Stimulated emission does not radiate in a pure dipole pattern

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.