Detecting nuclear polarization dynamics by spin noise spectroscopy
Spin noise spectroscopy (SNS) gained a lot of popularity in the last decade, when it was proven to be a highly efficient method of non-perturbative exploration of charge carrier dynamics and magnetic resonance in semiconductor systems. The fluctuation-dissipation theorem manifests the possibility of detection of linear susceptibility resonances in the medium by registration of the noise spectrum of this system. As applied to magnetic resonance spectroscopy, this principle can be realized by detecting fluctuations of the Faraday rotation of the linearly polarized probe beam passing through a transparent paramagnet. The detected noise signal is proportional to fluctuations of the medium magnetization. In the presence of magnetic field which is perpendicular to the light propagation axis the Faraday rotation noise spectrum reveals a peak at the magnetic resonance frequency corresponding to precession of spontaneous fluctuations of the spin ensemble at the Larmor frequency. In the absence of absorption in the sample the system remains in thermodynamical equilibrium, so the method can be regarded as essentially non-perturbative. The features like the possibility of three-dimensional tomographic measurements; no necessity for conventional ESR microwave equipment; widely extendable (up to hundreds of GHz) detection range; experimental simplicity are the most noticeable advances of this method among the others.
Due to very small magnitude of spontaneous magnetization fluctuations, spin noise spectroscopy necessitate the highest polarimetric sensitivity, which is fundamentally limited (for classical light emitters) by photon flux shot noise level. Higher sensitivity of measurements in this limit can be achieved by the increase of the probe light power density, which leads to increase of nonlinear light-matter interaction effects. In such conditions the experiment cannot be considered as non-perturbative, however nonlinear SNS experiments brings a lot of information about optical and magnetic effects in a semiconductor structures.
In the present work we uncover the magnetometric potential of the SNS. It is based on the known fact that the electron spin noise (SN) spectrum at arbitrary orientation of the external magnetic field generally reveals two components. The one of them is centered at zero frequency and reflects fluctuations of the longitudinal magnetization, while the other is centered at Larmor frequency and results from fluctuations of the transverse magnetization. Its magnitudes ratio is determined by mutual orientation of the light beam and magnetic field and therefore can be used to monitor direction of the effective magnetic field acting upon the spin system. In our work we show that SNS can be used for direct detection of the Overhauser field of the nuclei, optically oriented by the high-power elliptically polarized probe beam.
Host lattice nuclear spins occupy a special place among spin systems in semiconductors. Its exceptional robustness to effects of environment opens up prospects to use nuclear spins for information processing. On the other hand, they play major role in electron spin dynamics and decoherence. Mechanisms of the nuclear spin relaxation in n-type semiconductors, even in the best studied like GaAs, remain rather poorly understood. Weak interaction of nuclear spins with light significantly complicates direct optical studies of nuclear spin dynamics. In the present work, we involve SNS to investigate nuclear spin dynamics in n-doped semiconductors. The technique is applied to n-type GaAs layers embedded into a high-finesse microcavity. We show that nuclear spins can be optically polarized by the circularly polarized beam due to the residual absorption of the sample. The detection is achieved via Kerr rotation spectra recorded with time resolution of several seconds. A shift of the electron precession peak with respect to its unperturbed position, when nuclei are not polarized, is a direct measure of the Overhauser field BN created by optically cooled nuclei. SNS spectroscopy provides a direct measurement of BN with time resolution given by the SN spectrum acquisition time. The proposed method is applied to both metallic and insulating samples, revealing full potential of this technique, which allows to investigate nuclear spin relaxation in the presence of either donor-bound localized electrons, or mobile Fermi-edge electrons.
In addition to the retarded response, ascribed to nuclear system polarization buildup and decay, SN spectra also reveals an instantaneous shift of the precession peak and appearance of the non-magnetic component, similar to appearance of the additional longitudinal magnetic field. This effect is attributed to ac Stark effect acting upon spin system in the electromagnetic field of elliptically polarized high power density light. The effect is also illustrated by corresponding SNS measurements and described in more detail in the recent work of the team of contributors.