1. Introduction
Polarization measurement has gained broad applications in many research topics, including magnetic anisotropy[1], spin dynamics[2,3] in magnetic material, birefringence in chiral media[4], and electro-optic sampling technique[5]. In magnetism, one convenient and popular analytical tool is based on the magneto-optical effect, which alters the polarization of the reflected (Kerr effect) and the transmitted light (Faraday effect) through the asymmetric dielectric tensor induced by magnetization[1]. Since its first application to surface magnetism[6], the magneto-optical Kerr effect (MOKE) has been developed as a non-intrusive and versatile probe for remote measurements on static or dynamic properties of spin systems with very high sensitivity, e.g., spin Hall effect[7,8], ultrafast spin dynamics[9], imaging magnetic domain and nanostructure[10,11], as well as magneto-optic information storage[12]. However, because the polarization of light is very sensitive to a large variety of noise sources, it is difficult to achieve a sensitivity of in MOKE measurement, especially in the DC detection scheme[1,8,13–15]. This hampers the application of MOKE in many emerging subjects, such as spin Hall effect[7], time-reversal-symmetry-breaking (TRSB) states in a superconductor[16,17], where a sensitivity of is urgently needed.