Showing papers on "Photoexcitation published in 2010"
TL;DR: Attosecond pump–probe strategies are established as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born–Oppenheimer approximation.
Abstract: For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10(-15)-s) timescale. However, studying the inner workings of atoms and molecules on the electronic timescale has become possible only with the recent development of isolated attosecond (10(-18)-s) laser pulses. Such pulses have been used to investigate atomic photoexcitation and photoionization and electron dynamics in solids, and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H(2) and D(2) was monitored on femtosecond timescales and controlled using few-cycle near-infrared laser pulses. Here we report a molecular attosecond pump-probe experiment based on that work: H(2) and D(2) are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends-with attosecond time resolution-on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump-probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born-Oppenheimer approximation.
584 citations
TL;DR: It is shown that charge generation primarily occurs 2-10 ns after photoexcitation, which supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction.
Abstract: Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.
311 citations
TL;DR: In this article, the authors investigated the visible photon accelerated negative bias instability (NBI) in amorphous In-Ga-Zn-O (a-IGZO) thin film transistor (TFT).
Abstract: We investigated the visible photon accelerated negative bias instability (NBI) in amorphous In–Ga–Zn–O (a-IGZO) thin film transistor (TFT). As reported in previous works, the rigid shift in transfer curves with insignificant changes in field-effect mobility and subthreshold swing was observed. On the other hand, there is substantial change in capacitance-voltage characteristics caused by created subgap states. The suggested nature of created states is the ionized oxygen vacancy (VO2+) by the combination of visible light and negative bias. The generated VO2+ states enhance the NBI under illumination as increased deep hole trapping centers. Furthermore, the photoexcitation of VO to stable VO2+ yields excess free carriers in conduction band. The increased carrier density also enhances the negative shift in turn-on voltage of a-IGZO TFT.
302 citations
TL;DR: In this paper, the authors studied the relaxation dynamics of hot optical phonons in few-layer and multilayer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates.
Abstract: Using ultrafast optical pump-probe spectroscopy, we study the relaxation dynamics of hot optical phonons in few-layer and multilayer graphene films grown by epitaxy on silicon carbide substrates and by chemical vapor deposition on nickel substrates. In the first few hundred femtoseconds after photoexcitation, the hot carriers lose most of their energy to the generation of hot optical phonons which then present the main bottleneck to subsequent cooling. Optical phonon cooling on short time scales is found to be independent of the graphene growth technique, the number of layers, and the type of the substrate. We find average phonon lifetimes in the 2.5–2.55 ps range. We model the relaxation dynamics of the coupled carrier-phonon system with rate equations and find a good agreement between the experimental data and the theory. The extracted optical phonon lifetimes agree very well with the theory based on anharmonic phonon interactions.
283 citations
TL;DR: Colloidal manganese-doped semiconductor nanocrystals have been developed that show pronounced intrinsic high-temperature dual emission and this unique combination of properties makes this new class of intrinsic dual emitters attractive for ratiometric optical thermometry applications.
Abstract: Colloidal manganese-doped semiconductor nanocrystals have been developed that show pronounced intrinsic high-temperature dual emission. Photoexcitation of these nanocrystals gives rise to strongly temperature dependent luminescence involving two distinct but interconnected emissive excited states of the same doped nanocrystals. The ratio of the two intensities is independent of nonradiative effects. The temperature window over which pronounced dual emission is observed can be tuned by changing the nanocrystal energy gap during growth. This unique combination of properties makes this new class of intrinsic dual emitters attractive for ratiometric optical thermometry applications.
277 citations
TL;DR: High-yield triplet generation by singlet fission upon photoexcitation of a new aggregate of the carotenoid all-trans 3R,3'R-zeaxanthin is reported and it is revealed that triplets form within picoseconds.
Abstract: We report high-yield triplet generation by singlet fission upon photoexcitation of a new aggregate of the carotenoid all-trans 3R,3′R-zeaxanthin. The yield is determined by picosecond time-resolved resonance Raman spectroscopy, which allows direct characterization and quantification of triplet excited-state signatures and ground-state depletion. The technique and analysis reveals that triplets form within picoseconds. A quantum yield of 90−200% is derived with the assumption of weak exciton-coupling in the zeaxanthin aggregate.
206 citations
TL;DR: Simulation of the electronic structure and dynamics of the hydrated electron using a rigorously derived pseudopotential to treat the electron-water interaction resulted in a hydrated electrons that did not reside in a cavity but instead occupied a ~1-nanometer-diameter region of enhanced water density.
