Abstract
Confocal microscopy is an essential imaging tool for biological systems, solid-state physics and nanophotonics. Using confocal microscopes allows performing resonant fluorescence experiments, where the emitted light has the same wavelength as the excitation laser. These challenging experiments are carried out under linear cross-polarization conditions, rejecting laser light from the detector. In this work, we uncover the physical mechanisms that are at the origin of the yet-unexplained high polarization rejection ratio which makes these measurements possible. We show in both experiment and theory that the use of a reflecting surface (i.e., the beam splitter and mirrors) placed between the polarizer and analyzer in combination with a confocal arrangement explains the giant cross-polarization extinction ratio of and beyond. We map the modal transformation of the polarized optical Gaussian beam. We find an intensity “hole” in the reflected beam under cross-polarization conditions. We interpret this hole as a manifestation of the Imbert-Fedorov effect, which deviates the beam depending on its polarization helicity. This result implies that this topological effect is amplified here from the usually observed nanometer to the micrometer scale due to our cross-polarization dark-field methods. We confirm these experimental findings for a large variety of commercially available mirrors and polarization components, allowing their practical implementation in many experiments.
- Received 29 April 2020
- Revised 14 January 2021
- Accepted 15 February 2021
DOI:https://doi.org/10.1103/PhysRevX.11.021007
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Confocal microscopy is an essential imaging tool for biological systems, solid-state physics, and nanophotonics. Whereas conventional microscopes flood their targets with light, confocal microscopes peer through pinholes at points of illumination to enhance their resolution. The improved resolution has been of great help in studying light-matter interactions via resonant fluorescence experiments, which require a laser to excite light emission in a material. These experiments employ a technique for suppressing the laser light, and yet it is not clear why this technique works. Here, we uncover the physical mechanism that makes this suppression possible.
To reject the laser light, confocal setups employ polarization filters, which, in theory, should reduce the laser brightness by a factor of up to . In practice, they go up to 6 orders of magnitude beyond that. This is, to say the least, surprising. In our experiments, we show that the effect finds its origin in what is known as the Imbert-Fedorov shift, a subtle sideways shift of a polarized beam of light when it is reflected off a smooth surface, such as the beam splitter and mirrors in these setups.
Our work opens the door to a methodical design of sensitive laser resonant fluorescence microscopes with extreme background extinction for a broad range of applications in quantum optics and solid-state physics. The new experimental methods developed for this work can also be used for measuring material optical properties.