Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactions

Polyatomic molecules have several electronic states that have similar energies. Consequently, their chemical dynamics often involve nonadiabatic transitions between multiple potential energy surfaces. Elucidating the complex reactions of polyatomic molecules is one of the most important tasks of theoretical and experimental studies of chemical dynamics. This paper describes our recent experimental studies of the multidimensional multisurface dynamics of polyatomic molecules based on two-dimensional ion/electron imaging. It also discusses ultrafast photoelectron spectroscopy of liquids for elucidating nonadiabatic electronic dynamics in aqueous solutions.     

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.     

Femtosecond infrared spectroscopy of semiconductors and semiconductor nanostructures

Infrared spectroscopy on ultrafast time scales represents a powerful technique to investigate the nonequilibrium dynamics of elementary excitations in bulk and nanostructured semiconductors. In this article, recent progress in this field is reviewed. After a brief introduction into electronic excitations below the fundamental bandgap and ultrafast processes in semiconductors, infrared pulse generation and the methodology of time-resolved infrared spectroscopy are reviewed. The main part of this paper is devoted to coherent optical polarizations and nonequilibrium excitations of the electronic system in the spectral range below the fundamental band gap. The focus is on the physics of single component plasmas, i.e. electrons or holes. In particular, intraband, inter-valence and intersubband transitions are considered. Processes of phase relaxation, carrier and energy redistribution are analyzed. The potential of ultrafast infrared technology and spectroscopy for future applications is discussed in the final part.     

超快光声光谱技术的进展和前景

回顾了超快光声光谱学的发展过程并介绍了各种测量手段，论述了文章作者建立的飞秒时间分辨的光声光谱系统的工作原理及实验结果，并展望了超快光声光谱学的未来发展方向和应用前景。     

Optical pump-probe scanning tunneling microscopy for probing ultrafast dynamics on the nanoscale

The development of a method for exploring the ultrafast transient dynamics in small organized structures with high spatial resolution is expected to be a basis for further advances in current science and technology. Recently, we have developed a new microscopy technique by combining scanning tunneling microscopy (STM) with ultrashort-pulse laser technology, which enables the visualization of ultrafast carrier dynamics even on the single-atomic level. A nonequilibrium carrier distribution is generated using ultrashort laser pulses and its relaxation processes are probed by STM using the optical pump-probe method realized in STM by the pulse-picking technique. In this paper, the fundamentals of the new microscopy technique are overviewed.     

Radiation generated by the ultrafast migration of a positive charge following the ionization of a molecular system

Electronic many-body effects alone can be the driving force for an ultrafast migration of a positive charge created upon ionization of molecular systems. Here we show that this purely electronic phenomenon generates a characteristic IR radiation. The situation when the initial ionic wave packet is produced by a sudden removal of an electron is also studied. It is shown that in this case a much stronger UV emission is generated. This emission appears as an ultrafast response of the remaining electrons to the perturbation caused by the sudden ionization and as such is a universal phenomenon to be expected in every multielectron system.     

Amplified Emissions Spectroscopy for Sensing Applications

Amplified emission spectroscopy is promising for applications of qualitative and quantitative sensing of elements and their states. This technique has the characteristics of a narrow spectral line, high spectral resolution, and high signal‐to‐noise ratio. Recently, this technology has advanced tremendously with progress in excitation methods, that is, resonance multiphoton absorption and filamentation with ultrafast lasers. The mechanism, conditions, excitation, signal processing, and applications of amplified emission spectroscopy are reviewed herein. In particular, the different formation patterns of the amplified emission spectra produced by an ultrafast laser are analyzed in detail. In addition, a brief introduction is given to the existing applications of amplified emission. Finally, both a conclusion and a perspective of the future for amplified emission are presented.     

雪崩晶体管的导通方式讨论

本文详细讨论了雪崩晶体管各种不同的导通方式,并发现由于雪崩晶体管C—E极间电压变化速率太大时,造成雪崩晶体管导通,论述了产生这种现象的原因,及此现象的存在对超快速电路的影响。     

超快速激光光谱学技术在晶态半导体低维材料中的应用

本文结合作者近年来的工作，评述了用超快速激光光谱学技术对晶态半导体低维材料研究的进展，分别讨论了用超快光谱研究晶态半导体低维材料中载流子各种非平衡过程的优势和原理，提出了需要进一步探讨的方面。     

Ultrafast laser spectroscopy of semiconductor nanocrystals

A large amount of work has been worldwide directed to understand the properties of semiconductor nanostructures. Ultrafast lasers with pulsewidths of a few femtoseconds allowed the investigation of the dynamics of elementary excitations in semiconductor structures on ultrashort time scales. Recent progress in technology made it possible to fabricate semiconductor nanocrystals (i.e. crystals of nanometer dimensions) of well-defined properties. The purpose of this paper is to review the understanding of carrier relaxation and recombination processes in semiconductor nanocrystals as studied by ultrafast laser spectroscopy. The up-to-date techniques of ultrafast laser spectroscopy as well as the fabrication of semiconductor nanocrystals are discussed in some detail.     

