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The development of efficient nanoscale photon absorbers, such as plasmonic or high-index dielectric nanostructures, allows the remotely controlled release of heat on the nanoscale using light. These photothermal nanomaterials have found applications in various research and technological fields, ranging from materials science to biology. However, measuring the nanoscale thermal fields remains an open challenge, hindering full comprehension and control of nanoscale photothermal phenomena. Here, we review and discuss existent thermometries suitable for single nanoparticles heated under illumination. These methods are classified in four categories according to the region where they assess temperature: (1) the average temperature within a diffraction-limited volume, (2) the average temperature at the immediate vicinity of the nanoparticle surface, (3) the temperature of the nanoparticle itself, and (4) a map of the temperature around the nanoparticle with nanoscale spatial resolution. In the latter, because it is the most challenging and informative type of method, we also envisage new combinations of technologies that could be helpful in retrieving nanoscale temperature maps. Finally, we analyze and provide examples of strategies to validate the results obtained using different thermometry methods.

Optical antennas are nanostructures designed to manipulate light-matter interactions by interfacing propagating light with localized optical fields. In recent years, numerous devices have been realized to efficiently tailor the absorption and/or emission rates of fluorophores. By contrast, modifying the spatial characteristics of their radiation fields remains challenging. Successful phased array nanoantenna designs have required the organization of several elements over a footprint comparable to the operating wavelength. Here, we report unidirectional emission of a single fluorophore using an ultracompact optical antenna. The design consists of two side-by-side gold nanorods self-assembled via DNA origami, which also controls the positioning of the single-fluorophore. Our results show that when a single fluorescent molecule is positioned at the tip of one nanorod and emits at a frequency capable of driving the antenna in the antiphase mode, unidirectional emission with a forward to backward ratio of up to 9.9 dB can be achieved.

Diffraction of finite sized laser beams imposes a limit on the control that can be exerted over ultrafast pulses. This limit manifests as spatio-temporal coupling induced in standard implementations of pulse shaping schemes. We demonstrate the influence this has on coherent control experiments that depend on finite excitation, sample, and detection volumes. Based on solutions used in pulse stretching experiments, we introduce a double-pass scheme that reduces the errors produced through spatio-temporal coupling by at least one order of magnitude. Finally, employing single molecules as nanoscale probes, we prove that such a double pass scheme is capable of artifact-free pulse shaping at dimensions two orders of magnitude smaller than the diffraction limit.

Gold nanostars provide high sensitivity for single nanoparticle label-free biosensing. The nanostars present multiple plasmon resonances of which the lower energy ones, corresponding to the nanostar tips and core-tip interactions, are the most sensitive to environmental changes. Streptavidin molecules are detected upon binding to individual, biotin-modified gold nanostars by spectral shifts in the plasmon resonances. Concentrations as low as 0.1 nM produce a shift of the tip related plasmon resonances of about 2.3 nm (5.3 meV).