Intravital Imaging Study on Photodamage Produced by Femtosecond Near‐infrared Laser Pulses In Vivo

Ultrashort femtosecond pulsed lasers may provide indispensable benefits for medical bioimaging and diagnosis, particularly for noninvasive biopsy. However, the ability of femtosecond laser irradiation to produce biodamage in the living body is still a concern. To solve this biosafety issue, results of theoretical estimations as well as the in vitro and in situ experiments on femtosecond biodamage should be verified by experimental studies conducted in vivo. Here, we analyzed photodamage produced by femtosecond (19, 42 and 100 fs) near‐infrared (NIR; ~800 nm) laser pulses with an average power of 5 and 15 mW in living undissected Drosophila larvae (in vivo). These experimental data on photodamage in vivo agree with the results of theoretical modeling of other groups. Femtosecond NIR laser pulses may affect the concentration of fluorescent biomolecules localized in mitochondria of the cells of living undissected Drosophila larva. Our findings confirm that the results of the mathematical models of femtosecond laser ionization process in living tissues may have a practical value for development of noninvasive biopsy based on the use of femtosecond pulses.     

Ultrafast photodissociation dynamics of acetone at 195 nm: II. Unraveling complex three-body dissociation dynamics by femtosecond time-resolved photofragment translational spectroscopy

As a continuation of the preceding paper in this issue (J. Phys. Chem. A 2005, 109, 6805), we studied photodissociation dynamics of the acetone S2 (n, 3s) Rydberg state excited at 195 nm using femtosecond time-resolved photofragment translational spectroscopy. The technique, which is implemented by the combination of fs pump-probe ionization spectroscopy and kinetic energy resolved time-of-flight mass spectrometry (KETOF), measured temporal evolutions of the product kinetic energy distributions (KEDs) with a time resolution limited only by the laser pulse widths. Two methyl product KED components were resolved and assigned to the primary and secondary methyl products on the basis of their temporal behaviors. The results support the mechanism in which the primary dissociation occurs on the acetone S1 surface and provide complementary dynamical information to that discussed in the preceding paper.     

Baseline correction methods to deal with artifacts in femtosecond transient absorption spectroscopy

In femtosecond transient absorption spectroscopy, artifact contributions are usually observed at ultra-short time scale. These complex signals are very challenging because of their nature, related to ultrafast phenomena, and because they strongly distort the structure of the spectrokinetic data. The purpose of this work is to evaluate the potential of baseline correction methods for femtosecond transient absorption spectroscopy data pre-processing. Indeed, artifacts removal should ideally be performed before multivariate data analysis. The work is thus mainly focused on two different approaches which are filtering by discrete wavelet transform, on the one hand, and smoothing by asymmetric least squares, on the other hand. The results obtained both on simulated data and on femtosecond pump-probe spectroscopy data are discussed. It can be concluded that asymmetric least squares smoothing procedure turns out to perform satisfactory for artifacts removal. Indeed, only mild discrepancies are observed in the transient spectra and, most important, good recovery of the kinetics is obtained at ultra-short time scale.     

Fast- and ultra-fast laser pulse induced reactions between carbon dioxide and methane

The direct excitation of CO_2 using fast (nanosecond) and ultrafast (femtosecond) pulsed lasers was investigated. A gas reaction cell was used to excite CO_2 in a 50:50 mixture of CO_2 and CH_4 using nano- and femtosecond laser systems, to induce a reaction between these two compounds. FT-IR spectra showed that CO was formed using the nanosecond and femtosecond laser systems. It was also found that hydrocarbons, containing C-C bonds were formed. For both the nanosecond and femtosecond laser, it was found that more C-C higher hydrocarbons were formed after 5 h compared to 3 h of irradiation. Irradiation at pressures of 250, 350 and 500 kPa with the femtosecond laser system showed the expected increase in the amount of CO formed with an increase in pressure. Results from this study showed that carbon dioxide and methane can be activated successfully using nanosecond laser pulses at 2000 run and femtosecond laser pulses at 795 or 2000 nm and that these activated species react to form CO and C-C containing products.     

