2.1.

Samples

The random gold nanostructures are prepared by thermal evaporation under ultrahigh vacuum conditions (${10}^{-9}\mathrm{Torr}$), just below the percolation threshold (the metallic surface coverage is equal to 60%). Continuous gold films ($\text{thickness}=25\mathrm{nm}$, roughness rms of 1 nm) are elaborated by the same method. Plasmon resonances for the random gold film appear from 550 nm to far-infrared due to the large distribution of sizes and shapes of metal clusters. The morphology and the absorption spectra of the disordered samples are shown in Fig. 1. Using spectrophotometric and AFM measurements, we checked that these characteristics are the same for the several gold films that were used in the following experiments.

Fig. 1

(a) AFM topographic image (AFM) of a semicontinuous gold film. (b) Absorbance of the semicontinuous gold film. The red lines show the center wavelength of the three types of NCs, while the green line represents the laser wavelength.

The NCs are CdSe/CdS core shell NCs synthesized following the method described in Ref. 22. Their fluorescence corresponds to the one of two incoherent dipoles perpendicular to the NC $c$-axis. The emission of these NCs is within the plasmon resonances of the gold film (Fig. 1). Previous experiments have shown that the emission of the NCs is not quenched when they are directly deposited on the gold film, the shell acting as a spacer.²³ Three sizes of CdSe/CdS NCs are used to probe the properties of near-field modes at different heights. The first NCs (type 1) have a total diameter of 12 nm. Their wavelength emission is around 630 nm with a full-width at half-maximum of 30 nm [a typical spectrum of a single NC deposited on a glass coverslip and a semicontinuous film is shown in Figs. 2(a) and 2(b)]. The type 2 NCs exhibit a total diameter of 22 nm and their emission is around 660 nm. The third kind of NCs (type 3) presents a mean diameter of 30 nm and a wavelength also centered at 660 nm. The CdS surface of the NCs is surrounded by organic ligands that are mainly hexadecylamines and oleates. Their length is about 2 nm. Since the difference between the emission wavelengths of the different types of NC is of the order of the emission and plasmon linewidths and since the absorption of the semicontinuous film is nearly constant between 600 and 700 nm (Fig. 1), we consider that the coupling between the NCs and the gold structures does not depend on the type of NCs.

Fig. 2

Fluorescence spectrum of a single NC deposited on (a) a glass coverslip and (b) a semicontinuous gold film.

2.2.

Experimental Setup

The NCs are spin coated on the films and their fluorescence is individually analyzed with a confocal microscope and a standard Hanbury Brown and Twiss setup based on two avalanche photodiodes (MPD, time resolution of 50 ps). The optical excitation is provided by a laser diode ($\lambda =485\mathrm{nm}$ and pulse duration $\sim 100\mathrm{ps}$, Picoquant LDH D-C). The excitation is far from resonance that means that the laser polarization has no influence on the NCs emission and the following polarization measurements. In order to create only one (e–h) pair, a low power excitation is used. A rotating polarizer is placed on one arm of the Hanbury Brown and Twiss in order to analyze the polarization of the emission. By normalizing the measured intensity on this arm by the intensity detected on the other arm, the polarization can be determined precisely even if the sample slightly drifts and the collected intensity fluctuates. The fluorescence intensity is recorded during the rotation of the polarizer and the polarization ratio is defined as

Fig. 3

(a) PL decay of a single NC deposited on glass. The red line is a biexponential fit (lifetime of 14 and 78 ns). (b) PL decay of a single NC deposited on a random gold film. The red line is a biexponential fit (lifetime of 0.36 and 2.4 ns).

Before studying the coupling between the emitters and the gold films, we characterized their PL decay rate at their single emitter level when they are spin-coated on a glass coverslip. In agreement with previous results,⁸ the mean exciton lifetime is around 60 ns (see Fig. 4 for the typical PL decay we obtained for an NC deposited on a glass coverslip or a random gold film) and does not change significantly for the three different samples. This value will be taken as a reference in the following.

We first considered the small NCs (${\mathrm{NC}}_1$ type) deposited on a flat gold film [red ▪ of Fig. 5(a)]. The Purcell factors (defined as the ratio between the PL decay rate measured on the gold structures and on glass) are between 4 and 15 [Fig. 5(a)]. This dispersion on flat gold can be explained on one side by the different orientations of the NCs but also by the shell size dispersion of the NCs itself. Concerning the polarization ratio, it ranges between 10% and about 30%. This result is compared to calculations for various NCs orientations.²⁴ The NC is then modeled with two incoherent dipoles perpendicular to the $c$-axis. The NC emits at 630 nm and is placed 10 nm above the surface with 25 nm gold thickness. The calculation predicts a linear polarization ratio ranging from 0% to 25%, in agreement with the experimental results.

