Graphene-Based Tunable Dual-Frequency Terahertz Sensor.

A tunable dual-band terahertz sensor based on graphene is proposed. The sensor consists of a metal bottom layer, a middle dielectric layer, and single-layer graphene patterned with four strips on the top. The numerical simulations results show that the proposed sensor exhibits two significant absorption peaks at 2.58 THz and 6.07 THz. The corresponding absorption rates are as high as nearly 100% and 98%, respectively. The corresponding quality factor (Q) value is 11.8 at 2.58 THz and 29.6 at 6.07 THz. By adjusting the external electric field or chemical doping of graphene, the positions of the dual-frequency resonance peak can be dynamically tuned. The excitation of plasma resonance in graphene can illustrate the mechanism of the sensor. To verify the practical application of the device, the terahertz response of different kinds and different thicknesses of the analyte is investigated and analyzed. A phenomenon of obvious frequency shifts of the two resonance peaks can be observed. Therefore, the proposed sensor has great potential applications in terahertz fields, such as material characterization, medical diagnosis, and environmental monitoring.

A Polarization-Insensitive Broadband Terahertz Absorber Using Patterned Graphene.

A polarization-insensitive broadband terahertz absorber with a sandwich structure of metal−dielectric-graphene is designed and simulated. The graphene is patterned as an array of graphene square blocks with circular apertures. The results of the simulations and theoretical analysis show that the absorption exceeds 99% from 0.93 to 1.65 THz while 90% from 0.80 to 1.87 THz, and a broad relative bandwidth of 80.2% is achieved. The absorption performance is passively enhanced by altering physical dimensions of the graphene pattern and actively adjusted by changing the chemical potential of graphene. When the chemical potential increases from 0.1 eV to 0.7 eV, the corresponding terahertz absorption increases from 59.1% to 99%. The mechanism of absorption is disclosed by analyzing the impedance matching theory and distribution of electric-field intensity. In addition, different polarization modes and incident angles are also studied. The proposed absorber has the superiorities of broad relative bandwidth, high absorption rate, polarization insensitivity, and a wide incident angle, which offers some potential applications in the field of terahertz technology such as imaging, detection, and cloaking.

Experimental apparatus for investigating the propagation characteristics of the low-frequency electromagnetic waves in hypersonic plasma fluid generated by shock tube.

The shock tube generates a near real hypersonic plasma sheath environment with high temperature and high pressure for investigating the propagation characteristics of the electromagnetic (EM) waves in a hypersonic plasma fluid. With existing methods, it is difficult to measure the propagation characteristics from the transmitted component of low-frequency (LF) EM waves due to large-size LF focusing antennas and LF shielding structure. In this paper, a novel experimental apparatus is proposed to measure the propagation characteristics of the LF EM waves in a shock-tube-generated hypersonic plasma fluid. The tested plasma is utilized as a dynamic fluid EM shield of a receiver during the experiment. This individual receiver is placed in the center of the experimental segment tube of the shock tube so that it is enveloped completely by the hypersonic plasma fluid during the shock, thereby only allows the transmitted component of the LF EM waves to reach the receiver. The proposed method guarantees good measurement accuracy without requiring large LF focusing antennas, and the complex LF shielding structure extends to the shock tube. Both experiments and simulations were performed to evaluate its performance. The results indicated that the propagation characteristics of the transmitted magnetic field component meet that of the numerical simulations faithfully, where the shock wave velocity reached approximately 5 km/s, the plasma layer thickness was 80 mm, the electron density was 1012-1013/cm3, and the collision frequency was approximately 36 GHz. The proposed experimental apparatus is also suitable in studying the EM wave propagation, testing communication system performances, and testing the properties of transmitting and receiving antennas in the hypersonic plasma fluid.

Nonlinear optical properties of sodium copper chlorophyllin in aqueous solution.

BACKGROUND: Sodium copper chlorophyllin (SCC), as one of the derivatives of chlorophyll - with its inherent green features; good stability for heat, light, acids and alkalies; unique antimicrobial capability; and particular deodori zation performance - is widely applied in some fields such as the food industry, medicine and health care, daily cosmetic industry etc. SCC, as one of the metal porphyrins, has attracted much attention because of its unique electronic band structure and photon conversion performance. To promote the application of SCC in materials science; energy research and photonics, such as fast optical communications; and its use in nonlinear optical materials, solar photovoltaic cells, all-optical switches, optical limiters and saturable absorbers, great efforts should be dedicated to studying its nonlinear optical (NLO) properties. METHODS: In this study, the absorption spectra and NLO properties of SCC in aqueous solution at different concentrations were measured. The Z-scan technique was used to determine NLO properties. RESULTS: The results indicated that the absorption spectra of SCC exhibit 2 characteristic absorption peaks located at the wavelengths 405 and 630 nm, and the values of the peaks increase with increasing SCC concentration. The results also showed that SCC exhibits reverse saturation absorption and negative nonlinear refraction (self-defocusing). CONCLUSIONS: It can be seen that SCC has good optical nonlinearity which will be convenient for applications in materials science, energy research and photonics.

A novel pentanuclear zinc coordination polymer with one-dimensional chain: synthesis, structure and luminescent property.

One interesting pentanuclear zinc coordination polymer, [Zn(5)(OH)(2)(DTBA)(4)](n) (1) (DTBA=2,2'-dithiobisbenzoic acid) has been synthesized by hydrothermal method, and characterized by X-ray single crystal diffraction, elemental analyses, IR and TGA. The pentanuclear zinc unit exhibits pentagon configuration, which acts as secondary building unit to construct a one-dimensional chain structure by DTBA ligands. As far as we know, this kind of pentagon zinc unit is unprecedented. In addition, upon the excitation at 390 nm, the blue-fluorescence at 448 nm of the compound 1 was observed.