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Physical Review Letters

Ultrasensitive Surface Spectroscopy with a Miniature Optical Resonator

Andrew C. R. Pipino

Published Oct 11, 1999• 89 citations
Publisher

Abstract

The number density and orientation of molecules at the surface of a total-internal-reflection-ring minicavity are probed with extremely high sensitivity in a novel realization of the cavity ring-down optical absorption experiment. The modes of the ultralow-loss cavity, which are excited by photon tunneling, have long lifetimes that are sensitive to the presence of absorbing species in the evanescent field near a cavity facet. The total-internal-reflection-ring cavity extends cavity ring-down spectroscopy to surfaces and condensed matter, permitting a wide range of novel fundamental studies and applications. Routine single molecule detection may ultimately be feasible.

DOI
10.1103/physrevlett.83.3093

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The number density and orientation of molecules at the surface of a total-internal-reflection-ring minicavity are probed with extremely high sensitivity in a novel realization of the cavity ring-down optical absorption experiment. The modes of the ultralow-loss cavity, which are excited by photon tunneling, have long lifetimes that are sensitive to the presence of absorbing species in the evanescent field near a cavity facet. The total-internal-reflection-ring cavity extends cavity ring-down spectroscopy to surfaces and condensed matter, permitting a wide range of novel fundamental studies and applications. Routine single molecule detection may ultimately be feasible.
A significant limitation in the detection and spectroscopy of individual dye molecules in liquid room temperature samples is the loss of fluorescence signal through chromophore photobleaching. Although often considered a fundamental restriction on sensitivity, it has been shown that the probability per excitation cycle of photobleaching may be reduced by enhancing the spontaneous emission rate of the chromophore located inside a microdroplet, which acts as a optical cavity. Studies of glycerol microdroplets containing R6G dye molecules, and levitated in a three-dimensional electrodynamic trap, have demonstrated a significant enhancement of both the fluorescence decay rate [1] and the total fluorescence yield [2] of the chromophore. Modification of the emission rate is greatest for molecules located near the surface of small (< 10 μm) microdroplets, where the influence of the droplet cavity modes of the droplet is most significant. To exploit this surface sensitivity, surfactant forms of rhodamine dyes were recently studied [3]. Fluorescence emission of the dye molecules, constrained to the droplet surface and with fixed transition moments perpendicular to the surface normal, were shown to be significantly enhanced at small droplet sizes. Additionally, at larger droplet sizes corresponding to higher droplet Q’s, preferential emission into the cavity modes was observed, however the detection of fluorescence photons was delayed by the storage time of the cavity, which may be on the same order as the fluorescence lifetime. These measurements, performed on relatively concentrated dye solutions, suggest that microcavity effects may be exploited to greatly enhance single molecule detection sensitivity in glycerol microdroplets. Furthermore, the large cavity storage time of the high-Q droplets opens the possibility for re-excitation of the molecule following emission into the droplet resonance, and subsequent stimulated emission, that is, single-molecule lasing.
We describe recent results of experiments probing single molecules in the surface of spherical microcavities. Effects of spatial photoselection have been recently observed through changes in the single-molecule fluorescence photon statistics. In addition to being a sensitive probe of the highly interesting boundary region between classical and quantum mechanics, cavity-modified spontaneous emission and the possibility of single-molecule stimulated emission hold exciting promise as a means of exceeding sensitivity limitations such as photobleaching and absorption saturation in single molecule fluorescence detection. Ongoing efforts such as fluorescence imaging of single molecules on the surface of microspheres and attempts to observe single- molecule stimulated emission will also be described.
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Selective reflection spectroscopy at normal incidence provides signals with sub-Doppler resolution, linear with optical intensity. Frequency modulated (FM) selective reflection probes atomic vapours at distances comparable to the excitation wavelength (~λ/2π) and is used extensively to probe the Casimir-Polder interactions between an excited state atom and a macroscopic surface. Extending selective reflection spectroscopy to molecular gases allows probing a thin layer of molecular gas next to a surface. This is a very attractive prospect that allows envisaging high-resolution molecular spectroscopy and molecular frequency references in a compact and miniaturised apparatus, such as a thin cell [1] or a photonic crystal [2]. Additionally, it paves the way for spectroscopic probing of the Casimir-Polder interaction with molecules. The molecule-surface interaction has been the object of extensive theoretical investigations, focusing on the effects of molecular orientation and chirality. However, experimental tests are few and comparison with theoretical predictions has been challenging [3].
We use the optical resonance of a spherical microcavity (quality factor ∼106) to excite a single molecule. By attaching a p-terphenyl crystal doped with pentacene and terrylene molecules to a dielectric sphere, we detect individual molecules excited by the near field of the cavity. A low-temperature optical microscope is utilized to image the emission from the single molecule, determine its position on the sphere surface, and measure its absorption linewidth. These results demonstrate the feasibility of single-molecule cavity quantum electrodynamics.