Single-emitter super-resolved imaging of radiative decay rate enhancement in dielectric gap nanoantennas

High refractive index dielectric nanoantennas strongly modify the decay rate via the Purcell effect through the design of radiative channels. Due to their dielectric nature, the field is mainly confined inside the nanostructure and in the gap, which is hard to probe with scanning probe techniques. Here we use single-molecule fluorescence lifetime imaging microscopy (smFLIM) to map the decay rate enhancement in dielectric GaP nanoantenna dimers with a median localization precision of 14 nm. We measure, in the gap of the nanoantenna, decay rates that are almost 30 times larger than on a glass substrate. By comparing experimental results with numerical simulations we show that this large enhancement is essentially radiative, contrary to the case of plasmonic nanoantennas, and therefore has great potential for applications such as quantum optics and biosensing.


Excitation intensity enhancement simulations
We report FDTD simulations for the calculation of the field intensity enhancement with respect to the incident intensity for different wavelengths.The structure is either a GaP monomer or a dimer and is excited with a p-polarized plane wave impinging perpendicularly to the structure.In the case of the dimer, the polarization is set parallel to the dimer axis.

smFLIM experimental setup
The smFLIM experimental setup is reported in Figure S4.The principle of the smFLIM approach is also sketched.

LDOS enhancement simulations
We report on the simulated LDOS enhancement for dipoles located at a distance of 4 nm from the dimer surface.

Figure S1 .
Figure S1.Calculations of the normalized near-field intensity map (a,c) for the illumination wavelength of the experiment 625 nm and (b,d) for the maximum of the magnetic resonance at 750 nm for the GaP pillar with 200 nm diameter and 200 nm height with (c,d) and without (a,b) the 10 nm SiO2 layer.Illuminated with a p-polarized plane wave as illustrated by the arrow in a).

Figure S2 .
Figure S2.Calculations of the normalized near-field intensity map (a) for the illumination wavelength of the experiment 625 nm and (b) for the maximum of the magnetic resonance at 690 nm for the GaP dimer.Each disk has a diameter of 200 nm and a height of 200 nm.The two disks are separated by a 40 nm gap.The structure is illuminated with a p-polarized plane wave with the incoming electric field oriented along the dimer axis.

Figure S3 .
Figure S3.Calculations of the normalized near-field intensity map (a) for the illumination wavelength of the experiment 625 nm, (b,c) for wavelength in the area of detection 650 nm and (c) for the maximum of the magnetic resonance at 690 nm for the GaP dimer with 200 nm diameter and 200 high separated by a 40 nm gap capped with a 10 nm SiO2 layer.The structure is illuminated with a p-polarized plane wave oriented along the dimer axis.The enhancement shown in Figure S3a is a combination of the ED and MD resonances.

Figure S4 .
Figure S4.Experimental setup and sketch of the smFLIM approach.The sample is excited in widefield by a pulsed laser, through the substrate (a glass coverslip).Fluorescence photons are collected through a high NA objective and are split into two paths by a 50:50 beam-splitter.Half of the photons are detected by an EM-CCD camera and are used to image the PSF of each emitting molecule for its localization.The other half of the photons are detected on a SPAD array with a TCSPC system for the measurement of the lifetime of each emitting molecule.

Figure S5 .
Figure S5.Simulated LDOS enhancement for different positions of the dipole around a GaP dimer: (a) Schematic top view with the dashed line indicating the Y position of simulated dipoles.Each pillar has a diameter of 200 nm and a height of 200 nm and is coated with 10 nm of SiO2.(b-d) Simulated LDOS enhancement (side view) for dipoles with the dipole moment oriented along the X, Y and Z axis respectively.The LDOS is normalized to the value for a dipole in air.

Figure S6 .
Figure S6.Simulated total LDOS enhancement maps around the GaP dimer: (a) Top view of the simulated LDOS for dipoles positioned at a distance of 4 nm from the GaP pillars surface, normalized by the LDOS of a dipole on glass.All the dipoles are oriented parallel to the X axis.(b) Simulated LDOS enhancement plotted against the apparent dipole position.The apparent position is obtained by fitting the dipole's recorded PSF with a 2D Gaussian function.The white circles indicate the edges of the GaP pillars.