Plasmonic nanoantennas are efficient devices to concentrate light in spatial regions much smaller than the wavelength. Only recently, their ability to manipulate photons also on a femtosecond timescale has been harnessed. Nevertheless, designing the dynamical properties of optical antennas has been difficult since the relevant microscopic processes governing their ultrafast response have remained unclear. Here, we exploit frequency resolved optical gating to directly investigate plasmon response times of different antenna geometries resonant in the near infrared. Third-harmonic imaging is used in parallel to spatially monitor the plasmonic mode patterns. We find that the few-femtosecond dynamics of these nanodevices is dominated by radiative damping. A high efficiency for nonlinear frequency conversion is directly linked to long plasmon damping times. This single parameter explains the counter-intuitive result that rod-type nanoantennas with minimum volume generate by far the strongest third-harmonic emission as compared to the more bulky geometries of bowtie-, elliptical- or disc-shaped specimens.

We present a theoretical analysis of femtosecond pump-probe experiments performed on a single negatively charged quantum dot. The influence of Coulomb-correlation effects as well as carrier relaxation on transient transmission change signals is investigated. Our model describes ultrafast disappearance of the fundamental trion absorption due to instantaneous Coulomb renormalizations and a delayed onset of gain at the same frequency, as found in the measurements. Going beyond previous experimental information, we predict that after spin-conserving carrier relaxation, new optical transitions exhibiting either gain or absorption should emerge that build up on a picosecond timescale. The time dependence of these new transitions provides insight into details of the carrier relaxation processes.

We present two applications of a single nitrogen vacancy center in a nanodiamond as quantum probe for plasmonic nanostructures. Coupling to the nanostructures is achieved in a highly controlled manner by picking up a pre-characterized nanocrystal with an atomic force microscope and placing it at the desired position. Local launching of single excitations into a nanowire with a spatial control of few nanometers is demonstrated. Further, a two dimensional map of the electromagnetic environment of a plasmonic bowtie antenna was derived, resembling an ultimate limit of fluorescence lifetime nanoscopy.

Individual nanometer-sized plasmonic antennas are excited resonantly with few-cycle laser pulses in the near infrared. Intense third-harmonic emission of visible light prevails for fundamental photon energies below 1.1 eV. Interband luminescence and second harmonic generation occur solely at higher driving frequencies. We attribute these findings to multiphoton resonances with the d-band transitions of gold. The strong third-order signal allows direct measurement of a subcycle plasmon dephasing time of 2 fs, highlighting the efficient radiation coupling and broadband response of the devices.

The ability to coherently manipulate single electron and photon states is vital for quantum information processing. However,
typical quantization and correlation energies restrict processing rates in real implementations owing to the time–energy
uncertainty. Here we report optical initialization, manipulation and probing of a single CdSe/ZnSe semiconductor quantum dot
on femtosecond timescales, the ultimate limit for clean quantum operations in such ‘artificial atoms’. Resonant pump–probe
measurements on a donor-charged quantum dot reveal that the fundamental exciton absorption is switched off through
instantaneous Coulomb renormalization. Optical gain builds up following ultrafast intraband relaxation, with a thermalization
rate determined by the electron spin. Operating the system in a nonlinear regime, we demonstrate the ability to change the
number of quanta in a femtosecond light pulse by exactly ±1. This deterministic single-photon amplifier is characterized by a
flat gain spectrum.

We demonstrate femtosecond plasmonic interferometry with a novel geometry. The plasmonic microinterferometer consists of a tilted slit-groove pair. This arrangement allows for (i) interferometric measurements at a single wavelength with a single microinterferometer and (ii) unambiguous discrimination between changes in real and imaginary parts of the metal dielectric function. The performance is demonstrated by monitoring the sub-picosecond dynamics of hot electrons in gold.

Resonant optical nanoantennas hold great promise for applications
in physics and chemistry1–6. Their operation relies on their ability
to concentrate light on spatial scales much smaller than the
wavelength. In this work, we mechanically tune the length and
gap between two triangles comprising a single gold bow-tie
antenna by precise nanomanipulation with the tip of an atomic
force microscope. At the same time, the optical response of the
nanostructure is determined by means of dark-field scattering
spectroscopy. We find no unique single ‘antenna resonance’.
Instead, the plasmon mode splits into two dipole resonances for
gap sizes on the order of a few tens of nanometres, governed by
the full three-dimensional shape of the antenna arms. This result
opens the door to new nano-optomechanical devices, where
mechanical changes on the nanometre scale control the optical
properties of artificial structures.