  We use a three-pulse ultrafast optical spectroscopy to study the relaxation
processes in a frustrated Mott insulator Na$_2$IrO$_3$. By being able to
independently produce the out-of-equilibrium bound states (excitons) of
doublons and holons with the first pulse and suppress the underlying
antiferromagnetic order with the second one, we were able to elucidate the
relaxation mechanism of quasiparticles in this system. By observing the
difference in the exciton dynamics in the magnetically ordered and disordered
phases we found that the mass of this quasiparticle is mostly determined by its
interaction with the surrounding spins.
