Stark spectroscopy of excited-state transitions in a conjugated polymer

Stark spectroscopy, which is well established for probing transitions between the ground and excited states of many material classes, is extended to transitions between transient excited states. To this end, it is combined with femtosecond pump-probe spectroscopy on a conjugated polymer with appropriately introduced traps which harvest excitation energy and build up a sufficient excited state population. The results indicate a significant difference in the effective dipole moments between two short lived excited states.     

Excitonic relaxation in a conjugated polymer: Initial coherence beyond energy-dispersive population transfer

In this contribution, we deal with the pathways of femtosecond excitonic relaxation in conjugated polymers, focusing on the prominent, ground-state nondegenerate model system poly(p-phenylenevinylene) (PPV). After a brief discussion of the molecular exciton picture of optical excitations in organic semiconductors, we present exemplary results, categorized in two regimes. On early picosecond time scales, the dynamics of incoherent population relaxation is investigated by combining selectively tuned femtosecond excitation pulses with the technique of fluorescence up-conversion. As an initially prepared excitonic distribution gradually lowers its energy by diffusion to the bottom of the density of states (DOS), an excitation energy-dependent bathochromic shift dynamics is observed, accompanied by spectral intensity rearrangements. Motivated by the perception that intersite electronic coupling is absent in the lowest energy regime of the DOS, we further devise a strategy that enables us to measure the very early electronic oscillations and their quantum-stochastic relaxation within approximately the first 200 fs. By using femtosecond wave packet interferometry with appropriately tuned but otherwise freely propagating pulses, we observe fluorescence interferograms with strongly damped, low-frequency beatings, which stem from the spatial interference of two wave packets launched by two-pulse excitation. The results can be explained, semiquantitatively, in terms of a second-order perturbational approach and open up a new perspective on the complex puzzle of PPV optical dynamics.     

Excitonic effects and optical properties of passivated CdSe clusters

We calculate the optical properties of a series of passivated nonstoichiometric CdSe clusters using two first-principles approaches: time-dependent density functional theory within the local-density approximation, and by solving the Bethe-Salpeter equation for optical excitations with the GW approximation for the self-energy. We analyze the character of optical excitations leading to the first low-energy peak in the absorption cross section of these clusters. Within time-dependent density functional theory, we find that the lowest-energy excitation is mostly a single-level to single-level transition. In contrast, many-body methods predict a strong mixture of several different transitions, which is a signature of excitonic effects. The majority of the clusters have a series of dark transitions before the first bright transition. This may explain the long radiative lifetimes observed experimentally.     

Coherent control of optical emission from a conjugated polymer

We have observed that resonant Rayleigh scattering dominates the emission from poly(p-phenylene vinylene) excited with photons at energies below the threshold at which excitonic migration is reduced. The intensity of the resonant emission decays exponentially with a lifetime of up to 450 fs after pulsed excitation. The coherent nature of the emission was confirmed by angular variations in the far-field emission intensity-bright and dark speckles. Persistence of a coherent polarization was demonstrated by coherent control using phase-locked pulses.     

Excitonic effects and optical spectra of single-walled carbon nanotubes

Many-electron effects often dramatically modify the properties of reduced dimensional systems. We report calculations, based on an ab initio many-electron Green's function approach, of electron-hole interaction effects on the optical spectra of small-diameter single-walled carbon nanotubes. Excitonic effects qualitatively alter the optical spectra of both semiconducting and metallic tubes. Excitons are bound by approximately 1 eV in the semiconducting (8,0) tube and by approximately 100 meV in the metallic (3,3) tube. These large many-electron effects explain the discrepancies between previous theories and experiments.     

共轭高分子链中电荷迁移的热效应

从理论上研究了共轭高聚物链中在电场作用下极化子运动的热效应.基于SSH模型以及通过绝热动力学演化的方法,模拟了共轭高聚物链中极化子在电场作用下从链左端向右端运动的过程.晶格受到的热扰动作用假设为通过局域的晶格范围内原子位移的随机涨落来实现.结果发现,晶格中的局域热涨落对于运动中的极化子而言等效于一个势垒.势垒高度由高分子中受到热扰动的区域的范围大小以及该区域与其周围环境的温差来决定.当分子中存在热吸收不均匀的现象时,链内极化子迁移率在低电场范围内随电场的变化遵循对数曲线变化规律.     

Nonlinear optical and optical power limiting studies on a new thiophene-based conjugated polymer in solution and solid PMMA matrix

An experimental investigation of the third-order nonlinear optical properties of new poly{2-{5-[3,4-ditetradecyloxy-5-(1,3,4-oxadiazol-2-yl)thiophen-2-yl]-1,3,4-oxadiazol-2-yl}pyridine} (P) in tetrahydrofuran (THF) solution and in solid poly(methylmethacrylate) (PMMA) matrix, by Z-scan technique is reported. The Z-scan traces reveal that the composite films exhibit large negative nonlinear refractive index of the order 10⁻¹⁰ esu. The excited-state absorption cross-section was found to be larger than the ground-state absorption indicating that the operating nonlinear process is reverse saturable absorption (RSA). The new polymer P exhibits good optical power limiting properties in the nanosecond regime in solution and as well in solid PMMA matrix.     

