05 Sep. Stamp-like microscopy: a resonator improves the image
In microscopy, it is often problematic when too much light hits a sample – for example, when trying to image sensitive biological structures or when examining quantum particles. The goal is therefore to gather as much information as possible about the object under observation with a given amount of light. At TU Wien, a new approach was used in collaboration with the University of Vienna and the University of Siegen: the light is stored in a resonator in which the sample is also located. This allows a better signal to be achieved than with other methods.
Better signal through multiple light scattering
“In a conventional microscope, the light falls on the sample once and then enters an objective lens,” says Maximilian Prüfer, who led the study at the Atomic Institute of TU Wien as part of his Esprit Fellowship from the Austrian Science Fund FWF. “In our microscope, we place the sample in an optical resonator – between two mirrors.” To enable the resonator to function as a microscope, the team developed an unusual experimental setup with additional lenses: after the light beam has illuminated the sample, it is guided in a circle and strikes the sample again. Oliver Lueghamer, who built the microscope at TU Wien, explains: “Now the sample is illuminated again, but not with a conventional, uniform beam of light as at the beginning, but with a beam of light that already contains the image of the sample, so to speak.” Similar to a stamp that is pressed several times on the same spot, producing a clearly visible image even with faint ink at the end, the image of the sample becomes clearer and clearer when it is directed to the sample several times in the resonator.
Both theoretical calculations, developed in collaboration with Thomas Juffmann from the University of Vienna and Stefan Nimmrichter from the University of Siegen, and experiments show that this method provides more information than other microscopy techniques at a given light intensity. “The key figure is the signal-to-noise ratio, the ratio of desired signal to unwanted noise,” explains Prüfer. “This ratio is better than with other methods due to multiple scattering with the same disturbance of the sample.”
Precise despite unstable resonator
However, the practicality of the new method also depends on how susceptible it is to interference: “When using optical resonators, as we do, it is often important to keep their length extremely constant,” emphasizes Prüfer. “Usually, you have to go to great lengths to ensure that the distance between the two mirrors varies only minimally, otherwise the desired effect is lost. With our method, however, this is not the case.” The distance between the mirrors can also show a certain degree of instability without the effect disappearing. “This is important because it means that the method not only works in theory, but can also be used in practice with manageable effort,” says Prüfer.
According to the scientists, one of the goals of the new microscopy technique is to image ultracold Bose-Einstein condensates and thereby study their quantum physical behavior.
Original publication:
[O. Lueghammer et al.: Cavity-enhanced continuous-wave microscopy with potentially unstable cavity length; Nature Scientific Reports 15, 27676, 2025, https://www.nature.com/articles/s41598-025-13589-w]
Source and image: www.tuwien.at