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An analytic model is used to calculate the Raman and fluorescence enhancement of a molecule in between two closely spaced gold nanospheres. Instead of using the conventional approach that only the dipolar plasmonic mode is considered, we calculate the electric field enhancement in the nanometre sized gap, by taking account of the higher order modes in one gold sphere, which couples to the dipolar mode of the other sphere. The experimental confirmation is performed by gap-dependent tip-enhanced Raman spectroscopy (TERS) measurements. The photoluminescence and Raman enhancement are both observed with different growing trends as the gap width decreases. Red-shift of the background spectra is observed and implies the increasing coupling between the nanospheres. This analytic model is shown to be able to interpret the enhancement mechanisms underlying gap-dependent TERS experimental results.

Multi-resonant plasmonic simple geometries like nanocylinders and nanorods are highly interesting for two-photon photoluminescence and second harmonic generation applications, due to their easy fabrication and reproducibility in comparison with complex multi-resonant systems like dimers or nanoclusters. We demonstrate experimentally that by using a simple gold nanocylinder we can achieve a double resonantly enhanced two-photon photoluminescence of quantum dots, by matching the excitation wavelength of the quantum dots with a dipolar plasmon mode, while the emission is coupled with a radiative quadrupolar mode. We establish a method to separate experimentally the enhancement factor at the excitation and at the emission wavelengths for this double resonant system. The sensitivity of the spectral positions of the dipolar and quadrupolar plasmon resonances to the ellipticity of the nanocylinders and its impact on the two-photon photoluminescence enhancement are discussed.

Mechanically controllable break junctions are one suitable approach to generate atomic point contacts and ultrasmall and controllable gaps between two metal contacts. For constant bias voltages, the tunneling current can be used as a ruler to evaluate the distance between the contacts in the sub-1-nm regime and with sub-Å precision. This ruler can be used to measure the distance between two plasmonic nanostructures located at the designated breaking point of the break junction. In this work, an experimental setup together with suitable nanofabricated break junctions is developed that enables us to perform simultaneous gap-dependent optical and electrical characterization of coupled plasmonic particles, more specifically bowtie antennas in the highly interesting gap range from few nanometers down to zero gap width. The plasmonic break junction experiment is performed in the focus of a confocal microscope. Confocal scanning images and current measurements are simultaneously recorded and exhibit an increased current when the laser is focused in the proximity of the junction. This setup offers a flexible platform for further correlated optoelectronic investigations of coupled antennas or junctions bridged by nanomaterials.

Phthalocyanines are frequently used as probing molecules in the field of single-molecule surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). In this work, we systematically compare the SERS and TERS spectra from a thin cobalt phthalocyanine (CoPc) film that is deposited on a Au film. The contributions from electromagnetic (EM), resonance, and charge-transfer enhancements are discussed. Radially and azimuthally polarized vector beams are used to investigate the influences of molecular orientation and the localized surface plasmon resonance (SPR). Furthermore, two different excitation wavelengths (636 and 532 nm) are used to study the resonant excitation effect as well as the involvement of the charge-transfer processes between CoPc and the Au substrate. It is shown that the Raman peaks of CoPc are mostly enhanced by 636 nm excitation through a combination of resonant excitation, high EM enhancement, and chemical enhancement via charge transfer from the metal to the molecule. At 532 nm excitation, however, the SERS and TERS spectra are dominated by photoluminescence, which originates from a photo-induced charge-transfer process from the optically excited molecule to the metal. The contributions of the different enhancement mechanisms explain the optical contrasts seen in the TERS images of Au nanodisks covered by the CoPc film. The insight achieved in this work will help to understand the optical contrast in sub- or single-molecule TERS imaging and apply SERS or TERS in the field of photocatalysis.

In this review, we focus on the experimental demonstration of enhanced emission from single plasmonic tunneling junctions consisting of coupled nano antennas or noble metal tips on metallic substrates in scanning tunneling microscopy. Electromagnetic coupling between resonant plasmonic oscillations of two closely spaced noble metal particles leads to a strongly enhanced optical near field in the gap between. Electron beam lithography or wet chemical synthesis enables accurate control of the shape, aspect ratio, and gap size of the structures, which determines the spectral shape, position, and width of the plasmonic resonances. Many emerging nano-photonic technologies depend on the careful control of such localized resonances, including optical nano antennas for high-sensitivity sensors, nanoscale control of active devices, and improved photovoltaic devices. The results discussed here show how optical enhancement inside the plasmonic cavity can be further increased by a stronger localization via tunneling. Inelastic electron tunneling emission from a plasmonic junction allows for new analytical applications. Furthermore, the reviewed concepts represent the basis for novel ultra-small, fast, optically, and electronically switchable devices and could find applications in high-speed signal processing and optical telecommunications.

Frequency conversion plays an important role in both fundamental and applied nano-optics. Doubling the frequency of light by second harmonic generation (SHG) is a vital process e.g. in laser optics or high-resolution microscopy. SHG can be created through symmetry breaking at plasmonic nanostructures, or the local high electric near-fields of plasmonic nanoantennas can be utilized to further enhance the SHG e.g. from nonlinear crystals. Examples of SHG microscopy using cylindrical vector beams in combination with tilted nanocones and radially symmetric oligomers are shown as well as enhancement studies of the SHG from nonlinear crystals decorated with polarization-matched nanostructures.

We investigate the dynamics of the multi-photon excitation of single, isolated gold nanoparticles and show that the one/two/three emission can be actively switched by changing the excitation pulse duration.

Embedded in a tuneable λ/2-FabryPérot micro-resonator the radiative relaxation of a dye molecule or quantum dot can reproducibly be modified allowing to determine their quantum yield, control Förster energy-transfer or localize them with nanometer precision.

