Hongming Shen

Photoluminescence of a single complex plasmonic nanoparticle

We report detailed investigations of the photoluminescence (PL) generated from an individual gold nanoflower, a highly branched plasmonic nanoparticle. Compared to nanostructures with simple shapes, such as spheres, nanorods, and bipyramids, nanoflowers exhibit more distinct features, i.e., the PL spectra and far-field emission patterns are strongly dependent on the wavelength and polarization of the excitation light. The experimental results are qualitatively explained using theoretical calculations. In addition, the intrinsic PL signal is highly dominated by localized surface plasmon resonances. The crucial role of plasmonic coupling in complex nanostructures during the plasmon-enhanced PL process is highlighted. The findings contribute to a deeper understanding of the PL properties of metallic nanoparticles. This study will be beneficial for several potential applications, including optical imaging and sensing in the fields of materials science and biology.

Shape effect on a single-nanoparticle-based plasmonic nanosensor

Plasmonic refractometric nanosensors based on single nanostructures, i.e. spherical, nanorodand bipyramid-shaped gold nanoparticles, are investigated and compared numerically by employing the finite-difference time-domain method. The results show that the plasmonic sensing ability is distributed anisotropically around the nanorod and bipyramid, even for spherical nanoparticles when the illumination light is linearly polarized. To optimize nanosensor performance, some anisotropy in the shape of nanoparticles is required, this latter serving as an intrinsic light polarization filter to suppress the disturbance from localized surface plasmon resonance in other directions. The plasmonic near-field can be engineered by controlling the shape to achieve a concentrated and localized electromagnetic field, in direct relation with the sensing ability. Taking these factors into account, the gold bipyramid nanoconstruct which is easily available in experiment is proposed as an efficient plasmonic sensing platform. The bipyramid presents both highly localized sensitivity and high scattering cross-section, thus avoiding the trade-off during the selection of the widely used nanorod-shaped sensors. The parameters of the bipyramid structure can be optimized by numerical simulation to improve the plasmonic sensing. Our findings permit a deeper understanding of single-nanoparticle-sensor behavior, and the study provides an opportunity to optimize the plasmonic sensor.

Strongly enhanced Raman scattering of graphene by a single gold nanorod

Individual gold nanorods (AuNRs) and monolayer graphene hybrid system is investigated experimentally. Surface enhanced Raman scattering (SERS) signal of the graphene is observed due to a single AuNR with enhancement factor up to ∼1000-fold. The SERS intensity is strongly polarization dependent and the enhancement effect varies with the detuning between the excitation laser and the AuNR resonance. The SERS effect is highest when the resonant wavelength of the AuNRs matches well with the excitation light. By correlating the scattering and photoluminescence, it is demonstrated that the conventional background in SERS ascribes to the photon emission of metallic nanostructures.

Strong two-photon fluorescence enhanced jointly by dipolar and quadrupolar modes of a single plasmonic nanostructure

A single gold nano-cylinder presenting multipolar plasmon resonances to enhance two-photon fluorescence is investigated employing three dimensional finite-difference time-domain method. Cylinders of large dimension usually display dipolar and quadrupolar plasmonic resonances. We demonstrate that the dipolar resonance can couple with the incident light resulting in a large localized field enhancement which increases the molecular excitation rate. At the same time, the radiative quadrupolar mode overlaps with the emission band of excited fluorophores to assist the fluorescence emission due to an enhancement in the quantum efficiency. Such dipole-quadrupole jointly enhanced two-photon fluorescence exhibits exceptionally promise in brighter label design.

A hybrid nanoantenna for highly enhanced directional spontaneous emission

Spontaneous emission modulated by a hybrid plasmonic nanoantenna has been investigated by employing finite-difference time-domain method. The hybrid nanoantenna configurations constituted by a gap hot-spot and of a plasmonic corrugated grating and a metal reflector sandwiching a SiO2 thin layer which appears promising for high spontaneous emission enhancement devices. Simulation assays show that the coupling between the gap-antenna and plasmonic corrugations reaches an ultra-high near-field enhancement factor in the excitation process. Moreover, concerning the emission process, the corrugations concentrate the far-field radiated power within a tiny angular volume, offering unprecedented collection efficiency. In the past decades, many kinds of optical antennas have been proposed and optimized to enhance single molecule detection. However, the excitation enhancement effect for single individual or dimmer plasmonic nanostructure is limited due to intrinsic nonradiative decay of the nanoparticle plasmon and ...

