Exciton-phonon scattering and photo-excitation dynamics in J-aggregate microcavities

We have developed a model accounting for the photo-excitation dynamics and the photoluminescence of strongly coupled J-aggregate microcavities. Our model is based on a description of the J-aggregate film as a disordered Frenkel exciton system in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. In a strongly coupled microcavity exciton-polaritons are formed, mixing superradiant excitons and cavity photons. The calculation of the microcavity steady-state photoluminescence, following a CW non resonant pumping, is carried out. The experimental photoluminescence intensity ratio between upper and lower polariton branches is accurately reproduced. In particular both thermal activation of the photoluminescence intensity ratio and its Rabi splitting dependence are a consequence of the bottleneck in the relaxation, occurring at the bottom of the excitonic reservoir. The effects due to radiative channels of decay of excitons and to the presence of a paritticular set of discrete optical molecular vibrations active in relaxation processes are investigared.Comment: 8 pages, 6 figure

Plasmonic gold nanodiscs fabricated into a photonic-crystal nanocavity

We fabricate and characterise an optical structure consisting of a photonic crystal L3 nanocavity containing two gold nanodisks placed close to a field antinode. We use finite difference time domain (FDTD) modelling to show that the optical properties of the nanocavity are sensitive to the physical separation between the gold nanodisks, and that at reduced separation, the q-factor of a cavity mode polarised parallel to the dimer long-axis is reduced, indicating coupling between the cavity mode and a localised plasmon. Preliminary experimental measurements indeed indicate a damping of the cavity mode in the presence of the dimer; a result consistent with the FDTD modelling. Such a scheme may be used to integrate plasmonic systems into all-optical photonic circuits

Optical properties of nanowires based on a blend of poly (9,9-dioctylfluorene) [PFO] and poly(9,9-dioctyl fluorene-alt-benzothiadiazole) [F8BT]

Near-field and far-field optical microscopy are used to study the optical properties of nanowires based on a blend of 95% poly (9,9-dioctylfluorene) [PFO] doped with 5% poly (9,9-dioctylfluorene-alt-benzothiadiazole) [F8BT]. Single nanowires were imaged and optical investigations revealed that they act as nanoscale optical waveguides. Investigation using polarization-resolved far-field PL spectroscopy revealed emission from the nanowires was strongly anisotropic having preferred axial polarization. This suggests that a significant number of the polymer chains are oriented along the nanowire axis

Monitoring the Formation of a CH<inf>3</inf>NH<inf>3</inf>PbI<inf>3-</inf><inf>x</inf>Cl<inf>x</inf> Perovskite during Thermal Annealing Using X-Ray Scattering

Grazing incidence wide and small angle X-ray scattering (GIWAXS and GISAXS) measurements have been used to study the crystallization kinetics of the organolead halide perovskite CH3NH3PbI3-xClx during thermal annealing. In situ GIWAXS measurements recorded during annealing are used to characterize and quantify the transition from a crystalline precursor to the perovskite structure. In situ GISAXS measurements indicate an evolution of crystallite sizes during annealing, with the number of crystallites having sizes between 30 and 400 nm increasing through the annealing process. Using ex situ scanning electron microscopy, this evolution in length scales is confirmed and a concurrent increase in film surface coverage is observed, a parameter crucial for efficient solar cell performance. A series of photovoltaic devices are then fabricated in which perovskite films have been annealed for different times, and variations in device performance are explained on the basis of X-ray scattering measurements

Intrinsic optical bistability of thin films of linear molecular aggregates: The two-exciton approximation

We generalize our recent work on the optical bistability of thin films of molecular aggregates [J. Chem. Phys. 127, 164705 (2007); arXiv:0707.1264v1 [cond-mat.dis-nn]] by accounting for the optical transitions from the one-exciton manifold to the two-exciton manifold as well as the exciton-exciton annihilation of the two-exciton states via a high-lying molecular vibronic term. We also include the relaxation from the vibronic level back to both the one-exciton manifold and the ground state. By selecting the dominant optical transitions between the ground state, the one-exciton manifold, and the two-exciton manifold, we reduce the problem to four levels, enabling us to describe the nonlinear optical response of the film. The one- and two-exciton states are obtained by diagonalizing a Frenkel Hamiltonian with an uncorrelated on-site (diagonal) disorder. The optical dynamics is described by means of the density matrix equations coupled to the electromagnetic field in the film. We show that the one-to-two exciton transitions followed by a fast exciton-exciton annihilation promote the occurrence of bistability and reduce the switching intensity. We provide estimates of pertinent parameters for actual materials and conclude that the effect can be realized.Comment: 11 two-column pages, 6 figures, to appear in the Journal of Chemical Physic

