DOI: 10.1007/s00340-002-1006-5 Appl. Phys. B 75, 591–594 (2002) Rapid communication Lasers and Optics Applied Physics B s. astilean 1,2, ✉ w.l. barnes 1 Quantum efficiency and the photonic control of molecular fluorescence in the solid state 1 G13 Thin Film Photonics, School of Physics, University of Exeter, EX4 4QL, UK 2 Optics and Spectroscopy Department, Babes-Bolyai University, 3400 Cluj-Napoca, Romania Received: 31 May 2002/Revised version: 27 June 2002 Published online: 25 October 2002 • © Springer-Verlag 2002 ABSTRACT We investigate the role of quantum efficiency in determining the extent to which the local photonic environment may alter the spontaneous emission rate of organic dye molecules with broad luminescence spectra. Comparison of theory with experimental results shows that the quantum efficiency is a key determining fac- tor in such control, low quantum efficiencies leading to poor control. These results help establish a firm basis for characterising near-fields in nano-optics and controlling fluorescing species. PACS 78.67.-n; 32.70.Cs; 85.60.Jb 1 Introduction The spontaneous emission of light by an emitter is not an intrinsic property of the emitter; rather, it de- pends on the local optical density of states or photonic mode density (PMD) into which photons produced by spon- taneous emission may be released. The pioneering work of Drexhage et al. [1] 600 650 700 750 4 3 2 1 b a Fluorescence Intensity [au] Wavelenght [nm] FIGURE 1 Emission spectra of the two dyes used, R700 (a) and DiD (b), together with the schematic of the Drexhage sample geometry used in this work: 1, dye sub-monolayer; 2, polystyrene spacer; 3, silver mirror; and 4, silica substrate. (The sharp peak at 633 nm is the He-Ne laser excitation line) ✉ Fax: +44-1392/264-111, E-mail: S. Astilean@exeter.ac.uk demonstrated that the fluorescence life- time of Eu 3+ ions in front of a metallic mirror can be altered by varying the dis- tance between the ions and the mirror (see schematic shown in Fig. 1). This type of control over spontaneous emis- sion has been demonstrated for a wide range of emitters, ranging from quan- tum dots [2] to Rydberg atoms [3]. In the Drexhage geometry, the mirror pro- duces a reflected field which acts to drive the dipole moment associated with the emission. If the reflected field is in phase with the source then emission is enhanced; if it is out of phase, emission is inhibited. The effect of the reflected field on the spontaneous emission rate can be written as [4, 5], Γ Γ 0 = (1 - q) + q u=∞  u=0 I(u ) d u (1) where Γ is the modified spontaneous emission rate and Γ 0 is the emission rate in the absence of boundaries. The vari- able u is the normalised in-plane wave vector: the normalisation is with respect to the wave vector a photon (a plane wave) would have in the medium in which the emitter is embedded. The in- trinsic quantum efficiency of the emitter is given by q, whilst the integrand I(u ) is the power dissipated by the emitter as a function of the normalised in-plane wave vector. The (1 - q) term in the equation corresponds to non-radiative decay and is assumed to be indepen- dent of the local optical environment. The second term encompasses the mod- ification to the lifetime brought about by the local optical environment. It is clear from the equation above that theory pre- dicts that the extent of such modifica- tions is linearly dependent on the quan- tum efficiency q. This dependence has not explicitly been demonstrated before and is the focus of this communication. There are at least two reasons why it is important to understand the role the quantum efficiency plays. Firstly, the increasing interest in nano-optics and scanning near-field optical microscopy demands an ability to quantitatively