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Excited-state interactions between organic and inorganic components in hybrid metal halide semiconductors open up the possibility of moving charge and energy in deliberate ways, including energy funneling, triplet energy harvesting, or long-lived charge separation. In this work, we utilize π-conjugated naphthalene diimide electron accepting molecules to fabricate a hybrid one-dimensional (1D) lead iodide semiconductor ((NDIC2)Pb2I6) with an internal charge separating junction. Despite recent efforts on the synthesis of 1D metal halide semiconductors, little is known about their electronic structure, optical properties, and excited-state dynamics. Steady-state and time-resolved spectroscopy measurements of ((NDIC2)Pb2I6) thin films elucidate discrete optical features from the lead iodide and naphthalene diimide components of this heterostructure, along with a weakly bound optically active charge transfer state. The type-II heterojunction between the organic NDIC2 and inorganic Pb2I6 moieties facilitates rapid separation of photogenerated charges, where charge recombination is hindered by the spatial separation of charges across the organic/inorganic interface. Our study also provides some important insights into the ways in which Coulomb interactions between the organic and metal halide moieties and π–π interactions between the organic cations can affect the crystallization of these hybrid semiconductors with large, optically active π-conjugated chromophores. We believe our findings will further enable the rational design of low-dimensional organic–inorganic heterostructures where the dielectric environment, charge transfer states, and exciton behavior may be modulated.

The photo-driven process of singlet fission generates coupled triplet pairs (TT) with fundamentally intriguing and potentially useful properties. The quintet 5 TT 0 sublevel is particularly interesting for quantum information because it is highly entangled, is addressable with microwave pulses, and could be detected using optical techniques. Previous theoretical work on a model Hamiltonian and nonadiabatic transition theory, called the JDE model, has determined that this sublevel can be selectively populated if certain conditions are met. Among the most challenging, the molecules within the dimer undergoing singlet fission must have their principal magnetic axes parallel to one another and to an applied Zeeman field. Here, we present time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy of a single crystal sample of a tetracenethiophene compound featuring arrays of dimers aligned in this manner, which were mounted so that the orientation of the field relative to the molecular axes could be controlled. The observed spin sublevel populations in the paired TT and unpaired (T+T) triplets are consistent with predictions from the JDE model, including preferential 5 TT 0 formation at z ‖ B 0 , with one caveat—two 5 TT spin sublevels have little to no population. This may be due to crossings between the 5 TT and 3 TT manifolds in the field range investigated by TR-EPR, consistent with the intertriplet exchange energy determined by monitoring photoluminescence at varying magnetic fields.

Time-resolved photoluminescence (TRPL) with two excitation wavelengths – 670 (standard) and 405 nm – was used to examine the effect on carrier lifetime of two significant recent advances in CdTe: the incorporation of Se to form graded CdSe x Te 1-x and the use of Mg y Zn 1-y O buffers. The two excitation wavelengths probe depths of approximately 130 and 35 nm, respectively, and their comparison helps differentiate interface and bulk contributions to carrier lifetime. It was found that x = 0.2 Se was required to obtain lifetime improvements, primarily in the bulk. Additionally, TRPL traces for Mg y Zn 1-y O/CdSe x Te 1-x samples showed fast initial decay followed by a long-lived tail, which may be indicative of trap-dominated recombination. This behavior was not present for CdSe x Te 1-x films grown on Al 2 O 3 , which is currently state-of-the-art for surface passivation in CdTe. This indicates that further work is required to sufficiently passivate the front interface.

The Higgs amplitude mode is a collective excitation studied and observed in a broad class of matter, including superconductors, charge density waves, antiferromagnets, 3He p-wave superfluid, and ultracold atomic condensates. In all the observations reported thus far, the amplitude mode was excited by perturbing the condensate out of equilibrium. Studying an exciton-polariton condensate, here we report the first observation of this mode purely driven by intrinsic quantum fluctuations without such perturbations. By using an ultrahigh quality microcavity and a Raman spectrometer to maximally reject photoluminescence from the condensate, we observe weak but distinct photoluminescence at energies below the condensate emission. We identify this as the so-called ghost branches of the amplitude mode arising from quantum depletion of the condensate into this mode. These energies, as well as the overall structure of the photoluminescence spectra, are in good agreement with our theoretical analysis.

