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The coherence of collective modes, such as phonons and polarons, and their modulation of electronic states is long sought in complex systems, which is a crosscutting issue in photovoltaics and quantum electronics. In photovoltaic cells and lasers based on metal halide perovskites, the presence of polarons, i.e., photocarriers dressed by the macroscopic motion of charged lattice, assisted by terahertz (THz) longitudinal-optical (LO) phonons, has been intensely studied yet is still debated. This may be key for explaining the remarkable properties of the perovskite materials, e.g., defect tolerance, long charge lifetimes, and diffusion lengths. Here we use the intense single-cycle THz pulse with peak electric field up to ${E}_{\mathrm{THz}}=1000$ kV/cm to drive coherent polaronic band-edge oscillations at room temperature in ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}\phantom{\rule{4pt}{0ex}}({\mathrm{MAPbI}}_{3})$. We reveal the oscillatory behavior is dominated by a specific quantized lattice vibration mode at ${\ensuremath{\omega}}_{\mathrm{LO}}\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{THz}$, which is both dipole and momentum forbidden. THz-driven coherent polaron dynamics exhibits distinguishing features: room temperature coherent oscillations at ${\ensuremath{\omega}}_{\mathrm{LO}}$ longer than 1 ps in both single crystals and thin films, mode-selective modulation of different band-edge states assisted by electron-phonon interaction, and dynamic mode splitting at low temperature due to entropy and anharmonicity of organic cations. Our results demonstrate intense THz-driven coherent band-edge modulation is a powerful probe of electron-lattice coupling phenomena and polaronic quantum control in perovskites.

Self-absorption of internally radiated photons (``photon recycling'') is common in high-quality, direct-gap semiconductors, and can yield increased photovoltage in solar cells and enhanced emission efficiency in LEDs. For perovskite semiconductors, photon recycling is relatively unexplored, especially for devices under operating conditions. The authors develop a model to quantify the extent of photon recycling in state-of-the-art perovskite solar cells of varying nonradiative loss and geometry. They present clear experimental targets for material optoelectronic quality to harness photon recycling in solar cells, with major implications for achieving low-threshold lasing and efficient LEDs.

We demonstrate an unexpected tenfold reduction in the lasing threshold of an organic vertical microcavity under subpicosecond optical excitation. In contrast to conventional theory of lasing, we find that the lasing threshold depends on the rate at which excitons are created rather than the total energy delivered within the exciton lifetime. The threshold reduction is discussed in the context of microcavity-enhanced super-radiant coupling between the excitons. The interpretation of super-radiance is supported by the temporal relaxation dynamics of the microcavity emission, which follows the super-radiance time rather than the cavity lifetime. This demonstration suggests that room-temperature super-radiant effects could generally lower the threshold in four-level lasing systems of similar relaxation dynamics.

We present an integrated classical and quantum-mechanical theory of weak microcavity effects in layered media that treats both radiative and waveguided modes. The electromagnetic field of radiative modes is determined using classical field quantization, with the transition probability into each mode given by Fermi's ``golden rule.'' We apply this theory to model the dependence of the electroluminescence spectral intensity and polarization of organic light-emitting devices (OLED's) on emission angle, organic layer thickness, and applied voltage. Light propagation in the OLED layers and the substrate is described by both ray and wave optics. Theoretical predictions are compared to experimental observations on single heterostructure, and multiple layer stacked red-green-blue OLEDs. Analysis of the polarization, spectral shape, and intensity of the electroluminescence spectrum in the forward-scattered half plane accurately fits the experimental data. The theory predicts, and the experimental measurements confirm, that the in-plane emission from conventional OLED structures is strongly TM polarized, and can be redshifted by as much as 60 nm with respect to the peak emission in the normal direction. Measurements coupled to our analysis also indicate that the efficiency of generating singlet excitons in aluminum tris(8-hydroxyquinoline) $({\mathrm{Alq}}_{3})$-based OLED's is $5\ifmmode\pm\else\textpm\fi{}1%,$ with a \ensuremath{\sim}500-\AA{}-thick ${\mathrm{Alq}}_{3}$ layer corresponding to the highest external power efficiency.

