The slowing-down of ultrafast relaxation in C60 films at high femtosecond pump intensities

60 thin film was investigated in the energy range between 1.6 eV and 3.4 eV by a pump-supercontinuum probe (PSCP) technique with a 40 fs time resolution. The relaxation rate showed pronounced spectral dependence, with a maximum at 2 eV in the region of photo-induced darkening and at 2.4 eV in the region of photo-induced bleaching. The ultrafast relaxation rate decreased with increasing pump-pulse intensity. We suggest that the observed decrease results from extraheating of the carriers in the h_u and t_1u bands due to internal conversion from higher excited states, which are populated by two-step photon absorption of the intense pump pulse. Received: 2 May 1997/Revised version: 4 August 1997     

Impacting materials by light and seeing their structural dynamics

The emergence of the field of photo-induced transformation triggered by intense and ultra-short light pulse in a material has opened a new route to acting on matter. It combines many of today’s issues such as far away from equilibrium phenomena, coherence and quantum control, complex correlations and non linear responsiveness, inter-conversion of energy and information. The advent of ultrafast structural dynamics motivated developing new tools to directly watch how matter works, by probing the time- and length-scales on which photo-induced processes take place. Different points will be briefly overviewed: (i) the development of laser pump and X-ray probe for watching ultrafast structural dynamics, (ii) the physical features resulting from this way of impacting materials by light, (iii) the presentation of some examples investigated by time-resolved X-ray diffraction and diffuse scattering experiments to illustrate the complex structural dynamics of a photo-induced transformation.     

Ultrafast softening in InMnAs

We have used two-color time-resolved magneto-optical Kerr effect spectroscopy to manipulate and detect dynamic processes of spin/magnetic order in a ferromagnetic semiconductor InMnAs. We observed ultrafast photo-induced “softening” (i.e., transient decrease of coercivity) due to spin-polarized transient carriers. This transient softening persists only during the carrier lifetime (2 ps) and returns to its original value as soon as the carriers recombine to disappear. Our data clearly demonstrates that magnetic properties, e.g., coercivity, can be strongly and reversibly modified in an ultrafast manner. We attribute the origin of this unusual phenomenon to carrier-mediated ferromagnetic exchange interactions between Mn ions. We discuss the dependence of data on the pump polarization, pump intensity, and sample temperature. Our observation opens up new possibilities for ultrafast optical manipulation of ferromagnetic order as well as providing a new avenue for studying the dynamics of long-range collective order in strongly correlated many-body systems.     

Photo-induced ultrafast relaxation process in polymers: effect of lattice fluctuation

Our simulation shows that the lattice fluctuation has an essential effect on the photo-induced ultrafast relaxation process in a polymer and the relaxation process deviates significantly from that given in previous works which overlooked the lattice fluctuation.     

Femtosecond X-ray Studies of Photo-induced Structural Phase Transitions

We summarize recent advancements in the application of femtosecond X-ray scattering to experimental studies of photo-induced, structural phase transitions. Dynamic measurements of atomic rearrangement during solid-liquid and solid-solid transitions, as measured by ultrafast X-ray diffraction using plasma sources of line radiation are reported.     

捕光天线和反应中心的超快光物理研究

文章主要介绍了紫细菌Rhodobacter sphaeroide光合作用原初过程中的两种重要的功能体——捕光天线和反应中心的超快光物理研究，利用飞秒抽运一探测和飞秒瞬态吸收探测技术已经对捕光天线内两种色素（B80 HD和B850）内部或之间的能量传递过程作了广泛的研究和测试，其能量传递过程发生在几百飞秒至几个皮秒时域；而在反应中心中，在几种色素间的电荷分离以及电子传递过程也仅为皮秒量级。     

Study of photo-induced phenomena in VO2 by terahertz pump-probe spectroscopy

The ultrafast terahertz response to the photoexcitation in vanadium dioxide was investigated using the optical-pump terahertz-probe technique at room temperature. The optical excitation at 790 nm induced an ultrafast decrease of the transmittance of the terahertz pulses corresponding to the increase of the electronic conductivity within 0.7 ps, and then the transmittance decreases gradually up to 100 ps. This two-step behavior is very similar to the previous reports of the time resolved X-ray and electron diffractions. This fact indicates that the increase of the electronic conductivity and the change of the lattice structure proceed in parallel, and the photo-induced insulator-metal phase transition is of the Peierls type.     

