Adaptive optics (AO) was developed to solve a stubborn problem facing astronomers: The same atmospheric aberrations that cause stars to twinkle lead to blurry images as the path of light from those stars to a telescope is distorted. To correct for aberrations from both the atmosphere and the telescope's optics, observers monitor a guide star, either a real one or an artificial one created in the upper atmosphere by a laser, to measure how the final image is being distorted. Then they adjust their optical equipment in a way that counteracts the distortion, ideally resulting in a point-like star and in-focus astronomical images.
AO systems are also used in optical microscopy, where the lenses and the complex structure of specimens can induce significant aberrations. Without a sky full of stars as candidate guide stars for homing in on, scientists must identify a point-like part of their specimen to use. That is difficult to do, especially in label-free microscopy, in which the sample needs to be illuminated because it lacks fluorescent probes. Additionally, the correction blueprint for the optics needs to be adjusted any time the specimen or the arrangement of lenses changes.
Now researchers at the University of Glasgow and the Paris Institute of Nanosciences at Sorbonne University have developed a novel AO method to correct label-free microscopy images. Rather than adding to the optics, they take advantage of the photons that illuminate the sample. They use entangled photons, created by shining a laser through a nonlinear crystal. That allows photons reaching the camera to be used first to derive the aberration corrections and then to obtain a corrected image of the specimen. In each case, the photons captured by the camera are subject to the same aberrations.
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