Fangrong Hu

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.

Graphene as broadband terahertz antireflection coating

We examined the potential of stacked multilayer graphene as broadband terahertz (THz) antireflection coating based on the impedance matching effect in experiment and theory. The reflected pulses from the quartz and silicon substrates were observed to change with the layer number and doping concentration of the graphene coating. Remarkable broadband impedance matching was achieved due to optimized THz conductivity. Theoretical analysis based on Drude model and thin film Fresnel coefficients have been used to explain the experimental phenomena, which indicated the shift of Fermi level caused by chemical doping. This work paves the way for graphene-based broadband THz antireflection coating.

Discrimination of genetically modified sugar beets based on terahertz spectroscopy

The objective of this paper was to apply terahertz (THz) spectroscopy combined with chemometrics techniques for discrimination of genetically modified (GM) and non-GM sugar beets. In this paper, the THz spectra of 84 sugar beet samples (36 GM sugar beets and 48 non-GM ones) were obtained by using terahertz time-domain spectroscopy (THz-TDS) system in the frequency range from 0.2 to 1.2 THz. Three chemometrics methods, principal component analysis (PCA), discriminant analysis (DA) and discriminant partial least squares (DPLS), were employed to classify sugar beet samples into two groups: genetically modified organisms (GMOs) and non-GMOs. The DPLS method yielded the best classification result, and the percentages of successful classification for GM and non-GM sugar beets were both 100%. Results of the present study demonstrate the usefulness of THz spectroscopy together with chemometrics methods as a powerful tool to distinguish GM and non-GM sugar beets.

Hyper sausage neuron: Recognition of transgenic sugar-beet based on terahertz spectroscopy

This paper presents a novel approach for identification of terahertz (THz) spectral of genetically modified organisms (GMOs) based on Hyper Sausage Neuron (HSN), and THz transmittance spectra of some typical transgenic sugar-beet samples are investigated to demonstrate its feasibility. Principal component analysis (PCA) is applied to extract features of the spectrum data, and instead of the original spectrum data, the feature signals are fed into the HSN pattern recognition, a new multiple weights neural network (MWNN). The experimental result shows that the HSN model not only can correctly classify different types of transgenic sugar-beets, but also can reject identity non similar samples in the same type. The proposed approach provides a new effective method for detection and identification of GMOs by using THz spectroscopy.

Manipulating Magnetoinductive Coupling with Graphene-Based Plasmonic Metamaterials in THz Region

Coupling between physical modes can trigger some new physical phenomena such as frequency shift, new mode, and Rabi splitting. Resonant modes in graphene-based metamaterials provide a new platform for the research of coupling. In this work, we demonstrate that the plasmonic coupling of split ring resonator (SRR) dimer in graphene-based metamaterials can be easily manipulated. This magnetoinductive coupling can switch on/off the dark modes easily, which is usually done by symmetry-breaking structure previously. Furthermore, the dark mode can also be activated by Fermi energy as well as carrier concentration changing with either physical or chemical methods conveniently. In addition, different graphene-based SRR dimer configurations present different coupling strengths, which benefits the designing and optimizing of graphene-based metamaterials. The demonstration could enhance the versatility of both coupling studies in terahertz (THz) region and graphene-based metamaterials for THz devices.

Sensitive distinction between herbs by terahertz spectroscopy and a metamaterial resonator

ABSTRACT Terahertz spectroscopy, combined with a metamaterial resonator, was used to distinguish varieties of herbs that look alike to the naked eye. The metamaterial resonator, based on an ohm-ring, was experimentally demonstrated to exhibit two significant resonant peaks in the range from 0.2 to 1.4u2009THz. The resonant peaks were shifted to lower frequency with changes of the surface permittivity, and this feature was applied to identify varieties of herbs. In the experiment, different borneols were effectively identified according to the different degree of redshift. This work demonstrates that metamaterial resonator is a sensitive tool for terahertz spectral sensing, and shows a new detection method for different herbs.

