We study the modification of fluorescence emission and decay rate of single fluorescent molecules in the near field of a periodic plasmonic nanostructure formed by a square lattice of Au hollow conical pillars with a periodicity of 250 nm. We perform nanometer-resolved imaging of the LDOS by simultaneously mapping the position and the decay rate of photoactivatable single-molecules with a novel super-resolved microscopy approach which enables multiplexed and super-resolved fluorescence lifetime imaging at the single-molecule level (smFLIM) with a field of view of ~10 µm2. We observe the LDOS modification of such optically rich material at different illumination conditions and we measure a large Purcell factor enhancement which increases for oblique illumination of the nanostructure.
Enhancement of local electromagnetic fields is instrumental for engineering of light absorption, emission, scattering, chemical reactions, and other processes. Nanostructured composites with plasmonic inclusions have been shown as promising candidates to concentrate electromagnetic waves in nanometer-sized “hot spots”. Unfortunately, majority of high-performance plasmonic structures are resonance-based, and therefore their performance is relatively narrow-band. Here we present a novel material system that has potential to realize broadband enhancement of local intensity and explain the origin of this behavior.
The proposed material platform comprises an array of aligned plasmonic cones arranged in a periodic planar lattice. From the effective medium standpoint, such structure represents a uniaxial material whose effective permittivity varies along the cone. Importantly, there exists a relatively wide range of wavelengths where one component of the effective permittivity tensor crosses zero within the composite. According to previous research, strong enhancement of local field is expected in the vicinity of epsilon-near-zero point in homogeneous materials with spatially varying permittivity, often called transitional metamaterials. We show, however, that due to strong structural nonlocality electromagnetic response of nanocone media does not follow this recipe. In fact, we demonstrate that the incoming radiation is coupled into an additional electromagnetic wave that propagates towards the tip of the cone causing a strong enhancement to the local field. We present a comprehensive description of this phenomenon.
One of the central goals of the field of nonlinear optics is to bring the control of light to ultrafast time scales using structures that are easily integrated into nano-optic devices. The ability to design the polarization state of a signal light pulse, with a second control light pulse, at THz rates, will allow new techniques to be developed such as ultrafast polarimetry and quantum state manipulation.
Here we all-optically control, with a femtosecond pulse, the anisotropy of a metamaterial to change the polarization state of signal light at a switching rate of 0.3THz, which is found to be closely linked to the electron temperature distribution within the structure and so can be tuned with the control light wavelength. We experimentally measure more than 60° rotation of the polarization orientation of the signal light. This effect is due to an induced phase shift of the extraordinary wave compared to the ordinary wave of the signal light. Polarization control is observed in both transmission and reflection and shown to be general to any anisotropic metamaterial. Considering only the signal light, its leading edge can alter the polarization state of the pulse allowing the pulse’s incident intensity to be encoded in its transmitted polarization state.
We present a numerical study of the interaction of light with isolated nanoparticles of various symmetry shapes described by the Gielis superformula as well as nanoparticle arrays composed from them. Using the discrete dipole approximation and finite element numerical methods the effects of particle shape symmetry on the spectral properties of gold and silver nanoparticles were investigated. Starting from the spherical and cylindrical geometries and progressing to star-like polygonal shapes, we demonstrate that the variation of the symmetry can significantly enhance the strength of the dipolar resonance and shift the resonance to the red spectral range by hundreds of nanometres. Thus, is possible to tune the optical properties of the nanostructures all across the visible spectral range only by changing their shapes. Finally, we investigate the collective resonances of arrays of interacting nanoparticles of different shapes, elucidating the role of the particle symmetry in the collective response.
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