Fast-and ultra-fast laser pulse induced reactions between carbon dioxide and methane

The direct excitation of CO2 using fast (nanosecond) and ultrafast (femtosecond) pulsed lasers was investigated.A gas reaction cell was used to excite CO2 in a 50:50 mixture of CO2 and CH4 using nano-and femtosecond laser systems,to induce a reaction between these two compounds.FT-IR spectra showed that CO was formed using the nanosecond and femtosecond laser systems.It was also found that hydrocarbons,containing C-C bonds were formed.For both the nanosecond and femtosecond laser,it was found that more C-C higher hydrocarbons were formed after 5 h compared to 3 h of irradiation.Irradiation at pressures of 250,350 and 500 kPa with the femtosecond laser system showed the expected increase in the amount of CO formed with an increase in pressure.Results from this study showed that carbon dioxide and methane can be activated successfully using nanosecond laser pulses at 2000 nm and femtosecond laser pulses at 795 or 2000 nm and that these activated species react to form CO and C-C containing products.     

Short-Lived Radical Intermediates in the Photochemistry of Glucose Oxidase

Glucose oxidase is a flavoprotein that is relatively well-studied as a physico-chemical model system. The flavin cofactor is surrounded by several aromatic acid residues that can act as direct and indirect electron donors to photoexcited flavin. Yet, the identity of the photochemical product states is not well established. We present a detailed full spectral reinvestigation of this issue using femtosecond fluorescence and absorption spectroscopy. Based on a recent characterization of the unstable tyrosine cation radical TyrOH^•+, we now propose that the primary photoproduct involves this species, which was previously not considered. Formation of this product is followed by competing charge recombination and radical pair stabilization reactions that involve proton transfer and radical transfer to tryptophan. A minimal kinetic model is proposed, including a fraction of TyrOH^.+ that is stabilized up to the tens of picoseconds timescale, suggesting a potential role of this species as intermediate in biochemical electron transfer reactions.     

Matrix‐assisted laser desorption/ionization by using femtosecond laser pulses in the near‐infrared wavelength regime

We observe a substantial matrix‐assisted laser desorption/ionization (MALDI) signal when irradiating femtosecond laser pulses in the near‐infrared spectral range centered around 800 nm and using standard MALDI matrices with absorption bands in the ultraviolet (UV) regime. The laser pulse energy dependence of this novel phenomenon is investigated in comparison with MALDI with near‐UV laser pulses. Our observations show that multiphoton absorption/ionization could be a major issue among the MALDI processes when the sample is irradiated with ultra‐short laser pulses. Copyright © 2009 John Wiley & Sons, Ltd.     

Optical far- and near-field femtosecond laser ablation of Si for nanoscale chemical analysis

Extending spatial resolution in laser-based chemical analysis to the nanoscale becomes increasingly important as nanoscience and nanotechnology develop. Implementation of femtosecond laser pulses arises as a basic strategy for increasing resolution since it is associated with spatially localized material damage. In this work we study femtosecond laser far- and near-field processing of silicon (Si) at two distinct wavelengths (400 and 800 nm), for nanoscale chemical analysis. By tightly focusing femtosecond laser beams in the far-field, we were able to produce sub-micrometer craters. In order to further reduce the crater size, similar experiments were performed in the near-field through sub-wavelength apertures, resulting in the formation of sub-30-nm craters. Laser-induced breakdown spectroscopy (LIBS) was used for chemical analysis with a goal to identify the minimum crater size from which spectral emission could be measured. Emission from sub-micrometer craters (full width at half maximum) was possible, which are among the smallest ever reported for femtosecond LIBS.     

Ultrafast cooling of photoexcited electrons in gold nanoparticle-thiolated DNA conjugates involves the dissociation of the gold-thiol bond

Using UV-visible extinction spectroscopy and femtosecond pump-probe transient absorption spectroscopy, we have studied the effect of femtosecond laser heating on gold nanoparticles attached to DNA ligands via thiol groups. It is found that femtosecond pulse excitation of the DNA-modified nanoparticles at a wavelength of 400 nm leads to desorption of the thiolated DNA strands from the nanoparticle surface by the dissociation of the gold-sulfur bond. The laser-initiated gold-sulfur bond-breaking process is a new pathway for nonradiative relaxation of the optically excited electrons within the DNA-modified gold nanoparticles, as manifested by a faster decay rate of the excited electronic distribution at progressively higher laser pulse energies. The experimental results favor a bond dissociation mechanism involving the coupling between the photoexcited electrons of the nanoparticles and the gold-sulfur bond vibrations over one involving the conventional phonon-phonon thermal heating processes. The latter processes have been observed previously by our group to be effective in the selective photothermal destruction of cancer cells bound to anti-epidermal growth factor receptor-conjugated gold nanoparticles.     

