Plasmonics is a rapidly developing field at the boundary of physical optics and condensed matter physics. It studies phenomena induced by and associated with surface plasmons-elementary polar excitations bound to surfaces and interfaces of nanostructured good metals. This Roadmap is written collectively by prominent researchers in the field of plasmonics. It encompasses selected aspects of nanoplasmonics. Among them are fundamental aspects such as quantum plasmonics based on quantum-mechanical properties of both underlying materials and plasmons themselves (such as their quantum generator, spaser), plasmonics in novel materials, ultrafast (attosecond) nanoplasmonics, etc. Selected applications of nanoplasmonics are also reflected in this Roadmap, in particular, plasmonic waveguiding, practical applications of plasmonics enabled by novel materials, thermo-plasmonics, plasmonic-induced photochemistry and photo-catalysis. This Roadmap is a concise but authoritative overview of modern plasmonics. It will be of interest to a wide audience of both fundamental physicists and chemists and applied scientists and engineers.

We present a review on the advances of pulse control and ultrafast coherent excitation of both plasmonic nanoantennas and individual molecular systems, primarily based on the achievements in our group. Essential concepts from coherent control of ultrashort broadband laser pulses are combined with nanoscale diffraction limited detection and imaging of single photon emitters, i.e. the central area of this work is where ultrafast meets ultrasmall. First, the critical role of dedicated pulse shaping and phase control is discussed, which is crucial to realize free of spatio-temporal coupling Fourier limited pulses inside a high numerical aperture microscope at the diffraction limited spot. Next we apply this scheme to plasmonic antennas, exploiting broadband two-photon excitation, to determine amplitude and phase of plasmonic resonances, to achieve ultrafast switching of nanoscale hotspots, and multicolor second harmonic detection for imaging applications. Subsequently we address single molecules with phase-shaped pulses to control the electronic state population and retrieve single molecule vibrational dynamics response. We compare the response of a molecule to phase-locked with free phase multipulse excitation. Furthermore, we discuss phase control of excited state energy transfer in photosynthetic molecular complexes. Finally we combine nanoscale plasmonics with single molecule detection, to attain strong enhancement of both excitation and emission, with fluorescence lifetime shortening to the ps range. In conclusion, we anticipate that this review on ultrafast plasmonics and single emitter control will provide a useful view of the status of ultrafast nanophotonics and its application potential.

Optical wavefront shaping is a powerful technique to control the distribution of light in the focus of a microscope. This ability, combined with optogenetics, holds great promise for precise manipulation of neuronal activity with light. However, a deeper understanding of complex brain circuits requires pushing light-shaping methods into a new regime: the simultaneous excitation of several tens of targets, arbitrarily distributed in the three dimensions, with single-cell resolution. To this end, we developed a new optical scheme, based on the spatio-temporal shaping of a pulsed laser beam, to project several tens of spatially confined two-photon excitation patterns in a large volume. Compatibility with several different phase-shaping strategies allows the system to be optimized towards flexibility, simplicity, or multiple independent light manipulations, thus providing new routes for precise three-dimensional optogenetics. To validate the method, we performed multi-cell volumetric excitation of photoactivatable GCaMP in the central nervous system of drosophila larvae, a challenging structure with densely arrayed neurons, and photoconversion of the fluorescent protein Kaede in zebrafish larvae.

The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin flip from the lowest bright state to dark-exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark-exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.

The ultrafast coherent control of light localization in resonant plasmonic nanostructures is intricately related to the phase response of the involved plasmon resonances. In this work, we exploit the second harmonic signal generated by single optical nanoantennas subject to broadband phase-controlled femtosecond pulses to study and tailor the coherent resonance response. Our results reveal that both the spectral phase and the amplitude components associated with the plasmon resonance of arbitrary individual nanoantennas can be accurately determined.

Investigations of ultrafast processes occurring on the nanoscale require a combination of femtosecond pulses and nanometer spatial resolution. However, controlling femtosecond pulses with nanometer accuracy is very challenging, as the limitations imposed both by dispersive optics on the time duration of a pulse and by the spatial diffraction limit on the focusing of light must be overcome simultaneously. In this paper, we provide a universal method that allows full femtosecond pulse control in subdiffraction-limited areas. We achieve this aim by exploiting the intrinsic coherence of the second harmonic emission from a single nonlinear nanoparticle of deep subwavelength dimensions. The method is proven to be highly sensitive, easy to use, quick, robust and versatile. This approach allows measurements of minimal phase distortions and the delivery of tunable higher harmonic light in a nanometric volume. Moreover, the method is shown to be compatible with a wide range of particle sizes, shapes and materials, allowing easy optimization for any given sample. This method will facilitate the investigation of light-matter interactions on the femtosecond-nanometer level in various areas of scientific study.

We have measured the intrinsic exciton dephasing in high-quality zinc blende CdSe/CdS colloidal quantum dots in the temperature range from 5 to 170 K using a sensitive three-beam photon echo technique in heterodyne detection, which is not affected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing. From the temperature dependence of the zero-phonon line dephasing, the exciton lifetime, and the exciton thermalization within its fine structure, we show that the zero-phonon line dephasing of the lowest bright state originates from the phonon-assisted spin–flip to dark exciton states. Importantly, we can control the dephasing by tailoring the exciton fine structure through its dependence on the dot core size and shell thickness, as expected from the spin–flip mechanism. By reducing the electron–hole exchange interaction with increasing core size and delocalization of the electron wave function in the quasi-type-II core/shell band alignment, we find the longest zero-phonon line dephasing time of ∼110 ps at 5 K in dots with the largest core diameter (5.7 nm) and the thickest CdSe shell (9 monolayers) in the series studied.

We report on triggered single photon emission from GaAs quantum dots, grown on Si substrates and obtained by means of fabrication protocols compatible with the monolithic integration on Si based microelectronics. Very bright and sharp individual exciton lines are resolved in the spectra and can be followed up to 150 K. The nature of quantum emitters of single photon pulses can be measured up to liquid nitrogen temperature by Hanbury Brown and Twiss interferometric correlations.