Coherent excitons at different orientation arrangements of local transition dipole moments in circular light-harvesting complexes

The coherent exciton plays an important role in the photosynthetic primary process, and its functions are deeply dependent on the orientation arrangements of local transition dipole moments (TDMs). We theoretically and systematically study the physical property of the coherent exciton at different orientation arrangements of the local TDMs in circular light-harvesting (LH) complexes. Especially, if the orientation arrangements are different, the delocalized TDMs of the coherent excitons and the energy locations of the optically active coherent excitons (OACEs) can be obviously different, and then there are more manners to capture, store and transfer light energy in and between LH complexes. Similarly, if the orientation arrangements are altered, light absorption and radiative intensities can be converted fully between the OACEs in the upper and lower coherent exciton bands, and then the blue and red shifts of the absorption and radiative bands of the pigment molecules can occur simultaneously at some orientation arrangements. If the systems are in the vicinities of the critical orientation arrangements, the weak static disorder or small thermal excitation can destroy the coherent electronic excitations, and then the coherent exciton cannot exist any more.     

Sustainability of Environment‐Assisted Energy Transfer in Quantum Photobiological Complexes

It is shown that quantum sustainability is a universal phenomenon which emerges during environment‐assisted electronic excitation energy transfer (EET) in photobiological complexes (PBCs), such as photosynthetic reaction centers and centers of melanogenesis. We demonstrate that quantum photobiological systems must be sustainable for them to simultaneously endure continuous energy transfer and keep their internal structure from destruction or critical instability. These quantum effects occur due to the interaction of PBCs with their environment which can be described by means of the reduced density operator and effective non‐Hermitian Hamiltonian (NH). Sustainable NH models of EET predict the coherence beats, followed by the decrease of coherence down to a small, yet non‐zero value. This indicates that in sustainable PBCs, quantum effects survive on a much larger time scale than the energy relaxation of an exciton. We show that sustainable evolution significantly lowers the entropy of PBCs and improves the speed and capacity of EET.     

叶绿素a、b分子体系激子相干迁移的时间特性

本文利用激发能传递的广义主方程理论和光谱实验结果,首次研究了相同和相异叶绿素分子体系的激子相干迁移的时间特性.Chla分子间的相干时间为350fs,Chlb分子间为160fs,Chla与Chlb分子间为200fs.主要叶绿素分子间的激子相干迁移属弛豫前转移过程.并分析了该迁移的作用及影响相干时间长短的主要因素.     

Energy transfer between two aggregates in light-harvesting complexes

Energy transfer processes between two aggregates in a coupled chromophoric-pigment(protein)system are studied via the standard master equation approach.Each pigment of the two aggregates is modeled as a two-level system.The excitation energy is assumed to be transferred from the donor aggregate to the acceptor aggregate.The model can be used to theoretically simulate many aspects of light-harvesting complexes(LHCs).By applying the real bio-parameters of photosynthesis,we numerically investigate the efficiency of energy transfer(EET)between the two aggregates in terms of some factors,e.g.,the initial coherence of the donor aggregate,the coupling strengthes between the two aggregates and between different pigments,and the effects of noise from the environment.Our results provide evidence for that the actual numbers of pigments in the chromophoric rings of LHCs should be the optimum parameters for a high EET.We also give a detailed analysis of the effects of noise on the EET.     

分子聚集体中激子-激子湮灭过程

分子的激发能量转移和电荷转移是提高光伏电池和发光二极管效率的关键问题,其中分子聚集体中的激子-激子湮灭过程是影响分子激发能量转移的重要方面,细致研究激子-激子湮灭的动力学过程并与相关的瞬间吸收谱信号对比对相关的理论和实验都有重要意义.本文在分子间弱耦合近似下,用经典的率方程,应用方酸分子的基本参数对激子-激子湮灭过程做了微观描述,通过改变相关参数,研究了外场激发强度、聚集体的偶极矩位形、分子内的衰变率等因素对激子-激子湮灭过程的影响,分析了激子在第一激发态和高阶激发态的驰豫时间、电荷转移相干时间、激子融合和湮灭时间之间的关系,得到的结论适用于高阶激发态能级能量约为第一激发态能级能量的2倍的分子组成的分子聚集体.研究发现,J型聚集体由于相干能量转移时间较短,比H型聚集体有更高的湮灭率.激发场强越强,激子-激子湮灭的效率越高.分子高阶激发态的衰变率是激子-激子湮灭过程的关键因素.     

