High-Resolution Spectroscopy of Individual Quantum Dots in Wells
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duced to submicroscopic dimensions. These PL spikes arise from excitons localized in individual QD potentials. Remarkably the linewidth decreases from several meV in the ensemble-averaged spectrum (25-/J.m aperture) to 10s of /xeV in the single QD spectra, corresponding to an effective improvement in resolution of two orders in magnitude. By probing individual QDs, it becomes possible to resolve directly a number of phenomena that previously were hidden in the inhomogeneous linewidth. These phenomena include exciton excited states,1'3 fine-
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Introduction Currently spectroscopists are studying many semiconductor quantum-dot (QD) systems in great detail because of their scientific and technological importance. However as in all nanostructure systems in which significant confinement energies exist, size fluctuations lead to inhomogeneous broadening of the spectral lines. This blurring of the spectra severely reduces the amount of information obtainable from spectroscopy. The finding has initiated an effort to isolate optically and study spectroscopically individual QDs. Studies involving individual QDs in most QD systems have been published.1"8 Here the results of a series of experiments are reviewed on GaAs QDs defined by interface fluctuations in narrow GaAs quantum wells.9"13 These experiments demonstrate the elegance and potential of single-QD spectroscopy. An example of single-QD photoluminescence (PL) spectroscopy appears in Figure 1.'" The spectra shown were obtained at a temperature of 6 K by successively reducing the size of the laser spot on a GaAs quantum-well sample through the use of small apertures in a metal mask.5 The bottom trace is a PL spectrum obtained with a macroscopic laser spot diameter of 25 fim. The spectrum shows two broad peaks corresponding to the recombination of excitons in parts of the quantum well that are either 10 or 11 monolayers wide (2.8 or 3.1 nm). The spectrum is strongly inhomogeneously broadened as shown most directly by a reduction in the aperture size. The relatively broad lines break up into a decreasing number of extraordinarily narrow PL spikes as the aperture is re-
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Figure 1. Photoluminescence (PL) spectra excited and detected through apertures in an aluminum mask with diameters ranging from 25 /j.m to 0.2 ixm. In this way, the emission from individual quantum dots (QDs) is resolved. (From Reference 10.)
structure splitting,10 hyperfine shifts,11 and time-dependent spectral jumping.14 Moreover novel techniques have been developed, leading to the measurement of the Raman nuclear magnetic resonance (NMR) and coherent nonlinear optical spectra of individual QDs in addition to the PL spectra reported by many groups. These advanced measurement techniques provide extremely local and selective probes of composition, strain, and other local fields. They also create new opportunities for detailed study of the
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