Molecule fluorescence modified by a slit-based nanoantenna with dual gratings

In this study, molecule fluorescence modified by slit-based nanoantennas surrounded with metal gratings was investigated by employing the finite-difference time-domain method. We quantified the relative contribution of excitation and emission gains to the total fluorescence enhancement. The simulation results show that the asymmetric dual-grating (DG) antenna provides an efficient way to control the local excitation enhancement, the collection efficiency, and the quantum efficiency separately for bright emission and beaming light. We also investigated the dependence of fluorescence enhancement on the geometric parameters of the antenna, such as the nano-slit width and number of grooves. The asymmetric DG structure greatly improves the flexibility of the nanostructure design to further optimize the plasmonic enhancement effect and provides a promising route to manipulate single-molecule fluorescence emission.

Enhanced Single-Molecule Spontaneous Emission in an Optimized Nanoantenna with Plasmonic Gratings

In this study, we theoretically investigated the single-molecule fluorescence enhancement in a slit–groove structure. The excitation field enhancement, modified quantum efficiency, collection efficiency, and position dependence of a single-molecule spontaneous emission coupled to a plasmonic antenna are explored together. Simulation results revealed that the metal gratings play a crucial role in strengthening the local electromagnetic field enhancement in the excitation process and beaming the radiation light into a tiny angular volume in the emission process, whereas the total radiative decay rate and quantum efficiency are mainly determined by the slit cavity. Our findings provide an intuitive guideline to further optimize the plasmonic antenna for single-molecule detection and the results make a promising route to the development of photonic devices for the manipulation of single-molecule spontaneous emission.

Plasmonic nano-resonator enhanced one-photon luminescence from single gold nanorods

Strong Stokes and anti-Stokes one-photon luminescence from single gold nanorods is measured in experiments. It is found that the intensity and polarization of the Stokes and anti-Stokes emissions are in strong correlation. Our experimental observation discovered a coherent process in light emission from single gold nanorods. We present a theoretical mode, based on the concept of cavity resonance, for consistently understanding both Stokes and anti-Stokes photoluminescence. Our theory is in good agreement of all our measurements.

Light Emission from Plasmonic Nanostructures Enhanced with Fluorescent Nanodiamonds

In the surface-enhanced fluorescence (SEF) process, it is well known that the plasmonic nanostructure can enhance the light emission of fluorescent emitters. With the help of atomic force microscopy, a hybrid system consisting of a fluorescent nanodiamond and a gold nanoparticle was assembled step-by-step for in situ optical measurements. We demonstrate that fluorescent emitters can also enhance the light emission from gold nanoparticles which is judged through the intrinsic anti-Stokes emission owing to the nanostructures. The light emission intensity, spectral shape, and lifetime of the hybrid system were dependent on the coupling configuration. The interaction between gold nanoparticles and fluorescent emitter was modelled based on the concept of a quantised optical cavity by considering the nanodiamond and the nanoparticle as a two-level energy system and a nanoresonator, respectively. The theoretical calculations reveal that the dielectric antenna effect can enhance the local field felt by the nanoparticle, which contributes more to the light emission enhancement of the nanoparticles rather than the plasmonic coupling effect. The findings reveal that the SEF is a mutually enhancing process. This suggests the hybrid system should be considered as an entity to analyse and optimise surface-enhanced spectroscopy.

Luminescence quantum yields of gold nanoparticles varying with excitation wavelength

Luminescence quantum yields (QYs) of gold nanoparticles including nanorods, nanobipyramids and nanospheres are measured elaborately at single nanoparticle level with different excitation wavelengths. It is found that the QYs of the nanostructures are essentially dependent on the excitation wavelength. The QY is higher when the excitation wavelength is blue-detuned and close to the nanoparticles’ surface plasmon resonant peak. A phenomenological model based on plasmonic resonator concept is proposed to understand the experimental findings. The excitation wavelength dependent of QY is attributed to the wavelength dependent coupling efficiency between the free electrons oscillation and the intrinsic plasmon resonant radiative mode. These studies should contribute to the understanding of one-photon luminescence from metallic nanostructures and plasmonic surface enhanced spectroscopy.