Anomalous energy transfer dynamics due to torsional relaxation in a conjugated polymer

In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scale-exciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation. In order to resolve this question we perform femtosecond time-resolved transient absorption measurements of the energy relaxation of poly[3-(2,5-dioctylphenyl)thiophene] in toluene solution. We show that torsional relaxation can be distinguished from EET by site-selectively exciting low-energy conjugated segments. We present a unified model that integrates EET and torsional dynamics. In particular, comparison to ultrafast depolarization measurements shows that torsional dynamics cannot be neglected when analyzing EET dynamics and furthermore reveals that the exciton extends itself by about 2 monomer units during torsional relaxation.     

General properties of nuclear resonant scattering

The process of nuclear resonant scattering resonant scattering is considered on the basis of an optical model. The coherent properties coherent properties of the radiation and scattering mechanism are described. The complementary pictures of γ-ray resonant scattering in energy and time domains are presented. Special attention is paid to scattering of a γ quantum by an ensemble of nuclei. The central concept of the theory of nuclear resonant scattering, the nuclear exciton, nuclear exciton as a delocalized nuclear excitation, is described in detail. It is shown that both temporal and spatial aspects of coherence play a crucial role in the evolution of the nuclear exciton. A large place is given to the analysis of resonant scattering of synchrotron radiation by nuclear ensembles.

     

Non-radiative exciton recombination through excitation energy transfer in quantum dot clusters

Excitation energy transfer (EET) processes in CdSe/CdZnS quantum dot (QD) clusters have been investigated in this study by measuring their time-resolved and spectrally resolved fluorescence intensities. The contributions of radiative and non-radiative exciton recombination through EET are evaluated, where the latter is expected to occur in a large class of QD ensembles because of the presence of nonluminescent QDs. It appears that the fluorescence decay in larger QDs serving as acceptor does not show an initial rise, in addition the lifetime of the acceptor QD is independent of the excitation wavelength, suggesting that an EET is followed mostly by non-radiative recombination.     

Molecular motion under the influence of a coherent infrared-laser field

The quantum dynamics of molecular vibrations under the influence of coherent infrared-laser multiphoton excitation is irregular, in most cases, on the time scale of experimental interest. This irregularity arises from the anharmonic nature of the molecular force fields and is already present in one dimensional anharmonic oscillators. We analyse the onset of the irregularity in the model of a Morse oscillator and discuss the role of statistical or “chaotic” behaviour of the time dependent probability density. We further discuss the role of direct multiphoton excitation in these multilevel systems and compare the results with exact solutions for the harmonic oscillator.     

Direct-current induced dynamics in Co90 Fe10/Ni80 Fe20 point contacts

We have directly measured coherent high-frequency magnetization dynamics in ferromagnetic films induced by a spin-polarized dc current. The precession frequency can be tuned over a range of several gigahertz by varying the applied current. The frequencies of excitation also vary with applied field, resulting in a microwave oscillator that can be tuned from below 5 to above 40 GHz. This novel method of inducing high-frequency dynamics yields oscillations having quality factors from 200 to 800. We compare our results with those from single-domain simulations of current-induced dynamics.     

Dynamics of resonant energy transfer in a cold Rydberg gas

We investigate excitation transfer and migration processes in a cold gas of rubidium Rydberg atoms. Density-dependent measurements of the resonant population exchange for atoms initially excited into the 32P_3/2(|m_J|=3/2) state are compared with a Monte Carlo model for coherent energy transfer. The model is based on simulations of small atom subensembles involving up to ten atoms interacting via coherent pair processes. The role of interatomic mechanical forces due to the resonant dipole-dipole interaction is investigated. Good agreement is found between the experimental data and the predictions of the model, from which we infer that atomic motion has negligible influence on the energy transfer up to Rydberg densities of 10⁸ cm^-3, that the system has to be described in terms of many-body dynamics, and that the energy transfer preserves coherence on microsecond timescales.     

Ab initio calculations of exciton effects in optical spectra of an α-B<Subscript>12</Subscript> crystal

The dielectric functions and energy loss spectrum of electrons of a rhombohedral α-B₁₂ crystal are studied both in the single-particle and many-particle approximations using Bethe–Salpeter equations. The opposite roles of different contributions to exciton effects are discussed. The anisotropy of dielectric functions is shown, which demonstrates their high sensitivity with respect to the three-dimensional packing of icosahedrons. The influence of the coherent mixing of electronic and exciton states on the redistribution of oscillator strengths is found. The position of the plasmon of valence electrons and the high-frequency permittivity are found to be consistent with experimental data. The correlation between the distribution of oscillator strengths and features of the density of electronic states is discussed.     