The addition of Bi isoelectronic dopants to GaAs provides an attractive avenue for tailoring its electronic band structure, yet it also introduces less appealing and very strong hole localization. The origin of the localization is still not thoroughly understood, which has in part inhibited the practical use of $\mathrm{GaA}{\mathrm{s}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{i}}_{x}$ alloys. In this study, the evolution of hole localization was evaluated as a function of composition. We find that spatial overlap of Bi-related bound states at concentrations $g0.6%$ Bi effectively enables holes to be channeled to those at the lowest energies, thereby aiding localization of excitons $\ensuremath{\ge}150\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ below the band gap. The large energy gap between these bound states and the GaAs valence-band edge combined with the slow upward movement of the valence band with composition causes deep localization to persist to high concentrations $g6%$ Bi. The results provide important insight into the optical and transport behavior of $\mathrm{GaA}{\mathrm{s}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{i}}_{x}$ and its implications for device applications.

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.

While bismuth-related states are known to localize carriers in $\mathrm{GaA}{\mathrm{s}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{i}}_{x}$ alloys, the localization behavior of distinct Bi pair, triplet, and cluster states bound above the valence band is less well understood. We probe localization at three different Bi complex states in dilute $\mathrm{GaA}{\mathrm{s}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{i}}_{x}$ alloys using magnetophotoluminescence and time-resolved photoluminescence spectroscopy. The mass of electrons Coulomb-bound to holes trapped at Bi pair states is found to increase relative to the average electron mass in the alloy. This increase is attributed to enhanced local compressive strain in the immediate vicinity of the pairs. The dependence of energy transfer between these states on composition is also explored.

A lattice modulated by a purely geometrical potential leads to localized states or to spiral states for electrons and photons., The key to the development of advanced materials is to understand their electronic structure-property relationship. Utilization of this understanding to design new electronic materials with desired properties led to modern epitaxial growth approaches for synthesizing artificial lattices, which for almost half a century have become the mainstay of electronic and photonic technologies. In contrast to previous scalar modulation approaches, we now study synthetic crystal lattices that have a tensor artificial modulation and develop a theory for photons and conduction band states in these lattices in a regime with an unusual departure from the familiar consequences of translational symmetry and Bloch’s theorem. This study reveals that a nonmagnetic crystal lattice modulated by a purely geometrical orientational superlattice potential can lead to localized states or to spiral states for electrons and photons, as well as weakly or strongly localized states that could be used to markedly slow down the propagation of light and for optical energy storage applications.

We demonstrate condensation of microcavity polaritons with a very sharp threshold occuring at two orders of magnitude lower pump intensity than previous demonstrations of condensation. The long cavity-lifetime and trapping and pumping geometries are crucial to the realization of this low threshold. Polariton condensation, or "polariton lasing" has long been proposed as a promising source of coherent light at lower threshold than traditional lasing, and these results suggest methods to bring this threshold even lower., Comment: 4 pages, one figure, one table

The exciton bound to a pair of nitrogen atoms situated at nearby lattice sites in dilute GaAs:N provides an energetically uniform electronic system, spectrally distinct from pairs with larger or smaller separations, and can even be grown with a uniform pair orientation in the crystal. We use photoluminescence excitation spectroscopy on an ensemble of N pairs to study the narrow continuous energy distribution within two of the individual exchange- and symmetry-split exciton states. Inhomogeneous linewidths of 50–60 μeV vary across the crystal on a mesoscopic scale and can be 30 μeV at microscopic locations indicating that the homogeneous linewidth inferred from previous time-domain measurements is still considerably broadened. While excitation and emission linewidths are similar, results show a small energy shift between them indicative of exciton transfer via phonon-assisted tunneling between spatially separated N pairs. We numerically simulate the tunneling in a spatial network of randomly distributed pairs having a normal distribution of bound exciton energies. Comparing the ensemble excitation-emission energy shift with the measured results shows that the transfer probability is higher than expected from the dilute pair concentration and what is known of the exciton wavefunction spatial extent. Both the broadening and the exciton transfer have implications for the goal of pair-bound excitons as a single- or multi-qubit system.

