Yongkang Gao

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.

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.

Rapid and highly sensitive detection using Fano resonances in ultrathin plasmonic nanogratings

We developed a nanoplasmonic sensor platform employing the extraordinary optical properties of one-dimensional nanogratings patterned on 30 nm-thick ultrathin Ag films. Excitation of Fano resonances in the ultrathin Ag nanogratings results in transmission spectra with high amplitude, large contrast, and narrow bandwidth, making them well-suited for rapid and highly sensitive sensing applications. The ultrathin nanoplasmonic sensor chip was integrated with a polydimethylsiloxane microfluidic channel, and the measured refractive index resolution was found to be 1.46 × 10−6 refractive index units with a high temporal resolution of 1 s. This compares favorably with commercial prism-based surface plasmon resonance sensors, but is achieved using a more convenient collinear transmission geometry and a significantly smaller sensor footprint of 50 × 50 μm2. In addition, an order-of-magnitude improvement in the temporal and spatial resolutions was achieved relative to state-of-the-art nanoplasmonic sensors, for com...

Plasmonic interferometers for label-free multiplexed sensing

We report a plasmonic interferometric biosensor based on a simple slit-groove metallic nanostructure that monitors the phase changes of surface plasmon polaritons resulting from biomolecular adsorptions. The proposed sensing scheme integrates the strengths of miniaturized plasmonic architectures with sensitive optical interferometry techniques. Sensing peak linewidths as narrow as 7 nm and refractive index resolutions of 1 × 10(-5) RIU were experimentally measured from a miniaturized sensing area of 10 × 30 µm(2) using a collinear transmission setup and a low-cost compact spectrometer. A high-density array of such interferometric sensors was also fabricated to demonstrate its potential for real-time multiplexed sensing using a CCD camera for intensity interrogation. A self-referencing method was introduced to increase the sensitivity and reduce sensor noise for multiplexing measurements. The enhanced sensing performance, small sensor footprint, and simple instrumentation and optical alignment suggest promise to integrate this platform into low-cost label-free biosensing devices with high multiplexing capabilities.

Short-Range Surface Plasmon Polaritons for Extraordinary Low Transmission Through Ultra-Thin Metal Films with Nanopatterns

We provide both experimental and theoretical investigation on extraordinary low transmission through one-dimensional nanoslit and two-dimensional nanohole arrays on ultra-thin metal films. Unambiguous proofs demonstrate that short-range surface plasmon polaritons play a key role leading to this novel phenomenon, which could be useful for creating new polarization filters and other integrated plasmonic components.

A metal-insulator-metal plasmonic Mach-Zehnder interferometer array for multiplexed sensing

A multi-layered metal-insulator-metal plasmonic Mach-Zehnder interferometer (MZI) is proposed to work as an array for multiplexed sensing. The interference patterns based on wavelength modulation and intensity modulation are modeled analytically and numerically, showing a high figure of merit over 170 for intensity-interrogated sensing. The proposed structure can overcome the one-slit illumination limitation of previously reported single-layered double-slit plasmonic MZI and will enable portable, high-throughput and sensitive biosensing applications.

Breakthroughs in Photonics 2013: Research Highlights on Biosensors Based on Plasmonic Nanostructures

Label-free biomolecular sensing is by far the most common and successful application area in the emerging field of nanoplasmonics. This review paper highlights the latest progress and achievements made in this area. Key aspects of the nanoplasmonic sensor development, including performance enhancement, efforts to increase multiplexing capacity, and the progress in sensor integration and miniaturization, are discussed.

Experimental verification of the “rainbow” trapping effect in adiabatic plasmonic gratings

Direct measurements on graded grating structures show that light of different wavelengths in the 500–700nm region is “trapped” at different positions along the grating, consistent with theoretical predictions, thus verifying the “rainbow” trapping effect.

Phase change dispersion of plasmonic nano-objects

The phase change dispersion during the surface plasmon wave coupling process was extracted experimentally using a slit-groove interferometer and validated through numerical simulation, enriching the fundamental understanding of plamsonic subwavelength optics on a chip.