Frequency Combs Rev Up Imaging

For label-free, nondestructive imaging of complex systems, such as biological samples, coherent anti-Stokes Raman spectroscopy (CARS) is a popular technique. A serious drawback of CARS for such imaging applications, however, is its slow spectral acquisition speed. To improve this and spectral resolution in CARS microscopy, Takuro Ideguchi at the Max Planck Institute for Quantum Optics, in Germany, and coworkers devised a technique using laser frequency combs (Nature 2013, DOI: 10.1038/nature12607). Frequency combs are trains of laser pulses that are used to precisely control the frequency of light. Ideguchi and coworkers used two frequency combs that are slightly offset in frequency to rapidly generate CARS interferograms that can be converted via Fourier transformation to a CARS spectrum. The researchers were able to acquire CARS spectra in as few as 15 microseconds. In a demonstration, the team used the method to acquire CARS images of a capillary plate with holes filled with a mixture of hexafluorobenzene, nitrobenzene, nitromethane, and toluene. The work “is significant for CARS microscopy, where image acquisition speed is the single most important experimental parameter,” says CARS expert Eric O. Potma of the University of California, Irvine. “The beauty is that such fast acquisition times were obtained with a single optical detector, which is the preferred detection approach in CARS imaging applications.”