Abstract: Since the discovery of the hydrated electron more than 40 years ago, a general consensus has emerged that the hydrated electron occupies a quasispherical cavity in liquid water. We simulated the electronic structure and dynamics of the hydrated electron using a rigorously derived pseudopotential to treat the electron-water interaction, which incorporates attractive oxygen and repulsive hydrogen features that have not been included in previous pseudopotentials. What emerged was a hydrated electron that did not reside in a cavity but instead occupied a ~1-nanometer-diameter region of enhanced water density. Both the calculated ground-state absorption spectrum and the excited-state spectral dynamics after simulated photoexcitation of this noncavity hydrated electron showed excellent agreement with experiment. The relaxation pathway involves a rapid internal conversion followed by slow ground-state cooling, the opposite of the mechanism implicated by simulations in which the hydrated electron occupies a cavity.
204 citations
01 Apr 2010
TL;DR: In this article, the broadband optical conductivity and ultrafast carrier dynamics of epitaxial graphene in the few-layer limit were studied, and it was shown that the terahertz and near-infrared absorption of the material is dominated by excess hole carriers, with a 1.2-ps mono-exponential decay that refects the minority carrier recombination time.
Abstract: We study the broadband optical conductivity and ultrafast carrier dynamics of epitaxial graphene in the few-layer limit. Equilibrium spectra of nominally buffer, monolayer, and multilayer graphene exhibit significant terahertz and near-infrared absorption, consistent with a model of intra- and interband transitions in a dense Dirac electron plasma. Non-equilibrium terahertz transmission changes after photoexcitation are shown to be dominated by excess hole carriers, with a 1.2-ps mono-exponential decay that refects the minority-carrier recombination time.
201 citations
TL;DR: A new principle for controlling the coupling among mechanical, photonic, and electrical properties of ZnO nanowires is described, which could be potentially useful for fabricating piezo-phototronic devices.
Abstract: The localized coupling between piezoelectric and photoexcitation effects of a ZnO micro/nanowire device has been studied for the first time with the goal of designing and controlling the electrical transport characteristics of the device. The piezoelectric effect tends to raise the height of the local Schottky barrier (SB) at the metal−ZnO contact, while photoexcitation using a light that has energy higher than the band gap of ZnO lowers the SB height. By tuning the relative contributions of the effects from piezoelectricity via strain and photoexcitation via light intensity, the local contact can be tuned step-by-step and/or transformed from Schottky to Ohmic or from Ohmic to Schottky. This study describes a new principle for controlling the coupling among mechanical, photonic, and electrical properties of ZnO nanowires, which could be potentially useful for fabricating piezo-phototronic devices.
194 citations
TL;DR: In this paper, the energy levels involved in photogeneration and charge transport within conjugated polymers are investigated. But the authors focus on a specific case of poly(3-hexylthiophene) and do not consider the general applicability of their approach.
Abstract: There is an enormous potential in applying conjugated polymers in novel organic optoelectronic devices such as light-emitting diodes and solar cells. Although prototypes and first products exist, a comprehensive understanding of the fundamental processes and energetics involved during photoexcitation is still lacking and limits further device optimizations. Here we report on a unique analysis of the excited states involved in charge generation by photoexcitation. On the model system poly(3-hexylthiophene), we demonstrate the general applicability of our approach. From photoemission spectroscopy of occupied and unoccupied states, we determine the transport gap to 2.6 eV, which we show to be in agreement with the onset of photoconductivity by spectrally resolved photocurrent measurements. For photogenerated singlet exciton at the absorption edge, 0.7 eV of excess energy are required to overcome the binding energy; the intermediate charge-transfer state is situated only 0.3 eV above the singlet exciton. Our results give direct evidence of energy levels involved in the photogeneration and charge transport within conjugated polymers.
180 citations
TL;DR: In this paper, transient emission dynamics of CdSe/CdS/ZnS semiconductor nanocrystals using time-resolved photon counting have been investigated and it was shown that the size and intensity dependence of the observed decay dynamics seem inconsistent with those expected from Auger processes.
Abstract: The observed intermittent light emission from colloidal semiconductor nanocrystals has long been associated with Auger recombination assisted quenching. We test this view by observing transient emission dynamics of CdSe/CdS/ZnS semiconductor nanocrystals using time-resolved photon counting. The size and intensity dependence of the observed decay dynamics seem inconsistent with those expected from Auger processes. Rather, the data suggest that in the "off" state the quantum dot cycles in a three-step process: photoexcitation, rapid trapping, and subsequent slow nonradiative decay.
