Baogang Quan

Design of a polarization insensitive multiband terahertz metamaterial absorber

We design a terahertz (THz) metamaterial absorber having four narrowband high absorptivities of 98%, 97%, 98% and 97% at frequencies of 0.68 THz, 1.27 THz, 2.21 THz and 3.05 THz, respectively. The absorber consists of three metallic layers, which are separated by two dielectric spacers. The absorption performances are simulated using a commercialized full-wave electromagnetic simulation software, and the mechanism of absorption is theoretically investigated. The result shows that the absorber is insensitive to the polarization of THz wave and the position of every absorption peak can be effectively tuned by the geometries of the absorber. The potential applications of the absorber include spectrally selective detecting, THz sensing and thermal imaging.

Three Dimensional Hybrids of Vertical Graphene-nanosheet Sandwiched by Ag-nanoparticles for Enhanced Surface Selectively Catalytic Reactions

Three dimensional (3D) plasmonic nanostructure is perfect for the surface-enhanced Raman scattering (SERS) and also very suitable for surface catalytic reaction, but how to design and fabricate is still a robust task. Here, we show a 3D plasmonic nanohybrid of vertical graphene-nanosheet sandwiched by Ag-nanoparticles on the silicon nanocone array substrate for enhanced surface catalytic reaction. By SERS detection, we find that this hierarchical nanohybrid structure is highly efficient in the enhancement of catalytic reaction, even at a very low concentration of 10−11 M, which is far better than previous reports by four orders of magnitude. A strong electric field enhancement produced in the 3D framework nanohybrids of graphene nanosheet/Ag-nanoparticles is responsible for this great enhancement of catalytic reaction, due to larger electron collective oscillation in the composite system. Especially the oxygen adsorbed on the graphene and Ag nanoparticles can be excited to triplet excited states, and the electrons on the graphene and the nanoparticles can be effectively transferred to the oxygen, which plays very important role in molecular catalytic reactions. Our results demonstrate the contribution of graphene in plasmon-driven catalytic reactions, revealing a co-driven reaction process.This excellent SERS substrate can be used for future plasmon and graphene co-catalytic surface catalytic reactions, graphene-based surface plasmon sensors and so on.

Alkanethiol-functionalized terahertz metamaterial as label-free, highly-sensitive and specific biosensor.

Specific biorecognition is essential for many biological processes, for which highly sensitive and label-free biosensors are strongly demanded. The recently developed metamaterials are a potential choice for biosensing due to their exotic properties. In the current work, a label-free and specific sensor for streptavidin-agarose (SA) was fabricated based on terahertz metamaterial functionlized by octadecanthiols and biotins. Both low and high frequency resonant modes from the metamaterials are found applicable for the detection of SA, and a redshift up to 6.76 GHz for the high frequency mode was measured in the undiluted commercial solution. The low frequency mode is attributed to inductor-capacitor (LC) oscillation, while the high frequency mode originates from the plasmonic dipole oscillator, both of which are highly sensitive to the micro-environment change. Adsorption of SA of different concentrations causes different redshifts, and the replacement of high refractive-index substrate with low refractive-index substrate can efficiently promote the sensitivity, well agreeing with the numerical simulation. Moreover, for a particular biomolecule, the sensitivity can be further improved by optimizing the metamaterial design. This method might be very helpful for desirable biorecognition in biology, medicine, and drug industry.

Self-referenced sensing based on terahertz metamaterial for aqueous solutions

We demonstrated a self-referenced sensing method in reflection geometry for characterizing aqueous solutions based on terahertz metamaterials. The sensing signal and the reference signal are taken in one measurement from different interfaces of the substrate. For ethanol-water mixture and aqueous solution of NaCl, the line-shape of the modulated response shows distinct polarity, while the peak-valley value near resonant region depends linearly on the solution concentration. These observations result from the variation of dielectric environment near the interface between the metamaterials and the aqueous solutions. This method holds promise for future application in monitoring real aqueous biosystems and ecological water systems.

