Spin noise spectroscopy in semiconductors has matured during the past nine years into a versatile and well developed
technique being capable to unveil the intrinsic and unaltered spin dynamics in a wide range of semiconductor systems.
Originating from atom and quantum optics as a potential true quantum non-demolition measurement technique, SNS is
capable of unearthing the intricate dynamics of free or localized electron and hole spins in semiconductors being
eventually coupled to the nuclear spin bath as well. In this contribution, we review shortly the major steps which inspired
the success of spin noise spectroscopy in semiconductors and present the most recent extensions into the low-invasive
detection regime of the spin dynamics for the two extreme limits of very high and extremely low rates of spin
decoherence, respectively. On the one hand, merging ultrafast laser spectroscopy with spin noise spectroscopy enables
the detection of spin noise with picosecond resolution, i.e., with THz bandwidths yielding access to otherwise concealed
microscopic electronic processes. On the other hand, we present very high sensitivity SNS being capable to measure the
extremely long spin coherence of single holes enclosed in individual quantum dots venturing a step forward towards true
optical quantum non-demolition experiments in semiconductors. In addition, higher-order spin noise statistics of, e.g.,
single charges can give information beyond the linear response regime governed by the fundamental fluctuationdissipation
theorem and thereby possibly shed some light on the nested coupling between electronic and nuclear spins.
Spin noise spectroscopy can be an extraordinary efficient, all-optical and low-perturbing tool to study the equilibrium
spin dynamics in semiconductors. However, great care is necessary for studying the spin dynamics in inhomogeneous
quantum dot ensembles. First, we show measurements on the spin dynamics of localized holes in (InGa)As quantum dots
ensembles. The experiments reveal a very slow longitudinal spin relaxation time Tl and a moderately slow transverse spin relaxation time T2 * which results from the finite hyperfine interaction of the hole spins due to heavy-light hole mixing in (InGa)As quantum dots. The longitudinal spin relaxation rate shows a linear dependence on the probe intensity which suggests a linear extrapolation to zero intensity for the extraction of the intrinsic spin relaxation rate. However, calculations reveal that the intrinsic heavy-hole spin relaxation is easily shadowed in quantum dot ensembles by effects of finite absorption even if the majority of quantum dots is well out of resonance of the probe laser. For typical laser
intensities and very long spin relaxation times, a linear extrapolation to zero intensity is therefore not allowed. What is
more, the line shape of the spin noise spectra changes from Lorentzian to non-Lorentzian with increasing laser intensity
which can be easily misinterpreted as an intrinsic non-exponential spin relaxation process.
We demonstrate spin noise spectroscopy as an efficient and surprisingly sensitive experimental tool to measure
the spin dynamics of free and localized carriers in semiconductors. The technique suppresses perturbations and
gives access to intrinsic spin relaxation times by omitting optical excitation. We show the power of spin noise
spectroscopy for basic physics by measurements on n-type modulation doped (110) GaAs quantum wells. The
measurements reveal that the spin relaxation times are limited by stochastic spin-orbit fields and that the spin can
be used as marker for the observation of electron diffusion processes at thermal equilibrium. We show the power
of spin noise spectroscopy for applied physics, by three dimensional measurements of the doping distribution in
direct semiconductors.
We use capacitance and photoluminescence spectroscopy to study the energy splitting of electron and hole states in InAs self assembled quantum dots embedded in GaAs bulk material. In our photoluminescence spectra, measured with high excitation, we observe five peaks below the wetting layer transition which we attribute to electron hole recombination from quantum dot levels of the same quantum number. Resonant excited photoluminescence experiments show clearly the existence of phonon enhanced carrier relaxation if the energy splitting between two different quantum dot levels matches a multiple of the available phonon energies. Therefore a maximum in the intensity of the resonantly excited photoluminescence does not necessarily occur when most of the dots are pumped resonantly into an excited state, the main criterion, however, is that the energy distance between the pumped levels and the levels below matches a multiple of the available phonon energies.
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