Spin Noise Spectroscopy in Application to Dielectric and Semiconductor Paramagnets

Spin noise spectroscopy (SNS) has matured from a subtle effect in atomic vapors into a uniquely versatile optical tool for probing spin dynamics in condensed matter. Unlike conventional spectroscopic methods, SNS requires no resonant excitation or optical pumping—it passively probes the stochastic Faraday rotation arising from spontaneous magnetization fluctuations of an equilibrium spin ensemble. This nonperturbative nature, combined with a spin sensitivity that is proportional to the square root of the total probed spins, makes SNS ideal for studying small spin ensembles, local environments, and weakly coupled subsystems. In this review, we illustrate the power of SNS through recent accomplishments. First, we present the balance between nonperturbative measurement and optically induced phenomena, observed in a one-beam stationary experiment. This includes the observation of optically induced effective magnetic field (AC Stark effect) in n-GaAs microcavities, the direct detection of nuclear spin polarization buildup and decay, and the local optical electron to hole recharging of perovskite nanocrystals. Second, we turn to power-independent SNS abilities. We highlight SNS’s remarkable sensitivity to crystalline symmetry—from resolving anisotropic paramagnetic centers in cubic and birefringent crystals to uncovering hidden twinning domains in bulk halide perovskites. Finally, we demonstrate how the optical selectivity of SNS allows one to address individual groups of paramagnetic defects by probe wavelength tuning, a capability unattainable by conventional EPR. These examples establish SNS not only as a powerful diagnostic for fundamental spin physics, but also as a promising tool for future spintronic, magnetometric, and material engineering applications.