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Analytical Chemistry

Raman Imaging In A Different Light

Microscopy: Modulating the frequency of laser beams allows researchers to pull out multiple Raman shifts to capture images of tissue samples

by Celia Henry Arnaud
November 2, 2015 | A version of this story appeared in Volume 93, Issue 43

Ji-Xin Cheng and coworkers at Purdue University have developed an improved method for obtaining stimulated Raman scattering images of dense, highly scattering samples (Sci. Adv. 2015, DOI: 10.1126/sciadv.1500738). In the new method, the researchers modulate the pump laser beam at 16 frequencies in the megahertz range by dispersing the laser beam and scanning it onto a reflective pattern with 16 different densities. They then combine the pump beam with another laser beam, called the Stokes beam, and aim both at the sample. They record a time trace of the stimulated Raman signal from the sample and use a Fourier transform to demodulate the signal into 16 Raman shifts. With this approach, the Purdue team obtained stimulated Raman spectra of dimethyl sulfoxide that matched the spontaneous Raman spectra, even when pieces of dense tissue were placed in front of the detector to block the scattered photons from the sample. The team then mapped the active diffusion of α-tocopherol, also known as vitamin E, through mouse skin tissue. They also imaged intact human breast cancer tissue and live Caenorhabditis elegans worms.

Schematic of the optics used for an improved stimulated Raman scattering microscope.
Credit: Sci. Adv.
In the stimulated Raman scattering microscope, the pump laser beam is modulated at 16 frequencies and combined with a Stokes beam. The improved method yields better imaging of dense, opaque samples.


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