Shunping Zhang

Substrate-Induced Fano Resonances of a Plasmonic Nanocube: A Route to Increased-Sensitivity Localized Surface Plasmon Resonance Sensors Revealed

Symmetry-breaking introduced by an adjacent semi-infinite dielectric can introduce coupling and hybridization of the plasmon modes of a metallic nanostructure. This effect is particularly large for entities with a large contact area adjacent to the dielectric. For a nanocube, a nearby dielectric mediates an interaction between bright dipolar and dark quadrupolar modes, resulting in bonding and antibonding hybridized modes. The Fano resonance that dominates the scattering spectrum arises from the interference of these modes. This analysis provides a strategy for optimizing the sensitivity of nanostructures, whether chemically synthesized or grown by deposition methods, as high-performance localized surface plasmon resonance sensors.

Quantum Dot-Based Local Field Imaging Reveals Plasmon-Based Interferometric Logic in Silver Nanowire Networks

We show that the local electric field distribution of propagating plasmons along silver nanowires can be imaged by coating the nanowires with a layer of quantum dots, held off the surface of the nanowire by a nanoscale dielectric spacer layer. In simple networks of silver nanowires with two optical inputs, control of the optical polarization and phase of the input fields directs the guided waves to a specific nanowire output. The QD-luminescent images of these structures reveal that a complete family of phase-dependent, interferometric logic functions can be performed on these simple networks. These results show the potential for plasmonic waveguides to support compact interferometric logic operations.

Plasmonic Properties of Gold Nanoparticles Separated from a Gold Mirror by an Ultrathin Oxide

That a nanoparticle (NP) (for example of gold) residing above a gold mirror is almost as effective a surface enhanced Raman scattering (SERS) substrate (when illuminated with light of the correct polarization and wavelength) as two closely coupled gold nanoparticles has been known for some time. The NP-overmirror (NPOM) configuration has the valuable advantage that it is amenable to top-down fabrication. We have fabricated a series of Au-NPOM substrates with varying but thin atomic layer-deposited oxide spacer and measured the SERS enhancement as a function of spacer thickness and angle of incidence (AOI). These were compared with high-quality finite-difference time-domain calculations, which reproduce the observed spacer thickness and AOI dependences faithfully. The SERS intensity is expected to be strongly affected by the AOI on account for the fact that the hot spot formed in the space between the NP and the mirror is most efficiently excited with an electromagnetic field component that is normal to the surface of the mirror. Intriguingly we find that the SERS intensity maximizes at ~60° and show that this is due to the coherent superposition of the incident and the reflected field components. The observed SERS intensity is also shown to be very sensitive to the dielectric constant of the oxide spacer layer with the most intense signals obtained when using a low dielectric constant oxide layer (SiO(2)).

Tunable SERS in Gold Nanorod Dimers through Strain Control on an Elastomeric Substrate

In this Letter we provide experimental verification of the interparticle distance dependence of the SERS enhancement factor in 1 μm gold gapped nanorods. Au dimers are fabricated from electrochemically grown heterogeneous Au-Ag-Au nanorods and deposited on a stretchable elastomer film which allows active and reversible tuning of the interparticle gap on the sub-5-nm level. Significantly, this technique allows the distance dependence to be tracked using a single dimer, thereby avoiding enhancement factor reproducibility issues arising from morphological differences in disparate nanoparticle pairs.

Chiral Surface Plasmon Polaritons on Metallic Nanowires

Chiral surface plasmon polaritons (SPPs) can be generated by linearly polarized light incident at the end of a nanowire, exciting a coherent superposition of three specific nanowire waveguide modes. Images of chiral SPPs on individual nanowires obtained from quantum dot fluorescence excited by the SPP evanescent field reveal the chirality predicted in our theoretical model. The handedness and spatial extent of the helical periods of the chiral SPPs depend on the input polarization angle and nanowire diameter as well as the dielectric environment. Chirality is preserved in the free-space output wave, making a metallic nanowire a broad bandwidth subwavelength source of circular polarized photons.

Highly tunable propagating surface plasmons on supported silver nanowires

Surface plasmons, the quanta of the collective oscillations of free electrons at metal surface, can be easily tuned by changing the surrounding dielectric materials, which is well known for metal nanoparticles and metal surfaces, but less is known for one-dimensional metal nanowires. Here, we find an extremely large tunability of surface plasmons on Ag nanowires with a beat period of the near-field distribution pattern increasing by 90 nm per nanometer of Al2O3 coating, or by 16 µm per refractive index unit change in the surrounding medium. Such high sensitivity is crucial to directly control the optical signal distribution for various routing and demultiplexing functions in plasmonic circuits and may pave the way to the development of on-chip ultrasensitive biosensing.

Recent Advances in Plasmonic Sensors

Plasmonic sensing has been an important multidisciplinary research field and has been extensively used in detection of trace molecules in chemistry and biology. The sensing techniques are typically based on surface-enhanced spectroscopies and surface plasmon resonances (SPRs). This review article deals with some recent advances in surface-enhanced Raman scattering (SERS) sensors and SPR sensors using either localized surface plasmon resonances (LSPRs) or propagating surface plasmon polaritons (SPPs). The advances discussed herein present some improvements in SERS and SPR sensing, as well as a new type of nanowire-based SPP sensor.

Coherent Modulation of Propagating Plasmons in Silver-Nanowire-Based Structures

MOST [2009CB930700]; NSFC [10904171, 10625418, 10874233, 11004237]; CAS; Robert A. Welch Foundation [C-1220, C-1222]

Ultrasensitive Size-Selection of Plasmonic Nanoparticles by Fano Interference Optical Force

In this paper, we propose a solution for the ultrasensitive optical selection of plasmonic nanoparticles using Fano interference-induced scattering forces. Under a Gaussian beam excitation, the scattering of a plasmonic nanoparticle at its Fano resonance becomes strongly asymmetric in the lateral direction and consequently results in a net transverse scattering force, that is, Fano interference-induced force. The magnitude of this transverse scattering force is comparable with the gradient force in conventional optical manipulation experiments. More interestingly, the Fano scattering force is ultrasensitive to the particle size and excitation frequency due to the phase sensitivity of the interference between adjacent plasmon modes in the particle. Utilizing this distinct feature, we show the possibility of size-selective sorting of silver and gold nanoparticles with an accuracy of about ±10 nm and silica-gold core-shell nanoparticles with shell thickness down to several nanometers. These results would add to the toolbox of optical manipulation and fabrication.

Reduced linewidth multipolar plasmon resonances in metal nanorods and related applications

Using dark-field scattering spectroscopy, we study the multipolar plasmon resonances in single crystallized silver nanorods. The lineshapes and homogenous linewidths of the surface plasmon resonances (SPRs) of different orders are analyzed and compared. The high-order resonances are found to sustain asymmetric Fano lineshapes and their linewidths are narrower than the dipolar resonance. A quantitative comparison using the finite element method reveals more than a three times reduction in the linewidth for the third order resonance, as compared with the dipolar one. These narrow linewidths result from the smaller radiative damping of the multipolar SPRs. Benefiting from the reduced damping, multipolar SPRs in nanorods are better candidates for many plasmonic applications, including increased-sensitivity single particle SPR sensors and reduced-threshold nanolasers.