Bianisotropic response of microfabricated metamaterials in the terahertz region

Terahertz time-domain spectroscopy was used to investigate the bianisotropic response of microfabricated split-ring resonators (SRRs) and the in-plane SRR-wire metamaterials. We observed a strong polarization dependence of the transmission with a polarized incident terahertz electromagnetic wave. Two transmission minima, one from the bianisotropic contribution and the other from the anisotropic plasmonic contribution, are discussed. The evolution of the two transmission minima as a function of polarization angle is also demonstrated. The effective-medium theory combined with the dispersion relations in metamaterials is employed to elucidate the experimental data effectively.

Patterned Growth of Polyaniline Nanowire Arrays on a Flexible Substrate for High-Performance Gas Sensing

Uniform patterning of polyaniline nanowire arrays on a wafer-sized flexible substrate is achieved by combining photolithography and in situ polymerization techniques. Chemical gas sensors based on the patterned polyaniline nanowire arrays exhibit excellent performance because of their highly ordered morphology and large specific surface area.

Visible transmission response of nanoscale complementary metamaterials for sensing applications

Metamaterials (MMs) have shown huge potential in sensing applications by detecting their optical properties, which can be designed to operate at frequencies from visible to mid-IR. Here we constructed complementary split ring resonator (CSRR) based metamaterials in nanoscale with unit length of 100 nm and slit width of 30 nm, and observed obvious responses in the visible waveband from 600 to 900 nm. These visible responses show a good tunability with the structures geometry, and are well suited for dielectric detection. We demonstrated good refractive index sensing of CSRR based metamaterials in the visible region under both 0° and 90° polarized incidence. Our results extend the study of CSRR based metamaterials to the visible region, which is expected to deepen the understanding of the response mechanism of CSRRs and benefit their sensing applications in the visible region.

Optical modulation of terahertz behavior in silicon with structured surfaces

Optically modulated terahertz (THz) transmittance through Si with various resistivities, in particular the high-resistivity samples with a structured surface showing nanosized pillars or split-ring resonators (SRRs), was investigated. The samples with nanosized pillars display an increased transmittance and an accordingly reduced modulation depth. With SRRs on the surface, strongly selective modulation can be realized at the resonant frequencies where the transmittance is vanishingly small, whereas at the non-resonant frequencies, where the transmittance is large, the modulation depth is much greater. These results demonstrate an alternative route for the modulation of THz wave in the all-optical devices.

Sensing self-assembled alkanethiols by differential transmission interrogation with terahertz metamaterials

Surface-enhanced electromagnetic response in the resonant regions of split-ring resonators offers a sensitive way to probe the surface dipoles formed by alkanethiol molecules with a terahertz wave by a differential transmission (DT) interrogation method. The DT signal mainly comes from the interaction between alkanethiols and metamaterials by electron transfer and/or the variation of the dielectric constant. The Lorentz model is used to demonstrate the principle of DT interrogation theoretically, which suggests the variation of both frequency and damping of resonance can be captured cooperatively. This method has been employed to experimentally demonstrate the sensing feasibility for the chain length dependence of the alkanethiol molecules. Numerical simulations confirm that the enhancement is large at the gap and corner regions of this kind of metamaterials.

Wavelength de-multiplexing metasurface hologram.

A wavelength de-multiplexing metasurface hologram composed of subwavelength metallic antennas is designed and demonstrated experimentally in the terahertz (THz) regime. Different character patterns are generated at the separated working frequencies 0.50 THz and 0.63 THz which determine a narrow frequency bandwidth of 130 GHz. The two working frequencies are around the central resonance frequency of the antennas where antennas behave strong wavefront modulation. Each antenna is fully utilized to control the wavefront of the metasurface at different frequencies by an optimization algorithm. The results demonstrate a candidate way to design multi-colors optical display elements.