Adaptive subwavelength control of nano-optical fields

Adaptive shaping of the phase and amplitude of femtosecond laser pulses has been developed into an efficient tool for the directed manipulation of interference phenomena, thus providing coherent control over various quantum-mechanical systems. Temporal resolution in the femtosecond or even attosecond range has been demonstrated, but spatial resolution is limited by diffraction to approximately half the wavelength of the light field (that is, several hundred nanometres). Theory has indicated that the spatial limitation to coherent control can be overcome with the illumination of nanostructures: the spatial near-field distribution was shown to depend on the linear chirp of an irradiating laser pulse. An extension of this idea to adaptive control, combining multiparameter pulse shaping with a learning algorithm, demonstrated the generation of user-specified optical near-field distributions in an optimal and flexible fashion. Shaping of the polarization of the laser pulse provides a particularly efficient and versatile nano-optical manipulation method. Here we demonstrate the feasibility of this concept experimentally, by tailoring the optical near field in the vicinity of silver nanostructures through adaptive polarization shaping of femtosecond laser pulses and then probing the lateral field distribution by two-photon photoemission electron microscopy. In this combination of adaptive control and nano-optics, we achieve subwavelength dynamic localization of electromagnetic intensity on the nanometre scale and thus overcome the spatial restrictions of conventional optics. This experimental realization of theoretical suggestions opens a number of perspectives in coherent control, nano-optics, nonlinear spectroscopy, and other research fields in which optical investigations are carried out with spatial or temporal resolution.     

Quantum control of energy flow in light harvesting

Coherent light sources have been widely used in control schemes that exploit quantum interference effects to direct the outcome of photochemical processes. The adaptive shaping of laser pulses is a particularly powerful tool in this context: experimental output as feedback in an iterative learning loop refines the applied laser field to render it best suited to constraints set by the experimenter. This approach has been experimentally implemented to control a variety of processes, but the extent to which coherent excitation can also be used to direct the dynamics of complex molecular systems in a condensed-phase environment remains unclear. Here we report feedback-optimized coherent control over the energy-flow pathways in the light-harvesting antenna complex LH2 from Rhodopseudomonas acidophila, a photosynthetic purple bacterium. We show that phases imprinted by the light field mediate the branching ratio of energy transfer between intra- and intermolecular channels in the complex's donor acceptor system. This result illustrates that molecular complexity need not prevent coherent control, which can thus be extended to probe and affect biological functions.     

泵浦光中有效频段对CARS信号选择激发作用的实验分析

运用基于遗传算法的自适应飞秒脉冲整形技术,实现了苯(C6H6,992 cm-1)和氘代苯(C6D6,945 cm-1)混合溶液中相干反斯托克斯(CARS)信号峰高分辨率的选择激发.谐波频率分辨光学开关(SHG-FROG)痕迹显示出飞秒CARS的选择激发主要是通过对泵浦光中与某待测振动模式相对应的有效频段的裁剪来实现的.通过在傅立叶平面对泵浦光进行频谱裁剪进一步在实验上验证了这一结论.该项研究成果对于进一步提高遗传算法搜索效率提供了重要思路.对于复杂分子系统飞秒CARS的选择激发研究具有深远意义.     

飞秒激光脉冲经小孔衍射后的近场光强节点

应用数值求解格林函数的方法，计算模拟了飞秒脉冲通过亚波长小孔的衍射光场. 首先将具有高斯时间轮廓的飞秒脉冲展开成一系列简谐光波的叠加，对每一个频率的简谐光波进行数值计算，得到该频率的衍射光场的空间分布, 然后对所有频率的衍射光场进行反傅里叶变换得到光场随时间的变化.分析发现,飞秒脉冲通过小孔后沿界面向小孔两边传播，并且在该传播过程中, 衍射光场会在介质的分界面上和在接近界面的区域中形成光强的节点.     

整形飞秒激光耦合静电场导致的分子取向增强研究

为了进一步提高分子在复合场中的取向度,提出采用整形飞秒激光耦合静电场取向分子.飞秒激光采用3次方相位调制整形,整形后的激光脉冲类似于缓慢上升快速下降的激光脉冲,但是与后者相比,整形后的激光场更加利于分子取向,能够有效提高分子取向度.这种脉冲上升沿变化导致的取向增强有助于进一步分析复合场的取向机制,并且改变整形参数、电场强度和分子转动温度,可以进一步提高分子取向度.     

