ABSTRACT
We demonstrate an electrically tunable polarizer for terahertz (THz) frequency electromagnetic waves formed from a hybrid graphene-metal metasurface. Broadband (>3 THz) polarization-dependent modulation of THz transmission is demonstrated as a function of the graphene conductivity for various wire grid geometries, each tuned by gating using an overlaid ion gel. We show a strong enhancement of modulation (up to â¼17 times) compared to graphene wire grids in the frequency range of 0.2-2.5 THz upon introduction of the metallic elements. Theoretical calculations, considering both plasmonic coupling and Drude absorption, are in good agreement with our experimental findings.
ABSTRACT
We demonstrate a significant enhancement in the sensitivity of split ring resonator terahertz metamaterial dielectric sensors by the introduction of etched trenches into their inductive-capacitive gap area, both through finite element simulations and in experiments performed using terahertz time-domain spectroscopy. The enhanced sensitivity is demonstrated by observation of an increased frequency shift in response to overlaid dielectric material of thicknesses up to 18 µm deposited on to the sensor surface. We show that sensitivity to the dielectric is enhanced by a factor of up to â¼2.7 times by the incorporation of locally etched trenches with a depth of â¼3.4 µm, for example, and discuss the effect of the etching on the electrical properties of the sensors. Our experimental findings are in good agreement with simulations of the sensors obtained using finite element methods.
ABSTRACT
We report terahertz (THz) diffuse reflectance measurements of bulk powdered samples at a frequency of 2.83 THz using a narrowband quantum cascade laser. Samples studied comprise polydisperse powders with absorption coefficients extending over two orders of magnitude from â¼3 cm(-1) to >200 cm(-1). Diffuse reflectance measurements are used to obtain the effective absorption coefficient of these samples from the backscattering cross-section, predicted under the quasi-crystalline approximation (QCA) in the T-matrix formulation and in conjunction with the Percus-Yevick pair distribution function. Results are compared with effective absorption coefficients obtained from THz time-domain spectroscopy measurements on pressed pellet samples, and show good agreement over the range of effective absorption coefficients studied. We observe that the backscattering cross-section predicted under the QCA is strongly dependent on both the real and imaginary components of the complex permittivity of the sample, and we show that reliable determination of the absorption coefficient from diffuse reflectance measurements therefore requires knowledge of the sample's refractive index. This work demonstrates the applicability of diffuse reflectance measurements, using a THz frequency quantum cascade laser, to the high-resolution spectroscopic analysis of bulk powdered samples at THz frequencies.
ABSTRACT
Recently, cocaine has been concealed by dissolving it in alcohol and then transporting it in solution through ports and airports. At the present time it is very difficult to detect cocaine in this form in these environments. However, it has been shown that Raman spectroscopy can successfully detect the presence of these drugs without removing specimens from their containers. Using two portable 785 nm instruments and a 1064 nm laboratory-based instrument, several common containers used in smuggling were analyzed with varying concentrations of cocaine in ethanol solutions. The presence of cocaine is detectable to about 6% w/v in most containers. Green glass presents a problem at 785 nm due to fluorescence but by switching to 1064 nm this can be removed. To apply this technique to real samples as met within law enforcement scenarios, cocaine was dissolved in a selection of dark and white rums including Lamb's Navy Rum(™) , Brugal Añejo(™) , Bacardi(™) and Captain Morgan(™) ; cocaine was detected in all these rum solutions.