Exploration of magnetic field generation of H32+ by direc ionization and coherent resonant excitation

Coherent electronic dynamics are of great significance in photo-induced processes and molecular magnetism. We theoretically investigate electronic dynamics of triatomic molecule H₃²⁺ by circularly polarized pulses, including electron density distributions, induced electronic currents, and ultrafast magnetic field generation. By comparing the results of the coherent resonant excitation and direct ionization, we found that for the coherent resonant excitation, the electron is localized and the coherent electron wave packet moves periodically between three protons, which can be attributed to the coherent superposition of the ground A' state and excited E⁺ state. Whereas, for the direct single-photon ionization, the induced electronic currents mainly come from the free electron in the continuum state. It is found that there are differences in the intensity, phase, and frequency of the induced current and the generated magnetic field. The scheme allows one to control the induced electronic current and the ultrafast magnetic field generation.     

Computational prediction on photophysical properties of two excited state intramolecular proton transfer (ESIPT) fluorophores bearing the benzothiazole group

ABSTRACT

In this contribution, the photophysical properties of two excited state intramolecular proton transfer (ESIPT) fluorophores of 2,6-dibenzothiazolyl-4-methylphenol (I) and 2-benzothiazolyl-6-(2-(benzothiazolyl)vinyl)-4-methylphenol (II) were studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods at the PBE0 theoretical level. To probe into the origin of the absorption and emission bands observed experimentally, the absorption and emission spectra of I and II were simulated by the TD-PBE0/6-311?+?G(d,p) calculations. In addition, the photo-induced proton enol–keto tautomerization of the two targeted molecules was also explored. The present studies indicate that a good agreement is found between theoretical predictions and experimental data. Moreover, both of these molecules can undergo an ultrafast ESIPT process, which should be responsible for the single proton-transfer tautomer emission.     

Adsorption patterns of caffeic acid on titania: affinity,charge transfer and sunscreen applications

ABSTRACT

Several photo-induced processes contribute to titania-catalysed photodegradation of organic UV-absorbing compound, caffeic acid (CA), utilised as skin photoprotector in sunscreens containing titania. Optical transitions, channels of photoexcitations, and charge transfer state formation in anatase TiO₂ nanowire sensitised by CA are computed and analysed. Trends of charge transfer state formation are explored through modelling of non-adiabatic dynamics using reduced density matrix approach combined with density functional theory with Perdew–Burke–Ernzerhof functional, in the basis of plane waves, using Vienna Ab initio Simulation Package software. The electronic structure of the explored model shows signatures of quantum confinement of TiO₂ nanowire. A change of excitation energy provides control of either one of the possible channels for photoexcitations: (1) local excitation at either nanowire or CA-adsorbate or (2) photo-induced electron transfer from CA-adsorbate to titania. The conclusions obtained from this computational modelling deliver further understanding of the adsorption of catechol groups on the surface of titania nanowire that will help in designing titania-based materials least damaging to CA and skin.     

Photo-induced tunneling spectroscopy of ReS2: Dramatic increase of the quantum efficiency by chemical treatment

In this work we present a systematic study of the local photovoltaic properties of ReS₂, using a scanning tunneling microscope (STM). The tunneling junction of the STM was optically illuminated during the tunneling process. The phase sensitive detected photo-induced tunneling current (PITC) was studied as a function of wavelength and surface topography. In order to improve the performance of ReS₂ solar cells, the samples were treated with NaI/I₂ and EDTA solutions. Relative to the untreated sample, the EDTA-treated samples show an increase in the photo-induced tunneling current by a factor of 8–10 in the whole spectral range, the NaI/I-treated samples by 2–3. Two dimensional mapping of the PITC was performed on an atomic scale and compared to the surface topography.     

利用可见-近红外-中红外超快光谱揭示离子交换法制备的少层MoS2中缺陷介导的载流子动力学

本文结合可见-近红外-中红外瞬态吸收光谱技术对离子交换法制备的少层MoS₂中缺陷介导的载流子动力学进行了详细的解析. 在近红外瞬态吸收光谱中观察到的宽带漂白信号表明少层MoS₂纳米片带隙中分布着大量的缺陷态. 实验结果明确揭示了载流子被缺陷态的快速捕获以及进一步的复合过程,证明带隙中的缺陷态对MoS₂光生载流子动力学过程起着至关重要的作用. 在中红外瞬态吸收光谱中观察到的正信号到负信号的转变进一步证实了在导带下小于0.24 eV处存在被载流子占据的缺陷态. 这些在少层MoS₂纳米片中存在的缺陷态可以作为有效的载流子捕获中心来辅助光生载流子在皮秒时间尺度内完成非辐射复合过程.     

