A method to improve the resolution of optical technologies, including microscopes and bioimaging systems, has been developed. The discovery is based on a greater understanding of wave propagation, which is responsible for the transmission of information and energy in optical technologies.
According to researchers at the Center for Soft and Living Matter, within the Institute for Basic Science, if light passes through asymmetric apertures, astigmatism arises and can degrade image resolution. In addition to identifying this previously unknown problem, the researchers also showed how to remedy it. Their findings and suggestions were published in the Proceedings of the National Academy of Sciences (PNAS) earlier this month.
The IBS team explained that, as the aperture gets smaller, the focus shifts more and more backwards towards the lens, such that the initial focus is no longer "in focus." As a consequence, if the aperture is not equal in the vertical and horizontal planes, focal shifts will differ between these directions, leading to astigmatism. "Astigmatism can occur even with the most perfect lens if it is used with a non-circular aperture," explained Kai Lou, first author of the study.
The team applied the idea to improve line-temporal focusing microscopy (LTFM), which makes use of a naturally asymmetric input beam. As LTFM is a method used to visualize deep biological structures, the researchers tested their focal shift correction strategy with mouse lung tissues. The resolution obtained reportedly outperformed point scanning microscopy (PSM).
Even though this effect is very small and can be insignificant in some applications, correcting for aperture-induced astigmatism could make a significant difference in delicate systems, like advanced microscopy used to acquire a large volume of images, the team reported.
"In the future, we plan to apply aperture-induced astigmatism to even more complex information transfer technologies," said Steve Granick, co-correspondent author of this study. "Moreover, the study opens avenues to basically improve the design of any equipment handling electromagnetic waves, ultrasounds, or particles beams. For example, it also applies to waves, used with space antennas to focus on satellite or spaceship. We believe it can contribute to design better systems in synthetic microscopic eyesight, telecommunications, and even microwave devices."
Image. (A) A stone falling in a pond produces full circular waves that are centered on the impact point, and those waves would back propagate onto that same point if time could be reversed. Using this time-reversal argument, if one generates back propagating circular waves from a limited arc, they will not necessarily focus at the center. (B) Representation of how diffraction effects compete with focusing for a beam of different initial size, that is different aperture. Going from left to right on the x-axis, the input beam size (aperture size , orange line) increases. For a large beam, focusing is strong, leading to a small cross section in the focal plane (blue line). If the aperture is reduced, a critical situation is reached (dashed line) where is same as . At the critical point where the two lines cross, focusing and diffraction effects are equal, and energy (red) is best focused between the lens and the initial focal plane, which means that the effective focal plane has shifted towards the lens. Extremely small apertures correspond to a point source and produce diffraction without focusing. Image courtesy of ISB.