Using the technique of two-photon photoemission, we have observed image-potential resonances on Cu(110). The image-potential resonances have a short, bulklike lifetime, resulting in a weak, but observable, photoemission signal that is broad in energy; the peak of the density of states for the n=1 resonance is 0.68±0.06 eV below the vacuum level, with a width of 0.66±0.06 eV. This investigation demonstrates the versatility of using relatively long-pulse, two-photon photoemission for observing short-lived surface resonances, which are excited from bulk initial states

We investigate the incoherent diffusion of excitons in thin films (5.1±0.1 nm thick) of a highly absorbing J-aggregated cyanine dye material (106 cm-1 absorption constant) as the excitonic component of a polariton microcavity. Under high-intensity pulsed laser excitation, the J-aggregated molecular films exhibit significant exciton-exciton annihilation, indicating a large exciton diffusion radius of more than 100 nm. When the material is strongly coupled to a cavity, the polaritonic structure also shows exciton-exciton annihilation, which is a competing process against the establishment of a threshold population of polaritons needed for polariton lasing. This study suggests that exciton-exciton annihilation is a loss process which can significantly increase the lasing threshold in polariton microcavities.

We demonstrate intracavity optical pumping of J-aggregate microcavity exciton polaritons. The use of ultrathin layer-by-layer J-aggregate thin films as the strongly coupled exciton medium allows for inclusion of a thermally evaporated luminescent cavity spacer layer, through which the lower-branch exciton-polariton states are resonantly pumped. We present a measurement of the lower-branch exciton-polariton occupation in room-temperature microcavity devices containing J-aggregated molecular thin films under low-density steady-state excitation. The observed exciton-polariton occupation shows a Maxwell-Boltzmann distribution at $T=300\text{ }\text{K}$, indicating thermalization of exciton polaritons in the lower energy branch. This device design enables us to propose a new type of ``polariton laser'' architecture for microcavity exciton polaritons.

We use a scanning tunneling microscope to probe single-electron charging phenomena in individual CdSe/ZnS (core/shell) quantum dots (QDs) at room temperature. The QDs are deposited on top of a bare Au thin film and form a double-barrier tunnel junction (DBTJ) between the tip, QD, and substrate. Analysis of room-temperature hysteresis in the current-voltage $(IV)$ tunneling spectra, is consistent with trapped charge(s) presenting an additional potential barrier to tunneling, a measure of the Coulomb blockade. The paper describes the first direct electrical measurement of the trap-state energy on individual QDs. Manipulation of the charge occupation of the QD, verified by measuring the charging energy, $(61.4\ifmmode\pm\else\textpm\fi{}2.4)\text{ }\text{meV}$, and analysis of the DBTJ, show trap states $\ensuremath{\sim}1.09\text{ }\text{eV}$ below the QD conduction-band edge. In addition, the detrapping time, a measure of the tunneling barrier thickness, is determined to have an upper time limit of 250 ms. We hypothesize that the charge is trapped in a quantum-dot surface state.

We report a comprehensive study of photoluminescence (PL) quenching of tris-(8-hydroxyquinoline) aluminum $({\text{Alq}}_{3})$ at interfaces with thin films of tin oxide $({\text{SnO}}_{2})$ using both steady-state and time-resolved measurements. Quenching of excitons generated in the ${\text{Alq}}_{3}$ layer increased with increased conductivity of the ${\text{SnO}}_{2}$ films, which we relate with the presence of nonradiative energy transfer from excitons in ${\text{Alq}}_{3}$ to transitions in ${\text{SnO}}_{2}$. In addition, due to the semitransparency of ${\text{SnO}}_{2}$, the effects of optical interference on the steady-state PL quenching of ${\text{Alq}}_{3}$ are determined. We demonstrate that without accounting for the interference effects in the excitation, the extracted exciton diffusion length $({L}_{d})$ in ${\text{Alq}}_{3}$ is in the range of 10--20 nm. However, when using a numerical model to account for the optical interference effects, we find that ${L}_{d}$ is in the range of 5--9 nm, which agrees with ${L}_{d}$ extracted from time-resolved measurements (4--6 nm). We conclude that time-resolved measurements are least affected by optical interference, yielding the most accurate measurement of ${L}_{d}$.

We demonstrate that linear dispersion theory accurately predicts the linear optical response of strongly coupled microcavity exciton-polariton devices containing thin films of $J$ aggregates of the cyanine dye [5,6-dichloro-2-[3-[5,6-dichloro-1-ethyl-3-(3-sulfopropyl)-2(3H)-benzimidazolidene]-1-propenyl]-1-ethyl-3-(3-sulfopropyl) benzimidazolium hydroxide, inner salt, sodium salt] (TDBC) when a spectrally resolved complex index of refraction derived by model-free quasi-Kramers-Kronig regression is used for the $J$-aggregate thin film. We show that the Rabi splitting can be finely tuned through the $J$-aggregate film thickness, and we estimate that the minimum linewidth of lower branch exciton polaritons at room temperature in TDBC $J$-aggregate-based structures is 8.4 meV.

