Mingxia He

Low-loss ultra-high-Q dark mode plasmonic Fano metamaterials

We experimentally demonstrate a planar terahertz Fano metamaterial with an ultrahigh quality (Q) factor of 227. This is achieved by the excitation of the nonradiative dark modes by introducing a tiny asymmetry in the metamaterial structure. The extremely sharp quadrupole and Fano resonances are excited at normal incidence for orthogonal polarizations of the electric field. In order to capture the narrow linewidth of the dark resonance modes, we perform high resolution terahertz time-domain measurements with a scan length of 200 picoseconds and frequency resolution of 5 GHz. These high-Q metamaterials can be used in ultrasensitive label-free terahertz sensing, dense photonic integration, and narrowband filtering.

Terahertz superconductor metamaterial

We characterize the behavior of split ring resonators made up of high transition temperature yttrium barium copper oxide superconductor using terahertz time-domain spectroscopy measurements and numerical simulations. The superconductor metamaterial is found to show a remarkable change in the transmission spectra at the fundamental inductive-capacitive resonance as the temperature dips below the critical transition temperature. This resonance switching effect is normally absent in traditional metamaterials made up of regular metals. The temperature-dependent resonance behavior of the superconducting metamaterial would lead to development of low loss terahertz switches at cryogenic temperatures.

Effect of dielectric properties of metals on terahertz transmission subwavelength hole arrays

We study the influence of dielectric function of metals on the transmission properties of terahertz pulses through periodically patterned subwavelength holes. Because of a drastic increase in the value of dielectric constants, most metals become highly conductive at terahertz frequencies. Extraordinary terahertz transmission is observed in subwavelength hole arrays made from both good and poor electrical conductors. The measured transmittance of terahertz pulses is found to be enhanced with increasing ratio of the real to the imaginary dielectric constant of the constituent metals, for which the dielectric function follows the Drude model.

Spiral-type terahertz antennas and the manifestation of the Mushiake principle

We report on the experimental and theoretical study of the resonant eigenmodes of spiral-type terahertz antennas. The analysis is carried out for a varying number of spiral windings. For larger numbers the structure possesses a self-complementary property which allows the application of the Mushiake principle predicting that the impedance of such structures is half the impedance of free space. This permits to observe an equal and frequency independent reflection and transmission coefficient. This property makes the spiral-type terahertz antenna not only a fascinating example of a medium supporting strong resonances in the long wavelength limit but also a medium which can be easily and reasonably homogenized at higher frequencies. This is in stark contrast to most of the existing metamaterials.

Optical and dielectric properties of ZnO tetrapod structures at terahertz frequencies

The low-frequency optical and dielectric properties of ZnO tetrapod structures prepared by thermophysical method were studied by terahertz time-domain spectroscopy. The power absorption, refractive index, and the complex dielectric function were measured in the frequency range from 0.2to3.5THz. Based on a simple effective medium theory, the low-frequency dielectric properties of ZnO tetrapods were found to be associated with the transverse optical E1 phonon mode, which is consistent with that observed in bulk single-crystal ZnO.

A Broadband THz-TDS System Based on DSTMS Emitter and LTG InGaAs/InAlAs Photoconductive Antenna Detector

We demonstrate a 4-f terahertz time-domain spectroscopy (THz-TDS) system using an organic crystal DSTMS as the THz emitter and a low temperature grown (LTG) InGaAs/InAlAs photoconductive antenna as the receiver. The system covers a frequency range from 0.2 up to 8 THz. The influences of the pump laser power, the probe laser power and the azimuthal angle of the DSTMS crystal on the time-domain THz amplitude are experimentally analyzed. The frequency accuracy of the system is verified by measuring two metamaterial samples and a lactose film in this THz-TDS system. The proposed combination of DSTMS emission and PC antenna detection realizes a compact and low-cost THz-TDS scheme with an ultra-broad bandwidth, which may promote the development and the applications of THz-TDS techniques.

Terahertz Response of Bulk and Nanostructured ZnO

We present experimental characterization of far-infrared optical and dielectric prop- erties of single-crystal ZnO and nanostructured ZnO of difierent morphologies by terahertz time- domain spectroscopy. Frequency dependent complex dielectric function, power absorption, and refractive index of single-crystal, nanowire, tetrapod, tubular, and prismlike ZnO structures are experimentally measured in the terahertz regime, respectively. The experimental results are analyzed and well flt with dielectric theories and efiective medium models.

Quantitative analysis of Kerr nonlinearity and Kerr-like nonlinearity induced via terahertz generation in ZnTe

Kerr nonlinearity and Kerr-like nonlinearity induced via terahertz generation and the electro-optical effect in ZnTe crystal are investigated. In general, these nonlinear effects are concomitant and difficult to quantitatively analyze in the time domain. Even Z-scan technique, which is a simple and sensitive single-beam method to determine both the sign and magnitude of the nonlinear refractive index as well as the nonlinear absorption coefficient of a given material, cannot quantitatively analyze the contribution of each nonlinear effect directly. A method is proposed in the spectral domain to distinguish between Kerr nonlinearity and Kerr-like nonlinearity. Experimental results agree with theoretical analysis.

Surface plasmon-enhanced terahertz spectroscopic distinguishing between isomers in powder form

The effect of a dielectric overlayer on terahertz transmission through a freestanding metallic array of subwavelength holes is experimentally presented. There is a remarkable resonance redshift from 0.600 to 0.498 THz at the surface plasmon (SP) metal-dielectric resonance mode with increasing film thickness. When the overlayer film is thicker than a critical thickness, the resonance frequency becomes steady at the final resonance frequency ω(f). On the basis of the dispersion relation of SPs, two kinds of glutamic acid enantiomers are distinguished by use of SP-enhanced terahertz spectra of metallic array of subwavelength holes according to the result of ω(f). The terahertz plasmonic hole array with the sensitive nature provides an approach to distinguish trace amount of powder substances, which has a promising application prospect in the fields of public security and biomedical science, such as distinguishing between isomers and identifying expensive medicines and drugs.

Terahertz metasurface studies based on the near-field spectroscopy

Metasurface, as a kind of metamaterials with only one or two layers of functional structures, has arouses a world-wide research enthusiasm during the past several years. Especially, recent studies on metasurfaces working in the terahertz regime has partly fullfilled the emergent lack for the terahertz devices, which is a long-term bottleneck on the terahertz science and technology. Until now, most terahertz metasurfaces were designed, demonstrated and applied for the far-field utilization. However, in recent years, kinds of near-field terahertz technique have become more and more powerful and convenient to use, which facilities a new aspect of terahertz metasurface research. Here we presented the recent terahertz near-field studies on several metasurface-based terahertz devices, including two anomalous surface wave couplers and a meta-lens array. The near-field method not only gives a new tool for the terahertz metasurface study, but also makes the terahertz surface wave researches possible, which may broadly promote the development of the terahertz device technology.