Spontaneous Emission Control with Planar Dielectric Structures: An Asset for Ultrasensitive Fluorescence Analysis

We report on the interest of spontaneous emission control with planar dielectric multi-layer structures to increase the fluorescence collection efficiency in single-molecule detection experiments and ultrasensitive analysis. The effect of a very simple Fabry Perot microcavity on the radiation pattern of Eu chelate molecules is introduced and discussed. Then the fluorescence correlation spectroscopy (FCS) technique is used to detect Cyanine 5 molecules on a simple mirror and Rhodamine 6G (R6G) molecules in a microcavity, both in liquid solutions. The respective advantages of those dielectric structures for increasing the count rate per molecule are described. The detection enhancement is used to demonstrate photon antibunching on the nanosecond time scale with a dilute solution of R6G molecules (10 M) in the microcavity. Introduction The success of ultrasensitive analysis of fluorescent molecules in solution or on surfaces at ambient temperature has been built on two major experimental requirements [1]. First, optimizing the fluorescence collection efficiency of the apparatus, and secondly, discriminating as much as possible the relevant fluorescence signal against background. Great efforts have been devoted to the second point in order to achieve a sensitivity down to the single molecule level. In this respect, two spatial selection principles have been successfully applied in addition to traditional spectral filters to reduce the contribution from surrounding materials (mostly Rayleigh and Raman scattering). The first is the far-field confocal microscopy set-up which allows one to obtain a collection volume in solution as small as 0.2 fl with high numerical aperture (NA) microscope objectives [2, 3]. Even more selective is the method of near field scanning optical microscopy (NSOM), which defines sub-diffraction limit exciting spots on surfaces [4, 5]. In both cases, fluorescence is usually collected through a high NA microscope objective and the collection reaches about 30% of the total emitted light (1.2 NA, water immersion). The goal of this paper is to show that this value is not a limit and is likely to be notably increased by treating the problem at the very root of the spontaneous emission phenomenon. Spontaneous emission is often presented in the frame of quantum electrodynamics as stimulated emission, stimulated by vacuum field fluctuations. As a result, it is not an immutable property of the emitter, and Purcell first pointed out in 1946 that the confinement of an emitter in a cavity whose dimensions would be of the order of the emission wavelength should alter the spontaneous emission rate from its value in free space [6]. This possibility is summarised in Fermi’s golden rule which shows that the spontaneous emission rate of an electric dipole transition (the inverse of its spontaneous lifetime) 208 Single Molecules RESEARCH PAPER Single Mol. 1 (2000) 3 depends both on the local vacuum electric field amplitude and on the density of electromagnetic modes at the dipole location. The modification of electromagnetic boundaries in the vicinity of the source not only makes its radiation pattern non-isotropic but possibly induces changes of radiative lifetime. After the seminal experiment of Drexhage which demonstrated spontaneous emission control with a single mirror in the visible range [7], striking results of spontaneous emission inhibition or enhancement were obtained in the microwave domain with Fabry-Perot cavities which modify both the mode density and the vacuum field intensity [8, 9]. They initiated the very productive field of cavity quantum electrodynamics (CQED), and spontaneous emission control was thoroughly investigated with the almost ideal systems of atomic physics. The interest soon spread to the solid state community where spontaneous emission control appeared as a remarkable tool to increase the brightness of LEDs and make efficient monolithic microlasers. The progress in microfabrication techniques permitted the study of a wide variety of microcavity geometries focussing on their ability to control spontaneous emission and the different coupling regimes which can exist depending on the oscillator strength of the emitter [10]. With the existing structures, it is clear that notable lifetime modifications are only accessible for sources with narrow emission spectra, such as quantum boxes [11]. Nevertheless, with large spectrum emitters like Lanthanide ions in amorphous materials or organic dyes, it is still possible to use microcavities to tailor the radiation pattern and concentrate the emitted light in specific directions. This is therefore particularly attractive with a view to increasing fluorescence collection efficiency in biological ultra-sensitive analysis. In this context, the microstructure has to be robust enough in its principle to work on large wavelength domains and with sufficiently low tuning requirements to handle emitters in liquid phase. We present in the following section the assets of planar dielectric structures to fulfill those points. Then, two experimental realisations are discussed, a mirror and a microcavity, both used in confocal FCS apparatus to control the emission of a small number of molecules of Cyanine 5 and Rhodamine 6G. Theoretical Model and Preliminary Results Stacks of alternate highand low-index thin dielectric layers are commonly used to make interference filters and highly reflecting mirrors (also known as DBRs, Distributed Bragg Reflectors). Two properties of those planar thin film stacks are particularly interesting: depending on the wavelength range of interest, appropriate dielectric materials show extremely low absorption coefficients. Secondly, the spectral characteristics of any given stack can be very accurately monitored during the deposition process, layer by layer. It is therefore possible to build Fabry-Perot type microstructures whose resonance matches the emission wavelength of the source l0 [12]. This is the case if the optical thickness of the spacing layer between the two parallel mirrors is an integer multiple of l0/2. One has to place the source at an anti-node of the resonant electric field inside the cavity to obtain the maximum emission in this mode [13-15]. Fig. 1. Radiative and trapped light emitted by a source located inside a planar dielectric microcavity The emission of a source described as a monochromatic three-dimensional electromagnetic dipole in a planar dielectric microcavity grown on a glass substrate has been theoretically studied in [16], in the weak coupling regime approximation. For each particular position of the source in a dielectric stack, the emission distributed on a complete basis of orthonormal electromagnetic modes can be computed. As a result, the radiation pattern of the source and the ratio of the power extracted in the air on the total emitted power are also accessible. It turns out that even if the emitter is optimally coupled with the resonant mode of the Fabry-Perot cavity, a large amount of light remains trapped in the structure, either in the guided modes or in the substrate, as sketched on figure 1. This puts a practical limit on the light extraction achievable with planar microcavities [17, 18]. Nevertheless, one can still take advantage of the emission directivity and spectral selection which are classically obtained with planar microcavities. As a demonstrative example, let us describe briefly a very simple structure whose stack formula is {air-HLH4LHLHsilica substrate}, where H and L stand for highand lowrefractive index quarter wavelength layers respectively, at l = 620 nm. The corresponding dielectric materials are Zinc sulfide (nH = 2.36) and Cryolite (nL = 1.3). This stack forms