Competing mechanisms of polarization-controlled terahertz emission in Co/Mo spintronic bilayers

Abstract In this work, we reveal unconventional, polarization-dependent anisotropic terahertz (THz) emission from a sputtered Co/Mo bilayer attributed to competing mechanisms of THz generation. Unlike conventional spintronic THz emitters that operate solely via the inverse spin Hall effect (ISHE), the present system generates THz radiation through the combined action of ISHE and second-order nonlinear-optical processes. THz magnetometry shows pronounced asymmetries in the THz hysteresis loops compared with planar magneto-optical Kerr effect magnetometry. Systematic experiments on azimuthal rotation of the sample and controlled variation of the pump pulse polarization (linear and elliptical) enable a full decomposition of the THz field. Roughly 70% of the amplitude is an odd-in-field contribution generated by the ISHE in Mo, whereas the remaining 30% is an even-in-field component whose angular behavior points to a second-order interface nonlinearity governed by a frozen in-plane interfacial magnetization <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mi>M</mml:mi> <mml:mi>′</mml:mi> </mml:msup> </mml:mrow> </mml:math> that breaks inversion symmetry. These results demonstrate that the Co/Mo interface functions as a THz emitter whose amplitude is strongly sensitive to both the orientation of magnetization and the polarization of the incident light. The interplay between interfacial spin–orbit coupling and broken inversion symmetry gives rise to a distinct emission mechanism, which in turn results in the observed behavior not commonly found in conventional spintronic emitters. The symmetry-resolved separation of ISHE and optical rectification mechanisms provides clear design rules for tailoring interface-driven THz emitters based on ultrathin heterostructures.