The THz domain is a field of intense research. Today, we note that many sources and detectors exist but only a few modulators are available with low power consumption and weak footprint. In this perspective, previous works have demonstrated the possibility to optically modulate a THz beam passing through an InAs slab by photogenerating free carriers. In the continuity of this work, we seek to reduce the required optical pump power by carrying out a study on a numerical model implemented on MATLAB. We numerically show thay by reducing the size of the THz and IR beams we can reach up to 90% transmission modulation depth.