9 - Two-dimensional spectroscopy

Summary

Two-dimensional spectroscopy is an extension of conventional spectroscopic methods that characterises resonant systems as a function of two frequency variables. We use this technique to improve on the characterisation of atomic media along the lines discussed in the preceding chapter. On a microscopic level, the most important physical process for two-dimensional spectroscopy is the transfer of coherence between different transitions. We introduce the basics of the technique and discuss a few specific examples to outline its potential.

Fundamentals

Motivation and principle

Motivation

The preceding chapter showed how light drives the internal dynamics of resonant atomic media and how the measurement of optical anisotropies allows us to monitor these dynamics. The experiments discussed in the preceding chapter, however, can provide only limited information about the system. Most physical systems have more degrees of freedom than we can observe by measurements on transmitted light. As another limitation, we have primarily considered atoms that evolve under their internal Hamiltonian, only weakly perturbed by the probe laser beam. The example of light-induced spin nutation showed that the dynamics of optically pumped atoms differ significantly from those of a free atom. Although it is possible to observe spin nutation for systems with more than two ground-state sublevels, such an experiment suffers from the damping that accompanies optical pumping. The damping drives the system rapidly to an equilibrium, too fast for detailed dynamical observations.