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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.

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.

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.

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.

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.

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.

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.

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.

The aluminum concentration dependence of the energies of the direct and indirect bandgaps arising from the Γ and X conduction bands are measured at 1.7 K in the semiconductor alloy AlxGa1−xAs. The composition at which the bands cross is determined from photoluminescence of samples grown by molecular-beam epitaxy very close to crossover at x ≈ 0.4. The use of resonant laser excitation and the improved sample linewidth allows excitation intensities as low as 10−2 W/cm2, giving a precise determination of the bound exciton transition energies and their Γ and X crossover. Photoluminescence excitation spectroscopy is then used to measure the binding energies of the donor-bound excitons and the Γ free exciton binding energy. After correcting for the Γ- and X-dependence of these quantities, the crossover of the bandgap is determined to be at x = 0.401 and E = 2.086 eV.

COLLECTIVE excitations resulting from many-body Coulomb interactions have been studied extensively in the solid state1: for example, the exchange interaction between the electrons in two excitons (bound electron–hole pairs) can bind the excitons together, forming a biexciton. At the other extreme, if the number of excitons is sufficiently large (∼106), they can condense into a degenerate 'liquid' phase known as an electron–hole drop. But in conventional semiconductors, intermediate bound states, consisting of more than two excitons, are not formed. We show here, both theoretically and experimentally, that bound states of multiple excitons can form in the organic charge-transfer solid anthracene–(pyromellitic acid dianhydride). Coulomb interactions along the one-dimensional stacks of this material can stabilize trains of several charge-transfer excitons, and we refer to the resulting collective excitations as exciton strings.

Femtosecond differential absorption measurements of the quantum-confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The hole width increases with pump intensity and the magnitude of the induced transparency saturates at the highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition. >

ZnSe and ZnS are known to have a thermal nonlinearity. This has been used in the past for demonstrating optical bistability1,2,3, however the thermal nature of the nonlinearity makes these switches inherently slow and inefficient. Here we have measured the nonlinear spectra and decay times for a large, fast, electronic nonlinearity in MBE-grown thin films of ZnSe. These samples were grown on GaAs substrates which were then removed by chemical etching.