Ultrafast holography and transient-absorption spectroscopy in charge-transfer polymers

Charge-transfer polymers are a new class of nonlinear optical materials which can be used for generating femtosecond holographic gratings. Using semiconducting polymers sensitized with varying concentrations of C{sub 60}, holographic gratings were recorded by individual ultrafast laser pulses; the diffraction efficiency and time decay of the gratings were measured using non-degenerate four-wave mixing. Using a figure of merit for dynamic data processing, the temporal diffraction efficiency, this new class of materials exhibits between two and 12 orders of magnitude higher response than previous reports. The charge transfer range at polymer/C{sub 60} interfaces was further studied using transient absorption spectroscopy. The fact that charge-transfer occurs in the picosecond-time scale in bilayer structures (thickness 200 {angstrom}) implies that diffusion of localized excitations to the interface is not the dominant mechanism; the charge transfer range is a significant fraction of the film thickness. From analysis of the excited state decay curves, we estimate the charge transfer range to be 80 {angstrom} and interpret that range as resulting from quantum delocalization of the photoexcitations

Subgap absorption in conjugated polymers

Along with X{sup (3)}, the magnitude of the optical absorption in the transparent window below the principal absorption edge is an important parameter which will ultimately determine the utility of conjugated polymers in active integrated optical devices. With an absorptance sensitivity of < 10{sup {minus}5}, Photothermal Deflection Spectroscopy (PDS) is ideal for determining the absorption coefficients of thin films of transparent'' materials. We have used PDS to measure the optical absorption spectra of the conjugated polymers poly(1,4-phenylene-vinylene) (and derivitives) and polydiacetylene-4BCMU in the spectral region from 0.55 eV to 3 eV. Our spectra show that the shape of the absorption edge varies considerably from polymer to polymer, with polydiacetylene-4BCMU having the steepest absorption edge. The minimum absorption coefficients measured varied somewhat with sample age and quality, but were typically in the range 1 cm{sup {minus}1} to 10 cm{sup {minus}1}. In the region below 1 eV, overtones of C-H stretching modes were observed, indicating that further improvements in transparency in this spectral region might be achieved via deuteration of fluorination. 11 refs., 4 figs

Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers

We present femtosecond transient absorption measurements on $\pi$-conjugated supramolecular assemblies in a high pump fluence regime. Oligo(\emph{p}-phenylenevinylene) monofunctionalized with ureido-\emph{s}-triazine (MOPV) self-assembles into chiral stacks in dodecane solution below 75$^{\circ}$C at a concentration of $4\times 10^{-4}$ M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of $1^1$B$_{u}$-exciton spectral signatures, and by the sub-linear fluence dependence of time- and wavelength-integrated photoluminescence (PL) intensity. These two characteristics are much less pronounced in MOPV solution where the phase equilibrium is shifted significantly away from supramolecular assembly, slightly below the transition temperature. A mesoscopic rate-equation model is applied to extract the bimolecular annihilation rate constant from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimolecular annihilation rate is very high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramolecular nanostructures studied here have electronic properties that are intermediate between molecular aggregates and polymeric semiconductors

Mechanisms of a coniferous woodland persistence under drought and heat

Predictions of warmer droughts causing increasing forest mortality are becoming abundant, yet few studies have investigated the mechanisms of forest persistence. To examine the resistance of forests to warmer droughts, we used a five-year precipitation reduction (~45% removal), heat (+4 °C above ambient) and combined drought and heat experiment in an isolated stand of mature Pinus edulis-Juniperus monosperma. Despite severe experimental drought and heating, no trees died, and we observed only minor evidence of hydraulic failure or carbon starvation. Two mechanisms promoting survival were supported. First, access to bedrock water, or 'hydraulic refugia' aided trees in their resistance to the experimental conditions. Second, the isolation of this stand amongst a landscape of dead trees precluded ingress by Ips confusus, frequently the ultimate biotic mortality agent of piñon. These combined abiotic and biotic landscape-scale processes can moderate the impacts of future droughts on tree mortality by enabling tree avoidance of hydraulic failure, carbon starvation, and exposure to attacking abiotic agents.This project was supported by the Department of Energy, Office of Science, and Pacific Northwest National Lab’s LDRD program. DDB participation was supported via NSF EF-1340624; EF-1550756, and EAR-1331408, DEB-1824796 and DEB-1833502. CG was supported by a Director’s Fellowship from the Los Alamos National Laboratory and by the Swiss National Science Foundation SNF (PZ00P3_174068). AV was supported by a fellowship from Generalitat Valenciana (BEST/2016/289) and the project Survive-2 (CGL2015-69773-C2-2-P MINECO/FEDER) from the Spanish Government. DSM was supported via NSF IOS-1450679, IOS-1444571, and IOS-1547796

