Abstract
Vortex beams, which include phase or polarization singularities, have attracted attention and provided
new approaches to spectroscopic studies. We demonstrate in this paper the application of vortex pulses
to four-wave-mixing (FWM) spectroscopy which is a major technique to investigate the coherent
dynamics of excitons in semiconductors. The measurements provide some unique properties of GaN
excitons: FWM using optical vortex demonstrates the coherent orbital angular momentum dynamics
significantly exceeds the exciton dephasing time and its decay reflects inhomogeneity of rotational phase
gradient; FWM using polarization vortex realizes a snap-shot measurement of optical anisotropies in
GaN films, allowing us to evaluate the uniaxial strain energies and the spin-exchange interaction
constant with high accuracy and sensitivity.
