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