Proton radiation effects of conjugated polymer thin films

Polymeric-thin-film-based electronic and optoelectronic materials and devices are attractive for potential space and certain radiation-related applications due to their inherent features such as lightweight, flexible, biocompatible, etc. Proton radiation is a major form of ionizing radiation in space, particularly in the so-called inner Van Allen belt region where most near-earth satellites are orbiting, yet very few literature and data are available on proton radiation effects on conjugated polymer systems. In this report, the proton radiation effects on potential electronic/optoelectronic properties of several conjugated polymers and their composites are briefly evaluated. Specifically, UV–VIS absorption spectra of several conjugated polymers and/or their composite thin films were measured and compared right before and after the proton radiations at different dosages. The results revealed that proton radiation has very little or insignificant impact up to 800?rad on the optoelectronic properties of poly-3-hexyl-thiophene (P3HT), P3HT:PC₆₀BM blend, a light harvesting donor-bridge-acceptor (DBA) and a novel donor-bridge-fluorinated-acceptor (DBfA)-type block copolymer thin films.     

Excitonic effects in core-excitation spectra of semiconductors

Computer simulations of a model glass-forming system are presented, which study the correlation between the dynamics in real space and the topography of the potential energy landscape. This analysis clearly reveals that in the supercooled regime the dynamics is strongly influenced by the presence of deep valleys in the energy landscape, corresponding to long-lived metastable amorphous states. We explicitly relate nonexponential relaxation effects and dynamic heterogeneities to these metastable states and thus to the specific topography of the energy landscape.     

Positron annihilation studies of a conjugated polymer

Lineshapes of the Doppler broadened annihilation radiation (DBAR) of a conjugated polymer polydiacetylene, 4-butoxycarbonylmethylurethane (P-4BCMU), in its amorphous state, were measured over the temperature range of 30 to 190°C. A differential-scanning-calorimeter (DSC) was used to correlate the transition temperatures. The DBAR lineshapes show a distinct change during the structural transitions of the polymer. A two states trapping model is used to qualitatively describe the temperature dependence of the lineshape. The nature of formation of the trapped states of positrons above T_c, through the interplay of electronic structure/conformation change, is discussed.     

Excitonic optical absorption in semiconductors under intense terahertz radiation

The excitonic optical absorption of GaAs bulk semiconductors under intense terahertz （THz） radiation is investigated numerically. The method of solving initial-value problems, combined with the perfect matched layer technique, is used to calculate the optical susceptibility. In the presence of a driving THz field, in addition to the usual exciton peaks, 2p replica of the dark 2p exciton and even-THz-photon-sidebands of the main exciton resonance emerge in the continuum above the band edge and below the main exciton resonance. Moreover, to understand the shift of the position of the main exciton peak under intense THz radiation, it is necessary to take into consideration both the dynamical Franz-Keldysh effect and ac Stark effect simultaneously. For moderate frequency fields, the main exciton peak decreases and broadens due to the field-induced ionization of the excitons with THz field increasing. However, for high frequency THz fields, the characteristics of the exciton recur even under very strong THz fields, which accords with the recent experimental results qualitatively.     

Mesoscale structures in luminescent conjugated polymer thin films studied by near-field scanning optical microscopy

We report the studies of various conjugated polymer thin films with near-field scanning optical microscopy, NSOM. Firstly, it is shown that MEH-PPV thin film undergoes significant changes in film morphology upon thermal annealing. The once homogeneous morphology becomes inhomogeneous after annealing. Secondly, polarization near-field measurements reveal mesoscale polymer ordering in PPV thin film. The average domain size and the coefficient for linear dichroism were studied as a function of film thickness. Finally, phase separation in polymer blend film was directly observed by transmission NSOM. Time-resolved fluorescence spectra indicate that the phase domains are decomposed of different fractions of the two constituent polymers. The near-field optical microscopy was also used to write lithographic patterns with a resolution of 100 nm, exceeding the diffraction limit.     

GaN基量子阱的激子跃迁和光学性质

采用光致发光谱、光致发光激发谱以及拉曼光谱对GaN基量子阱材料进行了实验观察和分析 .实验结果表明样品中量子点结构不均匀及InGaN层中In成分分布不均匀 ,且其光致发光谱的波峰是由自由激子辐射复合发光引起的 .同时由室温下InGaN/GaN量子阱的拉曼谱可得知InGaN/GaN多量子阱的结构特征     

共轭高分子中激子演化过程的非绝热动力学研究

有机共轭高分子受光激发或被电荷掺杂后可能会产生各种激发状态的激子,激子的演化过程对有机发光光谱有着至关重要的影响.通过非绝热动力学演化的方法模拟了受光激发后有机高分子中激子驰豫的动力学过程,结果表明高激发态激子不稳定,由于电声耦合作用,高激发态激子会持续向低激发态激子演化,同时,低激发态激子的复合发光会发生红移.稳定的激子复合发光光谱中,基态激子发光强度最大,可高达70-80%;第一激发态激子及其它激发态激子发光强度的总和不超过20%.     

Excitonic effects in diluted magnetic semiconductor nanostructures

Excitonic properties and the dynamics are reported in quantum dots (QDs) and quantum wells (QW) of diluted magnetic semiconductors. Transient spectroscopies of photoluminescence and nonlinear-optical absorption and emission have been made on these quantum nanostructures. The Cd_1−x Mn_xSe QDs show the excitonic magnetic polaron effect with an increased binding energy. The quantum wells of the Cd_1−x Mn_xTe/ZnTe system display fast energy and dephasing relaxations of the free and localized excitons as well as the tunneling process of carriers and excitons in the QWs depending on the barrier widths. The observed dynamics and the enhanced excitonic effects are the inherent properties of the diluted magnetic nanostructures. Fiz. Tverd. Tela (St. Petersburg) 40, 846–848 (May 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.