Light propagation in two and three dimensional lattices for which the index of refraction exhibits spatial antisymmetry is investigated in the ray and photonic crystal regimes. In these regimes, all the two dimensional antisymmetry groups for which light fails to propagate are identified. In the ray-regime, it is observed that in tilings described by 7 of the 46 two dimensional antisymmetric groups, light is localized within a fundamental domain and does not propagate through the tiling, in contrast to the behavior in the other 39 groups. To understand the above phenomenon, a rule based on the number of anti-mirror planes passing through a single Bravais lattice point is derived. In the wave regime for photonic crystals, it is observed that there are no propagating eigensolutions for the same 7 tilings as above, whereas propagating solutions and energy pass band dispersion curves can be obtained for the other 39 groups. The reasons underlying this peculiar behavior are analyzed using the topological approach for modeling flow in dynamical billiards to shed light on the applicability of Bloch’s theorem for these periodic antisymmetric lattices.

We measure polarized photoluminescence emitted from excitons bound to the 1.508 eV N–N impurity pairs in the ultra-dilute semiconductor alloy GaAs:N grown by both metalorganic chemical vapor deposition (MOCVD) and MBE. In MOCVD-grown GaAs:N, the pair orientation is random, with pairs equally distributed over the two equivalent directions in the growth plane. In contrast, MBE results in a highly uniform ensemble of in-plane pairs preferentially aligned in a single 〈110〉 direction, and the population of out-of-plane pairs reduced. The results are important for quantum control of N pair qubits where observable energy levels depend on pair orientation.

Using inelastic light scattering, we measure the zone-center electronic excitation modes in a set of multiple quantum wells. The width of the wavefunction barriers was chosen such that it prevents significant coupling of the electron ground states between wells yet is transparent to electron tunneling under an electric field. Under these conditions, we find charge-density-like and spin-density-like plasmons whose energies do not correspond to the excitations calculated for either a single well or a set of Coulomb-coupled wells. The observed energies are proportional to the electric field strength and the lower energy modes agree with predictions for plasmons tunneling between the Wannier-Stark ladder states.Using inelastic light scattering, we measure the zone-center electronic excitation modes in a set of multiple quantum wells. The width of the wavefunction barriers was chosen such that it prevents significant coupling of the electron ground states between wells yet is transparent to electron tunneling under an electric field. Under these conditions, we find charge-density-like and spin-density-like plasmons whose energies do not correspond to the excitations calculated for either a single well or a set of Coulomb-coupled wells. The observed energies are proportional to the electric field strength and the lower energy modes agree with predictions for plasmons tunneling between the Wannier-Stark ladder states.

Resonant electronic Raman (ER) scattering is used to compare the below-gap excitations in molecular-beam epitaxially grown GaAs and in undoped semi-insulating GaAs substrates. The measurement geometry was designed to eliminate common measurement artifacts caused by the high optical transmission below the fundamental absorption edge. In epitaxial GaAs, ER is a clear Raman signal from the two-electron transitions of donors, eliminating an ambiguity encountered in previous results. In semi-insulating GaAs, ER occurs in a much broader dispersive band well below the bound exciton energies. The difference in the two materials may be due to the occupation of the substrate acceptor states in the presence of the midgap state EL2.

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.

Semiconductor strain engineering has become a critical feature of high-performance electronics because of the significant device performance enhancements that it enables. These improvements, which emerge from strain-induced modifications to the electronic band structure, necessitate new ultra-sensitive tools to probe the strain in semiconductors. Here, we demonstrate that minute amounts of strain in thin semiconductor epilayers can be measured using electronic Raman scattering. We applied this strain measurement technique to two different semiconductor alloy systems using coherently strained epitaxial thin films specifically designed to produce lattice-mismatch strains as small as 10−4. Comparing our strain sensitivity and signal strength in AlxGa1−xAs with those obtained using the industry-standard technique of phonon Raman scattering, we found that there was a sensitivity improvement of 200-fold and a signal enhancement of 4 × 103, thus obviating key constraints in semiconductor strain metrology., Engineering strain in semiconductor structures provides additional control over the optical and electronic properties, which is promising for device applications. Fluegel et al. show that electronic Raman scattering provides a route to sensitively measure the degree of strain in thin semiconductor layers.