TL;DR: Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics of the charge-density wave in the Mott insulator 1T-TaS2, opening the way to study the nonequilibrium dynamics of condensed matter systems with full elemental, chemical, and atomic- site selectivity.
Abstract: Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics of the charge-density wave in the Mott insulator 1T-TaS2. After strong photoexcitation, a prompt loss of charge order and subsequent fast equilibration dynamics of the electron-lattice system are observed. On the time scale of electron-phonon thermalization, about 1 ps, the system is driven across a phase transition from a long-range charge ordered state to a quasiequilibrium state with domainlike short-range charge and lattice order. The experiment opens the way to study the nonequilibrium dynamics of condensed matter systems with full elemental, chemical, and atomic-site selectivity.
TL;DR: In this paper, the ultrafast electron relaxation dynamics of anionic and neutral Au25(SR)18 nanoclusters were investigated using broad-band time-resolved optical spectroscopy.
Abstract: The ultrafast electron relaxation dynamics of anionic and neutral Au25(SR)18 nanoclusters are investigated using broad-band time-resolved optical spectroscopy. From an analysis of the wavelength-dependent transient absorption kinetics, we have obtained valuable information on the spectral features that originate from excitation of “core” and “core−shell” states. In both clusters, photoexcitation occurs into two nondegenerate states near the HOMO−LUMO gap that are derived from the core orbitals. A large difference in the lifetime of the core excitations is observed, with [Au25(SR)18]− exhibiting a decay rate more than 1000 times slower than the neutral cluster. Both clusters show strong coupling to two different coherent phonon modes, which are observed at 2.4 and 1.2 THz. The electron−phonon coupling is analyzed in terms of the spectral distribution and damping of the coherent modes.
TL;DR: This work measures the anisotropic midinfrared response of electrons and phonons in bulk YBa(2)Cu(3)O(7-δ) after femtosecond photoexcitation and introduces a powerful concept probing electron-lattice interactions in a variety of complex materials.
Abstract: We measure the anisotropic midinfrared response of electrons and phonons in bulk YBa(2)Cu(3)O(7-δ) after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs, demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials.
TL;DR: In this article, the effect of temperature on the quantum yield for charge carrier photogeneration in P3HT−PCBM blend films was studied using ultrafast transient absorption and microwave photoconductance techniques.
Abstract: The effect of temperature on the quantum yield for charge carrier photogeneration in P3HT−PCBM blend films was studied using ultrafast transient absorption and microwave photoconductance techniques. The quantum yield was found to be virtually independent of temperature for time scales up to tens of nanoseconds after photoexcitation of P3HT. Implications of this observation for the mechanism of free charge carrier generation are discussed. The decay of charges due to recombination and/or trapping on longer times becomes faster at higher temperature, as a result of thermally activated electron and hole mobilities. The magnitude of the quantum yield depends on the morphology of the blend film, which is determined by the spin-coating solvent and annealing conditions.
TL;DR: In this paper, the analysis of time-resolved photoluminescence data in the context of nanocrystal dynamics is discussed and the idea that dynamical processes might imprint easily identified signatures into the decays is introduced.
Abstract: It is becoming increasingly evident that to exploit nanocrystals in light-harvesting applications requires a high-level understanding of the interactions that link exciton states with surface states and the surrounding environment. Recent research has established time-resolved photoluminescence as a quantitative tool for the analysis of photoexcitation dynamics in colloidal semiconductor nanocrystals. Here, we discuss the analysis of time-resolved photoluminescence data in the context of nanocrystal dynamics. We introduce the idea that dynamical processes might imprint easily identified signatures into the decays and we suggest future work in the field.
TL;DR: In this article, photo-induced processes in phthalocyanine-functionalized gold nanoparticles (Pc-AuNPs) have been investigated by spectroscopic measurements.
Abstract: Photoinduced processes in phthalocyanine-functionalized gold nanoparticles (Pc-AuNPs) have been investigated by spectroscopic measurements. The metal-free phthalocyanines used have two linkers with thioacetate groups for bonding to the gold nanoparticle surface, and the attachment was achieved using a ligand exchange reaction. The absorption spectrum of the Pc-AuNPs shows a broadening of the phthalocyanine Q-band absorption, probably due to a tight packing of the phthalocyanines on the gold nanoparticle surface. For the attached phthalocyanines, fluorescence is strongly quenched, and the fluorescence lifetimes determined by time-correlated single photon counting (TCSPC) are strongly reduced. The quenching mechanisms were studied in detail with time-resolved absorption (pump−probe) measurements. A selective excitation of the gold cores in the pump−probe experiment results in an energy transfer from the gold nanoparticles to the attached phthalocyanines in ∼2.4 ps. Photoexcitation of mainly the phthalocyani...
