  Dilute-N GaPAsN alloys have great potential for optoelectronics
lattice-matched to Si. However, there is a lack of systematic calculation of
the optical response of these alloys. The present paper uses the sp^3d^5s^*s_N
tight-binding model to calculate the fullband electronic structure of dilute-N
GaPAsN, and then calculate the optical response functions considering direct
transitions within the electric dipole approximation. Good agreement is
obtained for the dielectric function in comparison to available optical data
for dilute nitrides. To achieve this, the sp^3d^5s^* parameters for GaP and
GaAs are optimized for their optical properties in comparison to published
data, which are then used as the basis for the sp^3d^5s^*s_N parameters for
dilute-N GaPN and GaAsN. The calculated absorption between the valence band and
the newly formed lowest conduction band of the dilute nitrides increases as the
N fraction increases, in agreement with experiments, mainly due to the net
increase in their coupling in the entire Brillouin zone, supported by the
calculated momentum matrix element in the present work.
