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The Raman frequency of the intersubband charge-density excitation plasmon in type-I and type-II quantum wells is known to be very sensitively controlled via a low-power optical pump signal. We find that, above a threshold electron density of approximately $4\ifmmode\pm\else\textpm\fi{}2\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$, the charge density plasmon mode splits first into two, and then with increasing density into three closely spaced frequencies. A similar splitting occurs in the spin-density wave plasmon associated with the same intersubband transition. We analyze the results including the coupling to the longitudinal optical phonon and hypothesize that the splittings arise from the special situation of a structure of spatially separated bipolar plasmons.

An electron-hole liquid (EHL), a condensed liquidlike phase of free electrons and holes in a semiconductor, presents a unique system for exploring quantum many-body phenomena. While the behavior of EHLs is generally understood, less attention has been devoted to systematically varying the onset of their formation and resulting properties. We report on an experimental approach to tune the conditions of formation and characteristics using a combination of low excitation densities and high magnetic fields up to 90 T. Demonstration of this approach was carried out in indirect-band-gap Al0.387Ga0.613As. EHL droplets can be nucleated from one of two multiexciton complex states depending on the applied excitation density. Furthermore, the excitation density influences the carrier density of the EHL at high magnetic fields, where filling of successive Landau levels can be controlled. The ability to manipulate the formation pathway, temperature, and carrier density of the EHL phase under otherwise fixed experimental conditions makes our approach a powerful tool for studying condensed carrier phases in further detail.

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{sub 1-x}N{sub 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. (c) 2000 The American Physical Society.

Two-color up-conversion time-resolved photoluminescence and time-resolved absorption are used to study spin relaxation of near-resonant heavy-hole excitons in spontaneously ordered Ga{sub x}In{sub 1{minus}x}P. The photoluminescence anisotropy is studied as a function of crystal direction, revealing a strong dependence on the light orientation with respect to the ordering axis. The maximum observed polarization is 0.8, and indicates that the polarization is near unity shortly after excitation. The carrier spin-relaxation time is 150 ps. {copyright} {ital 1999} {ital The American Physical Society}

We propose ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructures as a model system to study the phenomenon of photoluminescence (PL) up-conversion and demonstrate low-temperature up-converted PL (UPL) in these heterostructures. We find that a mechanism to prevent up-converted carriers in the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ layer from thermalizing back to the GaAs layer is essential for efficient UPL. We also find that neither a type-II band alignment nor existence of long-lived intermediate states is an essential requirement of UPL. The results of time-resolved UPL measurements are interpreted within the current models.

Reflection second harmonic generation (SHG) has been used to measure the rotation/reflection properties of spontaneously ordered Ga{sub 1{minus}x}In{sub x}P. Particular conditions are identified in which the SHG signal arises solely from the order-induced symmetry breaking of the {bar 4} operation in zinc blende. By extending the usual experimental techniques to the epilayer edges, the point group of ordered Ga{sub 1{minus}x}In{sub x}P was completely determined from optical methods, and the four components of the nonlinear susceptibility were measured. The case of double-variant samples is also discussed. {copyright} {ital 1998} {ital The American Physical Society}

We present evidence for a biexciton state in a molecular aggregate. Femtosecond pump-probe spectroscopy of the $H$ aggregate of an epitaxially grown trivalent metal-halogen-phthalocyanine gives distinct photoinduced absorptions to a continuum of two-exciton states and to a bound biexciton state. No signature of the biexciton is found in the $J$ aggregate of a very similar trivalent metal-halogen-phthalocyanine. The strong exciton-exciton interaction in the $H$-aggregate system has its origin in the unique cofacial stacking of the molecular components in this material.

