Evidence of the magneto-optical Kerr spectral shifts induced by quasi-static strain

We demonstrate a dynamical wavelength shift in the magneto-optical Kerr spectrum of a Permalloy thin film, excited by femtosecond laser pulses. Through precise measurements of magneto-optical dynamics with narrow-bandwidth probe pulses, we found systematic variations in the differential Kerr rotation and ellipticity, corresponding to the shape of the static Kerr spectrum. We attribute this behavior to a spectral shift caused by quasi-static strain driven by femtosecond laser-induced heating. Supporting this interpretation, we successfully observed a sign reversal in the static Kerr signal during the demagnetization process. By isolating two contributions to the observed dynamics—a reduction in magnetization due to increased spin temperature and an extra gain from the Kerr spectrum shift—we quantified a horizontal shift of the Kerr spectrum toward shorter wavelengths.