Process
STED functions by depleting specific regions of the sample while leaving a center focal spot active to emit fluorescence. This focal area can be engineered by altering the properties of the pupil plane of the objective lens. The most common early example of this diffractive optical elements, or DOEs, is a doughnut shape used in two dimensional lateral confinement shown below. The red zone is depleted, while the green spot is left active. This DOE is generated by a circular polarization of the depletion laser, combined with a helical phase ramp. The lateral resolution of this DOE is typically between 30 and 80 nm. However, values down to 2.4 nm have been reported. Using different DOEs, axial resolution on the order of 100 nm have been demonstrated. A modified Abbe’s equation describes this sub diffraction resolution as: D=λ/(2nsinα*sqrt(1+I/Isat)). To optimize the effectiveness of STED, the destructive interference in the center of the focal spot needs to be as close to perfect as possible. That imposes certain constraints on the optics that can be used.
Read more about this topic: STED Microscopy
Famous quotes containing the word process:
“If thinking is like perceiving, it must be either a process in which the soul is acted upon by what is capable of being thought, or a process different from but analogous to that. The thinking part of the soul must therefore be, while impassable, capable of receiving the form of an object; that is, must be potentially identical in character with its object without being the object. Mind must be related to what is thinkable, as sense is to what is sensible.”
—Aristotle (384–322 B.C.)
“I wish to see, in process of disappearing, that only thing which ever could bring this nation to civil war.”
—Abraham Lincoln (1809–1865)
“I’m not suggesting that all men are beautiful, vulnerable boys, but we all started out that way. What happened to us? How did we become monsters of feminist nightmares? The answer, of course, is that we underwent a careful and deliberate process of gender training, sometimes brutal, always dehumanizing, cutting away large chunks of ourselves. Little girls went through something similarly crippling. If the gender training was successful, we each ended up being half a person.”
—Frank Pittman (20th century)