...at JILA, which is a joint institute of the University of Colorado and the National Institute for Standards and Technology
Coherent Optical Phenomena in Very Dense Gases
Cartoon of two atoms colliding. The oscillations represent the superposition between ground and excited state. The phase of the two atoms after colliding is different from that of atoms that have not undergone collisions, representing the dephasing due to collisions.
Although frequency-domain lineshapes of vapors have been studied for decades, time-domain techniques provide a new window on dynamics otherwise obscured in the frequency domain. This is because, in the time-domain, the fastest dynamics dominate at short times and with high signal-to-noise ratio, while in the frequency domain, the fastest dynamics dominate the lineshape only in the wings and with low signal-to-noise ratio. Thus time-domain techniques enable an incredibly insightful picture of atom dynamics on the femtosecond time scale that heretofore were obscured in the wings of absorption lineshapes.
One major facet of our research is studying the dynamics of phase-interrupting interactions using two-pulse transient four-wave mixing (TFWM, see description under coherence in semiconductors). For a dense potassium vapor the excitation pulses are short compared to the duration of collisions and the time between collisions is long compared to the collision duration. Thus, there are distinct timescales in which the phase memory of the atoms is retained (called the non-Markovian regime) and in which the phase memory is lost (called the Markovian regime). This is not the case for most condensed phase systems, in which the time between interactions, the interaction duration and the pulse width are of similar order. Using the current theory of stochastic fluctuations of the energy levels due to interactions (in our case, collisions), we were able to simulate and fit the signatures of phase memory very well, providing support for the theoretical model, which is commonly used for more complex condensed phase systems. Currently three-pulse experiments and their time-resolved versions are also underway to shed more light on phase memory in atomic systems. A cartoon of two atoms colliding is shown at right.
< postdoc Xingcan Dai is currently involved in this research >