Index Dispersion above the Fundamental Band Edge in Nitrogen-Doped $\mathrm{Ga}{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ ($y=0.38\mathrm{}$, $E_{\mathrm{N}}<E_{\Gamma }$)

Data are presented on ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x{\mathrm{P}}_{1-z}{\mathrm{As}}_z-\mathrm{Ga}{\mathrm{As}}_{1-y}{\mathrm{P}}_y:\mathrm{N}$ heterojunctions showing that for the N trap located (∼ 5 meV) in the $\Gamma$ conduction band ($E_{\mathrm{N}}>E_{\Gamma }$) of $\mathrm{Ga}{\mathrm{As}}_{1-y}{\mathrm{P}}_y:\mathrm{N}$, band-to-band laser operation tends to be quenched locally. The index dispersion quantity ($n-\lambda \frac{\mathrm{dn}}{d\lambda }$) exhibits a large local increase due to the $k\ne 0$ components the trap introduces in the recombination process. These results are interpreted as the resonant-antiresonant behavior of the trap state interacting with the conduction-band density of states.