Dang Yuan Lei

Plasmonic light-harvesting devices over the whole visible spectrum.

On the basis of conformal transformation, a general strategy is proposed to design plasmonic nanostructures capable of an efficient harvesting of light over a broadband spectrum. The surface plasmon modes propagate toward the singularity of these structures where the group velocity vanishes and energy accumulates. A considerable field enhancement and confinement is thus expected. Radiation losses are also investigated when the structure dimension becomes comparable to the wavelength.

Hybrid nanoparticle–microcavity-based plasmonic nanosensors with improved detection resolution and extended remote-sensing ability

Optical nanosensors based on plasmonic nanoparticles have great potential for chemical and biological sensing applications, but their spectral detection resolution is severely constrained by their broad resonance linewidth, and their spatial sensing depth is limited to several tens of nanometres. Here we demonstrate that coupling a strong dipolar plasmonic resonance of a single metallic nanoparticle to the narrow bandwidth resonances of an optical microcavity creates a hybrid mode and discretizes the broad localized resonance, boosting the sensing figure-of-merit by up to 36 times. This cavity–nanoparticle system effectively combines the advantages of Fabry–Perot microresonators with those of plasmonic nanoparticles, providing interesting features such as remote-sensing ability, incident-angle independent resonances, strong polarization dependence, lateral ultra small sensing volume and strongly improved detection resolution. Such a hybrid system can be used not only to locally monitor specific dynamic processes in biosensing, but also to remotely sense important film parameters in thin-film nanometrology.

Role of Defects in the Phase Transition of VO2 Nanoparticles Probed by Plasmon Resonance Spectroscopy

Defects are known to affect nanoscale phase transitions, but their specific role in the metal-to-insulator transition in VO(2) has remained elusive. By combining plasmon resonance nanospectroscopy with density functional calculations, we correlate decreased phase-transition energy with oxygen vacancies created by strain at grain boundaries. By measuring the degree of metallization in the lithographically defined VO(2) nanoparticles, we find that hysteresis width narrows with increasing size, thus illustrating the potential for domain boundary engineering in phase-changing nanostructures.

Subgroup decomposition of plasmonic resonances in hybrid oligomers: modeling the resonance lineshape.

Plasmonic resonances with a Fano lineshape observed in metallic nanoclusters often arise from the destructive interference between a dark, subradiant mode and a bright, super-radiant one. A flexible control over the Fano profile characterized by its linewidth and spectral contrast is crucial for many potential applications such as slowing light and biosensing. In this work, we show how one can easily but significantly tailor the overall spectral profile in plasmonic nanocluster systems, for example, quadrumers and pentamers, by selectively altering the particle shape without a need to change the particle size, interparticle distance, or the number of elements of the oligomers. This is achieved through decomposing the whole spectrum into two separate contributions from subgroups, which are efficiently excited at their spectral peak positions. We further show that different strengths of interference between the two subgroups must be considered for a full understanding of the resulting spectral lineshape. In some cases, each subgroup is separately active in distinct frequency windows with only small overlap, leading to a simple convolution of the subspectra. Variation in particle shape of either subgroup results in the tuning of the overall spectral lineshape, which opens a novel pathway for shaping the plasmonic response in small nanoclusters.

Revealing Plasmonic Gap Modes in Particle-on-Film Systems Using Dark-Field Spectroscopy

Polarization-controlled excitation of plasmonic modes in nanometric Au particle-on-film gaps is investigated experimentally using single-particle dark-field spectroscopy. Two distinct geometries are explored: nanospheres on top of and inserted in a thin gold film. Numerical simulations reveal that the three resonances arising in the scattering spectra measured for particles on top of a film originate from highly confined gap modes at the interface. These modes feature different azimuthal characteristics, which are consistent with recent theoretical transformation optics studies. On the other hand, the scattering maxima of embedded particles are linked to dipolar modes having different orientations and damping rates. Finally, the radiation properties of the particle-film gap modes are studied through the mapping of the scattered power within different solid angle ranges.

