High-intensity Femtosecond Pulses Research Articles

Influence of pump laser fluence on ultrafast myoglobin structural dynamics.

High-intensity femtosecond pulses from an X-ray free-electron laser enable pump-probe experiments for the investigation of electronic and nuclear changes during light-induced reactions. On timescales ranging from femtoseconds to milliseconds and for a variety of biological systems, time-resolved serial femtosecond crystallography (TR-SFX) has provided detailed structural data for light-induced isomerization, breakage or formation of chemical bonds and electron transfer1,2. However, all ultrafast TR-SFX studies to date have employed such high pump laser energies that nominally several photons were absorbed per chromophore3-17. As multiphoton absorption may force the protein response into non-physiological pathways, it is of great concern18,19 whether this experimental approach20 allows valid conclusions to be drawn vis-à-vis biologically relevant single-photon-induced reactions18,19. Here we describe ultrafast pump-probe SFX experiments on the photodissociation of carboxymyoglobin, showing that different pump laser fluences yield markedly different results. In particular, the dynamics of structural changes and observed indicators of the mechanistically important coherent oscillations of the Fe-CO bond distance (predicted by recent quantum wavepacket dynamics21) are seen to depend strongly on pump laser energy, in line with quantum chemical analysis. Our results confirm both the feasibility and necessity of performing ultrafast TR-SFX pump-probe experiments in the linear photoexcitation regime. We consider this to be a starting point for reassessing both the design and theinterpretation of ultrafast TR-SFX pump-probe experiments20 such that mechanistically relevant insight emerges.

Dissociation of HeH+ in the electronic ground state using shaped mid-IR laser pulses.

Inspired by recent experimental work, we study the control over the laser-driven dissociation of the HeH+ ion in the electronic ground state. Shaped pulses with peak intensities below 1012 W cm-2 are obtained by phase modulation of high-intensity transform-limited femtosecond pulses. We investigate the performance of pulse shaping for a number of shaping parameters targeting both vibrational and rotational excitation pathways. The numerical results show that pulse shaping is most effective at low pulse energies and broad spectral bandwidths, while intense transform-limited pulses with narrow spectral bandwidths maximize dissociation. We show that the control achieved with a quadratic chirped pulse optimized for vibrational ladder climbing, a cascade excitation process of adjacent vibrational levels, is hindered by rotational motion leading to significantly reduced dissociation. Moreover, pulse shaping using higher-order polynomial phase functions is found to provide only a marginal increase in dissociation yields. Our results provide additional insights into the coherent control of bond breaking in diatomic molecules, and demonstrate the efficacy of pulse shaping for a range of pulse energies.

Protector-free, non-plasmonic silver quantum clusters by femtosecond pulse laser irradiation: in situ binding on nanocellulose filaments for improved catalytic activity and cycling performance.

This study introduces a new, facile method to synthesize silver clusters from aqueous silver ion solution by using high intensity femtosecond pulse laser irradiation. The particles obtained in the absence of reducing or capping agents are 1-17 nm in size and presented quantum properties, as characterized by fluorescence, but did not exhibit plasmon signals, which is not a common characteristic of conventional silver nanoparticles. In a further development, small silver quantum clusters (∼1 nm) were bound in situ to wet-spun filaments of cellulose nanofibrils by pulsed laser irradiation. The obtained hybrid filaments as well as free silver quantum clusters revealed a catalytic activity remarkably higher than that of free gold quantum clusters; moreover, the hybrid filaments were found to show improved stability and cycling performance for silver-based catalysis. The present results indicate the potential of femtosecond laser irradiation to generate clusters as well as hybrid systems with excellent performance and reactivity.

Isolated attosecond pulse generation from helium atom irradiated by a three-color laser pulse

Isolated attosecond pulses enables the studying and controlling of ultrafast electron processes in atoms and molecules. High-order harmonic generation (HHG) is the most promising way to generate such pulses, benefiting from the broad plateau structure of the typical HHG spectrum. We theoretically investigate high-order harmonic and attosecond pulse generation from helium atom in a three-color laser field, which is synthesized by 16 fs/1600 nm, 15 fs/1100 nm and 5 fs/800 nm pulse laser. Compared with harmonic spectrum generated by a two-color laser field synthesized by 16 fs/1600 nm and 15 fs/1100 nm, the harmonic spectrum generated from the synthesized three-color field exhibits high conversion efficiency and broadband supercontinuum characteristics. The continuous spectrum range covers from 230th to 690th harmonics, and the generation of 128 attosecond isolated short pulses with higher intensity is realized after Fourier transform. This result is attributable to the fact that the synthesized three-color electric field exhibits high-intensity and few-cycle mid-infrared femtosecond pulse laser characteristics, which can effectively control atomic ionization and recombination occurring within an effective optical period of the mid-infrared femtosecond pulse. This scheme solves the problems faced by the current femtosecond pulse laser technology, i.e. the few-cycle mid-infrared femtosecond pulse laser cannot have both carrier envelope phase stability and high power output.

