Optical frequency combs offer remarkable advantages as broadband sources of coherent light. On the one hand, they provide extremely high values of peak electric fields, leading to efficient frequency conversion all the way into the ultraviolet sector. On the other hand, they consist of a large number of narrow equidistant frequency modes, allowing direct measurements of optical frequency by mapping optical frequencies to radio frequencies. When interacting with a cold atomic cloud, both the pulsed nature of this radiation and the narrow linewidths of the individual comb lines can be utilized in a unique way [1].
In this talk I will discuss recent experimental and theoretical results demonstrating the effects of interaction between femtosecond light pulses and a cold atomic cloud. The interaction of a single comb line with an atomic transition enables frequency comb spectroscopy and direct laser cooling of Rubidium atoms [2]. Here, the spectroscopy is enabled by atomic linewidths being wider than the comb lines but narrower than their spectral distances, while the femtosecond pulses create coherent accumulation of optical coherences as the excited state populations have longer lifetimes than the pulse repetition periods. We have discovered that a similar mechanism can be used to generate long-lived (quantum) coherences in the atomic ground states, leading to the demonstration of electromagnetically induced transparency with a single frequency comb mode probe in a Lambda hyperfine level structure [3]. In the far-detuned regime, the interaction of cold atoms and many frequency modes of an ultrashort pulsed laser can be enhanced by a Fabry-Perot resonator, leading to self-organization and cavity cooling [4].
I will show how our results relate to the ongoing efforts towards laser cooling of atomic species with transitions in the vacuum-ultraviolet spectral region, and the designs for multimode quantum memories.
[1] A. Marian, M. C. Stowe, J. R. Lawall, D. Felinto, J. Ye, United Time-Frequency Spectroscopy for Dynamics and Global Structure, Science 306, 2063 (2004).
[2] N. Šantić, D. Buhin, D. Kovačić, I. Krešić, D. Aumiler, T. Ban, Cooling of atoms using an optical frequency comb, Scientific Reports 9, 1 (2019).
[3] I. Krešić, M. Kruljac, T. Ban, and D. Aumiler, Electromagnetically induced transparency with a single frequency comb mode probe, JOSA B 36, 1758 (2019).
[4] V. Torggler, I. Krešić, T. Ban, and H. Ritsch, Self-ordering and cavity cooling using a train of ultrashort pulses, New Journal of Physics 22, 063003 (2020).