Decay-rate enhancement of spontaneous emitters coupled to Bloch Surface Waves on one-dimensional photonic crystals

IntroductionThe ability of engineering the radiative properties of spontaneous emitters is one of the main targets in nanophotonics. In this framework, plasmonic cavities and nano-antennas have gained particular attention, due... [ view full abstract ]

Introduction

The ability of engineering the radiative properties of spontaneous emitters is one of the main targets in nanophotonics. In this framework, plasmonic cavities and nano-antennas have gained particular attention, due the large number of photonic states confined in subwavelength volumes accessible from the external environment. [1]

Unfortunately, plasmonic resonant structures working at visible wavelengths are affected by intrinsic high losses of metals that broaden the resonances reducing the quality factors.

Here we propose a fully dielectric structure exhibiting a resonant surface mode with well-defined dispersion relation (Bloch Surface Wave -BSW-). [2] By properly patterning the surface with a periodic structure we show a band gap opening in the dispersion relation of BSW and a defect state acting as a resonant cavity able to modify the spontaneous emission of fluorescent molecules located at the mode maxima

Methods

A dielectric multilayer composed by 10 pairs of SiO₂ (n=1.45, thickness 137 nm) and Ta₂O₅ (n=2.1, thickness 95 nm) thin films sustains resonant surface modes at the truncation interface. We fabricated 75 nm thick PMMA (n=1.48) concentric ring structures surrounding a central disk acting as a resonant cavity (Fig 1a) at the surface.

Results and Discussion

The ring structure acts as a resonant cavity surrounded by a Distributed Bragg Reflector (DBR). In Figure 1b a wide-field image shows the fluorescence radiation coupled to the fundamental cavity mode at the center of the inner spacer (fig. 1b). Excitation is provided by a 10ps pulsed laser, at . Fluorescence lifetime measurements performed collecting only the light coming from the cavity region through a pinhole show a decay rate enhancement of about one order of magnitude (fig. 1c). Moreover, the analysis of the emission spectrum, reported in figure 1d, reveals clearly the coupling to the resonant mode of the cavity, with the possibility to tune the resonant condition by modifying the grating period  and the inner spacer.

In conclusion, we present a purely dielectric photonic structure capable of controlling the emissive properties of fluorescent molecules both in terms of frequency emitted and decay rate.