Efficiency of radiative emission from thin films of a light-emitting conjugated polymer

J. A. E. Wasey, A. Safonov, I. D. W. Samuel, and William L. Barnes, Physical Review B, Vol. 64, article 205201 (2001). "Copyright © 2001 by the American Physical Society."We examine the efficiency of radiative emission from thin layers of light-emitting conjugated polymers. We compare our experimental results for photoluminescence of the conjugated polymer poly(2-methoxy, 5-(2′-ethyl-hexyloxy) 1,4 phenylenevinylene) (MEH-PPV) with those of a theoretical model, finding good agreement between the two. The specially developed model takes into account several factors including absorption in the emissive layer, a spread of emitter sites within the layer, and the broad emission spectrum of the polymer. We find that the photoluminescence quantum efficiency for radiative emission of a bare MEH-PPV film on a glass substrate is ∼25%. We then apply our model to study electroluminescent devices. We show that for these structures the efficiency of radiative emission is ∼10%. There is thus potential for considerable improvement in efficiency for both systems through recovery of some of the wasted waveguided light. Finally we use our model to reexamine some controversial results that indicate the probability of singlet exciton formation to be 0.4±0.05, and thus greater than the 0.25 expected from spin statistics. Our reanalysis supports a probability >0.25. We conclude by discussing the limitations of present models, including our own, in predicting the performance of realistic light-emitting diodes

Tree water dynamics in a drying and warming world

Disentangling the relative impacts of precipitation reduction and vapour pressure deficit (VPD) on plant water dynamics and determining whether acclimation may influence these patterns in the future is an important challenge. Here, we report sap flux density (FD), stomatal conductance (Gs), hydraulic conductivity (KL) and xylem anatomy in piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees subjected to five years of precipitation reduction, atmospheric warming (elevated VPD) and their combined effects. No acclimation occurred under precipitation reduction: lower Gs and FD were found for both species compared to ambient conditions. Warming reduced the sensibility of stomata to VPD for both species but resulted in the maintenance of Gs and FD to ambient levels only for piñon. For juniper, reduced soil moisture under warming negated benefits of stomatal adjustments and resulted in reduced FD, Gs and KL. Although reduced stomatal sensitivity to VPD also occurred under combined stresses, reductions in Gs, FD and KL took place to similar levels as under single stresses for both species. Our results show that stomatal conductance adjustments to high VPD could minimize but not entirely prevent additive effects of warming and drying on water use and carbon acquisition of trees in semi-arid regions.The Los Alamos Survival-Mortality (SUMO) Experiment was funded by the US Department of Energy, Office of Science, Biological and Environmental Research. C.G. and S.M. were supported by a Director’s Fellowship from the Los Alamos National Laboratory. A.V. was supported by a fellowship from Generalitat Valenciana (BEST/2016/289) and the project Survive-2 (CGL2015-69773-C2-2-P MINECO/FEDER) from the Spanish Government. CEAM is funded by Generalitat Valenciana

Femtosecond chirp-free studies of energy relaxation in semiconductor quantum dots: Search for a phonon bottleneck

Contrary to the predictions of phonon bottleneck theories, we observe very fast subpicosecond energy relaxation in strongly confined semiconductor nanocrystals with electron level spacing as large as 20 LO phonon energies

Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Quantum Dots

Low-dimensional semiconductors have attracted great interest due to the potential for tailoring their linear and nonlinear optical properties over a wide-range. Semiconductor nanocrystals (NC's) represent a class of quasi-zero-dimensional objects or quantum dots. Due to quantum cordhement and a large surface-to-volume ratio, the linear and nonlinear optical properties, and the carrier dynamics in NC's are significantly different horn those in bulk materials. napping at surface states can lead to a fast depopulation of quantized states, accompanied by charge separation and generation of local fields which significantly modifies the nonlinear optical response in NC's. 3D carrier confinement also has a drastic effect on the energy relaxation dynamics. In strongly confined NC's, the energy-level spacing can greatly exceed typical phonon energies. This has been expected to significantly inhibit phonon-related mechanisms for energy losses, an effect referred to as a phonon bottleneck. It has been suggested recently that the phonon bottleneck in 3D-confined systems can be removed due to enhanced role of Auger-type interactions. In this paper we report femtosecond (fs) studies of ultrafast optical nonlinearities, and energy relaxation and trap ping dynamics in three types of quantum-dot systems: semiconductor NC/glass composites made by high temperature precipitation, ion-implanted NC's, and colloidal NC'S. Comparison of ultrafast data for different samples allows us to separate effects being intrinsic to quantum dots from those related to lattice imperfections and interface properties