Pump Frequency Resonances for Light-Induced Incipient Superconductivity in YBa2Cu3O6.5

Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature, $T_C$, as evidenced by the terahertz frequency optical properties in the non-equilibrium state. In YBa$_2$Cu$_3$O$_{6+x}$ this phenomenon has so far been associated with the nonlinear excitation of certain lattice modes and the creation of new crystal structures. In other compounds, like La$_{2-x}$Ba$_x$CuO$_4$, similar effects were reported also for excitation at near infrared frequencies, and were interpreted as a signature of the melting of competing orders. However, to date it has not been possible to systematically tune the pump frequency widely in any one compound, to comprehensively compare the frequency dependent photo-susceptibility for this phenomenon. Here, we make use of a newly developed optical parametric amplifier, which generates widely tunable high intensity femtosecond pulses, to excite YBa$_2$Cu$_3$O$_{6.5}$ throughout the entire optical spectrum (3 - 750 THz). In the far-infrared region (3 - 25 THz), signatures of non-equilibrium superconductivity are induced only for excitation of the 16.4 THz and 19.2 THz vibrational modes that drive $c$-axis apical oxygen atomic positions. For higher driving frequencies (25 - 750 THz), a second resonance is observed around the charge transfer band edge at ~350 THz. These observations highlight the importance of coupling to the electronic structure of the CuO$_2$ planes, either mediated by a phonon or by charge transfer.

Ultrafast modification of oxide glass surface hardness

Application of femtosecond lasers is widely utilized in micromachining transparent materials. We have successfully altered the surface hardness of various commercial silicate glasses using a high-intensity femtosecond pulse laser. The femtosecond laser generates pulse energy of 500 nJ with a central wavelength of 800 nm. Using a peak power of 2.2 W and a repetition rate of 5.1 MHz, we observed an 18–20% increase surface hardness in glasses with low-modifier content and 16.6% decrease in glasses with high-modifier content. All laser exposed glasses show no detectable induced-crystallization or surface ablation. X-ray photoelectron spectroscopy results of our samples confirmed that the laser irradiation had no detectable effect on surface chemistry. X-ray reflectometry data showed the change in hardness was attributed to a thin layer with modified density. Experimental results suggest the strengthening mechanism derives from local structural transformation of interatomic bond distances and angles.

Virus Structures by X-Ray Free-Electron Lasers.

Until recently X-ray crystallography has been the standard technique for virus structure determinations. Available X-ray sources have continuously improved over the decades, leading to the realization of X-ray free-electron lasers (XFELs). They provide high-intensity femtosecond X-ray pulses, which allow for new kinds of experiments by making use of the diffraction-before-destruction principle. By overcoming classical dose constraints, they at least in principle allow researchers to perform X-ray virus structure determination for single particles at room temperature. Simultaneously, the availability of XFELs led to the development of the method of serial femtosecond crystallography, where a crystal structure is determined from the measurement of hundreds to thousands of microcrystals. In the case of virus crystallography this method does not require freezing of the crystals and allows researchers to perform experiments under non-equilibrium conditions (e.g., by laser-induced temperature jumps or rapid chemical mixing), which is currently not possible with electron microscopy.

Theory of SHG in a medium with combined nonlinear response

Second-harmonic generation (SHG) is used in many practical applications such as substance diagnostics and imaging of various processes as well as for frequency conversion. It is well known that frequency doubling may also be used for the generation of a tripled frequency wave due to mixing of optical radiation at basic frequency with optical radiation at doubled frequency. In this case, the relationship between phases of interacting waves with basic frequency and doubled frequency plays an important role. Therefore, the solution of the corresponding problem for wave phase evolutions is an important issue. In this paper, we provide such a solution for the SHG of high-intensity femtosecond pulses, taking into account the influence of a cubic nonlinear response on frequency doubling by using an original approach together with the long pulse duration approximation and plane wave approximation. The main feature of our approach is the absence of using the basic wave energy non-depletion approximation and using the problem invariants. It should be stressed that the frequency conversion process under consideration possesses a multi-stability property: there are two modes of SHG at certain values of intensity at basic frequency. We derive the wave intensity and phase evolution for both interacting waves for each of the modes. The derived formulas are verified by computer simulation based on using corresponding nonlinear Schrodinger equations. It is very important that we demonstrate the robustness of the derived formulas for the inhomogeneous shape of the pulse. They can be applied to computing of pulse shapes and beam profiles, even if reverse energy conversion occurs. As an example, we consider SHG for an incident Gaussian pulse with basic frequency.

Hit detection in serial femtosecond crystallography using X-ray spectroscopy of plasma emission.

