Collinear Advantage

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Collinear techniques are much more versatile than non-collinear techniques, though non-collinear techniques are the standard in nonlinear spectroscopy. Collinear techniques can go wherever you can shine a laser beam. One capability of collinear techniques is stand-off detection, which enables doing spectroscopy outside the lab or in a large vacuum chamber. The other major capability of a collinear technique is that it can be coupled with a microscope. That is to say, it is only possible to access the diffraction limit with collinear techniques, shown in the adjacent figure.

There are three major features of a diffraction-limited spot:

  • You can achieve high intensities with low power light sources (so consider ditching that amplifier)
  • You can select regions of highly inhomogeneous samples. Many samples are small and even the large ones often have short-range structure.
  • You can use the small spot to measure transport and diffusion of excitons, electrons, phonons, etc by scanning the positions of your excitation beams.

Collinear resonant spectroscopic techniques all have a major technical challenge to overcome: distinguishing the signal from everything else. The solution, heterodyne detection of frequency-shifted beams, is a coherent technique (though interestingly it does not require any of the exciting light sources to necessarily be lasers). This heterodyne technique can be applied to spectrally-resolved transient absorption (pump-probe) spectroscopy. It can also be applied to the set of phase-resolved third-order nonlinear spectroscopies called MDCS.

Steve Cundiff’s group has played a large role in advancing the collinear MDCS techniques. The group developed the first frequncy-comb based MDCS. Before that the group demonstrated heterodyne detected collinear spectroscopy and photocurrent detected collinear spectroscopy. These are the technologies that motivated the creation of MONSTR Sense.

a. Techniques using a non-collinear geometry are limited to have >30 micron spot size b. Techniques with a collinear geometry can get down to the diffraction limit.
Schematic for single interferometer in which the interference term between beams A and B is amplitude modulated by the difference in their tagging frequencies. By measuring the interference with respect to a reference laser that copropagates the beams in the interferometer, it is possible to measure the interference term with interferometric precision. By extending this scheme with multiple interferometers, we build up collinear MDCS. Below we show application of collinear schemes: stand-off detection and integration with a microscope.

About MONSTR Sense

We offer compact and collinear all-in-one spectroscopy systems for transient absorption (pump-probe), stimulated Raman, and multidimensional spectroscopies. We invite you to learn more about the techniques’ capabilities and check out our products.

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