Probing Single Molecules with Whispering Gallery Modes of Microspheres

Figure 1: 2-D image (60 microns x 65 microns) of the emission from a sample of p-terphenyl doped with terrylene, showing a single terrylene molecule at 1.8K.

D. J. Norris, M. Kuwata-Gonokami (Univ. of Tokyo), and W. E. Moerner

One goal of our research effort is to study the quantum nature of a single molecule and the fascinating "quantum optical" phenomena which arise. To approach this goal, we have built on previous single-molecule work which utilized the narrow lines available from rigid aromatic hydrocarbons at cryogenic temperatures.¹ When embedded as an impurity in a p-terphenyl crystal, molecules such as pentacene and terrylene exhibit extremely high peak absorption cross sections, high fluorescent quantum yields, and high photostability (See Figure 1). These molecules had previously been used to investigate the basic quantum optics of a single molecule,² which indicated that more complicated experiments should be possible. In particular, we have focused on studying the interaction between a single molecule and an optical cavity.

Cavity quantum electrodynamics (CQED) predicts that the spontaneous emission rate of a single molecule can be strongly enhanced if the molecular transition is in resonance with a small, low-loss, optical cavity (or microcavity).³ This enhancement has been observed in atomic beams, in solutions, and in semiconductors.^4-5 Experiments on a single molecule such as pentacene or terrylene would extend CQED measurements to a single quantum system embedded in a solid, in which the molecule is fixed spatially and can be studied for long periods. For these experiments, spherical microcavities are particularly convenient resonators. High quality dielectric (e.g. glass) spheres have optical resonances, or "whispering gallery modes", which occur when light circulates just inside the sphere’s surface due to total internal reflection.⁶

Single-molecule experiments with a dielectric sphere require not only the detection of a molecule but one which is within an optical wavelength of the cavity surface and in resonance with the cavity mode. As demonstrated in Fig. 2, we have satisfied these requirements by imaging single molecules which are attached to the surface of a 2mm diameter glass sphere at 1.8K. We have also shown that by measuring the optical linewidth of the molecule, one can determine if the molecular emission rate has been modified. Therefore, our results, the first reported in this area,⁷ represent a crucial step toward CQED measurements on a single molecule. In the relatively large sphere used in our preliminary measurements for demonstration, no enhancement of the molecular emission rate has been observed. The project will continue with experiments on much smaller spheres where modification of the molecular emission is expected.

Figure 2: Negative microscope image of a p-terphenyl crystal attached to a 2mm diameter glass sphere (microcavity) at 1.8K. The black dot in the center is the emission from a single terrylene molecule which is excited by the optical resonance of the cavity.

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