TL;DR: In this article, the authors describe studies aimed at effective excitation of molecules by localized surface plasmon polaritons and present several examples of improvements in the efficiencies of photochemical and photoelectrochemcal systems to overcome challenges for enhancement of photoenergy conversion in future.
Abstract: This Perspective describes studies aimed at effective excitation of molecules by localized surface plasmon polaritons Recently developed bottom-up and top-down techniques allow the controlled fabrication of well-defined metal structures exhibiting desirable localization of plasmon energy Under certain conditions, molecules display unique florescence and Raman scattering behavior in such localized fields, suggesting selective resonant excitation of specific electronic/vibrational modes Finally, several examples of improvements in the efficiencies of photochemical and photoelectrochemcal systems are briefly discussed to find a way to overcome challenges for enhancement of photoenergy conversion in future
TL;DR: Calculations for vibrational spectra of riboflavin, the parent molecule of biological blue-light receptor chromophores, in its electronic ground and lowest singlet excited states are presented, both in frequency and time domains.
Abstract: The photophysics and photochemistry of flavin molecules are of great interest due to their role for the biological function of flavoproteins. An important analysis tool toward the understanding of the initial photoexcitation step of flavins is electronic and vibrational spectroscopy, both in frequency and time domains. Here we present quantum chemical [(time-dependent) density functional theory ((TD-)DFT)] calculations for vibrational spectra of riboflavin, the parent molecule of biological blue-light receptor chromophores, in its electronic ground (S(0)) and lowest singlet excited states (S(1)). Further, vibronic absorption spectra for the S(0) --> S(1) transition and vibronic emission spectra for the reverse process are calculated, both including mode mixing. Solvent effects are partially accounted for by using a polarizable continuum model (PCM) or a conductor-like screening model (COSMO). Calculated vibrational and electronic spectra are in good agreement with measured ones and help to assign the experimental signals arising from photoexcitation of flavins. In particular, upon photoexcitation a loss of double bond character in the polar region of the ring system is observed which leads to vibronic fine structure in the electronic spectra. Besides vibronic effects, solvent effects are important for understanding the photophysics of flavins in solution quantitatively.
TL;DR: The finding that the heat capacity of gold nanoparticles can be changed upon UV laser excitation is important for understanding the fundamental nature of noble metal nanoparticles.
Abstract: An in situ spectroscopic study of the nanosecond laser-induced melting and size reduction of pseudospherical gold nanoparticles with 54 ± 7 nm diameter allowed the observation of a heating efficiency that was very dependent on the excitation wavelength. A remarkably greater efficiency was observed for the photothermal effect of interband excitation than that of intraband excitation. This noteworthy observation is ascribed to an altered electron heat capacity, ce, during photoexcitation depending on the excitation energy, which is a phenomenon that has not been realized previously. As a result, a 60% reduction of the specific heat capacity, cp, compared to that of bulk gold was obtained for interband excitation at 266 nm whereas the cp value for the excitation of the intraband transition at 532 nm was unaltered. A semiquantitative explanation was given for this striking phenomenon induced by interband excitation in which excitation−relaxation cycles of electrons upon excitation of 5d electrons to the 6sp b...
TL;DR: In this paper, the above-bandgap illumination of colloidal ZnO nanoparticles (NPs) in ethanol solutions is found to lead to reversible shifts of the absorption and photoluminescence (PL) excitation spectra, indicating charging of the nanoparticles with electrons.
Abstract: The above-bandgap illumination of colloidal ZnO nanoparticles (NPs) in ethanol solutions is found to lead to reversible shifts of the absorption and photoluminescence (PL) excitation spectra, indicating charging of the nanoparticles with electrons. A rapid drop of deep-level PL intensity at the early stage of illumination is observed simultaneously with the splitting-off and growth of a new red-shifted near-band-edge PL band. Such a splitting of the near-bandgap PL band under illumination is observed for the first time and corroborates with the previous assumptions about the behavior of the NP ensemble emission upon gradual NPs’ charging with electrons. The possible relation between the new PL band and photoinduced charging of NPs with excess electrons is discussed on the basis of the dependence of the PL spectrum evolution and absorption band shift relaxation on the NP size and controllable access of oxygen during illumination.
