  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 which 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.
