In this work, plasmonic nano-gaps consisting of a silver nanoparticle coupled to an extended silver film have been fully optimized for single molecule Surface-Enhanced Raman Scattering (SERS) spectroscopy. The SERS signal was found to be strongly dependent on the particle size and the molecule orientation with respect to the field inside the nano-gap. Using Finite Difference Time Domain (FDTD) simulations to complement the experimental measurements, the complex interplay between the excitation enhancement and the emission enhancement of the system as a function of particle size were highlighted. Additionally, in conjunction with Density Functional Theory (DFT), the well-defined field direction in the nano-gap enables to recover the orientation of individual molecules.
The ability to engineer the optical response of a plasmonic nano‐object is highly desired to achieve better control over light–matter interactions. Due to the sensitivity of plasmon resonances to the surrounding media, isotropic dielectric coating is an easy approach to modify the optical properties of a plasmonic nanostructure. However, the choice of coatings and the provided tunability is limited by the range of refractive indices of available materials. Here, it is shown that coating of plasmonic nano‐objects with an anisotropic metamaterial, which displays a hyperbolic dispersion and allows the design of refractive index on demand, provides greater flexibility in engineering their interaction with light. This is experimentally demonstrated by coating Au nanospheres with alternating SiO2 and Au multishells. This creates rich and highly tunable plasmonic modes covering a broad wavelength range (≈400–2200 nm) and produces high local field intensity enhancement (≈500‐fold). The concept is extended to hyperbolic coating of dielectric nano‐objects, confirming the nature of the modes to be related to the resonances in the hyperbolic layer. The implemented approach using a coating with an engineered effective refractive index may find applications in plasmon‐enhanced spectroscopy, nanolasers, design of nonlinear phenomena, photothermal conversions, and hot‐electron generation.
We demonstrate two-color nanoemitters that enable the selection of the dominant emitting wavelength by varying the polarization of excitation light. The nanoemitters were fabricated via surface plasmon-triggered two-photon polymerization. By using two polymerizable solutions with different quantum dots, emitters of different colors can be positioned selectively in different orientations in the close vicinity of the metal nanoparticles. The dominant emission wavelength of the metal/polymer anisotropic hybrid nanoemitter thus can be selected by altering the incident polarization.
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