Efficiency and temporal response of crystalline Kerr media in collinear optical Kerr gating

Optical Kerr gating is widely used in ultrafast measurements ranging from pulse characterization to spectroscopy and microscopy. We examined the efficiency and the temporal response of three cubic lattice Kerr media, YAG, GGG and BGO, and compared them with the well studied fused silica (fast response, low efficiency) and STO (high efficiency, slow response). YAG and GGG emerged as superior materials for ultrafast spectroscopy and microscopy applications thanks to their fast Kerr response and considerably higher gating efficiency than silica at low gating energies. Importantly, it was found that in collinear geometry all tested materials except STO are capable of reaching nearly 100% transmission.     

CdSeS合金结构量子点的多激子俄歇复合过程

多激子效应通常是指吸收单个光子产生多个激子的过程,该效应不仅可以为研究基于量子点的太阳能电池开拓新思路,还可以为提高太阳能电池的光电转换效率提供新方法.但是,超快多激子产生和复合机制尚不明确.这里以CdSeS合金结构量子点为研究对象,研究了其多激子生成和复合动力学.稳态吸收光谱显示, 510, 468和430 nm附近的稳态吸收峰,分别对应1S_(3/2)(h)-1S(e)(或1S), 2S_(3/2)(h)-1S(e)(或2S)和1P_P(3/2)(h)-1P(e)(或1P)激子的吸收带.通过飞秒时间分辨瞬态吸收光谱和纳秒时间分辨荧光光谱两种时间分辨光谱技术对CdSeS合金结构量子点的超快动力学进行了探究,结果显示, 1S激子的双激子复合时间大概是80 ps,这一时间比传统量子点的双激子复合时间(小于50 ps)延长了近一倍,结合最近发展的超快界面电荷分离技术,在激子湮灭之前将其利用起来,这一时间的延长将有很大的应用前景;其中,在2S和1P激子中除上述双激子复合外,还存在一个通过声子耦合路径的空穴弛豫过程,时间大概是5—6 ps.最后,利用纳秒时间分辨荧光光谱得到该样品体系单激子复合的时间约为200 ns.     

About the influence of friction and polydispersity on the jamming behavior of bead assemblies

We study the jamming of bead assemblies placed in a cylindrical container whose bottom is pierced with a circular hole. Their jamming behavior is quantified here by the median jamming diameter, that is the diameter of the hole for which the jamming probability is 0.5. Median jamming diameters of monodisperse assemblies are obtained numerically using the Distinct Element Method and experimentally with steel beads. We obtain good agreement between numerical and experimental results. The influence of friction is then investigated. In particular, the formation of concentric bead rings is observed for low frictions. We identify this phenomenon as a boundary effect and study its influence on jamming. Relying on measures obtained from simulation runs, the median jamming diameter of bidisperse bead assemblies is finally found to depend only on the volume-average diameter of their constituting beads. We formulate this as a tentative law and validate it using bidisperse assemblies of steel beads.     

A proposal for fs-electron microscopy experiments on high-energy excitations in solids

Recent advances in ultrafast technology enable both the study and the control of materials properties thanks to the ability to record high temporal resolution movies of their transformations, or the ability to generate new states of matter by selecting ad hoc an excitation to drive the system out of equilibrium. The holy grail of this type of experiments is to combine a high tuneability of the excitation with a wide observation window. For example, this is achieved in multidimensional optical spectroscopy where the response to several excitation energies is monitored in a broad energy range by a large bandwidth optical pulse. In this article, the possibility to combine the chemical sensitivity of intense tuneable X-rays pulses from a free electron laser, with the wide range of observables available in an ultrafast transmission electron microscope is discussed. The requirements for such experiments are quantified via estimates based on state of the art experiments and simulations, and it is proposed that ultrafast electron imaging, diffraction and spectroscopy experiments can be performed in combination with a chemically selective X-ray excitation of materials.     

