That is, the polarization is a convolution of the electric field at previous times with time-dependent susceptibility given by χ (Δ t). The upper limit of this integral can be extended to infinity as well if one defines χ (Δ t) = 0 for Δ t < 0. An instantaneous response would correspond to a Dirac delta function susceptibility χ (Δ t) = χδ (Δ t).