Qiaoqiang Gan

Experimental verification of the rainbow trapping effect in adiabatic plasmonic gratings

We report the experimental observation of a trapped rainbow in adiabatically graded metallic gratings, designed to validate theoretical predictions for this unique plasmonic structure. One-dimensional graded nanogratings were fabricated and their surface dispersion properties tailored by varying the grating groove depth, whose dimensions were confirmed by atomic force microscopy. Tunable plasmonic bandgaps were observed experimentally, and direct optical measurements on graded grating structures show that light of different wavelengths in the 500–700-nm region is “trapped” at different positions along the grating, consistent with computer simulations, thus verifying the “rainbow” trapping effect.

Plasmonic Mach-Zehnder interferometer for ultrasensitive on-chip biosensing.

We experimentally demonstrate a plasmonic Mach-Zehnder interferometer (MZI) integrated with a microfluidic chip for ultrasensitive optical biosensing. The MZI is formed by patterning two parallel nanoslits in a thin metal film, and the sensor monitors the phase difference, induced by surface biomolecular adsorptions, between surface plasmon waves propagating on top and bottom surfaces of the metal film. The combination of a nanoplasmonic architecture and sensitive interferometric techniques in this compact sensing platform yields enhanced refractive index sensitivities greater than 3500 nm/RIU and record high sensing figures of merit exceeding 200 in the visible region, greatly surpassing those of previous plasmonic sensors and still hold potential for further improvement through optimization of the device structure. We demonstrate real-time, label-free, quantitative monitoring of streptavidin-biotin specific binding with high signal-to-noise ratio in this simple, ultrasensitive, and miniaturized plasmonic biosensor.

Design of plasmonic back structures for efficiency enhancement of thin-film amorphous Si solar cells

Metallic back structures with one-dimensional periodic nanoridges attached to a thin-film amorphous Si (a-Si) solar cell are numerically studied. At the interfaces between a-Si and metal materials, the excitation of surface-plasmon polaritons leads to obvious absorption enhancements in a wide near-IR range for different ridge shapes and periods. The highest enhancement factor of the cell external quantum efficiency is estimated to be 3.32. The optimized structure can achieve an increase of 17.12% in the cell efficiency.

Plasmonic surface-wave splitter

The authors present an analysis of a plasmonic surface-wave splitter, simulated using a two-dimensional finite-difference time-domain technique. A single subwavelength slit is employed as a high-intensity nanoscale excitation source for plasmonic surface waves, resulting in a miniaturized light-surface plasmon coupler. With different surface structures located on the two sides of the slit, the device is able to confine and guide light waves of different wavelengths in opposite directions. Within the 15 mu m simulation region, it is found that the intensity of the guided light at the interface is roughly two to eight times the peak intensity of the incident light, and the propagation length can reach approximately 42 and 16 mu m and at the wavelengths of 0.63 and 1.33 mu m, respectively. (c) 2007 American Institute of Physics.

Bidirectional subwavelength slit splitter for THz surface plasmons.

We have conducted a feasibility study of a frequency splitter operating at THz frequencies, based on a bidirectional subwavelength slit simulated using two-dimensional finite difference time domain (FDTD) techniques. The near-field wave emanating from the narrow slit serves as a subwavelength-scaled excitation source. By placing two optimized grating structures on the opposite sides of the slit, the THz waves at different frequencies are guided in the two desired directions. Confinement of the optical field is illustrated for different surface structures.

Polymeric photovoltaics with various metallic plasmonic nanostructures

Broadband light absorption enhancement is numerically investigated for the active light harvesting layer of an organic photovoltaic (OPV), which consists of a blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Periodic plasmonic nanostructures placed above and below the active layer incorporate Ag, Al, Au, or a combination of two different metals. Three dimensional (3D) full-field electromagnetic simulations are applied to determine the effect of varying the metal employed in the plasmonic nanostructures on the absorption enhancement of the OPV. In addition, the geometric parameters (e.g., film thickness, period, and diameter) of the symmetrically distributed top and bottom metal (Ag, Al, or Au) nanostructures were varied to optimize the device structure and delineate the mechanism(s) leading to the absorption enhancement. A spectrally broadband, polarization-insensitive, and wide-angle absorption enhancement is obtained using a double pl...

Vertical Plasmonic Mach-Zehnder interferometer for sensitive optical sensing

By observing the wavelength shift of the peaks or valleys of the SPP interference pattern, a highly compact vertical plasmonic MZI with a potential to achieve a very high sensitivity is proposed.

From Waveguiding to Spatial Localization of THz Waves Within a Plasmonic Metallic Grating

In this paper, we have derived the dispersion relations for terahertz (THz) waves propagating in a direction parallel to the surface of a perfect conducting plane hosting an array of 1-D grooves having a constant depth. Since the dispersion relations for the fundamental surface mode resemble those for the surface plasmon polaritons (SPPs) at optical frequencies, such a uniform grating can be used to effectively guide THz waves. The dispersion relations for these spoof SPPs can be tailored by choosing the geometric parameters of the surface structures. We show that a graded metallic surface grating can be used to trap THz waves at different locations of the surface grating corresponding to the different wavelengths of the waves.

Bidirectional surface wave splitter at visible frequencies

We experimentally demonstrate a metal-film bidirectional surface wave splitter for guiding light at two visible wavelengths in opposite directions. Two nanoscale gratings were patterned on opposite sides of a subwavelength slit. The metallic surface grating structures were tailored geometrically to have different plasmonic bandgaps, enabling each grating to guide light of one wavelength and prohibit propagation at the other wavelength. The locations of the bandgaps were experimentally confirmed by interferometric measurements. Based on these design principles, a green-red bidirectional surface wave splitter is demonstrated, and the observed optical properties are shown to agree with theoretical predictions.

Rainbow trapping and releasing by chirped plasmonic waveguides at visible frequencies

We report on “trapping rainbow” on silver films covered by a one-dimensional chirped dielectric grating. We attribute the trapping effect to correlative dispersive relation between the excited surface plasmon polaritons (SPPs) and the lattice constant of the dielectric grating on the metal film, which results in localization of SPPs of different frequencies at different spatial positions. We further reveal that, by attaching another uniform dielectric grating on the other side of the metal film and real time tuning the refractive index of the grating, the trapped rainbow can be released in sequence. Analytical results are demonstrated by finite-difference time-domain simulation.