Spontaneous emission spots

Fig. 9 Schematic illustration of STED. (a) Jablonski diagram indicating the So and Si state of a molecule (left). A femtosecond pulse excites the fluorophore from So to a high vibrational level in Si. The following slow STED pulse induces the stimulated emission from the lowest Si level to a high vibrational level in So. The STED pulse depletes the Si state and quenches the fluorescence. The corresponding wavelength diagram is shown on the right-hand side the dotted and solid lines are the absorption and emission spectra, respectively. The spontaneous emission at A-oet is detected as the signal in the microscopy measurement, (b) The combination of the diffraction-limited excitation spot and the engineered STED pulse yields a narrowed point spread function...

In the experiment, the width of the atomic beam was compressed down to a spot diameter of 28 pm (Fig. 7.4). This minimum achievable spot diameter in the experiment was determined by the fluctuation of the momenta of the atoms due to spontaneous emission. In this experiment, every atom scattered a small number of photons because of optical pumping, so that the duration of the resonant interaction of each atom with the field was less than the flight time of the atoms through the laser beam. If the atoms interacted with the field all the time, the transverse motion of the atoms would be a periodic focusing and defocusing. 

Figure 25 The principle of STED after exciting a diffraction-limited zone in the sample (green spot), a donut-shaped spot is applied to the same area (red spot), which darirens or switches off peripheral excited molecules. Depletion of the fluorescent state is accomplished through stimulated emission from the excited state to the ground state by the red-shifted donut beam. By increasing the intensity in the depletion beam, spontaneous emission of a arbitrary small zone can be recorded (yellow spot).

Stimulated emission depletion (STED) microscopy is another microscopy approach to overcome the diffraction limitation of light microscopy by inhibiting fluorescence outside of the focal spot [9]. In this context, fluorescence is viewed as spontaneous light emission while stimulating emission via synchronized laser... 

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