamaterials by placing the metamaterials directly on a Si photodiode. Our measurement method enables the direct detection of transmitted light that appears as an evanescent wave in natural materials. Here, we report the transmittance measurements of a typical metamaterial using this method. The metamaterial was composed of Ag/Al(2)O(3) layers and was fabricated by direct evaporation on the Si photodiode. The measured transmittance agrees with the simulated transmittance. Our results confirmed that this measurement method can determine the transmittance properties of metamaterials and that it is applicable to other types of metamaterials.