Using time-resolved small-signal exciton absorption bleaching at low temperature as a spectroscopic technique, the optical transition energies from all three valence bands in spontaneously ordered Ga{sub x}In{sub 1{minus}x}P have been measured with high accuracy. The origin of the bleaching signal and the contributions of reflection, strain, and binding energy are discussed. With three measured energies from each sample, all parameters in the quasicubic perturbation model can be fitted. Good agreement is obtained with a spin-orbit-splitting parameter of 103 meV, nearly independent of the degree of ordering. The ratio of band-gap reduction to crystal-field-splitting parameter is found to be 2.7, slightly higher than previous works. This difference is attributed to a more accurate determination of light-hole-like band-gap energy. {copyright} {ital 1997} {ital The American Physical Society}

The evolution of individual nitrogen cluster bound states into an extended state infinite supercluster in dilute GaAs${}_{1\ensuremath{-}x}$N${}_{x}$ was probed through temperature and intensity-dependent, time-resolved and magnetophotoluminescence (PL) measurements. Samples with compositions less than 0.23$%$ N exhibit PL behavior that is consistent with emission from the extended states of the conduction band. Near a composition of 0.23$%$ N, a discontinuity develops between the extended state PL peak energy and the photoluminescence excitation absorption edge. The existence of dual localized/delocalized state behavior near this composition signals the formation of an N supercluster just below the conduction band edge. The infinite supercluster is fully developed by 0.32$%$ N.

Resonance Raman spectroscopy is used to probe the asymmetric broadening of the LO phonon linewidth in a dilute GaAs${}_{1\ensuremath{-}x}$N${}_{x}$ alloy ($x=0.41\phantom{\rule{-0.16em}{0ex}}%$). Electronic Raman scattering from a broad continuum is observed that gets enhanced concurrently with the LO phonon linewidth under resonance. The Fano interaction between the LO phonon and the electronic continuum is used to develop a model that satisfactorily explains the origin of the asymmetric LO phonon linewidth broadening in this abnormal alloy as arising due to coupling between the discrete and the continuum configurations.

Electrons in semiconductor alloys have generally been described in terms of Bloch states that evolve from constructive interference of electron waves scattering from perfectly periodic potentials, despite the loss of structural periodicity that occurs on alloying. Using the semiconductor alloy GaAs${}_{1\ensuremath{-}x}$N${}_{x}$ as a prototype, we demonstrate a localized to delocalized transition of the electronic states at a percolation threshold, the emergence of a mobility edge, and the onset of an abrupt perturbation to the host GaAs electronic structure, shedding light on the evolution of electronic structure in these abnormal alloys.

Exciton photoluminescence pattern generation is investigated in multiple quantum wells. High-contrast outer rings and localized bright spots are generated using efficient field-assisted upconversion of laser light whose photon energy lies below the energy of the luminescing quantum well transition. Time-resolved images of the bright spots reveal unexpected dynamics that are not explained by two-dimensional rate equations for generation and diffusion. This behavior can be understood as a result of three-dimensional transport in the vertically extended samples.

Resonant coherent lattice vibrations in the vicinity of the indirect absorption edge in a ${\mathrm{BiI}}_{3}$ layered crystal are observed in a femtosecond pump-probe experiment. Coherent optical phonons that are impulsively excited by femtosecond pump pulses modulate the phase of probe pulses, causing oscillation of the probe spectrum in time. This oscillation, which has a period given by the period of the oscillation, continues for more than 100 cycles. We speculate that the coherent phonon-assisted indirect exciton transitions also contribute to the data.

A wide-field microphotoluminescence study of ordered ${\text{Ga}}_{x}{\text{In}}_{1\ensuremath{-}x}\text{P}$ reveals spatially isolated submicron emission centers that emit in a spectral band 20 meV below previously studied PL features. The density of these centers is correlated with the presence of sharp PL lines reported in the low-energy PL band. Polarization analysis shows that many centers are very highly linearly polarized, contrary to models of quantum disks aligned at antiphase boundaries normal to the ordering direction. Orientations of the linearly polarized centers are found in some cases to be highly aligned along specific crystal directions that are not simply related to ordering.