Composition dependent tuning of electronic and optical properties in semiconducting two-dimensional (2D) transition metal dichalcogenide (TMDC) alloys is promising for tailoring the materials for optoelectronics. Here, we report a solution-based synthesis suitable to obtain predominantly monolayered 2D semiconducting Mo1P-xWxS2 nanosheets (NSs) with controlled composition as substrate-free colloidal inks. Atomic-level structural analysis by high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDXS) depicts the distribution of individual atoms within the Mo1-xWxS2 NSs and reveals the tendency for domain formation, especially at low molar tungsten fractions. These domains cause a broadening in the associated ensemble-level Raman spectra, confirming the extrapolation of the structural information from the microscopic scale to the properties of the entire sample. A characterization of the Mo1-xWxS2 NSs by steady-state optical spectroscopy shows that a band gap tuning in the range of 1.89-2.02 eV (614-655 nm) and a spin-orbit coupling-related exciton splitting of 0.16-0.38 eV can be achieved, which renders colloidal methods viable for upscaling low cost synthetic approaches toward applicationtaylored colloidal TMDCs. [ABSTRACT FROM AUTHOR]

Super-resolution microscopy (SRM) approaches revolutionize cell biology by providing insights into the nanoscale organization and dynamics of macromolecular assemblies and single molecules in living cells. A major hurdle limiting SRM democratization is post-acquisition data analysis which is often complex and time-consuming. Here, we present OneFlowTraX, a user-friendly and open-source software dedicated to the analysis of single-molecule localization microscopy (SMLM) approaches such as single-particle tracking photoactivated localization microscopy (sptPALM). Through an intuitive graphical user interface, OneFlowTraX provides an automated all-in-one solution for single-molecule localization, tracking, as well as mobility and clustering analyses. OneFlowTraX allows the extraction of diffusion and clustering parameters of millions of molecules in a few minutes. Finally, OneFlowTraX greatly simplifies data management following the FAIR (Findable, Accessible, Interoperable, Reusable) principles. We provide a detailed step-bystep manual and guidelines to assess the quality of single-molecule analyses. Applying different fluorophores including mEos3.2, PA-GFP, and PATagRFP, we exemplarily used OneFlowTraX to analyze the dynamics of plant plasma membrane-localized proteins including an aquaporin, the brassinosteroid receptor Brassinosteroid Insensitive 1 (BRI1) and the Receptor-Like Protein 44 (RLP44). [ABSTRACT FROM AUTHOR]

Hypericin tautomerization that involves the migration of the labile protons is believed to be the primary photophysical process relevant to its light-activated antiviral activity. Despite the difficulty in isolating individual tautomers, it can be directly observed in single-molecule experiments. We show that the tautomerization of single hypericin molecules in free space is observed as an abrupt flipping of the image pattern accompanied with fluorescence intensity fluctuations, which are not correlated with lifetime changes. Moreover, the study can be extended to a λ/2 Fabry–Pérot microcavity. The modification of the local photonic environment by a microcavity is well simulated with a theoretical model that shows good agreement with the experimental data. Inside a microcavity, the excited state lifetime and fluorescence intensity of single hypericin molecules are correlated, and a distinct jump of the lifetime and fluorescence intensity reveals the temporal behavior of the tautomerization with high sensitivity and high temporal resolution. The observed changes are also consistent with time-dependent density functional theory calculations. Our approach paves the way to monitor and even control reactions for a wider range of molecules at the single molecule level. [ABSTRACT FROM AUTHOR]

2D in‐plane ferroelectric NbOI2 exhibits strong second harmonic generation (SHG) and ultrahigh effective susceptibility. To push forward their applications in nonlinear photonics and optoelectronics, it is highly desirable to understand the emission dipole orientation and tunability of SHG, which is not achieved. Here, by integrating tight focusing from parabolic mirror with back focal plane (BFP) imaging technique, for the first time it is demonstrated that SHG emission of NbOI2 presents purely in‐plane dipole orientation in consistent with numerical simulations, suggesting the in‐plane components of the SHG susceptibility tensor in NbOI2 dominate the emission. Moreover, with the aid of ab‐initio calculations, it is found that the hydrostatic pressure can dramatically change the structure and resultant SHG intensity of NbOI2. Explicitly, SHG intensity endures a slight increase due to the distortion of octahedral at low pressure pressure, and then monotonously decreases due to the improvement of structural symmetry with further increasing pressure, and drastically quenching resulting from the ferroelectric to paraelectric phase transition. This work unambiguously demonstrates the dipole emission behavior of SHG and the relationship between structural evolution and SHG intensity, which provides an avenue for tunable nonlinear optics and optoelectronics. [ABSTRACT FROM AUTHOR]

Integrating anisotropy and nonlinearity of physical properties in 2D materials is of great importance to the progress of nanoelectronic, nanophotonic, and nano‐optoelectronic applications. In this work, Nb3SeI7, a newly discovered ternary layered material with non‐centrosymmetric lattices, is introduced, which exhibits strong second harmonic generation (SHG), in‐plane anisotropic electricity, and light absorption. Symmetry breaking of 2D Nb3SeI7 induced by the polar off‐center displacement of niobium leads to a high nonlinear optical response with a second‐order susceptibility of 6.71 pm V−1 and a high anisotropic factor of photoresponsivity up to 4.6, which are higher than most of the 2D materials. These properties of 2D Nb3SeI7 create new possibilities for chip‐integrated nonlinear optics devices and future‐proof polarization‐sensitive optoelectronic systems. [ABSTRACT FROM AUTHOR]