Although heavily N doped III-V semiconductors are frequently referred to as dilute nitrogen III-V-nitride alloys in general, it is important to realize that there are some subtle but important differences among them. In the indirect gap semiconductor GaP, since even a single N impurity can have a bound state, as far as the absorption near the band gap is concerned, it is difficult to describe GaP:N as an alloy. The N induced bound states (a hierarchy of impurity complexes) can in fact give rise to rather strong absorption below the indirect band gap, which effectively reduced the energy of the absorption edge, but the position of the absorption edge cannot be defined in a conventional way. In the direct gap semiconductor GaAs, a single N impurity does not form a bound state but instead has a resonant state above the conduction band edge. In this aspect, GaAs:N is similar to the situation in a conventional alloy, say, GaAs:P, except that the perturbation of N to the host is much stronger than that of P. However, because in reality N incorporation is typically in a random manner and a cluster as small as one N pair can generate bound states, the permissible region for GaAs:N to behave as a regular alloy is in fact rather limited, i.e., only when the N concentration is low enough so that the N pair and cluster states do not significantly interact with the host. In this study, a precise tracking of the evolution of the host and N induced impurity states will be offered for the two prototype systems, GaAs:N and GaP:N, with x varying from as low as ∼10 -5 to ∼10 -2 . Such a study gives valuable insight to the underlying physics of the material evolution, sets up a bench-mark for testing the theoretical modeling of this type of system, and serves to enhance our understanding of the behavior of isoelectronic impurities in semiconductors in general. The dissimilarity between GaP:N and GaAs:N indicates that seeking a unified model for all the isoelectronic doping systems is unrealistic. Our study also indicates that for strongly perturbed systems like GaP:N and GaAs:N, certain materials properties could be less well defined or not uniquely defined, as compared to those in other conventional semiconductor alloys.

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}

II - VI semiconductor (CdS, CdSe) nanocrystals with an average size of approximately one bulk exciton Bohr radius are embedded in a glass matrix. Due to the quantum confinement effect, they act as a quasi zero-dimensional system (quantum dots). Under strong nanosecond and femtosecond optical excitation, these quantum dots exhibit optical amplification (gain). We investigate the ultrashort gain dynamics of the strongly confined CdSe quantum dots by femtosecond pump-probe spectroscopy. From multiple-beam pump-probe measurements, we conclude that the gain mechanism is governed by biexciton to exciton transitions. Femtosecond dephasing measurements reveal a constant scattering rate across the gain region and confirm the two-electron-hole pair gain model. Nanosecond pump-probe measurements on CdS quantum dots in sol-gel glasses show optical gain up to room temperature. In all cases, the gain region is broad and stretches below the fundamental absorption of the nanocrystals. The reason is the multitude...

At present, wavelength-division-multiplexed fibre lines routinely operate at 10 Gbit s−1 per channel. The transition from static-path networks to true all-optical networks encompassing many nodes, in which channels are added/dropped and efficiently reassigned, will require improved tools for all-optical wavelength shifting. Specifically, one must be able to shift the carrier wavelength (frequency) of an optical data signal over tens of nanometres (a THz range) without the bottleneck of electrical conversion. Popular approaches to this problem make use of the nonlinear interaction between two wavelengths within a semiconductor optical amplifier1 whereas more novel methods invoke terahertz-frequency electro–optic modulation2 and polaritons3. Here we outline the principles and demonstrate the use of optically excited plasmons as a tunable frequency source that can be mixed with a laser frequency through Raman scattering. The scheme is all-optical and enables dynamical control of the output carrier wavelength simply by varying the power of a control laser.

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.

The low-temperature birefringence of spontaneously ordered GaInP is studied by using polarized transmission spectroscopy at 5 K. Refractive indexes n o , n e and the birefringence Δn oe = n o − n e are obtained by applying two independent techniques: (1) measuring the polarized transmission spectra with polarization parallel to each of the two cleaved edges individually; and (2) measuring one transmission spectrum with a cross-polarization configuration. The results of these two techniques are consistent, and both give Δn oe ≈ 0.02 near the band-gap of partially ordered GaInP. Exact and approximate formulae have been derived for analyzing the cross-polarization spectrum. A possible device-polarization rotator-is proposed based on the in-plane birefringence.

