Tri-comb Spectroscopy*

Optical multidimenisonal coherent spectroscopy (MDCS) has been developed over the last 20 years and proven very powerful at unfolding congested spectra, identifying coupling between resonances, making size-resolved measurements of nanoparticles and revealing many-body interactions [1]. However, MDCS has required a complex apparatus and has suffered from limited spectral resolution.

Inspired by developments in dual-comb spectroscopy, we developed comb-based MDCS, improving the spectral resolution by over an order of magnitude, which we demonstrated by resolving the hyperfine split transitions in a rubidium vapor [2]. With the addition of third frequency comb, which we call "tri-comb spectroscopy," we are able to eliminate all moving parts and further improve the resolution with a data record of less the 0.5 seconds long [3]. These results also demonstrate the separation of the two isotopies of rubidium, showing the promise of MDCS for chemical sensing where the separation of species is a critical challenge.

These results open a path towards an implementation of MDCS that can rapidly produce spectra using a compact and robust apparatus that can be used outside a laboratory.

[1] 1. S. T. Cundiff and S. Mukamel, "Optical multidimensional coherent spectroscopy," Phys. Today 66, 44 (July 2013).
[2] B. Lomsadze and S. T. Cundiff, "Frequency combs enable rapid and high-resolution multidimensional coherent spectroscopy," Science 357, 1389–1391 (2017).
[3] B. Lomsadze, B. C. Smith, and S. T. Cundiff, "Tri-comb spectroscopy," Nat. Photonics 12 , 676–680 (2018).