Rotationally reconfigurable metamaterials based on moiré phenomenon.

Exploiting moiré interference, we make a new type of reconfigurable metamaterials and study their transmission tunability for incident electromagnetic waves. The moiré pattern is formed by overlapping two transparent layers, each of which has a periodic metallic pattern, and the cluster size of the resulting moiré pattern can be varied by changing the relative superposition angle of the two layers. In our reconfigurable metamaterials, both the size and structural shape of the unit cell can be varied simultaneously through moiré interference. We show that the transmission of electromagnetic waves can be controlled from 90% to 10% at 11 GHz by experiments and numerical simulation. The reconfigurable metamaterial proposed here can be applied in bandpass filters and tunable modulation devices.

A metasurface composed of orifice-type unit cells for the redirection of acoustic waves.

To implement a sound wave redirection system, a two-dimensional (2D) slice of a three-dimensional (3D) metasurface is designed and fabricated using a one-dimensional (1D) face-centred orifice cubic (FCOC) unit cell. The metasurface consists of five identical periodic groups, of which one periodic group consists of eight unit-cell groups with a phase shift of [Formula: see text] adjacent to each other. One unit-cell group consists of four 1D FCOC unit cells with the same orifice diameter. From the numerical simulation results of the designed metasurface, we observed the redirections of sound waves and compared them with the expected theoretical results. It was confirmed that the experimental results agree well with the simulated results with respect to the different incident angles and frequencies. The used frequencies that satisfy the homogeneous medium condition of the metamaterial for the redirection of incident waves range between 1500 and 2700 Hz. At the characteristic frequency of 1540 Hz at normal incidence, it is considered that stationary evanescent waves exist at the boundary of the metasurface due to the characteristics of the surface wave and the limited end boundary. The FCOC-based metasurface provides a new method of metasurface fabrication and is expected to expand the applicability of the metasurface because it can be easily applied to a surface with any shape.

Hemispherical acoustic Luneburg lens with the acoustic Goos-Hänchen shift and Fresnel filtering effect.

A two-dimensional (2D) slice of a 3D hemispherical acoustic Luneburg lens using a quasi-conformal transformation and face-centred-orifice-cubic (FCOC) unit cells is designed and fabricated. With the system, the focusing characteristics of acoustic waves with frequencies that satisfy the homogeneous medium condition of the metamaterial are observed, such as focusing of acoustic plane waves at the antipodal point on the transformed surface of the opposite side for the incident direction and focus spreading due to total internal reflection at the focus point. The attenuation losses of the system are measured and compared with those of an untransformed system with respect to frequency. The value of the acoustic Goos-Hänchen shift is determined by comparing the experimental and theoretical and simulated values of the focus points with respect to the incident angle. The effect of acoustic Fresnel filtering due to the angular distribution of the incident waves at the flat surface boundary is verified by comparing the results of the experiment and a simulation.

Chaotic behaviors of operational amplifiers.

We investigate nonlinear dynamical behaviors of operational amplifiers. When the output terminal of an operational amplifier is connected to the inverting input terminal, the circuit exhibits period-doubling bifurcation, chaos, and periodic windows, depending on the voltages of the positive and the negative power supplies. We study these nonlinear dynamical characteristics of this electronic circuit experimentally.