Analysis of the extraordinary transmission properties of arrays of subwavelength holes on a metal film in the terahertz region

The extraordinary transmission properties of terahertz waves through arrays of subwavelength holes on a metal film have been investigated. The effects of the geometrical parameters and dielectrics filling the hole on transmission properties have been explored. The results show that the resonance peak shifts to long wavelength as the hole width and lattice period constant increase. As the refractive index of the dielectric materials filling the hole increases, the transmittance increases and the resonance peak wavelength changes to low frequency. The phase retardation can be flexibly changed by varying the geometrical parameters and dielectric materials filling in the hole.     

Transmission characteristics of surface plasmon polaritons through a metallic rectangle above a metallic film

The effects of a silver rectangle on the transmission characteristics of surface plasmon polaritons (SPPs) that propagate at the air–silver interface are investigated using the finite-element method. Results show that the structural parameters of the rectangle and distance between rectangle and film significantly influence SPP-transmission characteristics. These effects are due to the restriction of SPPs at the air–silver interface and resonance around the rectangle.     

Enhanced transmission through periodic arrays of sub-wavelength holes with different media

The enhanced transmission of a metallic photonic crystal slab filled with different linear media in the hole-structure is investigated theoretically. The results show that the transmission peaks exhibit a red shift when the dielectric constants of the media are increased. By varying the thickness of the linear media, the enhanced transmission peak generated by the holes has a great shift and the one generated by the surface plasmon moves slightly. By changing the position of the linear media, these two transmission peaks have no significant shift.     

Optical transmission through double-layer compound metallic gratings with subwavelength slits

Optical transmission through a double-layer compound metallic grating (DCMG) composed of two identical compound metallic gratings (CMGs) with two subwavelength slits filled with different dielectrics inside each period is investigated by using the finite-difference time-domain method. The results show that the transmission properties of the DCMG are dependent on both the separation G between the two metallic layers and the phase configurations of the electromagnetic waves at the exits of adjacent slits of each layer. When a suitable separation (G?～?300?nm) is chosen, for the DCMG a notable transmission peak emerges at a certain wavelength, at which phase resonance appears for the corresponding CMG, while the transmission spectra of the corresponding double-layer simple metallic gratings (DSMGs) with the separation (G?～?300?nm) exhibit unexpected transmission suppression in a broad spectral region. When G?>?340?nm, the intensity of the transmission around the wavelength for the DCMG gradually decreases down to almost zero as G increases, while the high transmission is nearly maintained for the corresponding DSMGs.     

Splitting and unidirectional excitation of surface plasmon polaritons by two uniform metallic nanoslits with a nanocavity antenna

A tunable splitter and source of surface plasmon polaritons (SPPs) is proposed. The structure is composed of two Ag films, with two uniform nanoslits fabricated in the lower Ag film and lying above is a movable Ag film. By changing the horizontal position of the top Ag film, the splitting ratio of the SPPs varies periodically based on SPP interference. The field distribution of the structure is investigated by using the finite-difference time-domain (FDTD) method. The period obtained by the FDTD agrees well with the Fabry–Pérot cavity model. When the SPP splitting ratio is large (or small), our structure acts as a low noise SPP source. Compared with other SPP sources, our model possesses better directionality.     

Anatomical analysis of the light transmission through a single subwavelength metal slit by the FDTD method

Transverse-magnetic (TM) polarized light transmission through a single subwavelength metal slit is re-examined with finite-difference time-domain (FDTD) simulations. In contrast to previous studies, we derive an anatomical view of the electromagnetic field distribution in different cross-sections and emphasize the generation of a field coupling mode in the slit. Numerical modeling reveals that both peak and dip transmissions are features of the field interference in the slit. The slit width and depth are mainly responsible for establishing the amplitude and phase, respectively, of the coupled mode. Moreover, it is found that the output energy dispensation between the radiative and surface components is actually determined by the slit width. Analysis of the physical properties of the slit, including the coupled mode structure in the transverse plane, the effective refractive index and Ohmic absorption losses, provides new insights into the light transmission processes.     

