  As optical two-dimensional coherent spectroscopy (2DCS) is extended to a
broader range of applications, it is critical to improve the detection
sensitivity of optical 2DCS. We developed a fast phase-cycling scheme in a
non-collinear optical 2DCS implementation by using liquid crystal phase
retarders to modulate the phases of two excitation pulses. The background in
the signal can be eliminated by combining either two or four interferograms
measured with a proper phase configuration. The effectiveness of this method
was validated in optical 2DCS measurements of an atomic vapor. This fast
phase-cycling scheme will enable optical 2DCS in novel emerging applications
that require enhanced detection sensitivity.
