  We attract attention to the fact that the ultimate (shot-noise-limited)
polarimetric sensitivity can be enhanced by orders of magnitude leaving the
photon flux incident onto the photodetector on the same low level. This
opportunity is of crucial importance for present-day spin noise spectroscopy,
where a direct increase of sensitivity by increasing the probe beam power is
strongly restricted by the admissible input power of the broadband
photodetectors. The gain in sensitivity is achieved by replacing the 45-deg
polarization geometry commonly used in conventional schemes with balanced
detectors by geometries with stronger polarization extinction. The efficiency
of these high-extinction polarization geometries with enhancement of the
detected signal by more than an order of magnitude is demonstrated by
measurements of the spin noise spectra of bulk n:GaAs in the spectral range
835-918 nm. It is shown that the inevitable growth of the probe beam power with
the sensitivity gain makes spin noise spectroscopy much more perturbative, but,
at the same time, opens up fresh opportunities for studying nonlinear
interactions of strong light fields with spin ensembles.
