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arXiv:2312.02333 · Optica

Stimulated emission does not radiate in a pure dipole pattern

Andrew E. S. Barentine, W. E. Moerner

Published Mar 5, 2024• 2 citations
arXivHTMLPDF

Abstract

Stimulated emission (StE) remains relatively unused as an image-forming signal despite having potential advantages over fluorescence in speed, coherence, and ultimately resolution. Several ideas for the radiation pattern and directionality of StE remain prevalent, namely, whether a single molecule would radiate StE itself in a pure dipole pattern, or whether its emission direction depends on the driving field. Previous StE imaging has been carried out in transmission, which would collect signal either way. Here, we introduce the StE driving field (the probe) at an angle, using total internal reflection to avoid incident probe light and its specular reflections in our detection path. In this non-collinear detection configuration that also collects some fluorescence from the sample, we observe fluorescence depletion even in the spectral window where an increase in detected signal from StE would be expected if StE radiated like a simple classical dipole. Because simultaneous direct measurement of the fluorescence represents a calibration of the potential size of StE were it spatially patterned like a classical dipole emitter, our study clarifies a critical characteristic of StE for optimal microscope design, optical cooling, and applications using small arrays of emitters.

DOI
10.1364/optica.515226

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Stimulated emission (StE) remains relatively unused as an image-forming signal despite having potential advantages over fluorescence in speed, coherence, and ultimately resolution. Several ideas for the radiation pattern and directionality of StE remain prevalent, namely, whether a single molecule would radiate StE itself in a pure dipole pattern, or whether its emission direction depends on the driving field. Previous StE imaging has been carried out in transmission, which would collect signal either way. Here, we introduce the StE driving field (the probe) at an angle, using total internal reflection to avoid incident probe light and its specular reflections in our detection path. In this non-collinear detection configuration that also collects some fluorescence from the sample, we observe fluorescence depletion even in the spectral window where an increase in detected signal from StE would be expected if StE radiated like a simple classical dipole. Because simultaneous direct measurement of the fluorescence represents a calibration of the potential size of StE were it spatially patterned like a classical dipole emitter, our study clarifies a critical characteristic of StE for optimal microscope design, optical cooling, and applications using small arrays of emitters.
This thesis presents the first application of the stimulated emission depletion (STED) technique to the field of single-molecule fluorescence spectroscopy. It is demonstrated that fluorescence quenching induced by STED is reversible and can be repeated a large number of cycles on a single molecule. Being ideal point-like probes, single molecules can therefore be used to characterize the resolution of STED microscopes. In a spectroscopic study, two simplifed models of the photophysical processes involved in STED are analyzed and applied to the experimental determination of the stimulated emission cross sections on a single-molecule level. In addition, the STED concept as applied in subdiffraction-resolution microscopy is transferred to fluorescence fluctuation spectroscopy. A successful implementation promises to expand the possibilities particularly of fluorescence correlation spectroscopy (FCS) which is already the most widely used fluctuation technique today but is restricted to concentrations on the nanomolar scale. Reducing the detection volume beyond the diffraction limit could render the micromolar range accessible and could thus open up new applications in the life sciences.
Zenodo hosts the archival version of this document; for convenient viewing, please visit andrewgyork.github.io/stimulated_emission_imaging Abstract Stimulated emission is often described as indistinguishable from the stimulating beam, sharing the same phase, frequency, polarization, and direction of travel. What direction does stimulated emission travel when the emitter is pointlike? Pointlike sources typically emit in all directions (a spherical wave), but if the stimulating beam is nearly monodirectional (a plane wave), will the stimulated emission propagate in all directions, or just one? We perform simple experiments to explore this apparent paradox, with an eye towards technological utility. In particular, does stimulated emission encode information about an emitter's location? Can lenses use this stimulated emission to form images of emitter distributions? If so, this suggests a way to combine the high signal rates of transmitted light microscopy with the specificity of fluorescence microscopy.
Nonlinear optical microscopy techniques have emerged as a set of successful tools for biological imaging. Stimulated emission microscopy belongs to a small subset of pump–probe techniques, which can image non-fluorescent samples without requiring fluorescent labeling. However, its sensitivity has been shown to be ultimately limited by the quantum fluctuations in the probe beam. We propose and experimentally implement sub-shot-noise limited stimulated emission microscopy by preparing the probe pulse in an intensity-squeezed state. This technique paves the way for imaging delicate biological samples that have no detectable fluorescence with sensitivity beyond standard quantum fluctuations.
Nonlinear optical microscopy techniques have emerged as a set of successful tools for biological imaging. Stimulated emission microscopy belongs to a small subset of pump-probe techniques which can image non-fluorescent samples without requiring fluorescent labelling. However, its sensitivity has been shown to be ultimately limited by the quantum fluctuations in the probe beam. We propose and experimentally implement sub-shot-noise limited stimulated emission microscopy by preparing the probe pulse in an intensity-squeezed state. This technique paves the way for imaging delicate biological samples that have no detectable fluorescence with sensitivity beyond standard quantum fluctuations.
Whereas the basic features of stimulated emission are easily seen to hold true for ensembles of dipoles, the same is not always true for a single dipole system. For example, symmetry requires that well localized, bound dipoles emit a dipole field which is isotropic in the plane defined by the dipole vector. Indeed, for this case, nothing in the interaction between the dipole and the field contains information on the propagation direction of the field. These observations seem at odds with the highly directional nature of stimulated emission. The authors therefore test for stimulated emission by an explicit calculation considering both a single Rabi type dipole and a damped dipole whose centre-of-mass coordinates are fixed at a point. Remarkably, despite the symmetrical dipole radiation pattern, stimulated emission into the original stimulating wave is shown to occur exactly. An explanation of this effect is then considered by investigating dipoles whose centre-of-mass coordinates are allowed to vary. In this case, photon recoil of the dipole introduces a directional sense to the interaction. In the extreme opposite and trivial case of a highly delocalized dipole (centre-of-mass momentum eigenstates) stimulated emission again occurs exactly. In all intermediate cases, however, stimulated emission occurs only partially or approximately. A condition imposed on a dipole's spatial extent is established for stimulated emission to occur approximately. Finally, the original bound and localized dipole is seen to be a special limiting case of recoiling dipoles for which stimulated emission occurs exactly.
We demonstrate a new method of determining the three-dimensional dipole orientations of single molecules by direct imaging of the emission patterns in the back focal plane of a high-numerical-aperture objective lens. We compare the reconstructed emission-dipole orientations with a previously established method of absorption-dipole mapping. We find that, for a given number of emitted photons, emission pattern imaging provides better accuracy (1°–2°) than absorption-dipole mapping of single molecules. Compared with some other methods for emission-dipole mapping, the presented method is (1) less sensitive to optical aberrations and adjustment and (2) data analysis is simplified because radiation patterns can be expressed in a simple analytical form.