Single-shot ultrabroadband two-dimensional electronic spectroscopy of the light-harvesting complex LH2

Here we present two-dimensional (2D) electronic spectra of the light-harvesting complex LH2 from purple bacteria using coherent pulses with bandwidth of over 100 nm FWHM. This broadband excitation and detection has allowed the simultaneous capture of both the B800 and B850 bands using a single light source. We demonstrate that one laser pulse is sufficient to capture the entire 2D electronic spectrum with a high signal-to-noise ratio. At a waiting time of 800 fs, we observe population transfer from the B800 to B850 band as manifested by a prominent cross peak. These results will enable observation of the dynamics of biological systems across both ultrafast (<1 ps) and slower (>1 ms) timescales simultaneously.     

The Paths of Excitation Energy Deactivation in LH1 Reduced Mutant and Wild-Type Strains of Rhodobacter sphaeroides

The P3 mutant of Rhodobacter sphaeroides had an altered ratio of reaction center to core (LH1) and peripheral (LH2) antenna complexes compared to the wild-type strain. Intracytoplasmic membranes from these two strains were purified and then resuspended in buffer or immobilized in isotropic and stretched polymer film. The absorption, photoacoustic, and delayed luminescence spectra were measured. The ratios of infrared absorption and photoacoustic bands (located at about 880 nm for LH1 and at 850 and about 800 nm for LH2) as well as the half-width of these bands are different for the LH2 and LH1 mutants and wild-type strain. The whole yields of thermal deactivation of the two strains were comparable, but in the absorption region of LH2 it was slightly lower in the case of the mutant than for the wild-type strain. The delayed luminescence main maxima were observed at about 860 and 700 nm. The first one could be due to emission of bacteriochlorophyll a of LH2 complexes. The emission at about 700 nm is probably due to dihydromesochlorophyll, which is usually, to some extent, produced from bacteriochlorophyll a in bacterial complexes. The delayed luminescence emission is competing with excitation energy transfer to the reaction center. The intensity of the delayed luminescence of the mutant strain was higher than that of the wild-type strain when both samples were excited in a region of carotenoid absorption. The mutant contains less carotenoids than the wild-type strain. Carotenoids work as efficient antenna. When they at a lower concentration the excitation can be trapped more easily by some chlorophyll-like pigment isolated from the excitation energy chain. The dependences of delayed luminescence spectra on the light polarization and excitation wavelengths for the wild-type strain and for the mutant were different. The anisotropy of delayed luminescence showed that bacteriochlorophyll a molecules of different orientations were contributing to the mutant and the wild-type strain emission. All the results suggest that the excitation energy transfer from the antenna to the reaction center in the mutant and the wild-type strain is similar.     

通过飞秒泵浦探测技术研究PbS纳米颗粒的激发态瞬态动力学过程

利用飞秒泵浦探测技术研究了PbS半导体纳米颗粒复合的SiO₂溶胶凝胶薄膜的瞬态动力学过程。通过改变激发探测波长和激发光强度,研究引起PbS半导体纳米颗粒的非线性吸收的两种机制。当激发探测波长选在激子吸收峰附近(620nm)时,由于激子的饱和吸收引起的光致漂白,当激发波长选在激子能态的低能侧(753,800nm),同时观察到激子的饱和吸收和双激子效应引起的光致吸收。研究了激子的饱和吸收和双激子效应引起的激发态吸收随激发态电子-空穴对浓度的变化关系,表明双激子效应与载流子浓度有很大关系。在高激发强度下,双激子效应引起的诱导吸收远远大于激子跃迁引起的光致漂白,双激子效应在非线性吸收中起着决定性作用。     

Ultrafast IR and THz spectroscopy of photo-induced insulator to metal transition in highly correlated organic system