Fig. 4

Measurement of the maximum pass axis direction $\theta$. (a) Intensity detected after the polarizer normalized to the intensity measured on the other arm of the HBT setup. The starting position of the rotating polarizer is the same for each NC. $\theta$ ranges between $-90\mathrm{deg}$ and 90 deg. If a minimum is first reached while scanning the polarizer direction, the angle is then negative. (b) Polar plot corresponding to (a).

Fig. 5

(a) Polarization ratio as a function of the Purcell factor for individual NCs (type 1, diameter of 12 nm) deposited on a flat gold film (red ▪) or a semicontinuous film (green ♦). (b) Polarization ratio as a function of polarization angle for the NC coupled to the semicontinuous film. The dashed line corresponds to a Purcell factor of 1.

In comparison with a flat gold film, drastic lifetime reductions observed on disordered films are induced by strong spatial localizations and enhancements of the field [green ♦ of Fig. 5(a)]. The disorder induces a strong modification of the optical modes. Discussions on the nature of these modes are wide.^25–27 It has been shown that these surfaces could support localized and delocalized modes with an increase of localized modes close to the percolation threshold.¹⁶

Because the LDOS and the spontaneous emission of the emitter are linked, this modification can be probed by the measurement of the decay rates on the surface. The CdSe/CdS NCs are characterized by two incoherent dipoles and their orientation is randomly spread over the surface. The decay rate is then proportional to the partial local LDOS

The polarization is a signature of the complexity of the modes on these kinds of samples due to the complex geometry and the existence of strongly polarized modes is striking. This could be explained by highly localized modes induced by a simple geometry corresponding to a dimer configuration. Polarization emission of molecules in a metallic dimer has been shown to be linear and its depolarization has been observed when introducing a third metallic particule.²⁸ Of course this model is too simple to explain all the electromagnetic modes on random surfaces but the polarized emission of some modes can be compared to a dimer mode emission, which can also exist on the complex structure.

No link between Purcell factors and the polarization degrees of the emission is observed. Even if the enhancement of the decay rate depends on the NC orientation, the 2D dipole structure and the large number of NCs studied suggest that the mode polarization and the mode localization are uncorrelated. The plot of the polarization ratio versus the polarization angle in Fig. 5(b) also illustrates the randomness of the coupling between the NCs and the semicontinuous gold film.

Bigger NCs (samples 2 and 3 with mean diameter 22 and 30 nm, respectively) have been used to probe the modes for a higher distance away from the surface. As expected, lifetime reductions are lower on both samples due to the higher distance to the surface (see Fig. 6). The Purcell factor is lower than 15 for the two samples and is clearly lower for the biggest NCs (red ▪). The PL decay rate of some of these NCs is very few accelerated. This decrease accounts for the very fast decay of the electromagnetic modes with the distance to the surface. The statistics of the polarization ratio is also modified by the increase of the NCs diameter. The maximum is 72% for the 22 nm diameter NCs and 60% for the 30 nm diameter ones. Moreover the standard deviation also decreases from 17% to 11%. These results confirm the drop of the coupling with the distance between the film and the emitter. They also demonstrate that the hot spot structure is confined in the near field of the film at the scale of few nanometers.

Fig. 6

Polarization ratio as a function of the Purcell factor for two kinds of individual NCs (type 2, diameter of 22 nm, green ♦, and type 3, 30 nm diameter, red ▪). The dashed line corresponds to a Purcell factor of 1.

These results confirm the decrease of the coupling with the distance between the film and the emitter. When the NC is close to the surface, the fluorescence carries the feature of the plasmonic mode. When the distance to the sample increases, the fluorescence tends to the emission of the NC dipole. This behavior has been observed for a molecule close to a gold nanorod where the plasmonic mode feature gets lost for distances to the nanorod larger than 20 nm.²⁹ This evolution is very fast and the plasmonic feature is quickly lost. Here is the interest of the NCs: the distance to the surface can be varied with their size and very short distances, allowing the ability to probe the plasmonic modes. Our analysis demonstrates that the hot spot structure is confined in the near field of the film at the scale of few nanometers.