Using time-resolved small-signal exciton absorption bleaching at low temperature, the optical transition energies from all three valence bands in spontaneously-ordered GaInP have been measured with high accuracy. The origin of the bleaching signal and the contributions of binding energy are discussed. With three measured energies from each sample, all parameters in the quasi-cubic 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 bandgap reduction to crystal-field splitting parameter is found to be 2.7, slightly higher than in previous works. This difference is attributed to a more accurate determination of light-hole-like bandgap energy.

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 article of record as published may be found at http://dx.doi.org/10.1063/1.4847635 The mobility of electrons in double heterostructures of p-type Ga{sub 0.50}In{sub 0.50}P has been determined by measuring minority carrier diffusion length and lifetime. The minority electron mobility increases monotonically from 300K to 5K, limited primarily by optical phonon and alloy scattering. Comparison to majority electron mobility over the same temperature range in comparably doped samples shows a significant reduction in ionized impurity scattering at lower temperatures, due to differences in interaction of repulsive versus attractive carriers with ionized dopant sites. These results should be useful in modeling and optimization for multi-junction solar cells and other optoelectronic devices.

Thin-film polycrystalline semiconductors are currently at the forefront of inexpensive large-area solar cell and integrated circuit technologies because of their reduced processing and substrate selection constraints. Understanding the extent to which structural and electronic defects influence carrier transport in these materials is critical to controlling the optoelectronic properties, yet many measurement techniques are only capable of indirectly probing their effects. Here we apply a novel photoluminescence imaging technique to directly observe the low temperature diffusion of photocarriers through and across defect states in polycrystalline CdTe thin films. Our measurements show that an inhomogeneous distribution of localized defect states mediates long-range hole transport across multiple grain boundaries to locations exceeding 10 μm from the point of photogeneration. These results provide new insight into the key role deep trap states have in low temperature carrier transport in polycrystalline CdTe by revealing their propensity to act as networks for hopping conduction., Understanding the role of defects on semiconductor carrier transport should help improve their performance in devices. Using photoluminescence techniques, Alberi et al. image the carrier diffusion in polycrystalline CdTe and find that long-range transport is mediated by the distribution of defect states.

We present a model for gain in a quasi zero-dimensional quantum confined semiconductor system. Due to a combination of a multitude of one-electron-hole pair and two-electron-hole pair transitions and inhomogeneous broadening, the gain region is broad, quasi-continuous and stretches below the absorption edge. Femtosecond experiments in the gain region of strongly confined CdSe quantum dots confirm our theoretical predictions.

In the previous paper we demonstrated novel multiexcitons in a neutral mixed‐stack charge‐transfer solid. The lowest multiexciton, the biexciton, has recently been of interest also in the context of quasi‐one‐dimensional organic materials that are different from the mixed‐stack solids. The nature and strength of the optical transition from the exciton to the two‐exciton states is of importance in understanding photoinduced absorption as well as two‐photon absorption. We show that within the diverse theoretical models that describe these different classes of materials, the excited state absorption from the optical exciton to the two‐exciton states changes in a fundamental way upon the formation of the biexciton. The identical nature of the exciton absorption within these models is a consequence of one dimensionality.

The role of localized states and their influence on the broader band structure remains a crucial question in understanding the band structure evolution in GaAs1− x Bi x . In this work, we present clear spectroscopic observations of recombination at several localized states in GaAs1− x Bi x . Sharp and recognizable photoluminescence features appear in multiple samples and redshift as a function of GaBi fraction between x = 0.16% and 0.4% at a linearized rate of 34 meV per % Bi, weaker than the redshift associated with band-to-band recombination. Interpreting these results in terms of radiative recombination between localized holes and free electrons sheds light on the relative movement of the conduction band minimum and the characteristics of localized bismuth-related trap states in GaAs1− x Bi x alloys.

Dilute bismide semiconductor alloys are a promising material platform for optoelectronic devices due to drastic impacts of bismuth on the electronic structure of the alloy. At the same time, the details of bismuth incorporation in the lattice are not fully understood. In this work, we conduct Raman scattering spectroscopy on GaP1− x Bi x epilayers grown by molecular beam epitaxy (MBE) and identify several bismuth-related Raman features including gap vibration modes at 296, 303, and 314 cm−1. This study paves the way for more detailed analysis of the local symmetry at bismuth incorporation sites in the dilute bismide alloy regime.

