Numerical investigations on the electromagnetic enhancement effect to tip-enhanced Raman scattering and fluorescence processes

Broadband Tip-Enhanced Nonlinear Optical Response in a Plasmonic Nanocavity

We report a significantly broad nonlinear optical response enhanced in a tip-substrate plasmonic nanocavity. Focusing on the near-field second harmonics of the wavelength-tunable femtosecond laser, we demonstrate that the tip-enhancement of nonlinear optical effects efficiently works over the broad wavelength range through the visible to infrared region. We also found that this broadband nonlinear optical property is directly affected not only by the nanometer-scale sharpness of the tip apexes but also by the micrometer-scale surface geometry of the tip shafts. While spatially nonlocal plasmonic modes excited throughout the micrometer-scale tip shafts enhance near-to-mid-infrared incoming light, the radiation of visible-to-near-infrared second harmonics is boosted by localized plasmons at the nanogap. These two plasmonic modes simultaneously affect the excitation and emission processes, realizing the strong and broad enhancement of second harmonic generation. Our results provide a new basis for the physical understanding and fine manipulation of nonlinear optical phenomena enhanced in plasmonic nanocavities.

Influence of an atomistic protrusion at the tip apex on enhancing molecular emission in tunnel junctions: A theoretical study

Light emission from the gap of a scanning tunneling microscope can be used to investigate many optoelectronic processes at the single-molecule level and to gain insight into the fundamental photophysical mechanisms involved. One important issue is how to improve the quantum efficiency of quantum emitters in the nanometer-sized metallic gap so that molecule-specific emission can be clearly observed. Here, using electromagnetic simulations, we systematically investigate the influence of an atomic-scale protrusion at the tip apex on the emission properties of a point dipole in the plasmonic nanocavity. We found that such an atomistic protrusion can induce strong and spatially highly confined electric fields, thus increasing the quantum efficiency of molecular fluorescence over two orders of magnitude even when its dipole is oriented parallel to the metal surface, a situation occurring in most realistic single-molecule electroluminescence experiments. In addition, our theoretical simulations indicate that due to the lightning rod effect induced by the protrusion in a plasmonic nanocavity, the quantum efficiency increases monotonically as the tip approaches the dipole to the point of contact, instead of being quenched, thus explaining previous experimental observations with ever-enhancing fluorescence. Furthermore, we also examine in detail how the protrusion radius, height, and material affect the protrusion-induced emission enhancement. These results are believed to be instructive for further studies on the optoelectronic properties of single molecules in tip-based plasmonic nanocavities.

Influence of nonlocal dielectric response on the Au tip-enhanced fluorescence effect

Tip-enhanced fluorescence (TEF) with ultra-high detection sensitivity and spatial resolution has been a powerful characterization technique in the study of surface science and life science. Herein, a systematically theoretical investigation in the visible range had been performed to study TEF properties of a single molecule located inside a nanogap formed by Au tip and substrate. In the strong localized surface plasmon coupling effect, the contribution of nonlocal dielectric response to the fluorescence quantum yield as well as radiative and energy dissipated decay rates were calculated. It is found that the nonlocal dielectric effects become comparable to the radiative and energy dissipated decay rates with the increasing of the tip-molecule distance, as a result, the nonlocal dielectric effect significantly suppresses the fluorescence process. The huge excitation enhancement at the shorter tip-molecule distance can efficiently compensate the low quantum yield, leading to the great fluorescence enhancement. The results show that the maximum enhancement obtained from the calculations can reach as high as four orders of magnitude by optimizing the tip-molecule distance. These results are not only helpful to our understanding of the TEF mechanism but also valuable for its further applications.