We investigate the mechanism of operation of hybrid organic/colloidal quantum dot light emitting devices (QD-LEDs). Novel quantum dot (QD) deposition methods allow us to change the location of an emissive QD monolayer within a QD-LED multilayer structure. We find that the quantum efficiency of devices improves by $g50%$ upon imbedding QD monolayers into the hole transporting layer $l10\text{ }\text{nm}$ away from the interface between hole and electron transporting layers. We consider two possible mechanisms responsible for this improvement: one based on a charge injection model of the device operation and the other based on an exciton energy-transfer model. In order to differentiate between the two suggested mechanisms, we fabricate a set of structures that enable control over charge injection into colloidal QDs. We find that the dominant process limiting QD-LED efficiency is charging of the QDs by trapped electrons. We demonstrate that with the set of organic materials implemented in this study, device efficiency is increased by maximizing energy transfer from organics to QDs and by limiting direct charge injection that contributes to QD charging.

We report on Monte Carlo calculations of the exciton energy level disorder in polar amorphous organic solids arising from electrostatic interactions between polarizable molecular charge distributions. In contrast to previous reports in which the discrete molecular polarizabilities are modeled as a dielectric continuum having dielectric constant $ϵ$, our calculations take account of the molecular-scale polarization inhomogeneity. We demonstrate that exciton energy level disorder is only weakly dependent on $ϵ$. For a range of $ϵ$ typical of organic solids, we report an empirical expression for the energy level disorder, and show that conventional dielectric continuum models are not consistent with the observed results. Results of our calculations are shown to be consistent with experimental measurements of exciton energy disorder in aluminum tris-(8-hydroxyquinoline).

Time-resolved photoluminescence spectroscopy of amorphous organic thin films of aluminum tris-(8-hydroxyquinoline) show emission spectra that redshift with time following excitation by ultrafast laser pulses. Based on reports of similar phenomena in other materials, we attribute this effect to the exciton diffusion between energetically dissimilar molecules by means of F\"orster transfer. In analyzing results at 295, 180, 75, and 35 K, we show that existing theoretical treatments of exciton diffusion require two modifications to self-consistently fit our data: one must include spatial disorder in the model, and the energy dependence of F\"orster transfer must be calculated using the donor-acceptor spectral overlap, instead of a Boltzman distribution. Monte Carlo simulations utilizing these changes yield results that are self-consistent with the observed spectral shifts.

The strong coupling limit of cavity QED is reached when matter inserted inside a microcavity exchanges energy with the resonant mode of the cavity more rapidly than the combined rate at which light leaves the cavity and the matter wave function loses its phase information [1,2]. In this limit, the microcavity and matter form a composite quantum system with two new eigenstates that are superpositions of the initial uncoupled states, with new eigenenergies separated in energy by the Rabi splitting. The matter component of the coupled system can be a gas of atoms trapped inside the cavity [3], a superconducting qubit [4], or a solid state thin film containing excitons, in the form of an inorganic quantum well [5], quantum dot [6,7], or organic material [8], in which case the superposition states are referred to as exciton polaritons. Applications of strong coupling in atomic and semiconductor systems have led to one-atom zero threshold lasers [9], high gain polariton parametric amplifiers [10], and predictions that strong coupling may play a key role in future quantum information processors [11]. These experiments have all relied on optical pumping. Here we demonstrate electrically pumped exciton-polariton emission, the first device in which strongly coupled states of light and matter are electrically excited. The matter component of our device is a 6 � 1n mthick film of J aggregated dye. The film consists of 4 bilayers [12] of the cationic polyelectrolyte PDAC (poly diallyldimethylammonium chloride) and J aggregates of the anionic cyanine dye TDBC (5,6-dichloro-2-[3-[5,6-dichloro-1-ethyl-3-(3-sulfopropyl)-2(3H)-benzimidazolidene]-1-propenyl]-1-ethyl-3-(3-sulfopropyl) benzimidazolium hydroxide, inner salt, sodium salt), molecular structures shown in Fig. 1(a). The J aggregates are crystallites of dye in which the transition dipoles of the constituent molecules strongly couple to form a collective narrow linewidth optical transition possessing oscillator strength derived from all the aggregated monomers [13]. The bilayer films contain a high density of J aggregated TDBC and therefore have a very large peak absorption

Exciton-polariton dynamics are studied in organic semiconductor microcavities operating in the strongly coupled polariton regime, with thin films of cyanine J-aggregates sandwiched between low-loss dielectric mirrors. The delocalized excitons are subject to polariton normal mode splittings which are modulated by near resonant femtosecond excitation pulses, while orientational disorder effects on the formation of polariton states play an important role in polariton dynamics.