用超短脉冲激光研究1,3-二氯苯激发态动力学

利用啁啾脉冲放大技术建立了一套掺钛蓝宝石飞秒激光放大系统,该系统输出中心波长808nm,单脉冲能量8mJ,脉冲宽度60fs,脉冲重复频率20Hz.利用飞秒激光泵浦-探测及分子束技术,结合飞行时间质谱,对1,3-二氯苯分子的激发态动力学过程进行了研究,实验中观察到该分子能级间的量子拍频现象,并获得了第一单重激发态寿命及其拍频频率,阐述了飞秒激光场下间位二氯苯分子的电离解离机理.     

Optical emission studies of plasma induced by single and double femtosecond laser pulses

Double-pulse femtosecond laser ablation has been shown to lead to significant increase of the intensity and reproducibility of the optical emission signal compared to single-pulse ablation particularly when an appropriate interpulse delay is selected, that is typically in the range of 50–1000 ps. This effect can be especially advantageous in the context of femtosecond laser-induced breakdown spectroscopy analysis of materials. A detailed comparative study of collinear double- over single-pulse femtosecond laser-induced breakdown spectroscopy has been carried out, based on measurements of emission lifetime, temperature and electronic density of plasmas, produced during laser ablation of brass with 450 fs laser pulses at 248 nm. The results obtained show a distinct increase of plasma temperature and electronic density as well as a longer decay time in the double-pulse case. The plasma temperature increase is in agreement with the observed dependence of the emission intensity enhancement on the upper energy level of the corresponding spectral line. Namely, intensity enhancement of emission lines originating from higher lying levels is more profound compared to that of lines arising from lower energy levels. Finally, a substantial decrease of the plasma threshold fluence was observed in the double-pulse arrangement; this enables sensitive analysis with minimal damage on the sample surface.     

Femtosecond laser-induced microstructures in glasses and applications in micro-optics

Femtosecond laser has been widely used in microscopic modifications to materials due to its ultra-short laser pulse and ultrahigh light intensity. When a transparent material e.g. glass is irradiated by a tightly focused femtosecond laser, the photo-induced reaction is expected to occur only near the focused part of the laser beam inside the glass due to the multiphoton processes. We observed various induced structures e.g. color center defects, refractive index change, micro-void and micro-crack, in glasses after the femtosecond laser irradiation. In this paper, we review the femtosecond laser induced phenomena and discuss the mechanisms of the observed phenomena. We also introduce the fabrication of various micro-optical components, e.g. optical waveguide, micro-grating, micro-lens, fiber attenuator, 3-dimensional optical memory by using the femtosecond laser-induced structures. The femtosecond laser will open new possibilities in the fabrication of micro-optical components with various optical functions.     

Coherently controlled ultrafast four-wave mixing spectroscopy

A novel approach to coherent nonlinear optical spectroscopy based on two-dimensional femtosecond pulse shaping is introduced. Multiple phase-stable output beams are created and overlapped at the sample in a phase-matched boxcars geometry via two-dimensional femtosecond pulse shaping. The pulse timing, shape, phase, and spectral content within all beams may be specified, yielding an unprecedented level of control over the interacting fields in nonlinear spectroscopic experiments. Heterodyne detection and phase cycling of the nonlinear signal are easily implemented due to the excellent phase stability among all output beams. This approach combines the waveform generation capabilities of magnetic resonance spectroscopy with the wavevector specification and phase matching of nonlinear optical spectroscopy, yielding the control capabilities and signal selectivity of both. Results on four prototype systems are used to illustrate some of the novel possibilities of this method.     

Quantum dynamics of solid Ne upon photo-excitation of a NO impurity: A Gaussian wave packet approach

A high-dimensional quantum wave packet approach based on Gaussian wave packets in Cartesian coordinates is presented. In this method, the high-dimensional wave packet is expressed as a product of time-dependent complex Gaussian functions, which describe the motion of individual atoms. It is applied to the ultrafast geometrical rearrangement dynamics of NO doped cryogenic Ne matrices after femtosecond laser pulse excitation. The static deformation of the solid due to the impurity as well as the dynamical response after femtosecond excitation are analyzed and compared to reduced dimensionality studies. The advantages and limitations of this method are analyzed in the perspective of future applications to other quantum solids.     

