Ya-Lun Ho

Coupling of localized surface plasmons to U-shaped cavities for high-sensitivity and miniaturized detectors

We report numerical analysis of the coupling of localized surface plasmons to the modes of U-shaped cavities. The coupling results in intense resonance for which the electric field is strongly enhanced on the cavity surfaces. As a result, an optical vortex in the power flow is formed in the cavities and a sharp and strong resonance dip is observed in the reflectance spectrum. High sensitivity of the dip wavelength to change in the refractive index of the surrounding medium is reported. The high sensitivity is realized with a small number of cavities, thus enabling miniaturization of detectors based on U-shaped cavities.

Plasmonic Hybrid Cavity-Channel Structure for Tunable Narrow-Band Optical Absorption

A hybrid plasmonic structure consisting of adjacent U-shaped cavities separated by a nanochannel is proposed for tunable and narrow-band selection of light. The hybrid cavity-channel structure achieves absorption resonance with a bandwidth, defined as the full-width at half-maximum, of 1.5 nm and tunable property in the near-infrared and infrared regions. The hybrid structure resonance originates in the coupling of horizontal surface plasmon mode of the U-cavity with channel mode, which sustains stationary-surface-plasmons in the channel with antinodes at the channel entrances enabling light concentration and nodes at the channel exits enabling light confinement. As a result of the coupling, a sharp and strong absorption resonance is readily adjustable by varying the geometrical parameters of the U-cavity while keeping the channel parameters unchanged.

Plasmon focusing in short gold sphere nanochains for surface-enhanced Raman scattering

Power-flow focusing in metal nanostructures is attracting growing attention to design efficient and tunable substrates for surface-enhanced Raman spectroscopy (SERS), and to propose a more reliable alternative to random surfaces for single-molecule sensing. In this paper, finite-difference time-domain simulations were used to explore the near-field amplification features of short chains of gold (Au) nanospheres. Short chains of gold spheres were found to induce stronger field enhancements than infinite chains due to a more efficient trapping and focusing of the incident energy. In addition, interaction with a suitably tuned SiO2/Au double-layer substrate was demonstrated to widen the resonances bandwidth, meeting another practical need for SERS.

Single-Step Electrophoretic Deposition of Non-noble Metal Catalyst Layer with Low Onset Voltage for Ethanol Electro-oxidation.

A single-step electrophoretic deposition (EPD) process is used to fabricate catalyst layers which consist of nickel oxide nanoparticles attached on the surface of nanographitic flakes. Magnesium ions present in the colloid charge positively the flakes surface as they attach on it and are also used to bind nanographitic flakes together. The fabricated catalyst layers showed a very low onset voltage (-0.2 V vs Ag/AgCl) in the electro-oxidation of ethanol. To clarify the occurring catalytic mechanism, we performed annealing treatment to produce samples having a different electrochemical behavior with a large onset voltage. Temperature dependence measurements of the layer conductivity pointed toward a charge transport mechanism based on hopping for the nonannealed layers, while the drift transport is observed in the annealed layers. The hopping charge transport is responsible for the appearance of the low onset voltage in ethanol electro-oxidation.

Spectrally Selective Photocapacitance Modulation in Plasmonic Nanochannels for Infrared Imaging

The optical response of subwavelength plasmonic structures can be used to monitor minute changes in their physical, chemical, and biological environments with high performance for sensing. The optical response in the far field is governed by the near-field properties of plasmon resonances. Sharp, tunable resonances can be obtained by controlling the shape of the structure and by using resonant cavities. However, microintegration of plasmonic structures on chips is difficult because of the readout in the far field. As such, structures that form an electrical microcircuit and directly monitor the near-field variation would be more desirable. Here, we report on an electronically readable photocapacitor based on a plasmonic nanochannel structure with high spectral resolution and a large modulation capability. The structure consists of metallic U-cavities and semiconductor channels, which are used to focus and confine light at the semiconductor-metal interfaces. At these interfaces, light is efficiently converted into photocarriers that change the electrical impedance of the structure. The capacitance modulation of the structure in response to light produces a light-to-dark contrast ratio larger than 10(3). A reflectance spectrum with a bandwidth of 16 nm and a 6% modulation depth is detected using a reactance variation of 3 kΩ with the same bandwidth. This photocapacitor design offers a practical means of monitoring changes induced by the near field and thus could be deployed in pixel arrays of image sensors for miniaturized spectroscopic applications.

