How does super resolution microscopy work?

The four common super resolution microscopy techniques are SIM, STED, PALM and STORM. 

1. Structured illumination microscopy (SIM) is utilized to increase the spatial resolution of light microscopy. It works by exciting a fluorescent sample several times with distinct striped illumination patterns; the orientation and location of the stripes are altered every time. The stripes interact with the high-frequency light as the sample is excited. This process results in a third pattern which is more straightforward to analyze. By combining several images, more detail is achieved and an image is regenerated with higher resolution. 
2. STED involves a method known as spatially patterned excitation. Two lasers are utilized on the focal plane. The excitation laser and the STED laser together diminish the effective point spread function (PSF). A lower PSF is correlated with a higher resolution. This type of microscopy functions by suppressing excited-state fluorophores found nearby to the focal point. This process is known as stimulated emission. It forms a ring-shaped pattern around the center of the image (where fluorophores are inactive) while leaving the center unaffected. This results in a PSF that is lower than the diffraction limit of light, enhancing resolution. Highly sensitive photon detectors are then used to capture signals emitted specifical by fluorophores in the center region of the image.  
3. PALM involves the use of photoactivatable fluorophores to display detailed images of molecules. The photoactive fluorophores are able to be switched on (in an active state) and off (in an inactive state) using light of different wavelengths. When exposed to the laser, the fluorophores become activated and emit for a brief period before they subsequently bleach. The laser randomly (stochastically) activates these fluorophores until they have all emitted light. Moreover, since these molecules are activated randomly and in smaller groups, a more accurate determination of positions can be obtained. Each fluorophore that emits light that is affected by the 300 nm diffraction limit. By analyzing the PSF of each activated fluorophore individually, it becomes possible to determine the locations down to approximately 20 nm. 
4. STORM uses specialized dyes that can switch between off states (dark) to on states (illuminated) when exposed to a low-power laser. Through producing images from these switching fluorophores over an extended period of time, STORM is accurately able to determine the location of these molecules. There are two types of STORM. One type is simply known as STORM and utilizes an activator dye and reporter dye (to switch fluorophores on and report a signal). The second type is known as direct STORM, which doesn’t use an activator dye. Fluorophores are instead used alongside buffers and lasers to generate photoswitching.