We measure the frequency spectra of random spin fluctuations, or "spin noise," in ensembles of (In,Ga)As/GaAs quantum dots (QDs) at low temperatures. We employ a spin noise spectrometer based on a sensitive optical Faraday rotation magnetometer that is coupled to a digitizer and field-programmable gate array, to measure and average noise spectra from 0-1 GHz continuously in real time with subnanoradian/sqrt[Hz] sensitivity. Both electron and hole spin fluctuations generate distinct noise peaks, whose shift and broadening with magnetic field directly reveal their g factors and dephasing rates within the ensemble. A large, energy-dependent anisotropy of the in-plane hole g factor is clearly exposed, reflecting systematic variations in the average QD confinement potential.
We show that in a magnetic field B the otherwise forbidden lowest exciton in Cu2O (paraexciton of Gamma(2)(+) symmetry) gives rise to a narrow absorption line of 80 neV at a temperature of 1.2 K. The B2 dependence of the field-induced oscillator strength and the low energy shift DeltaE with increasing field strength are measured. From two-phonon excitation spectroscopy measurements we derive by a merely kinematical analysis a very reliable value for the paraexciton mass. A blueshift and a broadening of the absorption line are observed for increasing excitation intensity. These observations are discussed in connection with a Bose-Einstein condensation of paraexcitons in Cu2O.
The lowest ortho-and paraexcitons of Cu 2 O are studied by high-resolution spectroscopy. After a detailed description of the experimental setup, the quadrupole polariton is extended to a two-oscillator resonance. As an example, the group velocity of the M = ±1 orthoexciton components in a magnetic field of 0.1 T is measured and fitted to a two-oscillator model with f (oscillator strength) as the only fit parameter. The paraexciton resonance is measured in a magnetic field of 10 T. Its oscillator strength is by almost a factor 200 lower than the oscillator strength of the orthoexciton.
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