Ultrafast control of slow light in THz electromagnetically induced transparency metasurfaces

In this paper,we experimentally demonstrate ultrafast optical control of slow light in the terahertz(THz) range by combining the electromagnetically induced transparency(EIT) metasurfaces with the cut wire made of P~+-implanted silicon with short carrier lifetime.Employing the optical-pump THz-probe spectroscopy,we observed that the device transited from a state with a slow light effect to a state without a slow light effect in an ultrafast time of 5 ps and recovered within 200 ps.A coupled oscillator model is utilized to explain the origin of controllability.The experimental results agree very well with the simulated and theoretical results.These EIT metasurfaces have the potential to be used as an ultrafast THz optical delay device.     

Optomechanical-organized multipulse dynamics in ultrafast fiber laser

A novel organized multipulse pattern and its birth dynamics under strong optomechanical effect in microfiber-assisted ultrafast fiber laser are investigated in this work. The background pulses are observed to obviously exhibit selectively amplifying self-organized process of evolving into quasi-stable equidistant clusters. The radio frequency spectrum of the multipulse pattern displays a harmonic mode-locking-like behavior with a repetition rate of 2.0138 GHz, corresponding to the frequency of torsional-radial (TR_2m) acoustic mode in microfiber. The results show the evidence of optomechanical effect in dominating the birth dynamics and pattern of multipulse.     

Conformational order of n-dodecanethiol and n-dodecaneselenol monolayers on polycrystalline copper investigated by PM-IRRAS and SFG spectroscopy

Self-assembled monolayers (SAMs) of n-dodecanethiol (C₁₂H₂₅SH) and n-dodecaneselenol (C₁₂H₂₅SeH) on polycrystalline copper have been elaborated with the purpose of achieving densely packed and crystalline-like assemblies. By combining the surface sensitivity of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and sum-frequency generation spectroscopy (SFG), the effect of the self-assembly time (15 min, 30 min, 1 h, 2 h and 24 h) on the formation of n-dodecanethiol and n-dodecaneselenol monolayers on untreated and electrochemically reduced polycrystalline copper has been investigated. On electrochemically reduced copper, PM-IRRAS spectroscopy shows that both molecules are able to form well organized layers. SFG spectroscopy indicates that the C₁₂H₂₅SeH SAMs are slightly better ordered than those achieved with C₁₂H₂₅SH. On untreated copper, the two molecules lead to different film organizations. Both PM-IRRAS and SFG indicate that C₁₂H₂₅SH SAMs are of the same film quality as those obtained on electrochemically reduced copper. On the contrary, C₁₂H₂₅SeH monolayers are invariably poorly organized at the molecular level.     

Applications of time-resolved terahertz spectroscopy in ultrafast carrier dynamics

Three time-resolved terahertz (THz) spectroscopy methods (optical-pump/THz-probe spectroscopy,THzpump/THz-probe spectroscopy,and THz-pump/optical-probe spectroscopy) are reviewed.These are used to characterize ultrafast dynamics in photo-or THz-excited semiconductors,superconductors,nanomaterials,and other materials.In particular,the optical-pump/THz-probe spectroscopy is utilized to investigate carrier dynamics and the related intervalley scattering phenomena in semiconductors.The recent development of intense pulsed THz sources is expected to affect the research in nonlinear THz responses of various materials.     

Phenotypic fluctuation rendered by a single genotype and evolutionary rate

Proteins are ideal systems to experimentally study stochastic complexity. They share many properties with supercooled liquids and glasses, but are functionally organized, perform biological work, and interact strongly with their surroundings. We describe the complexity of proteins, expressed through their hierarchically organized energy landscape, and characterize how fluctuations in the hydration shell and the bulk environment influence protein motions and functions.     

Spatial solitons in optofluidic waveguide arrays with focusing ultrafast Kerr nonlinearity

We present an optofluidic nonlinear waveguide array that is fabricated by selectively filling several strands of a photonic crystal fiber with the liquid CCl(4), which exhibits a large focusing ultrafast Kerr nonlinearity. We demonstrate a power dependent formation of a spatial soliton in this novel optofluidic device. The large thermo-optical effect of liquids enables us to control the characteristics of the spatial soliton formation in these nonlinear structures. This opens the road toward flexible designs and the realization of a new class of optofluidic devices with complex nonlinear landscapes and novel effects.