  The generation rate of photocarriers in optoelectronic materials is commonly
calculated using the Poynting vector in the frequency domain. In time-domain
approaches where the nonlinear coupling between electromagnetic (EM) waves and
photocarriers can be accounted for, the Poynting vector model is no longer
applicable. One main reason is that the photocurrent radiates low-frequency EM
waves out of the spectrum of the source, e.g., terahertz (THz) waves are
generated in THz photoconductive antennas. These frequency components do not
contribute to the photocarrier generation since the corresponding photon energy
is smaller than the optoelectronic material's bandgap energy. However, the
instantaneous Poynting vector does not distinguish the power flux of different
frequency components. This work proposes a material absorption-based model
capable of calculating the carrier generation rate accurately in the time
domain. Using the Lorentz dispersion model with poles reside in the optical
frequency region, the instantaneous optical absorption, which corresponds to
the power dissipation in the polarization, is calculated and used to calculate
the generation rate. The Lorentz model is formulated with an auxiliary
differential equation method that updates the polarization current density,
from which the absorbed optical power corresponding to each Lorentz pole is
directly calculated in the time domain. Examples show that the proposed model
is more accurate than the Poynting vector-based model and is stable even when
the generated low-frequency component is strong.
