Calculation of Photocarrier Generation from Optical Absorption for Time-domain Simulation of Optoelectronic Devices

Photocarrier generation rate in optoelectronic materials is often calculated using the Poynting vector in the frequency domain. However, this approach is not accurate in time-domain simulations of photoconductive devices because the instantaneous Poynting vector does not distinguish between power flux densities of optical and low-frequency electromagnetic fields. The latter is generated by photocurrents and is not supposed to contribute to the photocarrier generation since the corresponding photon energy is smaller than the bandgap energy of the optoelectronic material. This work proposes an optical absorption-based model to accurately calculate the generation rate in time-domain simulations. The proposed approach considers the material dispersion near the optical frequency corresponding to the bandgap energy of the optoelectronic material and calculates the instantaneous optical absorption from the polarization current density associated with this dispersion model. Numerical examples show that the proposed method is more accurate than the Poynting vector-based approach in calculating the instantaneous optical absorption. The method is further validated against experimental results via simulations of a photoconductive device, where the Poynting vector-based approach results in divergent carrier densities when the low-frequency fields are strong.