Abstract— <p>In this study, we conduct a theoretical analysis of the optical absorption coefficients (OACs) and refractive index changes (RICs) arising from intraband and interband transitions in monolayer germanene under the influence of a magnetic field. The findings reveal that both OACs and RICs blueshift as the magnetic field strength increases. Notably, the peaks associated with spin-up states consistently appear to the right of those corresponding to spin-down states. This distinction in peak positions is attributed to the strong spin–orbit coupling (SOC) characteristic of monolayer germanene. For intraband transitions, the OACs and RICs exhibit a single peak in the terahertz (THz) region. Moreover, interband transitions give rise to multiple peaks, spanning from the high THz range to the low far-infrared range, underscoring the promising potential of monolayer germanene for optoelectronic applications.</p>