Fluorescence enhancement via excitation of bright and dark modes in 2D plasmonic nanostructures

Introduction For the last decade, noble metal nanostructures are of the most studied ones, where light coupling to plasmonic modes of single nanoparticles and their periodic arrangement is under study. It is known that... [ view full abstract ]

Introduction

For the last decade, noble metal nanostructures are of the most studied ones, where light coupling to plasmonic modes of single nanoparticles and their periodic arrangement is under study. It is known that plasmonic lattices support so-called dark non-radiative modes of the higher Q-factor than that of the radiative (bright) modes. Indeed, such plasmonic structures are of interest for nanolasers [1,2] operating in the bright and dark modes’ regimes.

Methods

In this work, we experimentally studied 2D arrays of nanodisks having different periods embedded in a dye-doped polymer waveguiding layer (Rhodamin 101–Su8). The structural parameters of the samples were chosen so that to spectrally overlap the fluorescence spectrum with their optical resonances. For the fabricated dye-doped polymer waveguiding layer, the optical gain of 10–20 cm^-1 was measured by means of the variable stripe length method [3]. The samples have been studied for their optical responses, including measurement of the fluorescence lifetime, at excitation by pulse laser radiation.

Results and Discussion

Our studies showed that the optical spectra of the samples exhibit three main features: a waveguide mode, grating localized plasmon resonance and non-radiative (dark) modes. By their intrinsic nature, the dark mode is not observed in transmission or reflection spectra of the samples. However, the fluorescence of Rhodamin 101 in the waveguiding layer led to excitation and visualization of the dark modes. Moreover, the fluorescence intensity for wavelengths corresponding to the dark modes was found to increase by one order of magnitude as compared with the intensity of the single dye-doped polymer layer at the same wavelengths. Note, also, that the enhancement due to the dark modes was larger than that of the waveguide mode and the mode associated with grating localized plasmon resonance. Studies on the fluorescence lifetimes showed that they became somewhat longer for the fluorescence at the dark modes’ wavelengths. We believe that our results should prove useful for understanding and optimizing the nanolasers.

[1] Oulton R.F et al. (2009) Nature 461: 629–632. 

[2] Hakala T.K. et al. (2017) Nature communications.  Т.8. С. 13687.

[3] Shaklee K.L., Leheny R.F. (1971) AppliedPhysicsLetters. Т.18. №.11. С. 475-477.