Single-pulse coherently controlled nonlinear Raman spectroscopy and microscopy

Molecular vibrations have oscillation periods that reflect the molecular structure, and are hence being used as a spectroscopic fingerprint for detection and identification. At present, all nonlinear spectroscopy schemes use two or more laser beams to measure such vibrations. The availability of ultrashort (femtosecond) optical pulses with durations shorter than typical molecular vibration periods has enabled the coherent excitation of molecular vibrations using a single pulse. Here we perform single-pulse vibrational spectroscopy on several molecules in the liquid phase, where both the excitation and the readout processes are performed by the same pulse. The main difficulty with single-pulse spectroscopy is that all vibrational levels with energies within the pulse bandwidth are excited. We achieve high spectral resolution, nearly two orders of magnitude better than the pulse bandwidth, by using quantum coherent control techniques. By appropriately modulating the spectral phase of the pulse we are able to exploit the quantum interference between multiple paths to selectively populate a given vibrational level, and to probe this population using the same pulse. This scheme, using a single broadband laser source, is particularly attractive for nonlinear microscopy applications, as we demonstrate by constructing a coherent anti-Stokes Raman (CARS) microscope operating with a single laser beam.     

激光质量测量的自适应衰减控制技术研究

针对激光光束质量测量的激光光强高精度控制问题, 提出一种自适应衰减调节激光光强度的方法。该方法基于单片机控制算法, 利用PC机反馈的光斑图像灰度差值计算得出激光光强需要衰减的倍数, 并通过衰减轮电机控制两组衰减片组合完成对激光光强度的自适应控制, 实现了成像系统闭环控制激光光斑强度, 从而快速、 准确地得到特定光强度的激光光斑图像。实验结果表明, 该方法与以往衰减控制相比,能准确地得到高精度灰度值激光光斑图像, 激光光束质量单次测量时间为5 s, 系统测量时间为3 min。     

kHz 脉冲分子束的产生及其在强场原子分子物理实验中的应用

采用悬臂压电脉冲阀, 制备出最高重复频率达到3 kHz 的脉冲分子束。通过测量氮气分子束在飞秒激光作用下隧道电离生成的N2 +(B-X)荧光光谱, 对脉冲阀的性能做了表征。这种kHz 脉冲分子束搭配kHz 飞秒激光器, 将推动飞秒强激光驱动的原子分子动力学实验研究。     

强场原子分子物理实验研究中的符合测量技术及其应用

飞秒激光具有超短的脉冲宽度和超强的峰值功率，已经成为测量和操控原子分子超快动力学行为的重要工具．但是强激光场下，原子分子行为非常复杂，多个反应通道纠缠在一起．全微分符合测量技术能够提供特定反应通道精确的动力学数据，推动了强场原子分子物理研究的快速发展．本文结合北京大学新建的冷靶反冲离子动量谱仪，介绍全微分符合测量技术在强场原子分子物理实验研究中的重要应用以及在强场原子分子物理实验研究方面取得的一些重要进展．     

Real-time spectroscopy of transition states in bacteriorhodopsin during retinal isomerization.

Real-time investigations of the rearrangement of bonds during chemical transformations require femtosecond temporal resolution, so that the atomic vibrations within the reacting molecules can be observed. Following the development of lasers capable of emitting ultrashort laser flashes on this timescale, chemical reactions involving relatively simple molecules have been monitored in detail, revealing the transient existence of intermediate species as reactants are transformed into products. Here we report the direct observation of nuclear motion in a complex biological system, the retinal chromophore of bacteriorhodopsin (bR568), as it undergoes the trans-cis photoisomerization that is fundamental to the vision process. By using visible-light pulses of less than 5 femtosecond in duration, we are able to monitor changes in the vibrational spectra of the transition state and thus show that despite photoexcitation of the anti-bonding molecular orbital involved, isomerization does not occur instantly, but involves transient formation of a so-called 'tumbling state'. Our observations thus agree with growing experimental and ab initio evidence for a three-state photoisomerization model and firmly discount the initially suggested two-state model for this process.     