Enhancement of photoconduction in a conjugated polymer through doping with copper nanoparticles

We investigated the effect of doping with copper nanoparticles (Cu NPs) on photoconductivity of poly(3-hexylthiophene) P3HT films. ∼20 nm Cu NPs were fabricated in a simple one step reduction process in hydrophobic environment. Structure, morphology and optical properties of the Cu nanoclusters were characterized. Films of P3HT containing Cu NPs were fabricated and their optical and electrical properties investigated. The results indicate that the change of the efficiency of the light absorption brought about by the effect of plasmonic resonances is minor, but there is a substantial influence of the metal nanoparticles on the efficiency of the photogeneration of charges in P3HT.     

Fano resonances of a metal nanorod array with a symmetry breaking wedge and gain medium filling

Fano resonances of a metal nanorod array with a symmetry breaking wedge and gain medium filling have been explored using the finite-difference time-domain method. Results show that a periodic symmetry breaking nanorod array supports Fano resonance due to the interaction between a hybridized dipolar plasmon mode of the nanorod and a narrower quadrupolar mode of the slice. By a tiny increase of the imaginary part of the dielectric constant of the gain medium, the Ohmic loss can be counteracted, and Fano resonance dip gets deeper significantly. Additionally, the modulation depth can be improved by changing the real part of the gain medium dielectric constant. The results found are useful for further develop of the devices on the basis of the Fano resonance and its modulation.     

Formation and evolution mechanisms of transmission dips in a compound metallic grating with perpendicular cut

The transmission properties are investigated of a compound metallic grating in which each period is comprised of two slits but only one is engraved with a perpendicular cut are investigated. Based on Fabry–Parot (FP) resonances and phase resonances, the formation and evolution mechanisms of transmission dips are analyzed. We show that there is a corresponding relationship between the depth of transmission dips and the phase difference of magnetic fields in adjacent slits. In particular, the absorption intensity is powerfully enhanced as phase resonances take place and absorption peaks corresponding to transmission dips exhibit blue-shift or red-shift with an increase in depth of the perpendicular cut. We also show that the variation of transmission peaks and bandwidth of transmission dips can be interpreted by a compound of the transmission spectra of two corresponding simple gratings.     

Plasmonic-induced transparency in a metallic stub with two cuts and transmission line model

We investigate a metal–insulator–metal (MIM) structure, which is composed of a bus MIM waveguide and a stub modified by two cuts. A transmission line (TL) model is proposed to depict the propagation characteristics of surface plasmon polaritons (SPPs). The finite element method is conducted to calculate the transmission spectrum of SPPs. Plasmonic-induced transparency spectral response can be achieved when different cuts are introduced. Under different parameters, theoretical results based on the TL model match with the simulation results very well. It is believed that our findings provide a smart way to design MIM-based plasmonic sensors and slow light devices.     

Experimental investigation of heat transfer in a horizontal tube falling film absorber with aqueous solutions of LiBr with and without surfactants

To learn about the physical background of the absorption process, the accompanying heat transfer was studied experimentally. A horizontal tube heat exchanger, similar to commercial absorbers, was built. Two different types of tubes were tested, one with a plain, the other with a knurled surface. The solution volume flow, the temperatures of the cooling water and of the solution, and the concentration of the solution were varied to search for correlations of the heat-transfer coefficient with the physical properties of the solution. The heat-transfer coefficients decrease with increasing viscosity and augmenting surface tension. They also depend on the fluid regime of the solution film, i.e. they increase with growing solution volume flow. The influence of two surfactants — 1-octanol and 2-ethyl-1-hexanol in various concentrations — on the absorption process was examined quantitatively. An increase of 60–140% was measured for the heat-transfer coe coefficients.