Quantum dots of InP, GaP and GaInP{sub 2} with diameters ranging from 20-65 {Angstrom} were synthesized as well-crystallized nanoparticles with bulk zinc blende structure. The high sample quality of the InP and GaP QDs results in excitonic features in the absorption spectra. The GaP and GaInP{sub 2} QD colloids exhibited very intense (quantum yields of 15-25%) visible photoluminescence at room temperature. The photoluminescence for InP preparations showed two emission bands: one band in the visible at the band edge of the QD (50 nm Stokes shift), and a second band above 800 nm. The near-IR PL is attributed to deep traps, presumably phosphorus vacancies on the QD surface. This band disappears after activation of particles by controlled addition of etchant. In that case very intense band-edge emission (quantum yield 30% at room temperature and 60% at 10 K) was obtained. The QDs were characterized by TEM, SAXS, AFM, powder x-ray diffraction, steady state optical absorption and photoluminescence spectroscopy, ps to ns transient photoluminescence spectroscopy, and fs to ps pump-probe absorption (i.e., hole-burning) spectroscopy. Results will also be reported on efforts to produce ordered arrays of InP QDs.

The loss of phase coherence in II-VI quantum dots is investigated using experimental methods working both in the frequency and in the time domain. Femtosecond differential absorption experiments reveal narrow resonances at very early times after excitation and low excitation densities for CdSe quantum dots. A substructure is detected and explained by transitions from the series of confined hole states. By nondegenerate four-wave mixing experiments (NDFWM) the dephasing time is determined for CdS quantum dots with radii larger than the Bohr radius. Temperature and excitation intensity dependent dephasing times between 20 fs and some 100 fs are measured and attributed to electron-hole pair-phonon coupling and carrier-carrier scattering

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.

The use of a high magnetic field (57 T) to study the formation and evolution of nitrogen (N) cluster and supercluster states in GaAs:N is demonstrated. A magnetic field is used to lift the conduction band edge and expose resonant N cluster states so that they can be directly experimentally investigated. The reduction of the exciton Bohr radius also results in the fragmentation of N supercluster states, enabling a magnetic field induced delocalized to localized transition. The application of very high magnetic fields thus presents a powerful way to probe percolation phenomena in semiconductors with bound and resonant isoelectronic cluster states.

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.

Optical pumping of a type-I/type-II coupled asymmetric quantum well pair induces a spatially separated two dimensional charge carriers plasma in the well’s wide and narrow parts. Treating the two coupled wells as a single system we find that the eigenstate probability distribution localizes exclusively either in the wide or the narrow parts of the well pair. The energy of the narrow-well localized state determines the minimal excitation energy for optically pumped charge carriers separation. In a previously used design [Guliamov et al., PRB 64 035314 (2001)] this narrow well transition energy was measured to correspond to a wavelength of 646 nm. We propose modifications to the design suggested earlier with the purpose of pushing up the energy required for the optical pumping of the two-dimensional plasma into the green and blue regions of the visible spectrum.

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.

The bound states of n Frenkel excitons ( n = 2,3,…), which are called excitonic n-strings, are theoretically shown to exist in contrast with a system of Wannier excitons. The electronic structure and nonlinear optical responses of these excitonic n-strings are clarified. The first evidence for the excitonic 2- and 3-strings is provided from comparison of the calculated and observed differential transmission spectrum of an Anthracene-PMDA crystal pumped by femtosecond laser pulses.

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.

Femtosecond pump-and-probe spectroscopy was adopted to study the hot carrier distribution in a CdSe thin film. In the experiment, a nonthermalized hot carrier distribution was observed when the pump was turned on. The nonthermalized distribution was observed at the 200 meV broad energy range extending from the pump energy to the low energy side. The low energy tail of the nonthermalized distribution indicates that the carrier-LO phonon scattering competes with the carrier-carrier scattering in the thermalization process. In this sense, this observation is a unique example of the nonthermalized hot carrier distribution in semiconductors. From the ratio of the nonthermalized distribution to the total one, the thermalization time was estimated to be 16-40 fs. In the wake of the thermalization, cooling of photogenerated carriers was observed. With an increase of the excitation density up to 1019 cm-3, the carrier cooling rate was slower. The observed cooling rate was slower than the theoretical calculation, taking account of the screening of the carrier-phonon interaction. This is ascribed to the hot phonon effects of the LO phonon and the TO phonon.