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.