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Coherent Raman techniques provide a way to visualize the chemical groups that are present in samples without using any labelling. Coherent Raman uses two laser beams whose frequency difference is equal to the vibrational frequency of the targeted chemical bond.

Coherent Raman imaging is limited in speed and can damage the sample

The conventional approach uses single-point focused lasers that scan the sample using moving mirrors. The image is then reconstructed point by point and is limited by the time required for the laser beams to scan the sample. Increasing the scanning speed means increasing the power of the lasers, which inevitably leads to photo-damage to the sample.

Random illumination to go faster and minimize damage to samples 

Researchers from the CRIMSON consortium at the Institut Fresnel in Marseille, in collaboration with Lithuanian, English and American colleagues, have recently developed a new coherent Raman method for imaging the chemical bonds of molecules that does not use laser scanning of the sample but wide-field imaging using a camera. To do this, they illuminated the sample with random speckles, which have the advantage of keeping their statistical properties when propagating in a scattering medium such as biological tissue. By changing the speckle illumination, they were able to demonstrate (1) that it is possible to image chemical bonds that are located only in the imaging plane, thereby achieving virtual optical sectioning, and (2) that it is possible to improve the resolution by a factor of 2. In addition, this speckle illumination mode protects the sample from damage caused by laser illumination, making it possible to image the sample over the long term. This new imaging method should help to advance research in the fields of biology, pharmacology and medicine, in particular for the rapid detection of cancerous tissue.

Read more at: E. M. Fantuzzi, S. Heuke, S. Labouesse, D. Gudavičius, R. Bartels, A. Sentenac, and H. Rigneault, “Wide-field coherent anti-Stokes Raman scattering microscopy using random illuminations”, Nature Photonics (2023). https://doi.org/10.1038/s41566-023-01294-x