Transient establishment of the wavefronts for negative, zero, and positive refraction.

We quantitatively demonstrate transient establishment of wavefronts for negative, zero, and positive refraction through a wedge-shaped metamaterial consisting of periodically arranged split-ring resonators and metallic wires. The wavefronts for the three types of refractions propagate through the second interface of the wedge along positive refraction angles at first, then reorganize, and finally propagate along the effective refraction angles after a period of establishment time respectively. The establishment time of the wavefronts prevents violating causality or superluminal propagation for negative and zero refraction. The establishment time for negative or zero refraction is longer than that for positive refraction. For all three refraction processes, transient establishment processes precede the establishment of steady propagation. Moreover, some detailed characters are proven in our research, including infinite wavelength, uniform phase inside the zero-index material, and the phase velocity being antiparallel to the group velocity in the negative-index material.

Efficient generation and frequency modulation of quasi-monochromatic terahertz wave in Lithium Niobate subwavelength waveguide.

A kind of lateral excitation (LE) configuration is proposed for quasi-monochromatic terahertz generation via impulsive stimulated Raman scattering in a LiNbO3 (LN) slab waveguide by numerical simulation. In an individual waveguide, maximum efficiency frequency-selective excitation is achieved with linewidth narrower than 38 GHz when phase matching is fulfilled between the pump laser and the generated terahertz (THz) waves. As a result, the frequency and linewidth of narrowband THz waves can be tuned through changing the dispersion of THz waves, which is implemented by adjusting the thickness of host LN slab. Furthermore, Au-Air-LN-Air-Au multilayer LE structure is developed to realize a dramatic change of the dispersion to obtain quasi-monochromatic THz waves, of which the linewidth is achieved as narrow as 10 GHz. In addition, the frequency and linewidth of quasi-monochromatic THz waves are modulated dynamically by varying the distance between LN slab and Au mirrors flexibly. Consequently, the optimized LE structure is expected to boost the development of high-precision and real-time inspection and sensing.

Direct visualization of light confinement and standing wave in THz Fabry-Perot resonator with Bragg mirrors.

We report for the first time the ability to perform time resolved imaging of terahertz (THz) waves propagating within a Fabry-Perot resonator on a LiNbO3 slab. Electro-optic effect is used to record the full spatiotemporal evolution of THz fields inside the resonator. In addition to revealing the real-space behavior, the data further demonstrate the confinement and the standing wave modes of THz in the cavity in frequency domain. The experimental results are in good agreement with numerical simulations. Using the coherent imaging technique to gain real-time information about a resonator system provides a unique path to study the physics of optical cavity.

THz band-stop filter using metamaterials surfaced on LiNbO(3) sub-wavelength slab waveguide.

We designed and implemented periodic bar arrays metamaterials to select appropriate frequencies of terahertz (THz) waves propagating in a LiNbO(3) sub-wavelength waveguide. The spatial and temporal electric field profiles of the THz waves were recorded using a time-resolved phase-contrast imaging system. The metamaterials can operate as a band-stop filter to realize blocking back THz waves in a band range of 0.6-1.0 THz, while transparent transmission for the fundamental mode of the slab over a range of 0.3-0.6 THz.

Isotropic spiral plasmonic metamaterial for sensing large refractive index change.

We numerically investigate the optical properties of a spiral G-shaped metamaterial, which exhibits both a sharp resonance with a high-quality factor and a broad resonance with a long linear slope in the near-infrared region. By employing the sharp resonating mode, refractive index sensing with sensitivity up to 410 nm/RIU is demonstrated theoretically. In addition, the metamaterial shows single wavelength sensing ability, which allows fast determination of environmental refractive index changes as large as 0.5 by simply monitoring variations in the transmitted intensity of a certain wavelength, which will be beneficial to the development of refractive index sensors based on monochromatic light sources and detectors. As the resonances are independent of the polarization direction of a linearly polarized wave, the sensor manufacturing process will benefit as a result of the lack of an alignment requirement for the axis of polarizers with the metamaterials' orientation.

Giant circular dichroism of large-area extrinsic chiral metal nanocrecents.

In this work, we demonstrate the strong extrinsic chirality of the larger-area metal nanocrescents by experiments and simulations. Our results show that the metal nanocrescent exhibits giant and tunable circular dichroism (CD) effect, which is intensively dependent on the incident angle of light. We attribute the giant extrinsic chirality of the metal nanocrescent to the excitation efficiencies difference of localized surface plasmon resonance (LSPR) modes for two kinds of circularly polarized light at a non-zero incident angle. In experiment, the largest CD of 0.37 is obtained at the wavelength of 826 nm with the incident angle of 60°. Furthermore, the CD spectra can be tuned flexibly by changing the metal nanocrescent diameter. Benefitting from the simple, low-cost and mature fabrication process, the proposed large-area metal nanocrescents are propitious to application.

Surface enhancement of THz wave by coupling a subwavelength LiNbO3 slab waveguide with a composite antenna structure.

Highly intense terahertz electromagnetic field and efficiently surface localized terahertz field in subwavelength volumes are of vital importance for terahertz photonics integration, also will greatly accelerate the development for integrated applications in biochemical sensing, imaging, terahertz spectroscopy, enhancement of nonlinear effects and even quantum research. In this paper, we achieved large terahertz field enhancement and surface field localization through depositing a pair of Au composite antennas on a LiNbO3 subwavelength slab waveguide, which can serve as an excellent on-chip platform for terahertz research and application. The antennas consist of two opposing tip-to-tip triangles separated by a gap, and each triangle combines with a strip antenna. Time-resolved imaging and finite-difference time-domain method were used to resolve the characteristics of the designed antennas experimentally and simulatively. Through these methods, we demonstrated outstanding abilities of the platform: leading to a large electric field enhancement, concentrating almost full terahertz energy on the waveguide's surface when they are resonant with the terahertz waves and tunable resonant frequency. These abilities make the subwavelength waveguide coupling with the composite antennas be able to sever as a good integrated device to identify terahertz-sensitive small objects, or an excellent platform to terahertz spectroscopy and quantum research.