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We have used Optically Detected Resonance (ODR) spectroscopy to probe the electronic properties of undoped and barrier-doped GaAs/Al 0.3 Ga 0.7 As multiple-quantum-well (MQW) samples with well widths between 12.5 and 20 nm in magnetic fields up to 15 T at low temperatures. The simultaneous observation of electron and hole CR along with several internal transitions of neutral excitons (IETs) verifies the symmetry-related energy differences of the internal transitions to differences between electron and hole CR. The observed degeneracy of the 1s 3 2p + IET from the two radiative magneto-excitons is due to the very small electron g-factor. ODR measurements on 20 nm wide MQWs (not-intentionally-and barrier-doped) exhibit transitions of the negatively charged excitonic complex.

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The "lock-in" detection technique can extract, from a possibly noisy waveform, the amplitude of a signal that is synchronous with a known reference signal. This paper examines the effects of input noise on the output of a lock-in amplifier. We present quantitative predictions for the root-mean-square size of the resulting fluctuations and for the spectral density of the noise at the output of a lock-in amplifier. Our results show how a lock-in amplifier can be used to measure the spectral density of noise in the case of a noise-only input signal. Some implications of the theory, familiar and surprising, are tested against experimental data.