Band energy control of molybdenum oxide by surface hydration

EPSRC (Grants EP/M009580/1, EP/J017361/1, EP/I01330X/1, and EP/I028641/1), the Royal Society, and the European Research Council. The work benefited from the University of Bath's High Performance Computing Facility, and access to the HECToR supercomputer through membership of the UKs HPC Materials Chemistry Consortium, which is funded by EPSRC (Grant No. EP/F067496) and the UltraFOx grant

Thermally stable solution processed vanadium oxide as a hole extraction layer in organic solar cells

Low-temperature solution-processable vanadium oxide (V2Ox) thin films have been employed as hole extraction layers (HELs) in polymer bulk heterojunction solar cells. V2Ox films were fabricated in air by spin-coating vanadium(V) oxytriisopropoxide (s-V2Ox) at room temperature without the need for further thermal annealing. The deposited vanadium(V) oxytriisopropoxide film undergoes hydrolysis in air, converting to V2Ox with optical and electronic properties comparable to vacuum-deposited V2O5. When s-V2Ox thin films were annealed in air at temperatures of 100 °C and 200 °C, OPV devices showed similar results with good thermal stability and better light transparency. Annealing at 300 °C and 400 °C resulted in a power conversion efficiency (PCE) of 5% with a decrement approximately 15% lower than that of unannealed films; this is due to the relative decrease in the shunt resistance (Rsh) and an increase in the series resistance (Rs) related to changes in the oxidation state of vanadium

Room temperature broadband polariton lasing from a dye‐filled microcavity

A material system is proposed to generate polariton lasing at room temperature over a broad spectral range. The system developed is based on a boron‐dipyrromethene fluorescent dye (BODIPY‐G1) that is dispersed into a polystyrene matrix and used as the active layer of a strongly coupled microcavity. It is shown that the BODIPY‐G1 exciton polaritons undergo nonlinear emission over a broad range of exciton–cavity mode detuning in the green‐yellow portion of the visible spectrum, with polariton lasing achieved over a spectral range spanning 33 nm. The recorded linewidth of ≈0.1 nm corresponds to a condensate coherence lifetime of ≈1 ps. It is proposed that similar effects can be anticipated using a range of molecular dyes in the BODIPY family; a result that paves the way for tunable polariton devices over the visible and near‐infrared spectral region

Intrinsic optical bistability of thin films of linear molecular aggregates: The one-exciton approximation

We perform a theoretical study of the nonlinear optical response of an ultrathin film consisting of oriented linear aggregates. A single aggregate is described by a Frenkel exciton Hamiltonian with uncorrelated on-site disorder. The exciton wave functions and energies are found exactly by numerically diagonalizing the Hamiltonian. The principal restriction we impose is that only the optical transitions between the ground state and optically dominant states of the one-exciton manifold are considered, whereas transitions to other states, including those of higher exciton manifolds, are neglected. The optical dynamics of the system is treated within the framework of truncated optical Maxwell-Bloch equations in which the electric polarization is calculated by using a joint distribution of the transition frequency and the transition dipole moment of the optically dominant states. This function contains all the statistical information about these two quantities that govern the optical response, and is obtained numerically by sampling many disorder realizations. We derive a steady-state equation that establishes a relationship between the output and input intensities of the electric field and show that within a certain range of the parameter space this equation exhibits a three-valued solution for the output field. A time-domain analysis is employed to investigate the stability of different branches of the three-valued solutions and to get insight into switching times. We discuss the possibility to experimentally verify the bistable behavior.Comment: 13 two-column pages, 8 figures, accepted to the Journal of Chemical Physic

Ultralong-range polariton-assisted energy transfer in organic microcavities.

Non-radiative energy transfer between spatially-separated molecules in a microcavity can occur when an excitonic state on both molecules are strongly-coupled to the same optical mode, forming so-called ‘hybrid’ polaritons. Such energy transfer has previously been explored when thin-films of different molecules are relatively closely spaced (~100 nm). In this letter, we explore strong-coupled microcavities in which thin-films of two J-aggregated molecular dyes were separated by a spacer layer having a thickness of up to 2 μm. Here, strong light-matter coupling and hybridisation between the excitonic transition is identified using white-light reflectivity and photoluminescence emission. We use steady-state spectroscopy to demonstrate polariton-mediated energy transfer between such coupled states over ‘mesoscopic distances’, with this process being enhanced compared to non-cavity control structures