Dispersion-mediated modal reconfiguration in a binary photonic lattice

Dynamic control of on-site potentials is key to controlling band structures and light transport in lattice systems. Here, we demonstrate a dynamically tunable binary photonic lattice in a four-level atomic system, formed by superimposing two standing-wave fields with a 1:2 period ratio, under far-detuned electromagnetically induced transparency. In this regime, absorption is strongly suppressed, and the refractive-index response is dominated by its dispersive component. By tuning the laser frequency detuning, we continuously reshape the refractive-index contrast between two sublattices, enabling all-optical control of the on-site potentials. This control drives a clear reconfiguration of the optical modes, whereby the probe field shifts its concentration in real space, accompanied by an alternating distribution of reciprocal-space energy between the first- and second-order diffraction channels. Our results demonstrate a low-loss route to on-demand mode reconfiguration, opening new possibilities for programmable light steering and reconfigurable photonic functions in atomic media.