N2和O2分子的强场光电离

本研究工作主要采用超声分子束、飞秒激光与飞行时间(TOF)质谱等实验技术相结合的方法研究了N_2和O_2分子在飞秒强激光场作用下的光电离.通过分析N_2和O_2分子的飞行时间质谱及碎片离子产额,得到了离子产额的激光强度依赖关系、激光极化效应和偏振度效应,并结合强场理论知识对光电离机制进行了细致地分析.     

飞秒脉冲在磷酸二氢钾晶体中的色散特性

探讨飞秒脉冲在单轴晶体中的色散特性,根据主轴折射率色散方程,在不考虑晶体吸收及其他非线性作用的情况下,研究飞秒脉冲在磷酸二氢钾（monopotassium phosphate,KDP）晶体中的色散特性.由于晶体的色散,入射飞秒脉冲中不同频率的光波在晶体中传播时会引起不同的相位变化,从而改变出射脉冲的波形.通过数值计算得到飞秒脉冲在晶体中的传输特性,发现输出脉冲的脉宽、光强、展宽会随输入脉冲的中心波长、晶体的长度及脉冲光波的偏振方式等因素的变化而变化.所得到的结果,对于倍频研究、脉冲整形以及光学晶体器件的研发等具有一定的参考价值.     

Selectivity in vibrationally mediated single-molecule chemistry

The selective excitation of molecular vibrations provides a means to directly influence the speed and outcome of chemical reactions. Such mode-selective chemistry has traditionally used laser pulses to prepare reactants in specific vibrational states to enhance reactivity or modify the distribution of product species. Inelastic tunnelling electrons may also excite molecular vibrations and have been used to that effect on adsorbed molecules, to cleave individual chemical bonds and induce molecular motion or dissociation. Here we demonstrate that inelastic tunnelling electrons can be tuned to induce selectively either the translation or desorption of individual ammonia molecules on a Cu(100) surface. We are able to select a particular reaction pathway by adjusting the electronic tunnelling current and energy during the reaction induction such that we activate either the stretching vibration of ammonia or the inversion of its pyramidal structure. Our results illustrate the ability of the scanning tunnelling microscope to probe single-molecule events in the limit of very low yield and very low power irradiation, which should allow the investigation of reaction pathways not readily amenable to study by more conventional approaches.     

Ultrafast non-thermal control of magnetization by instantaneous photomagnetic pulses

The demand for ever-increasing density of information storage and speed of manipulation has triggered an intense search for ways to control the magnetization of a medium by means other than magnetic fields. Recent experiments on laser-induced demagnetization and spin reorientation use ultrafast lasers as a means to manipulate magnetization, accessing timescales of a picosecond or less. However, in all these cases the observed magnetic excitation is the result of optical absorption followed by a rapid temperature increase. This thermal origin of spin excitation considerably limits potential applications because the repetition frequency is limited by the cooling time. Here we demonstrate that circularly polarized femtosecond laser pulses can be used to non-thermally excite and coherently control the spin dynamics in magnets by way of the inverse Faraday effect. Such a photomagnetic interaction is instantaneous and is limited in time by the pulse width (approximately 200 fs in our experiment). Our finding thus reveals an alternative mechanism of ultrafast coherent spin control, and offers prospects for applications of ultrafast lasers in magnetic devices.     

Electrostatic trapping of ammonia molecules

The ability to cool and slow atoms with light for subsequent trapping allows investigations of the properties and interactions of the trapped atoms in unprecedented detail. By contrast, the complex structure of molecules prohibits this type of manipulation, but magnetic trapping of calcium hydride molecules thermalized in ultra-cold buffer gas and optical trapping of caesium dimers generated from ultra-cold caesium atoms have been reported. However, these methods depend on the target molecules being paramagnetic or able to form through the association of atoms amenable to laser cooling, respectively, thus restricting the range of species that can be studied. Here we describe the slowing of an adiabatically cooled beam of deuterated ammonia molecules by time-varying inhomogeneous electric fields and subsequent loading into an electrostatic trap. We are able to trap state-selected ammonia molecules with a density of 10(6) cm(-3) in a volume of 0.25 cm3 at temperatures below 0.35 K. We observe pronounced density oscillations caused by the rapid switching of the electric fields during loading of the trap. Our findings illustrate that polar molecules can be efficiently cooled and trapped, thus providing an opportunity to study collisions and collective quantum effects in a wide range of ultra-cold molecular systems.     