Hierarchical Porous Plasmonic Metamaterials for Reproducible Ultrasensitive Surface‐Enhanced Raman Spectroscopy

Hierarchical porous plasmonic metamaterials consisting of periodic nanoholes with tunable diameter and uniformly distributed mesopores over the bulk are developed as a new class of 3D surface-enhanced Raman spectroscopy (SERS) substrates. This multiscale architecture not only facilitates efficient cascaded electromagnetic enhancement but also provides an enormous number of Raman-active binding sites, exhibiting excellent reproducibility and ultrasensitive detection of aromatic molecules down to 10(-13) M.

Plasmonic Hybridization between Nanowires and a Metallic Surface: A Transformation Optics Approach

The interaction between metallic nanowires and a metal substrate is investigated by means of transformation optics. This plasmonic system is of particular interest for single molecule detection or nanolasers. By mapping such a plasmonic device onto a metal-insulator-metal infinite structure, its optical response can be fully derived analytically. In this article, the absorption cross-section of a nanowire placed close to a metallic surface is derived within and beyond the quasi-static limit. The system is shown to support several modes characterized by a different angular momentum and whose resonance red-shifts when the nanoparticle approaches the metal substrate. These resonances give rise to a drastic field enhancement (>10(2)) within the narrow gap separating the nanoparticle from the metal surface. The case of a nanowire dimer is also investigated and is closely related to the previous configuration. More physical insights are provided especially with respect to the invisibility dips appearing in the radiative spectrum. Numerical simulations have also been performed to confirm our analytical predictions and determine their range of validity.

Broadband nano-focusing of light using kissing nanowires

A strategy has been proposed recently to design plasmonic nano- structures capable of efficient harvesting of light over a broadband spectrum. Applying a singular conformal transformation to a metal-insulator-metal infinite structure, the optical response of two kissing nanowires can be deduced analytically. This nanostructure is shown to exhibit a large and continuous absorption cross-section relative to its physical size over the whole visible spectrum. Considerable field enhancement and confinement at the nano-scale are also expected at the touching point. Actually, instead of transporting the energy out to infinity, like in a metal slab geometry, the surface plasmon modes here propagate towards the singularity of the structure where their velocity vanishes and energy accumulates. The field enhancement is then a balance between this energy accumulation and dissipation losses. The asymptotic case of a nanowire placed on top of a metal plate is shown to be of great interest for nanofocusing. Finally, numerical simulations are performed to investigate the effect of radiative losses when the structure dimension becomes comparable to the wavelength.

Tunable surface plasmon mediated emission from semiconductors by using metal alloys

The authors have explored the possibility of using binary metal alloys on surface plasmon mediated emission from semiconductor. By adjusting the alloy composition, they have found that the surface plasmon resonance energy can be tuned to match with the emission energy of semiconductor so that the energy transfer process between the semiconductor and surface plasmons can be optimized. They have calculated the plasmonic density of states and Purcell factor for ZnO and ZnTe at different alloy compositions and the results support the argument. Experimentally, they have prepared AlxAg1−x∕ZnO films at different compositions and have measured their photoluminescence. The band-edge emission from Al0.8Ag0.2∕ZnO is found to be ∼60 times stronger than that of bare ZnO, which is consistent with the theoretical prediction. As a result, metal alloys can be considered as a simple and effective means in optimizing the surface plasmon mediated emission.

Single-particle plasmon resonance spectroscopy of phase transition in vanadium dioxide.

We demonstrate thermally controlled plasmon resonance modulation of single gold nanoparticles on vanadium dioxide thin films by performing dark-field spectroscopy measurements at different temperatures. The plasmon resonance of the nanoparticles exhibits a significant blueshift in the visible range when the vanadium dioxide film undergoes its insulator-to-metal phase transition around 67 °C. More importantly, the resonance shift shows a clear hysteresis, mirroring the behavior of the vanadium dioxide film. At a fixed wavelength, the scattering intensity of Au particles also shows a hysteretic behavior decorated with an overshoot before (after) the insulator-metal (metal-insulator) phase transition of the vanadium dioxide film, suggesting that the nanoparticle is probing local variations in the phase transition.