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Exciton photoluminescence (PL) is an important technique for the characterization of quantum wells (QW's). We discuss the etfect of localization on the diamagnetic energy shift of an exciton in a QW. It is shown how the diamagnetism of an electron in the ground state, with arbitrary geometry, depends on the dimensions of its wave packet. We consider the properties of free excitons in QW s, introducing an effective electron-hole interaction. We use dimensional analysis to relate the finitebarrier problem to the simpler case of perfect confinement. For a bound exciton, localization in the plane of the QW causes the diamagnetism to be smaller than for a free exciton. The efFect of localization is not important if the range is much larger than the free-exciton Bohr radius. The exciton diamagnetic shift is reduced by localization in 10 and 20 A (In, Ga)As/InP QW s grown by solidsource molecular-beam epitaxy. Uncertainty about the value of the free-exciton diamagnetic shift limits the sensitivity of the diamagnetism as a probe of exciton localization. Despite this, the method still provides valuable information on exciton localization, particularly when combined with studies of the phonon sideband of PL.

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We demonstrate, for the first time to our knowledge, the generation of second-harmonic pulses by use of a novel methodology for achieving first-order quasi-phase matching in a semiconductor waveguide crystal. This methodology is based on a periodic modulation of the susceptibility coefficient along the direction of light-beam propagation in which advantage is taken of the fact that chi((2))(GaAs)>chi((2))(Al(x)Ga(1-x)As) . Efficient second-harmonic generation at 975 nm of a pump wavelength of 1950 nm has been demonstrated for a crystal with a nonuniform domain dimension (duty cycle, ~39/61).