Yingbo He

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.

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.

DNA Origami Directed Assembly of Gold Bowtie Nanoantennas for Single‐Molecule Surface‐Enhanced Raman Scattering

Metallic bowtie nanoarchitectures can produce dramatic electric field enhancement, which is advantageous in single-molecule analysis and optical information processing. Plasmonic bowtie nanostructures were successfully constructed using a DNA origami-based bottom-up assembly strategy, which enables precise control over the geometrical configuration of the bowtie with an approximate 5 nm gap. A single Raman probe was accurately positioned at the gap of the bowtie. Single-molecule surface-enhanced Raman scattering (SM-SERS) of individual nanostructures, including ones containing an alkyne group, was observed. The design achieved repeatable local field enhancement of several orders of magnitude. This method opens the door on a novel strategy for the fabrication of metal bowtie structures and SM-SERS, which can be utilized in the design of highly-sensitive photonic devices.

Surface enhanced fluorescence by metallic nano-apertures associated with stair-gratings

Metallic nano-apertures associated with stair-gratings are proposed for surface enhanced fluorescence with high excitation enhancement and narrow emission beaming effect. Fluorescence correlation spectroscopy method was utilized to analyze the fluorescence trace and fluorescence enhancement, and the angular patterns of fluorescent emission were measured with the back focal plane imaging method. The stair-grating presents a strong optical response which covering well both the excitation and the emission bands of the photoluminescence process. Such high enhancement and narrow directionality by the stair-gratings would enable the detection of single molecules with low numerical aperture objective effectively.

Nano-gap between a gold tip and nanorod for polarization dependent surface enhanced Raman scattering

We demonstrate experimentally that a nano-gap could be constructed by using a scanning probe microscope to allow a gold tip to approach a gold nanorod immobilized on a glass coverslip. The nano-gap can enhance Raman scattering of graphene sandwiched between the tip and the nanorod. The Raman intensity was strongly dependent on the incident light polarization. Here, linear, radial, azimuthal, and intermediate states between radial and azimuthal polarization were investigated and compared in detail. The maximum surface-enhanced Raman scattering effect of the nano-gap occurred for the intermediate states between the radial and azimuthal polarized light.

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.

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.

Spectral shape of one-photon luminescence from single gold nanorods

Light emission from gold nanoparticles was investigated with ultra-narrow-band notch filters to obtain the complete spectral shape. The anti-Stokes emission band was observed at all excitation wavelengths. The spectral shape of the anti-Stokes emission could be well fitted by a Fermi–Dirac-like line shape, while the spectral profile of the Stokes emission could be fitted by a Lorentzian line shape. The electron distribution and local surface plasmon resonance jointly determined the spectral shape. Additionally, we found that the anti-Stokes emission intensity increased more rapidly compared with that of the Stokes emission as illumination power was increased. This phenomenon can be understood from the temperature dependence of the electron distribution owing to photothermal effects.

A New Model for Understanding One-Photon Luminescence from Single Gold Nanorods

We experimentally and theoretically studied the the photonluminescence from a single gold nanorod. Our theory explains the main features of the photon-luminescence radiation and is in good agreement with experimental observations.