量子相干态的二维电子光谱测量的原理、应用和发展

二维电子光谱是一种同时具有高的时间分辨率和光谱分辨率的非线性光谱学方法.它不仅可以对凝聚相分子复杂动力学过程进行直接测量,还可以测量不同电子态、电子态-振动态之间的量子相干过程.2007年,Flemming课题组利用二维电子光谱于低温77 K的条件下在捕光天线蛋白Fenna-Matthews-Olson中发现了能量传递过程存在量子相干现象.尽管后续的实验研究表明,该体系中实验观测到的量子相干现象不可能是由单纯的电子态相干引起的,然而这一实验现象的报道极大地激发了人们对天然或人工模拟光合系统中存在量子相干传能途径的探索,目前还是一个相当活跃的研究领域.本文旨在通过介绍二维电子光谱学原理、装置及其在光合作用体系能量传递中量子相干现象的应用,使二维电子光谱这种实验方法能够在更多的研究领域得以普及与推广.     

Quantum algorithm for distributed clock synchronization

We study theoretically the coherent nonlinear optical response of doped quantum wells with several subbands. When the Fermi energy approaches the exciton level of an upper subband, the absorption spectrum acquires a characteristic double-peak shape originating from the interference between the Fermi-edge singularity and the exciton resonance. We demonstrate that, for off-resonant pump excitation, the pump-probe spectrum undergoes a striking transformation, with a time-dependent exchange of oscillator strength between the Fermi-edge singularity and exciton peaks. This effect originates from the many-body electron-hole correlations which determine the dynamical response of the Fermi sea.     

Mechanism of THz emission from asymmetric double quantum wells

Impulsive interband optical excitation of the lowest two conduction subbands of a suitably engineered GaAs/AlGaAs double quantum well can lead to coherent THz emission. We demonstrate that, contrary to previous expectations, the dominant emission mechanism can involve the beating of either continuum or exciton states, depending on excitation conditions. The coherence of the continuum beats persists for several picoseconds even for excitation an optical phonon energy above the band edge. We attribute this to the small energy difference between the component eigenstates, which substantially reduces the number of relevant scattering events.     

Coherent exciton-plasmon interaction in the hybrid semiconductor quantum dot and metal nanoparticle complex

We studied theoretically the exciton coherent dynamics in the hybrid complex composed of CdTe quantum dot (QDs) and rodlike Au nanoparticles (NPs) by the self-consistent approach. Through adjusting the aspect ratio of the rodlike Au NPs, the radiative rate of the exciton and the nonradiative energy transfer rate from the QD to the Au NP are tunable in the wide range 0.05-4 ns(-1) and 4.4 x 10(-4) to 2.6 ns(-1), respectively; consequently, the period of Rabi oscillations of exciton population is tunable in the range 0.6 pi-9 pi.     

Master equation techniques for exciton motion,relaxation, capture,and annihilation

Techniques developed recently for the study of exciton dynamics in molecular solids are discussed. They include master equation methods for the analysis of prerelaxation energy transfer, the generalized master equation approach to coherence in exciton motion, and the defect technique as applied to exciton trapping and annihilation.     

Phonon-assisted excitation energy transfer in photosynthetic systems

The phonon-assisted process of energy transfer aiming at exploring the newly emerging frontier between biology and physics is an issue of central interest.This article shows the important role of the intramolecular vibrational modes for excitation energy transfer in the photosynthetic systems.Based on a dimer system consisting of a donor and an acceptor modeled by two two-level systems,in which one of them is coupled to a high-energy vibrational mode,we derive an effective Hamiltonian describing the vibration-assisted coherent energy transfer process in the polaron frame.The effective Hamiltonian reveals in the case that the vibrational mode dynamically matches the energy detuning between the donor and the acceptor,the original detuned energy transfer becomes resonant energy transfer.In addition,the population dynamics and coherence dynamics of the dimer system with and without vibration-assistance are investigated numerically.It is found that,the energy transfer efficiency and the transfer time depend heavily on the interaction strength of the donor and the high-energy vibrational mode,as well as the vibrational frequency.The numerical results also indicate that the initial state and dissipation rate of the vibrational mode have little influence on the dynamics of the dimer system.Results obtained in this article are not only helpful to understand the natural photosynthesis,but also offer an optimal design principle for artificial photosynthesis.     

Excitonic-vibronic coupled dimers: A dynamic approach

The dynamical properties of exciton transfer coupled to polarization vibrations in a two site system are investigated in detail. A fixed point analysis of the full system of Bloch-oscillator equations representing the coupled excitonic-vibronic flow is performed. For overcritical polarization a bifurcation converting the stable bonding ground state to a hyperbolic unstable state which is basic to the dynamical properties of the model is obtained. The phase space of the system is generally of a mixed type: Above bifurcation chaos develops starting from the region of the hyperbolic state and spreading with increasing energy over the Bloch sphere leaving only islands of regular dynamics. The behaviour of the polarization oscillator accordingly changes from regular to chaotic.