Plasmonic Sinks for the Selective Removal of Long-Lived States

Abstract

The use of plasmonic nanostructures for the removal of unwanted long-lived states is investigated. We show that the total decay rate of such a state can be increased by up to four orders of magnitude, as compared to its intrinsic radiative decay rate, while leaving other neighboring optical transitions unaffected. For the specific case of molecular triplet excited states, we show that the use of a "plasmonic sink" has the potential to reduce photobleaching and ground state depletion by at least two orders of magnitude. We consider, in addition, the impact of such structures on the performance of organic semiconductor lasers and show that, under realistic device conditions, plasmonic sinks have the capacity to increase the achievable laser repetition rate by a factor equal to the triplet decay rate enhancement. We conclude by studying the effect of exciton diffusion on the triplet density in the presence of metallic nanoparticles. KeywordsPlasmons, Sinks, Nanoshells, Purcell Effect, Quenching, Triplets, Organic Lasers, Photobleaching, Non-radiative decay Plasmonic nanoparticles and antennas have demonstrated significant potential for enhancing the luminescence of adjacent emitters. [1][2][3][4][5][6] The mechanisms for enhancement, however, can be varied and the origin of the increased brightness difficult to unravel. For example, the nanostructure can serve to increase the local electric field at the excitation wavelength resulting in an increased excitation rate, 7-9 or equivalently, absorption efficiency of the emitter. In addition, the scattering cross-section of the particle or radiation pattern of the antenna can lead to a higher collection or out-coupling efficiency for the emitted light. 2, 10 Finally, and most importantly for this work, the increased photonic density of states at the emission wavelength can result, via the Purcell effect, in an increased radiative decay rate, Γ R , for the emitter. [1][2][8][9][10] This effect, however, is typically concomitant with an unwanted and stronger increase in the non-radiative decay rate, Γ NR , due to Joule heating of the metal. [9][10] In the usual case where Γ NR >> Γ R , increases in quantum yield, φ F , are only possible for emitters with very low intrinsic yields 0 R NR φ < Γ Γ . 11 In this work, we show that instead of being detrimental, the high non-radiative decay rate typical of emitters in close proximity to plasmonic structures can be exploited to selectively remove unwanted excited states in a number of physical systems.In molecular systems, for example, the build-up of large triplet state populations prevents the efficient excitation of organic molecules with high repetition rates or continuous sources, 12-13 and can be a significant source of photobleaching. 14-17 Indeed,Hale et al. first proposed and demonstrated the possibility of using plasmonic nanoparticles to increase the photostability of organic thin films. 18 In a later section weshow that in that case, the effect fits into the "diffusive regime", which has several advantages, bu...

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