Photo-induced insulator to metal transitions (PIMT) in quarter filled layered organic conductors ET [bis(ethylenedithio)tetrathiafulvalene]-based salts α-(ET)₂I₃, θ-(ET)₂RbZn(SCN)₄, and κ-(d-ET)₂Cu[N(CN)₂Br] were investigated using ultrafast spectroscopy in the near, mid-infrared and terahertz (THz) regions. In charge ordered salts α-(ET)₂I₃ and θ-(ET)₂RbZn(SCN)₄, an immediate (<30 fs) generation of a microscopic metallic state is driven by the electronic process. Subsequently, condensation of the microscopic metallic domain to the macroscopic scale is accompanied by a small molecular rearrangement in α-(ET)₂I₃. However, in θ-(ET)₂RbZn(SCN)₄, a large structural difference between the insulator and metallic phases prevents stabilization of the macroscopic metallic state. In a dimer Mott insulator κ-(d-ET)₂Cu[N(CN)₂Br], photo-generation of the metallic state shows a finite rise time of ca. 1 ps, which is attributable to the photo-induced change of on-site Coulomb energy on each dimer (U_dim) through dimeric molecular rearrangement. Thus, the ultrafast dynamics of PIMT depend strongly on the molecular arrangement in the layer of ET salts.     

GaAs光生载流子动力学机制的超快光谱学分析

利用飞秒激光泵浦探测技术,通过改变光学参数,如中心波长、功率,分别对未故意掺杂高纯n型砷化镓的差分反射谱进行研究,进而分析室温下砷化镓光生载流子动力学过程.首先,当泵浦光功率恒为100mW,探测光功率恒为10mW时,随着中心波长的增大,差分反射率峰值随之增大,信噪比也随之增加.其次,通过拟合不同延迟时间下泵浦光功率和差分反射率的实验曲线,并和理论模型比较后发现,在一定范围内的泵浦功率和差分反射率呈线性相关,未故意掺杂高纯n型砷化镓的饱和载流子浓度为(3.590 1±0.310 3)×1017 cm~(-3).在此基础上,把光生载流子动力学过程分为3个过程:804±67fs的光激发过程、134~268fs的初始散射过程、1ps和3~6ps的复合过程.研究表明,差分反射率与探测功率不存在显著的依赖性,但差分反射谱的信噪比与探测功率存在相关性.     

锰位铁掺杂La0.67Sr0.33FexMn1-xO3薄膜的光诱导效应

分别采用溶胶-凝胶法和脉冲激光沉积的方法制备了La_067Sr_033Fe_xMn_1-xO₃(x=0, 005, 010, 015)系列块材和薄膜,研究了Fe部分替代对La_067Sr_033Fe_xMn_1-xO₃薄膜 关键词： 光诱导 Fｅ掺杂 晶格效应     

Changes in the 5-HT2A receptor system in the pre-mammillary hypothalamus of the ewe are related to regulation of LH pulsatile secretion by an endogenous circannual rhythm

Background  

We wanted to determine if changes in the expression of serotonin 2A receptor (5HT2_A receptor) gene in the premammillary hypothalamus are associated with changes in reproductive neuroendocrine status. Thus, we compared 2 groups of ovariectomized-estradiol-treated ewes that expressed high vs low LH pulsatility in two different paradigms (2 groups per paradigm): (a) refractoriness (low LH secretion) or not (high LH secretion) to short days in pineal-intact Ile-de-France ewes (RSD) and (b) endogenous circannual rhythm (ECR) in free-running pinealectomized Suffolk ewes in the active or inactive stage of their reproductive rhythm.     

锐钛矿和金红石二氧化钛中光生载流子的超快动力学：电声相互作用研究

本文结合玻尔兹曼输运方程和电声散射速率计算研究锐钛矿和金红石二氧化钛中光生载流子的超快动力学过程. 其中,动力学模拟所需的结构参数均通过第一性原理计算获得. 结果表明,由于存在强Fr?hlich型电声耦合,纵光学声子模对两个晶相的能量弛豫过程均有十分显著的影响,但是两个晶相的弛豫机理却表现出明显的差异. 对于单条导带内的弛豫过程,锐钛矿和金红石的能量弛豫时间分别为24.0 fs和11.8 fs,前者约为后者的二倍. 这一差异来源于两个晶相中不同的电子扩散分布以及不同的声学模散射贡献. 对于涉及多条导带的弛豫过程,预测的锐钛矿和金红石的总体弛豫时间分别为47 fs和57 fs,其相对大小与单条导带的情况相反. 分析表明金红石相弛豫较慢是因为存在多个速率控制步骤. 这些发现为调控电子动力学以及设计高效的二氧化钛器件提供了有价值的信息.