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Microscopic origins of charge currents and electromagnetic (EM) radiationgenerated by them in spintronic THz emitters -- such as, femtosecond laserpulse-driven single magnetic layer or its heterostructures with a nonmagneticlayer hosting strong spin-orbit coupling (SOC) -- remain poorly understooddespite nearly three decades since the discovery of ultrafast demagnetization.We introduce a first-principles method to compute these quantities, where thedynamics of charge and current densities is obtained from real-timetime-dependent density functional theory (TDDFT), which are then fed into theJefimenko equations for properly retarded electric and magnetic field solutionsof the Maxwell equations. By Fourier transforming different time-dependentterms in the Jefimenko equations, we unravel that in 0.1--30 THz range theelectric field of far-field EM radiation by Ni layer, chosen as an example, is{\em dominated by charge current pumped by demagnetization}, while ofteninvoked magnetic dipole radiation from time-dependent magnetization of a singlemagnetic layer is a negligible effect. Such overlooked case of charge currentpumping by time-dependent quantum system, whose magnetization is shrinkingwhile its vector does not rotate, does not require any spin-to-chargeconversion via SOC effects. In Ni/Pt bilayer, EM radiation remains dominated bycharge current within Ni layer, whose magnitude is larger than in the case ofsingle Ni layer due to faster demagnetization, while often invokedspin-to-charge conversion within Pt layer provides additional but smallercontribution. By using Poynting vector and its flux, we also quantifyefficiency of conversion of light into emitted EM radiation, as well as theangular distribution of the latter.

Origins of electromagnetic radiation from spintronic terahertz emitters: A time-dependent density functional theory plus Jefimenko equations approach