Tomographic imaging of molecular orbitals

Single-electron wavefunctions, or orbitals, are the mathematical constructs used to describe the multi-electron wavefunction of molecules. Because the highest-lying orbitals are responsible for chemical properties, they are of particular interest. To observe these orbitals change as bonds are formed and broken is to observe the essence of chemistry. Yet single orbitals are difficult to observe experimentally, and until now, this has been impossible on the timescale of chemical reactions. Here we demonstrate that the full three-dimensional structure of a single orbital can be imaged by a seemingly unlikely technique, using high harmonics generated from intense femtosecond laser pulses focused on aligned molecules. Applying this approach to a series of molecular alignments, we accomplish a tomographic reconstruction of the highest occupied molecular orbital of N2. The method also allows us to follow the attosecond dynamics of an electron wave packet.     

Correlated electron emission in multiphoton double ionization

Electronic correlations govern the dynamics of many phenomena in nature, such as chemical reactions and solid state effects, including superconductivity. Such correlation effects can be most clearly investigated in processes involving single atoms. In particular, the emission of two electrons from an atom--induced by the impact of a single photon, a charged particle or by a short laser pulse--has become the standard process for studies of dynamical electron correlations. Atoms and molecules exposed to laser fields that are comparable in intensity to the nuclear fields have extremely high probabilities for double ionization; this has been attributed to electron-electron interaction. Here we report a strong correlation between the magnitude and the direction of the momentum of two electrons that are emitted from an argon atom, driven by a femtosecond laser pulse (at 38 TW cm(-2)). Increasing the laser intensity causes the momentum correlation between the electrons to be lost, implying that a transition in the laser-atom coupling mechanism takes place.     

Genetic mechanisms and evolutionary significance of natural variation in Arabidopsis

Genomic studies of natural variation in model organisms provide a bridge between molecular analyses of gene function and evolutionary investigations of adaptation and natural selection. In the model plant species Arabidopsis thaliana, recent studies of natural variation have led to the identification of genes underlying ecologically important complex traits, and provided new insights about the processes of genome evolution, geographic population structure, and the selective mechanisms shaping complex trait variation in natural populations. These advances illustrate the potential for a new synthesis to elucidate mechanisms for the adaptive evolution of complex traits from nucleotide sequences to real-world environments.     

多光束超分辨显微成像系统的设计与光束重合度优化

为了提升超分辨显微成像系统的分辨能力,自主设计并搭建了一套多光束超分辨显微成像系统。该成像系统包括有3束激光束,分别为405 nm实心激发光束、488 nm实心激发光束以及488 nm空心损耗光束。通过Labview程序控制纳米位移台与单光子测器间的数据采集,采用80 nm金纳米颗粒探针扫描方式对该成像系统进行纳米尺度的光束调制,完成了多光束显微成像系统的光学调节,主要包括有多光束的光斑整形、多光束间的光束空间重合度优化以及对应光斑强度分布的仿真模拟。通过已知绿色荧光蛋白样品的测试结果,说明了该方法对于系统中光束空间重合度的提升作用与效果。     

Two-dimensional spectroscopy of electronic couplings in photosynthesis

Time-resolved optical spectroscopy is widely used to study vibrational and electronic dynamics by monitoring transient changes in excited state populations on a femtosecond timescale. Yet the fundamental cause of electronic and vibrational dynamics--the coupling between the different energy levels involved--is usually inferred only indirectly. Two-dimensional femtosecond infrared spectroscopy based on the heterodyne detection of three-pulse photon echoes has recently allowed the direct mapping of vibrational couplings, yielding transient structural information. Here we extend the approach to the visible range and directly measure electronic couplings in a molecular complex, the Fenna-Matthews-Olson photosynthetic light-harvesting protein. As in all photosynthetic systems, the conversion of light into chemical energy is driven by electronic couplings that ensure the efficient transport of energy from light-capturing antenna pigments to the reaction centre. We monitor this process as a function of time and frequency and show that excitation energy does not simply cascade stepwise down the energy ladder. We find instead distinct energy transport pathways that depend sensitively on the detailed spatial properties of the delocalized excited-state wavefunctions of the whole pigment-protein complex.