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Stimulated emission depletion microscopy narrows the region allowed to emit detectable fluorescence. A focused excitation pulse or beam populates the excited state. A red-shifted depletion beam with a central intensity minimum drives stimulated emission from the surrounding excited fluorophores. The stimulated photons are rejected from the fluorescence detection channel, leaving spontaneous emission concentrated near the depletion zero. Scanning this small effective emission region builds the image.^[ref]

Near an ideal zero, take depletion intensity proportional to squared radius. The radius at which it reaches a saturation scale decreases as the depletion amplitude grows. A common approximate resolution law iswhere $d_0$ is the undepleted characteristic width and $I_{\mathrm{sat}}$ depends on fluorophore kinetics and illumination timing. It is an idealized scaling, not a universal instrument calibration. For example, a threefold width reduction needs $I_{\max }/I_{\mathrm{sat}}\approx 8$ in this model. The illumination beams remain diffraction limited; nonlinear control of the emitting population creates the smaller effective response.

The cost is high depletion intensity, with possible photobleaching, phototoxicity and heating, demanding fluorophore photophysics, precise beam alignment and maintenance of the central zero. Point scanning limits speed and trades off field size, dwell time and photon statistics. Scattering and aberrations can fill the depletion minimum and degrade performance in depth. A simple lateral doughnut mainly improves lateral resolution; improved axial confinement requires an appropriate three-dimensional depletion pattern. Resolution gain is therefore conditional on usable photons, sample integrity and the actual depletion geometry.

Researcher in fluorescence microscopy. Hell and collaborators developed stimulated emission depletion microscopy, using controlled depletion of fluorescence to narrow the effective emitting region and improve spatial resolution.^[ref]

Stimulated emission depletion microscopy narrows the region from which detected fluorescence is allowed to originate. After excitation, a second beam drives stimulated emission in the surrounding region; a central minimum of that depletion beam leaves a smaller region able to emit the collected fluorescence.

Scanning this region builds an image with detail beyond an ordinary excitation-only diffraction limit. The depletion light must be rejected from the detection channel, and the central minimum must be well aligned with the excitation focus. Increasing depletion intensity can narrow the effective emitting region, but bleaching, background and specimen changes limit the useful improvement.^[ref]