Broadband Spintronic Detection of the Absolute Field Strength of Terahertz Electromagnetic Pulses

Abstract

We demonstrate the detection of broadband intense terahertz electromagnetic pulses by Zeeman-torque sampling (ZTS). Our approach is based on magneto-optic probing of the Zeeman torque that the terahertz magnetic field exerts on the magnetization of a ferromagnet. Using an 8-nm-thick iron film as a sensor, we detect pulses from a silicon-based spintronic terahertz emitter with a bandwidth of 0.1-11 THz and a peak field of >0.1 MV/cm. A simple static magneto-optic calibration measurement provides access to absolute transient terahertz field strengths. We show the relevant added value of ZTS compared to electro-optic sampling (EOS): an echo-free transfer function with simple frequency dependence, linearity even at high terahertz-field amplitudes, the straightforward calibration of EOS response functions, and modulation of the polarization-sensitive direction by an external ac magnetic field. Consequently, ZTS has interesting applications even beyond the accurate characterization of broadband high-field terahertz pulses for nonlinear terahertz spectroscopy.