Excitation Wavelength Dependence of Photocatalyzed Oxidation of Methanol on TiO2(110)

Post-irradiation temperature-programmed desorption (TPD) has been used to study the photocatalyzed oxidation of methanol on TiO₂(110) surface under the irradiation of 360, 380 and 400 nm light. The photocatalytic process initiated by ultraviolet light of different wavelength are similar. Methanol has been photocatalytically converted into formaldehyde, and the released hydrogen atoms transfer to the neighboring twofold coordinated oxygen to form bridging hydroxyls. The reaction rate, however, is strongly wavelength dependent. The reaction rate under 360 nm light irradiation is 4.8 times of that in the case of 400 nm exposure, consistent with a previous femtosecond time-resolved absorption measurement on TiO₂ which shows the faster charge carrier recombination in the near-band-gap than the over-band-gap excitation. So far, the underlying factors which govern the excitation wavelength dependence of photocatalytic activity of TiO₂ and other photocatalysts remain unclear, and future studies are needed to address this important issue.     

飞秒泵浦-探测实验中量子拍频现象的数据处理

The pump-probe experiments for 1,3-dichlorobenzene in gas phase have been performed by femtosecond multiphoton ionization mass spectra at a pump pulse of 270 nm and a probe pulse of 810 nm by using our homebuilt femtosecond laser system. The models for the exponential process and quantum beat are presented to extract the photodissociation dynamics information from the data. A fitting and analyzing software which is programmed by graphical language LabVIEW has been developed for the experimental data. The Convolution, Levenberg Marquardt,and Deconvolution mathematics VIs of LabVIEW are used in this program to make the program much simpler than using other programming languages. The experimental data have been well fitted with this program. The results provide the lifetime of excited-state S1 and the frequency of quantum beat.     

Vibrational molecular quantum computing: basis set independence and theoretical realization of the Deutsch-Jozsa algorithm

The phase of quantum gates is one key issue for the implementation of quantum algorithms. In this paper we first investigate the phase evolution of global molecular quantum gates, which are realized by optimally shaped femtosecond laser pulses. The specific laser fields are calculated using the multitarget optimal control algorithm, our modification of the optimal control theory relevant for application in quantum computing. As qubit system we use vibrational modes of polyatomic molecules, here the two IR-active modes of acetylene. Exemplarily, we present our results for a Pi gate, which shows a strong dependence on the phase, leading to a significant decrease in quantum yield. To correct for this unwanted behavior we include pressure on the quantum phase in our multitarget approach. In addition the accuracy of these phase corrected global quantum gates is enhanced. Furthermore we could show that in our molecular approach phase corrected quantum gates and basis set independence are directly linked. Basis set independence is also another property highly required for the performance of quantum algorithms. By realizing the Deutsch-Jozsa algorithm in our two qubit molecular model system, we demonstrate the good performance of our phase corrected and basis set independent quantum gates.     

Fragmentation of Neutral Amino Acids and Small Peptides by Intense, Femtosecond Laser Pulses

High power femtosecond laser pulses have unique properties that could lead to their application as ionization or activation sources in mass spectrometry. By concentrating many photons into pulse lengths approaching the timescales associated with atomic motion, very strong electric field strengths are generated, which can efficiently ionize and fragment molecules without the need for resonant absorption. However, the complex interaction between these pulses and biomolecular species is not well understood. To address this issue, we have studied the interaction of intense, femtosecond pulses with a number of amino acids and small peptides. Unlike previous studies, we have used neutral forms of these molecular targets, which allowed us to investigate dissociation of radical cations without the spectra being complicated by the action of mobile protons. We found fragmentation was dominated by fast, radical-initiated dissociation close to the charge site generated by the initial ionization or from subsequent ultrafast migration of this charge. Fragments with lower yields, which are useful for structural determinations, were also observed and attributed to radical migration caused by hydrogen atom transfer within the molecule.

Ion induced snowballs as a diagnostic tool to investigate the caging of metal clusters in large helium droplets

Metal clusters embedded in ultracold helium nanodroplets are exposed to femtosecond laser pulses with intensities of 10(13)-10(14) W/cm2. The influence of the matrix on the ionization and fragmentation dynamics is studied by pump-probe time-of-flight mass spectrometry. Special attention is paid to the generation of helium snowballs around positive metal ions (Me(z+)He(N), z=1,2). Closings of the first and second helium shells are found for silver at N(1)=10,12 and N(2)=32,44, as well as for magnesium at N1=19-20. The distinct abundance enhancement of helium snowballs in the presence of isolated atoms and small clusters in the droplets is used as a diagnostics to explore the cage effect. For silver, a reaggregation of the clusters is observed at 30 ps after femtosecond laser excitation.