Cleaning CARS: Cutting down on spectral noise pollution

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  • Published: Jan 5, 2012
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Cleaning CARS: Cutting down on spectral noise pollution

In CARS

Researchers at the University of Twente's MESA+ Institute for Nanotechnology and elsewhere have used a "reverse" approach to cleaning CARS, coherent anti-Stokes Raman spectroscopy, images that helps them eliminate background noise.

The interpretation of data from single-molecule optical techniques is fraught with danger. After all, fuzziness due to light interference and other effects distorts the view. Erik Garbacik, Jeroen Korterik, Cees Otto (MIRA Institute for Biomedical Technology and Technical Medicine), Shaul Mukamel (Department of Chemistry, University of California, Irvine, California, USA), Jennifer Herek and Herman Offerhaus have now suggested that taking an opposing view could help clean up the spectroscopic image by eliminating background noise. They detail their approach in the journal Physical Review Letters.

Conventionally, optical spectroscopy would begin with the laser beam incident on the sample and "read" the molecules within. Various methods have then been applied to reduce noise and improve contrast. For instance, the exploitation of polarization dependences of the resonant and non-resonant components of Chi, direct measurement or extraction of the vibrational phase of the oscillators, shaping the phase of a broadband optical pulse to match that of the molecule, or introducing time delays so that the the resonant vibrational state might be probed after the non-resonant coherence has decayed.

Theoretical CARS

However, what if one were instead to begin with the molecule in CARS spectroscopy? Mukamel and colleague Rahav in California have considered a focus on energy transfer from a molecular quantum mechanical point of view. Given that CARS is already an established and powerful technique used in food testing and medical imaging, which has the major advantage of requiring no fluorescent tags or labels to make manifest a molecular image, such a notion could be exploited to reduce background noise complications.

The research team begins by examining the the molecule's energy levels first to give us Vibrational Molecular Interferometry, which, the team says, will vastly expand the uses of CARS and related techniques. Essentially, the process allows them to carry out non-linear microspectroscopy in an intuitive way so that they can extract the vibrational response directly. "Three optical fields create a pair of Stokes Raman pathways that interfere in the same vibrational state," the team explains. "Frequency modulating one of the fields leads to amplitude modulations on all of the fields." They can thus undertake high-speed imaging without interference from the non-resonant background, the noise that reduces contrast, and so distinguish between the electronic and vibrational contributions to the overall signal.

The team has demonstrated proof of principle in imaging a mayonnaise sample in which the normal, strong non-resonant background due to water is effectively removed from the "equation" using their approach.

The researchers summarise their approach: "We have demonstrated a new quantum mechanical approach for non-linear microspectroscopy that exploits interference between two competing Stokes Raman pathways by analogy to coherent control." They add that the technique could make it possible to "convert CARS into a fully dissipative technique."


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Researchers at the University of Twente's MESA+ Institute for Nanotechnology have used a
Mayo in CARS? 

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