TL;DR: In this paper, the ultraviolet photodissociation of amorphous water ice at different ice temperatures was investigated using molecular dynamics simulations and analytical potentials, and the effect of ice temperature on UV photodiissociation processes was analyzed.
Abstract: The ultraviolet (UV) photodissociation of amorphous water ice at different ice temperatures is investigated using molecular dynamics (MD) simulations and analytical potentials. Previous MD calculations of UV photodissociation of amorphous and crystalline water ice at 10 K [S. Andersson et al., J. Chem. Phys. 124, 064715 (2006)] revealed—for both types of ice—that H atom, OH, and H2O desorption are the most important processes after photoexcitation in the uppermost layers of the ice. Water desorption takes place either by direct desorption of recombined water, or when, after dissociation, an H atom transfers part of its kinetic energy to one of the surrounding water molecules which is thereby kicked out from the ice. We present results of MD simulations of UV photodissociation of amorphous ice at 10, 20, 30, and 90 K in order to analyze the effect of ice temperature on UV photodissociation processes. Desorption and trapping probabilities are calculated for photoexcitation of H2O in the top four monolayers ...
TL;DR: In this paper, the use of Raman spectroscopy, in concert with density functional theory, as a strategy for understanding excited-state structure in metal polypyridyl complexes is discussed.
TL;DR: These model studies illustrate how the coupling between the electron-proton transfer and the solvent dynamics can be tuned by altering the solute and solvent properties.
Abstract: A theoretical formulation for modeling photoinduced nonequilibrium proton-coupled electron transfer (PCET) reactions in solution is presented. In this formulation, the PCET system is described by donor and acceptor electron-proton vibronic free energy surfaces that depend on a single collective solvent coordinate. Dielectric continuum theory is used to obtain a generalized Langevin equation of motion for this collective solvent coordinate. The terms in this equation depend on the solvent properties, such as the dielectric constants, relaxation time, and molecular moment of inertia, as well as the solute properties characterizing the vibronic surfaces. The ultrafast dynamics following photoexcitation is simulated using a surface hopping method in conjunction with the Langevin equation of motion. This methodology is used to examine a series of model photoinduced PCET systems, where the initial nonequilibrium state is prepared by vertical photoexcitation from the ground electronic state to the donor electronic state. Analysis of the dynamical trajectories provides insight into the interplay between the solvent dynamics and the electron-proton transfer for these types of processes. In addition, these model studies illustrate how the coupling between the electron-proton transfer and the solvent dynamics can be tuned by altering the solute and solvent properties.
TL;DR: In this paper, strong asymmetric field-emission intensity distributions are observed depending on three parameters: (i) the polarization of the light, (ii) the azimuthal, and (iii) the polar orientation of the tip apex relative to the laser incidence direction.
Abstract: Field-emission patterns from a clean tungsten tip apex induced by femtosecond laser pulses have been investigated. Strongly asymmetric field-emission intensity distributions are observed depending on three parameters: (i) the polarization of the light, (ii) the azimuthal, and (iii) the polar orientation of the tip apex relative to the laser incidence direction. In effect, we have realized an ultrafast pulsed field-emission source with site selectivity of a few tens of nanometers. Simulations of local fields on the tip apex and of electron emission patterns based on photoexcited nonequilibrium electron distributions explain our observations quantitatively. Electron emission processes are found to depend on laser power and tip voltage. At relatively low laser power and high tip voltage, field-emission after two-photon photoexcitation is the dominant process. At relatively low laser power and low tip voltage, photoemission processes are dominant. As the laser power increases, photoemission from the tip shank becomes noticeable.
© 2010 The American Physical Society
TL;DR: In this article, the authors investigated the mechanisms of rapid solidification of a nanoscale region of a metal film transiently melted by a localized photoexcitation in a large-scale atomistic simulation.
