We report on the first experimental observation of spin noise in a single semiconductor quantum well embedded into a microcavity. The great cavity-enhanced sensitivity to fluctuations of optical anisotropy has allowed us to measure the Kerr rotation and ellipticity noise spectra in the strong coupling regime. The spin noise spectra clearly show two resonant features: a conventional magneto-resonant component shifting towards higher frequencies with magnetic field and an unusual "nonmagnetic" component centered at zero frequency and getting suppressed with increasing magnetic field. We attribute the first of them to the Larmor precession of free electron spins, while the second one being presumably due to hyperfine electron-nuclei spin interactions.Introduction. In the present-day physics of semiconductor nanostructures, a considerable interest is shown for the fundamental spin-related properties which are also promising in applications. Among optical methods of spin dynamics studies, an important place is given to the Faraday-rotation-based spin noise spectroscopy (SNS) which became well-known and popular during the last several years [1]. The advantages of SNS are primarily owed to its nonperturbative nature because probing the sample response by a weak laser beam in the region of transparency does not lead to any real electronic transitions. Extreme smallness of the magnetization fluctuations detected with the SNS technique calls for the highest polarimetric sensitivity which is achieved by using various electronic or optical means. A real breakthrough occurred when the fast-Fourier-transform (FFT) spectrum analyzers were applied in electronics of the SNS technique [2]. The most straightforward optical way to enhance the polarimetric sensitivity implies increasing intensity of the probe light beam and, simultaneously, leaving the input power of photodetector on the admissible level. This can be implemented either by using highextinction polarization geometries [3] or by placing the sample inside a high-Q optical cavity [4]. In both cases, the light power density on the sample can be increased by a few orders of magnitude, with the light power on the photodetector and, therefore, the photocurrent shot noise remaining on the same low level.For low-dimensional semiconductor structures (quantum wells, wires and dots) the problem of polarimetric sensitivity is especially topical. In Ref.[5], in order to increase the signal, the spin noise spectra of n-doped GaAs quantum wells were studied in the samples containing ten identical quantum wells (QWs). The measurement of the spin noise spectrum of a layer of InAs/GaAs quantum dots (QDs) in a high-finesse microcavity allowed Dahbashi et al. [6] to perform unique investigation of spin dynamics of a single heavy hole localized in a selected QD. We are not aware of any experimental study of spin

We exploit the potential of the spin noise spectroscopy (SNS) for studies of nuclear spin dynamics in n-GaAs. The SNS experiments were performed on bulk n-type GaAs layers embedded into a high-finesse microcavity at negative detuning. In our experiments, nuclear spin polarisation initially prepared by optical pumping is monitored in real time via a shift of the peak position in the electron spin noise spectrum. We demonstrate that this shift is a direct measure of the Overhauser field acting on the electron spin. The dynamics of nuclear spin is shown to be strongly dependent on the electron concentration.

Rapid development of spin noise spectroscopy of the last decade has led to a number of remarkable achievements in the fields of both magnetic resonance and optical spectroscopy. In this report, we demonstrate a new – magnetometric – potential of the spin noise spectroscopy and use it to study magnetic fields acting upon electron spin-system of an n-GaAs layer in a high-Q microcavity probed by elliptically polarized light. Along with the external magnetic field, applied to the sample, the spin noise spectrum revealed the Overhauser field created by optically oriented nuclei and an additional, previously unobserved, field arising in the presence of circularly polarized light. This “optical field” is directed along the light propagation axis, with its sign determined by sign of the light helicity. We show that this field results from the optical Stark effect in the field of the elliptically polarized light. This conclusion is supported by theoretical estimates.

In the conventional spin noise spectroscopy, the probe laser light monitors fluctuations of the spin orientation of a paramagnet revealed as fluctuations of its gyrotropy, i.e., circular birefringence. For spins larger than 1/2, there exists spin arrangement of a higher order-the so-called spin alignment-which also exhibits spontaneous fluctuations. We show theoretically and experimentally that alignment fluctuations manifest themselves as the noise of the linear birefringence. In a magnetic field, the spin-alignment fluctuations, in contrast to those of spin orientation, show up as the noise of the probe-beam ellipticity at the double Larmor frequency, with the most efficient geometry of its observation being the Faraday configuration with the light propagating along the magnetic field. We have detected the spin-alignment noise in a cesium-vapor cell probed at the wavelength of D2 line (852.3 nm). The magnetic-field and polarization dependence of the ellipticity noise are in full agreement with the developed theory.

We develop a method of non-perturbative optical control over adiabatic remagnetisation of the nuclear spin system and apply it to verify the spin temperature concept in GaAs microcavities. The nuclear spin system is shown to exactly follow the predictions of the spin-temperature theory, despite the quadrupole interaction that was earlier reported to disrupt nuclear spin thermalisation. These findings open a way to deep cooling of nuclear spins in semiconductor structures, with a prospect of realisation of nuclear spin-ordered states for high fidelity spin-photon interfaces.The concept of nuclear spin temperature is one of the cornerstones of the nuclear magnetism in solids 1,2 . It has made possible realisation of the cryogenic cooling into the microKelvin range 3 and observation of nuclear spin ordering in metals and insulators 4,5 . Such degree of control of the nuclear spin system (NSS) in semiconductor heterostructures would allow enhancing the efficiency of spin-based information storage and processing 6-9 . However, proving the validity of the spin temperature concept for semiconductor nano-and microstructures is challenging due to the lack of techniques capable of precise sensing of weak nuclear magnetisation in a small volume. In addition, recent experiments showed that in quantum dots, where strong quadrupole-induced local fields have been reported, nuclear spin temperature failed to establish 10 . In this context, NSS thermalisation sensing in semiconductor heterostructures is one the central issues for both fundamental questions related to the realisation of nuclear spin-ordered states, and for potential applications, such as high fidelity spin-photon interfaces 6-9 .The basic postulates of the spin temperature theory are illustrated in Fig. 1(a). It is assumed that during the characteristic time T 2 determined by spin-spin interactions the NSS reaches the internal equilibrium. This means that properties of the NSS are governed by a single parameter, the spin temperature Θ N . When this temperature is made different from the lattice temperature Θ L (e.g. by the optical pumping), the thermalisation of the NSS with the crystal lattice usually requires a much longer characteristic time T 1 . Fig.1(b) illustrates one of the main predictions of the spin temperature theory: if the NSS is subjected to a slowly varying magnetic field, such that dB/dt < B L /T 2 , then Θ N and the nuclear spin polarisation P N change obeying universal expressions:(1) Here γ N is the gyromagnetic ratio of the nuclear spin I, angular brackets denote the averaging over all nuclear species, k B is the Boltzman constant, and Θ N i is the spin temperature at strong magnetic field B i >> B L , where B L is the local field induced by the fluctuating nuclear spins. These generic relations are based on the principle of entropy conservation in a thermodynamic system during adiabatic process. They constitute the basis for the nuclear spin cooling by adiabatic demagnetisation, a widely used cryogenic technique 4,11-13 . The nuclear spin tem...