We report a giant bowing of the spin-orbit splitting energy Delta0 in the dilute GaAs1-xBix alloy for Bi concentrations ranging from 0% to 1.8%. This is the first observation of a large relativistic correction to the host electronic band structure induced by just a few percent of isoelectronic doping in a semiconductor material. It opens up the possibility of tailoring the spin-orbit splitting in semiconductors for spintronic applications.

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In heavily nitrogen doped GaP, we show how isoelectronic doping results in an impurity band, and how this is manifested as a large band-gap reduction and an enhanced band-edge absorption. Heavily doped GaP:N or GaP 1Ϫx N x exhibits properties characteristic of both direct and indirect gap semiconductors. Exciton bound states associated with perturbed nitrogen pair centers and larger GaN clusters are observed. This paper indicates that to properly describe the properties of an impurity band, a hierarchy of impurity complexes needs to be considered. Our data also suggest that the excitonic effect plays a role in the impurity band formation and band-gap reduction.

It is found that there exists a category of material interfaces, readily available, that not only can provide total refraction (i.e., zero reflection) but can also give rise to amphoteric refraction (i.e., both positive and negative refraction) for electromagnetic waves in any frequency domain as well as for ballistic electron waves. These two unusual phenomena are demonstrated experimentally for the propagation of light through such an interface.

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We discuss theoretically a novel approach to tailoring the properties of a new family of organic-inorganic hybrid superlattices, using two isostructural materials, ZnSe(en)0.5 and ZnTe(en)0.5, as examples. Replacing Se with Te leads to a number of nontrivial changes: the conduction band parity, singularity type, conductivity in the superlattice direction, and the p-type dopability. Experimentally, we report the first unambiguous observation of exciton-polariton emission in a hybrid semiconductor, i.e., ZnTe(en)0.5 . The band-edge excitonic transitions in both emission and absorption are explained by the calculated electronic structures.

The coherent interaction of femtosecond laser pulses and thin CdSe and GaAs samples is investigated experimentally and theoretically. Oscillatory structures in the differential probe transmission around the exciton resonance and around the pump frequency are observed when the probe pulse precedes the pump. Comparison with theory attributes the oscillations to the coherent coupling between the light field and the electron-hole transitions in the semiconductor.For nonresonant excitation, the oscillatory structures around the exciton are identified as the early stages of the optical Stark eAect.PACS numbers: 73.60.n, 71.35.+z, 78.47.+p Coherent light-matter interactions have been extensively studied in atomic spectroscopy.These interactions are readily observable in atomic systems as a result of the relatively long dephasing times. In semiconductors, on the other hand, the coherent coupling of the exciting light to the electron-hole transitions disappears very rapidly because of the various interaction processes of the electronic excitations. Until now only a few coherent processes in semiconductors have been observed experimentally.The anisotropic state filling in Ge, and phase coherence and orientational relaxation of excitons in GaAs, as well as the study of coherent coupling effects in pump-probe spectroscopy in dyes, ' have been investigated using picosecond pulses. In the femtosecond time domain, carrier orientational relaxation and bandto-band spectral hole burning, ' as well as the exciton optical Stark effect, ' " have been reported.In this paper, we report the first observation of oscillatory structures in the probe transmission spectra around the exciton when the semiconductor is excited spectrally below the exciton resonance and the probe pulse precedes the pump pulse. For the case of resonant interband excitation, similar oscillations are also observed around the central frequency of the pump pulse. We refer loosely to this class of effects as "coherent transients, " including all the nonstationary features observed in semiconductors on time scales short enough that thermalization to quasiequilibrium due to carrier-carrier or carrier-phonon scattering has not yet occurred.Experiments were performed with a variety of different semiconductors to show that the observed coherent effects are quite general and do not depend on the specific semiconductor.A semiclassical theory is developed which qualitatively explains all of the experimentally observed features. The experiments were conducted in a pump-probe geometry with =80-fs pulses.The laser system consists of a colliding-pulse modelocked dye laser amplified in a dye amplifier using a 10-kHz copper-vapor laser or a 10-Hz neodymium-doped yttrium aluminum garnet laser. A portion of the amplified pulses was focused on an ethylene glycol jet to generate a white-light continuum probe pulse while the remainder was used as pump pulse. For below-bandgap excitation, with the 10-Hz system, the portion of the continuum was selected with interference filters and w...

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