Silver Nanoslit Research Articles

A Concave Blazed-Grating-Based Smartphone Spectrometer for Multichannel Sensing

A compact imaging smartphone spectrometer utilizing a lightweight aspheric concave blazed-grating is presented for multichannel spectral detection. The lightweight concave diffraction grating is fabricated on a plastic substrate using a two-step thermal-embossing method in a cost-effective and mass-productive way, which enables a large-area, smooth and aspheric concave surface while preserving a blazed-grating structure. The blazed-grating serves as a key single element for aberration-free light focusing and highly-efficient light diffraction. It thus eliminates extra moving optics in the spectrometer and improves the optical characteristics and system integration, which are proven by both simulation and experiments. Spectral linewidth of 1 nm is achieved by combining the concave grating with a smartphone camera. This compact imaging smartphone spectrometer is only about the size of a match-box. Spectra can be obtained directly from the detected images using a developed application (APP) interface. High-throughput plasmonic resonance sensing using microfluidic silver nanoslit arrays under various glycerin environments is demonstrated, and the detected resonance wavelength response is comparable with that detected by a commercial compact spectrometer.

Surface-Emitting Surface Plasmon Polariton Laser in a Second-Order Distributed Feedback Defect Cavity

The coupling between photons and lattice plasmons in metallic nanostructure arrays makes them good candidates to build radiative surface plasmon polariton (SPP) lasers. We report lasing action from the defect state in a second-order distributed feedback SPP cavity. In this device, a silver nanoslit array confines the SPPs in its second-order bandgap, while a phase shift section in the array introduces a high-quality-factor defect state for SPP lasing. At the same time, the fundamental order grating diffraction of the nanoslit array provides a high extraction efficiency from SPPs to photons. Using optical pumping, we demonstrate pulsed lasing at room temperature, with a collimated emission in the surface-normal direction and a high single-mode fiber coupling efficiency. Together with its compact two-dimensional geometry and lasing wavelength tunability, the proposed device is a promising candidate for small footprint and low-cost surface-emitting lasers.

Analysis of Dual-Core Photonic Crystal Fiber Based on Surface Plasmon Resonance Sensor with Segmented Silver Film

A dual-core photonic crystal fiber (PCF) based on surface plasmon resonance (SPR) sensor with segmented silver film (silver nanoslit) deposited in microfluidic channel is designed. The coupling characteristics and sensing performance of the proposed sensor are analyzed numerically by finite element method (FEM). The resonance intensity and wavelength sensitivity can be tuned by the segmented number and angle of the silver nanoslit, due to a strong coupling between propagation surface plasmon resonance (PSPR) on the silver film surface and local surface plasmon resonance (LSPR) at the silver nanoslit. When the segmented number is 40 and the segmented angle is 0.5°, an average sensitivity of 5100 nm/RIU is obtained in the refractive index (RI) range from 1.330 to 1.360, and is 1080 nm/RIU higher than that of continuous silver film structure under the same conditions. In addition, a maximum wavelength sensitivity of 7600 nm/RIU is achieved in the RI range from 1.355 to 1.360, corresponding to a high RI resolution of 1.32 × 10−5 RIU, which can be used to detect the slight change in the RI of an unknown analyte.

Enhancing Surface Sensing Sensitivity of Metallic Nanostructures using Blue-Shifted Surface Plasmon Mode and Fano Resonance

Improving surface sensitivities of nanostructure-based plasmonic sensors is an important issue to be addressed. Among the SPR measurements, the wavelength interrogation is commonly utilized. We proposed using blue-shifted surface plasmon mode and Fano resonance, caused by the coupling of a cavity mode (angle-independent) and the surface plasmon mode (angle-dependent) in a long-periodicity silver nanoslit array, to increase surface (wavelength) sensitivities of metallic nanostructures. It results in an improvement by at least a factor of 4 in the spectral shift as compared to sensors operated under normal incidence. The improved surface sensitivity was attributed to a high refractive index sensitivity and the decrease of plasmonic evanescent field caused by two effects, the Fano coupling and the blue-shifted resonance. These concepts can enhance the sensing capability and be applicable to various metallic nanostructures with periodicities.