Independent light-trapping cavity for ultra-sensitive plasmonic sensing

The sensing characteristics of an independent-plasmonic-cavity structure that traps light were investigated. The cavity structure traps light by generating self-contained optical vortices in each independent cavity without leakage or propagation of the light; therefore, strong and sharp resonance dips are obtained in a reflectance spectrum. Multiple optical vortices are generated in the independent cavities in higher-order plasmonic cavity modes at shorter wavelengths, realizing the resonance in a wide range from visible to near-infrared. Compared to the propagating surface plasmon resonance, the resonance of plasmonic cavity mode in the independent cavity does not depend on variation of the incident angle of light. The independent-cavity structure was fabricated by a simple process, and it experimentally demonstrated a high sensitivity (above 1500 nm per refractive index unit) and a figure of merit above 20.

Plasmonic tooth-multilayer structure with high enhancement field for surface enhanced Raman spectroscopy

The significant enhancement seen in surface-enhanced Raman scattering (SERS) heavily relies on the ability of plasmonic structures to strongly confine light. Current techniques used to fabricate plasmonic nanostructures have been limited in their reproducibility for bottom-up techniques or their feature size for top-down techniques. Here, we propose a tooth multilayer structure that can be fabricated by using physical vapor deposition and selective wet etching, achieving extremely small feature sizes and high reproducibility. A multilayer structure composed of two alternating materials whose thicknesses can be controlled accurately in the nanometer range is deposited on a flat substrate using ion-beam sputtering. Subsequent selective wet etching is used to form nanogaps in one of the materials constituting the multilayer, with the depth of the nanogaps being controlled by the wet etching time. Combining both techniques can allow the nanogap dimensions to be controlled at sub 10 nm length scale, thus achieving a tooth multilayer structure with high enhancement and tunability of the resonance mode over a broad range, ideal for SERS applications.

Plasmonic nanochannel structure for narrow-band selective thermal emitter

A plasmonic structure consisting of a periodic arrangement of vertical silicon nanochannels connected by U-shaped gold layers is demonstrated as a spectrally selective thermal emitter. The plasmonic nanochannel structure sustains a coupled mode between a surface plasmon polariton and a stationary surface plasmon resonance, which induces a strong and sharp resonance observed in the form of a reflectance dip in the far field. Upon heating the structure, a strong and narrow-bandwidth thermal emittance peak is observed with a maximum emittance value of 0.72 and a full-width-at-half-maximum of 248 nm at a wavelength of 5.66 μm, which corresponds to the reflectance dip wavelength. Moreover, we demonstrate the control of the emission peak wavelength by varying the period of the structure. The plasmonic nanochannel structure realizes a small-size and selective infrared thermal emitter, which is expected to be applicable as an infrared light source.

Fluid-controlled tunable infrared filtering in hollow plasmonic nanofin cavities

Subwavelength structures sustaining surface plasmons have been employed in numerous fields due to their small size and ability to manipulate light beyond the diffraction limit. Light filtering using small-size plasmonic devices is a promising means of portable spectroscopy for purposes such as on-site chemical analyses. However, most plasmonic filters can only tune the resonance band by modifying the geometry of the structure or changing the incident light angle. Here, we present a plasmonic nanofin-cavity structure having a narrow band with its resonance wavelength controlled by varying the fluid in the hollow cavities of the filter. Control of the narrow-band resonance is realized over a wide range because of the coupling between the stationary surface plasmons generated from the nanofin-cavity mode and the propagating surface plasmons. The hollow cavity design enables fluid to be easily injected and removed, so that the filtered band can be controlled without the need for a complex and bulky structure or application of an external voltage.

A 3D metallic structure array for refractive index sensing with optical vortex

A simple and large-scale fabrication technique for three dimensional structure arrays using a photolithography process was applied to realize an array of high-aspect-ratio metallic fins. The fin array enables light confinement between the high-aspect-ratio fins, thus generating optical vortices. The light confinement between the fins produces sharp dips in the reflection spectrum of the array. We show that the position of the dip wavelength is sensitive to change in the refractive index of the surrounding medium. Sensitivity to change in the refractive index was quantified by optical simulation and experimental measurements.