Abstract: The extreme cooling rates in material processing can be achieved in a number of current and emerging femtosecond laser techniques capable of highly localized energy deposition. The mechanisms of rapid solidification of a nanoscale region of a metal film transiently melted by a localized photoexcitation are investigated in a large-scale atomistic simulation. The small size of the melted region, steep temperature gradients, and fast two-dimensional electron heat conduction result in the cooling rate exceeding 1013 K/s and create conditions for deep undercooling of the melt. The velocity of the liquid/crystal interface rises up to the maximum value of ∼80 m/s during the initial stage of the cooling process and stays approximately constant as the temperature of the melted region continues to decrease. When the temperature drops down to the level of ∼0.6Tm, a massive homogeneous nucleation of the crystal phase inside the undercooled liquid region takes place and prevents the undercooled liquid from reaching th...
TL;DR: The authors' observations of the pH-dependent photocurrent are consistent with previous reports in a bR thin film configuration, which also showed a polarity inversion at pH 5-6.
TL;DR: In this article, a review on random lasing properties of organic nanofibers made of oligophenyl nanocrystals grown by molecular epitaxy on polar substrates is presented.
Abstract: We present a review on random lasing in organic nanofibers made of oligophenyl nanocrystals grown by molecular epitaxy on polar substrates. The nanofibers have sub-wavelength cross-sectional dimensions and can extend in length up to the millimeter scale. We report on random lasing properties of nanofibers, under subpicosecond photopumping, both in the coherent and incoherent regimes. With the aid of both optical and morphological studies on individual fibers, we get insight into one-dimensional coherent feedback taking place along the nanofibers' axes. Model calculations of light propagation in disordered media allow us to give a semiquantitative description of one-dimensional coherent random lasing near the lasing threshold. We also report on amplified simulated emission in individual nanofibers and demonstrate that nanoscale linear optical amplifiers can be obtained by molecular self-assembly at surfaces. Photophysical studies of nanofibers resorting to subpicosecond luminescence and pump–probe spectroscopy give us valuable information on temperature-dependent, excited-state nonlinear processes, such as exciton–exciton annihilation and photoinduced absorption. Excited-state effects strongly influence lasing thresholds under quasi-continuous-wave photoexcitation conditions, as demonstrated in photoexcitation experiments performed with nanosecond pulses. Last, we briefly discuss the potential of organic epitaxial nanofibers featuring low-threshold random lasing for photonic sensing applications.
TL;DR: In this paper, micro-and nano-structuring of a silver-containing zinc phosphate glass under high repetition rate femtosecond near-infrared laser exposure is reported.
Abstract: We report on the micro- and nano-structuring of a silver-containing zinc phosphate glass under high repetition rate femtosecond near-infrared laser exposure. Luminescent silver clusters are locally formed thanks to multi-photon absorption. The excitation mechanisms in the glass are investigated with a transient absorption pump-probe experiment. The free electron density of the femtosecond-laser-induced ionized material for irradiation conditions leading to structural modifications is measured. We show that the involved photo-excitation process in the laser–glass interaction is a four-photon absorption and the measured free electron density is on the order of 1017 cm− 3, four orders of magnitude below the critical electron density. The luminescence properties of these resulting structures have been investigated. Emission spectra are compared with those collected after different irradiations (γ and electron beams). The migration of silver species has been assigned to be responsible for local modifications and selective acid etching behavior of the structure.
TL;DR: Each of the platinum-containing organometallic dimer complexes exhibits efficient nonlinear absorption of nanosecond pulses in the near-infrared region (600-800 nm), and it is demonstrated that effect is most efficient in the chromophores where the 2PA cross section maxima coincides spectrally with the excited triplet state absorption.
Abstract: A series of platinum-containing organometallic dimer complexes has been synthesized and the photophysical properties have been investigated under one- and two-photon (2PA) absorption conditions. The complexes have the general structure [DPAF−C≡C−Pt(PBu3)2−C≡C−Ar−C≡C−Pt(PBu3)2−C≡C−DPAF], where Ar is a π-conjugated unit, Bu = n-butyl, and DPAF = diphenylamino-2,7-fluorenylene. The core Ar units include 1,4-phenylene, 2,5-thienylene, 5,5′-(2,2′-bithienylene), 2,5-(3,4-ethylenedioxythiophene, 2,1,3-benzothiadiazole, and 4,7-dithien-2-yl-2,1,3-benzothiadiazole. Absorption and photoluminescence spectroscopy indicates that the complexes feature low-lying excited states based on both the core [-Pt(PBu3)2−C≡C−Ar−C≡C−Pt(PBu3)2-] chromophore as well as the DPAF units. Photoexcitation of the complexes produces a singlet state excited state, which rapidly undergos intersystem crossing to afford a triplet state that has a lifetime in the microsecond time domain. In most cases, the lowest energy triplet state is localiz...