Experimental and theoretical investigations of the near-band-gap optical nonlinearities in type-II GaAs/AlAs quantum wells are reported. The temporal evolution of the optical nonlinearities of different samples grown at our two different laboratories is analyzed by comparing femtosecond results at different pump-probe time delays with nanosecond measurements at different sample temperatures. The quasi-cw measurements at low temperatures show exciton bleaching and a pronounced heavy-hole exciton blueshift, which is absent at elevated plasma temperatures (\ensuremath{\simeq}100 K). Femtosecond spectra show a delayed onset of the exciton blueshift consistent with hole-plasma cooling in the absence of electrons. The results are analyzed using many-body theory assuming complete electron-hole charge separation. We show that the presence of a one-component plasma modifies the nonlinear optical behavior of the GaAs layer and results in dramatically different properties compared to type-I quantum-well structures.

On observe aussi cet effet dans un coupleur directionnel non lineaire quand une transmission totale augmente les guides d'onde. Les equations de Bloch des semiconducteurs du systeme sont resolues numeriquement. La saturation d'absorption transitoire et le recouvrement observes sont expliques comme une suite adiabatique ultra-rapide de l'excitation des semiconducteurs

An above-band-gap transition ${E}_{+}$ is experimentally observed in the dilute $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$ alloy. Precise measurements at very low dilutions are made of the above-band-gap transition ${E}_{+}$ that is observed in $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$, making it possible to compare the behavior of the different isoelectronic traps Bi and N in the common host GaAs with respect to their perturbation to the host electronic structure. We suggest that the origin of the ${E}_{+}$ level observed in GaAs is not the isolated isoelectronic impurity level ${\mathrm{N}}_{x}$, as is presumed in the band-anticrossing model, but rather the isoelectronic-impurity-induced perturbation of the conduction band ${L}_{6}^{c}$.

A recently observed pinned peak in photoluminescence excitation spectroscopy studies PLE of GaNxP1−x epilayers, that remained stationary with nitrogen concentration, was attributed to a transition from the valence band edge to either the t2 X3 or t2 L conduction bands by Buyanova et al. Phys. Rev. B 69, 201303 R 2004 . Using absorption and PLE studies on carefully prepared samples, we show that this pinned peak is merely an artifact that arises from the GaP buffer layer and is not associated with the GaNxP1−x epilayers.

Hybrid semiconductors comprising networks of inorganic II--VI semiconductor segments bonded to organic solvent chains show promise as a versatile material system that offers the capability of synthesizing many new electronic structures that are desirable for photonic applications. Polarized optical absorption and reflection are measured in single crystal semiconductor nanostructures composed of arrays of monolayers or linear chains. This shows very sharp spectral signatures of excitonic transitions, providing evidence of an electronic bandstructure that is strongly modified from the bulk inorganic semiconductor. Together with x-ray diffraction structural analysis, it gives a clear image of a monolayer multiple quantum well structure in which a chemical growth process has achieved a quantum confinement larger than other known heterostructure types, together with near-zero layer width fluctuations, a very large oscillator strength and large exciton binding energy.

At sufficiently high doping levels, the impurities in a semiconductor are expected to perturb the host band structure, and the perturbed host is then expected to alter the impurity state from that of the dilute limit (a recoil). Despite many decades of studies on impurities, it has been impossible to simultaneously and accurately track the evolution of the host band structure and the impurity state. The isoelectronically doped system provides a unique opportunity to track this evolution. GaAs:N, as a prototype system, has been investigated both experimentally and theoretically for this purpose.

Narrow-line photoluminescence in the below-gap spectral region of ordered ${\mathrm{Ga}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}\mathrm{P}$ is investigated with high-spatial and -spectral resolution photoluminescence excitation and time-resolved spectroscopy. Sharp absorption lines are found both correlated and uncorrelated with emission lines, indicating multiple coupled systems. Saturation and temporal measurements support a quantum dot model.