Experimental and theoretical investigations of tartaric acid isomers by terahertz spectroscopy and density functional theory

The terahertz (THz) absorption spectra of l-, d-, and dl-tartaric acid have been measured in the frequency range from 0.2 to 2.0u202fTHz by terahertz time-domain spectroscopy (THz-TDS). The characteristic absorption peaks of these three tartaric acid isomers were obtained, which showed remarkable difference between enantiomers (l- and d-tartaric acid) and the racemic compound (dl-tartaric acid) in their peak frequencies. In parallel with the experimental study, theoretical calculations on isolated-molecule and unit cell of tartaric acids using density functional theory (DFT) were also performed for simulating the experimental THz spectrum features, which were in good agreement with the experimental data. Results demonstrate that THz-TDS can distinguish the tiny diversity between tartaric acid chiral isomers and its racemic compound, and provided an effective method for molecular identification in biological and biomedical engineering.

Coupling Between Metamolecular Modes and Lattice Diffraction Modes of Metamaterials in Terahertz Region

The coupling between plasmonic modes and lattice diffraction modes of circular split-ring resonators in terahertz (THz) region has been analyzed by changing the lattice constants. We discovered a blue shift of the individual eigenmode in short lattice constants, which is due to the longitudinal coupling of two adjacent eigenmodes from neighboring unit cells. We found an anti-crossing effect in long lattice constants for both TM and TE incidence as the lattice diffraction modes is fused with the plasmonic modes to form hybrid modes. Moreover, these hybrid modes are more sensitive to the refractive index change than the pure modes, which shows the promise to increase the sensitivity of metamaterials by coupling.

Intensity modulation of a terahertz bandpass filter: utilizing image currents induced on MEMS reconfigurable metamaterials

We experimentally demonstrated a tunable terahertz bandpass filter based on microelectromechanical systems (MEMS) reconfigurable metamaterials. The unit cell of the filter consists of two split-ring resonators (SRRs) and a movable bar. Initially, the movable bar situates at the center of the unit cell, and the filter has two passbands whose central frequencies locate at 0.65 and 0.96xa0THz. The intensity of the two passbands can be actively modulated by the movable bar, and a maximum modulation depth of 96% is achieved at 0.96xa0THz. The mechanism of tunability is investigated using the finite-integration time-domain method. The result shows that the image currents induced on the movable bar are opposite the resonance currents induced on the SRRs and, thus, weaken the oscillating intensity of the resonance currents. This scheme paves the way to dynamically control and switch the terahertz wave at some constant frequencies utilizing induced image currents.

Giant plasmonic mode splitting in THz metamaterials mediated by coupling with Lorentz phonon mode

Giant plasmonic mode splitting has been observed in THz metamaterials due to the mediation by the Lorentz phonon dielectric material. This splitting mode is confirmed by the surface current distribution, indicating that plasmonic modes behave like dipole resonances, while the phonon mode behaves like multipole resonance due to coupling. The splitting of the plasmonic modes demonstrates an anti-crossing behavior with the change in Lorentz central frequency, which suggests that there is energy redistribution between plasmon and phonon modes. Similar to the Stark effect, the splitting frequency difference increases with the increasing direct current dielectric function. We also propose an interaction Hamiltonian to understand the physical mechanism of the plasmonic splitting. Furthermore, the splitting is convincible for small Lorentz dielectrics such as sugar and amino acid in the THz region, which could be used for biomolecular sensing applications.Giant plasmonic mode splitting has been observed in THz metamaterials due to the mediation by the Lorentz phonon dielectric material. This splitting mode is confirmed by the surface current distribution, indicating that plasmonic modes behave like dipole resonances, while the phonon mode behaves like multipole resonance due to coupling. The splitting of the plasmonic modes demonstrates an anti-crossing behavior with the change in Lorentz central frequency, which suggests that there is energy redistribution between plasmon and phonon modes. Similar to the Stark effect, the splitting frequency difference increases with the increasing direct current dielectric function. We also propose an interaction Hamiltonian to understand the physical mechanism of the plasmonic splitting. Furthermore, the splitting is convincible for small Lorentz dielectrics such as sugar and amino acid in the THz region, which could be used for biomolecular sensing applications.