Ultrahigh-Q metallic nanocavity resonances with externally-amplified intracavity feedback.

We propose a mechanism of ultrahigh-Q metallic nanocavity resonances that involves an efficient loss-compensation scheme favorable for room-temperature operation. We theoretically show that surface plasmon-polaritons excited on the entrance and exit interfaces of a metallic nanocavity array efficiently transfer external optical gain to the cavity modes by inducing resonantly-amplified intracavity feedback. Surprisingly, the modal gain in the nanocavity with the externally amplified feedback is inversely proportional to the cavity length as opposed to conventional optical cavity amplifiers requiring longer cavities for higher optical gain. Utilizing this effect, we numerically demonstrate room-temperature nanocavity resonance Q-factor exceeding 104 in a 25-nm-wide silver nanoslit array. The proposed mechanism provides a highly efficient plasmonic amplification process particularly for subwavelength plasmonic cavities which are essential components in active nanoplasmonic devices.

Enhanced optical transmission through a nanoslit by coupling light between periodic strips and metal film

Surface plasmon resonance (SPR) is obtained by exciting the surface plasmon (SP) at the metal and dielectric interface, which can greatly enhance the extraordinary optical transmission (EOT) through a nanoslit milled in the metal film. We present a structure with a 50-nm-wide silver nanoslit for EOT by coupling light into the dielectric interlayer between periodic strips and a metal film. When the period of the metallic strips is equal to the wavelength of the SPR, the transmission efficiency of 187.6 through the nanoslit is enhanced. The metallic strip width over the nanoslit is optimized to improve transmission efficiency, and the maximal efficiency of 204.3 is achieved.

Nanocavity antenna array for enhancing extraordinary optical transmission of light through a metallic nanoslit

The transmission of light through a single nanoslit milled in the metallic film is enhanced by using an array of nanocavity antennas. It is shown that transmission efficiency through a 25-nm-wide silver nanoslit can be enhanced to η=151 when six pairs of nanostrips are placed 50 nm away from the input surface of the silver film at wavelength λ0=1 μm. The influences of both the nanoslit position and the slit-to-strip distance on the extraordinary transmission are investigated, and the physical mechanism is explained.

Near- to far-field imaging of free-space and surface-bound waves emanating from a metal nanoslit

The authors report the radiation pattern (radial and angular distribution of light intensity) of a silver nanoslit measured in the near- to far-field regimes. In most far fields, the 1∕r dependence of intensity distribution, expected from a cylindrical wave emanating from a line source, is clearly observed. The glancing angle regime is found to be governed by the presence of surface plasmons, showing higher intensity closer to the metal surface. From the radiation patterns measured with a tilted-probe, radial-scan method, a branching ratio is quantitatively determined for the free-space radiation and surface plasmon components, emerging from the nanoslit.

High-sensitivity surface plasmon resonance spectroscopy based on a metal nanoslit array

We have chemically modified metal nanoslit array surfaces with alkanethiol self-assembled monolayers and have characterized the resulting spectral shift of optical transmission. Adsorption of a self-assembled monolayer (1.5nm thick) on a silver nanoslit array (slit width of 30–50nm and grating period of 360nm) is found to cause an 11nm redshift of the main transmission peak. Strong confinement of optical fields in the narrow slit region allows sensitive transduction of surface modification into a shift of surface plasmon resonance wavelength.

Blueshift of surface plasmon resonance spectra in anneal-treated silver nanoslit arrays

Silver nanoslit arrays were anneal treated in vacuum, and the effects on the surface morphology of silver and the surface plasmon resonance characteristics were investigated. Optical transmission through nanoslit arrays shows a distinctive change in the spectral profiles after annealing: A clear blueshift of the transmission peaks and dips (20nm shift for an anneal temperature of 150°C). Scanning electron microscopy reveals a morphological change of silver: Increased grain sizes, and smooth and round surface profiles after the anneal treatment. The observed blueshift of transmission spectra correlates well with the geometrical and dimensional changes of silver islands defined between slits, which are found to alter the surface plasmon resonance conditions involving various mechanisms in different regimes.