Stimulated emission is next to Fluorescence an additional photon emission process and was theoretically discovered by Albert Einstein in 1917. When an atom or molecule is in an excited state (for example by absorbing a photon), it can relax to the ground state by emitting spontaneous light (fluorescence) or via non-radiative decay (for example by heat exchange). However, Einstein theoretically discovered that here is another possibility; when additional light (with specific energy) is interacting with the the atom or molecule, then there is a certain possibility that the atom/molecule will return to the ground state by emitting a photon with exactly the same energy as the additional light it was interacting with. The process of stimulated emission is also illustrated in figure 1.
Consider an atom or molecule in an excited energy state with energy gap ΔE = E1 − E0 that is stimulated to emit a photon with the same phase, frequency and polarization as an incident photon with an energy equal to the energy gap ΔE = hν, where h is the Planck constant and ν is the frequency of the photon. The rate of stimulated emission is given by kst = σΦ , where Φ is the number of photons per area per time unit and σ is the stimulated emission cross section for the related transition. Since σ for fluorescent dyes is typically very small (in the order of 10-17cm2), this process is not very efficient. Relatively high intensities (> 40 MW · cm-2) are needed to make the stimulated emission rate comparable to the spontaneous emission rate of ~ 10^9 s-1.
STED Microscopy is a super-resolution technique that uses stimulated emission process to effectively deplete a specific region of fluorophores, and is thereby able to circumvent the diffraction limit. In order for STED to work, high laser intensity is required in order to make the stimulated emission rate high enough for effective depletion. This is one of the reasons why Pulsed lasers have have an advantage over Continuous-wave lasers in STED microscopy.