Yuping Yang

Observing metamaterial induced transparency in individual Fano resonators with broken symmetry

Metamaterial induced transparency is demonstrated using individual split ring resonators with two gaps on opposite side. For the symmetric structure, only a low quality dipolar resonance is witnessed at a normal incidence excited with electric field along the resonator gaps. Displacement of one gap from the centre breaks the symmetry and a higher order mode, inaccessible in the symmetric structure, is excited. Coherent interaction among the modes in the split ring resonator forms an extremely sharp narrowband transparency window centred directly at the dipole resonance. Such metamaterial could facilitate coherent manipulation of terahertz signals for delay, storage, and nonlinear applications.

Ultra-high Q even eigenmode resonance in terahertz metamaterials

We report the simultaneous excitation of the odd and the even eigenmode resonances in a periodic array of square split-ring resonators, with four resonators per unit cell. When the electric field is parallel to their gaps, only the two well-studied odd eigenmodes are excited. As the resonators are rotated relative to one another, we observe the emergence and excitation of an extremely sharp even eigenmode. In uncoupled split-ring resonators, this even eigenmode is typically radiative in nature with a broad resonance linewidth and low Q-factor. However, in our coupled system, for specific range of rotation angles, our simulations revealed a remarkably high quality factor (Q � 100) for this eigenmode, which has sub-radiant characteristics. This type of quad-supercell metamaterial offers the advantage of enabling access to all the three distinct resonance features of the split-ring resonator, which consists of two odd eigenmodes in addition to the high-Q even eigenmode, which could be exploited for high performance multiband filters and absorbers. The high Q even eigenmode could find applications in designing label free bio-sensors and for studying the enhanced light matter interaction effects. V C 2015 AIP Publishing LLC.

Anomalous terahertz transmission in bow-tie plasmonic antenna apertures

Arrays of subwavelength dipole bow-tie apertures are designed and characterized at terahertz frequencies. For an incident terahertz field perpendicular to the longer axis of the bow tie, a strong resonance enhancement, line narrowing, and a nonmonotonic frequency shift were observed with increasing length of the tapered bow-tie arms. Such characteristic behaviors primarily originate from localized surface plasmon resonances. In addition, with a decreasing aperture size, the contribution of localized plasmons becomes prominent due to an increase in plasmonic lifetime as the terahertz pulses strongly couple with the metallic surface surrounding the bow-tie apertures.

Identification of Genistein and Biochanin A by THz (far-infrared) vibrational spectra

The temperature dependent vibrational spectra of Genistein and Biochanin A, major components in Mongolian medicines Agi, in the range of 0.5-4.5 THz (16.7-150 cm(-1)) are presented for the first time. Over the temperature range from 295 to 77K, 12 highly resolved spectral features for the Genistein and 13 features for Biochanin A were measured by THz-TDS and display strong linewidth narrowing and frequency blue-shift with cooling. Such narrow-line THz fingerprint spectra provide a rapid, nondestructive and reliable method for the identification of these Chinese traditional medicines.

Four-step pulses of fractional-order surface plasmons

The 0-order transmission of a 1D metallic grating, on a high-resistivity silicon wafer in optical contact with a silicon plate, has been characterized by terahertz time-domain spectroscopy with subpicosecond resolution over a 400 ps scan range. Two new long-time-delay, powerful pulses are observed after the second reflected pulse. In the frequency domain, these two strong and fast-ringing structures correspond to the bandwidth ranges between the [0, 1] and [0, 2] surface plasmon modes and the range above [0, 2], respectively. A physical optics ray analysis provides an intuitive understanding of these new four-step (reflection, diffraction, total reflection, and diffraction) pulses, caused by fractional-order surface plasmon type beam coupling.

High-Resolution THz Transmission and Reflection Measurements and Consequent Understanding of Resonant Hole-Arrays

We report high-resolution THz-TDS characterization of the transmission and reflection of a thin metal hole-array in optical contact with a 10 mm thick high-resistivity Si plate. It was found that the red-shift of the [±1, 0] spoof surface-plasmon (SSP) transmission peak from the Woods anomaly valley indicates the coupling strength of the SSP wave to the metal surface due to the filling factor and shape of the holes, and that the red-shift provides a measurement of the SSP wave velocity. Our measurements of the amplitude transmission peak, and the corresponding reflection dip, of the hole-array agreed with numerical simulation to an accuracy of ±2 GHz, and the measured phase was in good agreement with simulation. However, our reflection measurements showed a second unexplained dip at the frequency of Woods anomaly.

Understanding fractional-order surface plasmons

We show experimentally that diffraction-induced surface plasmon excitation can mimic enhanced transmission and cause a highly sensitive modulation by the coherent interference between zero-order and reflected first-order diffraction in select regions of the terahertz spectrum. Based on the study of a one-dimensional metallic grating, we obtain the physical mechanisms of the fractional-order surface plasmon resonances observed with the two-dimensional grating of the metallic hole array.

Dielectric sphere-coupled THz super-resolution imaging

A straightforward terahertz (THz) imaging method, analogous to microsphere optical nanoscope, is proposed and developed with spatial resolution beyond the diffraction limit. By attaching a 3 mm Teflon sphere directly onto a free-standing silicon grating, a subwavelength spatial resolution better than the Rayleigh limit is demonstrated. Furthermore, the dielectric sphere-coupled THz microscope not only enables far-field, large-area measurement, but also characterizes high-throughput and broad-bandwidth imaging properties. In addition, a numerical study of the THz pulse propagation through a dielectric sphere agrees well with the experimental results, showing frequency-dependent magnification and focal length.

High-resolution THz reflection measurements of resonant hole-arrays

The high-resolution THz amplitude spectrum as well as the phase shift of the reflection of a thin metal hole-array are measured, and an unexpected Woods anomaly dip, as well as an unusual step-like 2π phase shift are observed.

Terahertz spectroscopy of sub-wavelength size metallic particles

The terahertz wave transmission through ensembles of sub-wavelength size metallic particles is studied by THz-TDS and finite different time domain (FDTD). The result of the experiment shows that with the increase of the thickness of the sample, the terahertz pulses experience significant attenuation, delay and broadening. In addition, an obvious red-shift and narrowing of the amplitude spectrum are observed. Numerical simulations based on FDTD are performed to explain the details of the data and lead to an excellent agreement with the experimental results. Furthermore, the relation between the size of the particles and the transmission properties of terahertz radiation is analyzed.