Serial femtosecond crystallography is an emerging and promising method for determining protein structures, making use of the ultrafast and bright X-ray pulses from X-ray free-electron lasers. The upcoming X-ray laser sources will produce well above 1000 pulses per second and will pose a new challenge: how to quickly determine successful crystal hits and avoid a high-rate data deluge. Proposed here is a hit-finding scheme based on detecting photons from plasma emission after the sample has been intercepted by the X-ray laser. Plasma emission spectra are simulated for systems exposed to high-intensity femtosecond pulses, for both protein crystals and the liquid carrier systems that are used for sample delivery. The thermal radiation from the glowing plasma gives a strong background in the XUV region that depends on the intensity of the pulse, around the emission lines from light elements (carbon, nitrogen, oxygen). Sample hits can be reliably distinguished from the carrier liquid based on the characteristic emission lines from heavier elements present only in the sample, such as sulfur. For buffer systems with sulfur present, selenomethionine substitution is suggested, where the selenium emission lines could be used both as an indication of a hit and as an aid in phasing and structural reconstruction of the protein.

Ultrafast laser based hybrid methodology of silicon microstructure fabrication for optoelectronic applications

As an alternative approach to conventional lithography based fabrication, simple methodology of ultrafast laser writing followed by chemical processing for fabrication of silicon microstructures is studied and presented. Laser fluence and number of pulses dependent laser-matter interaction study reveals several concurrent extreme nonlinearities that influence the structural morphology in both longitudinal and transverse directions. High intensity femtosecond pulse propagation produces inevitable structural features, such as quasi aperiodic surface textures, V-shaped craters, re-casted melt and debris. To minimize such undesired effects, isotropic and anisotropic chemical etching processes have been systematically optimized. Such hybrid protocols resulted into definite microstructures, with surface quality comparable to those obtained from other lithographic fabrication methods. The proposed methodology is expected to provide control over desired feature sizes, for large-scale and cost-effective fabrication of microstructures for many optoelectronics applications.

Rainbow Stars—Multi-Color Laser Emission for Science and Art

Rainbow Stars—Multi-Color Laser Emission for Science and Art

Experimental Observation of Dynamical Localization in Laser-Kicked Molecular Rotors.

The periodically kicked rotor is a paradigm system for studying quantum effects on classically chaotic dynamics. The wave function of the quantum rotor localizes in angular momentum space, similarly to Anderson localization of the electronic wave function in disordered solids. Here, we observe dynamical localization in a system of true quantum rotors by subjecting nitrogen molecules to periodic sequences of femtosecond pulses. Exponential distribution of the molecular angular momentum-the hallmark of dynamical localization-is measured directly by means of coherent Raman scattering. We demonstrate the suppressed rotational energy growth with the number of laser kicks and study the dependence of the localization length on the kick strength. Because of its quantum coherent nature, both timing and amplitude noise are shown to destroy the localization and revive the diffusive growth of energy.

Generating long sequences of high-intensity femtosecond pulses.

We present an approach to creating pulse sequences extending beyond 150ps in duration, comprised of 100μJ femtosecond pulses. A quarter of the pulse train is produced by a high-resolution pulse shaper, which allows full controllability over the timing of each pulse. Two nested Michelson interferometers follow to quadruple the pulse number and the sequence duration. To boost the pulse energy, the long train is sent through a multipass Ti:sapphire amplifier, followed by an external compressor. A periodic sequence of 84 pulses of 120fs width and an average pulse energy of 107μJ, separated by 2ps, is demonstrated as a proof of principle.

Properties of femtosecond laser-induced defects in alkali metal fluoride crystals

The formation of various defects in the crystal lattice of lithium and sodium fluorides under the action of femtosecond laser pulses is described. The difference between the concentration distributions of aggregate color centers and light-scattering nanodefets in the channels is shown. The effect geometrical focusing has on color center formations is demonstrated. The observed phenomena are explained by the nonlinear processes induced by high-intensity femtosecond pulses.

Formation of color centers and light scattering structures by femtosecond laser pulses in sodium fluoride

Modification of sodium fluoride crystal lattice by means of femtosecond laser pulses with λmax=800nm, energy 0.5mJ, duration 30fs and repetition rate 1kHz has been considered in the paper. Effective formation of simple and complex aggregate color centers and light scattering nanodefects in the channel of a laser beam in NaF crystal have been shown for the first time. Dependences of color centers concentration on the distance between the channel center and its periphery and along the channel have been presented. Influence of external focusing on color centers creation has been revealed. Explanations of the observed phenomena have been presented on the basis of nonlinear processes taking place under the effect of high-intensity femtosecond pulses.

Time-Resolved Pump and Probe Experiment for Wide-Gap Semiconductors Using Free Electron Laser and Synchronously-Operated Femtosecond Laser

Pump–probe technique is an important means of studying the structure and dynamics of a single protein molecule. X-ray free electron lasers (XFELs) such as the Linac Coherent Light Source (LCLS) and SPring-8 Angstrom Compact free electron Laser (SACLA) provide the Angstrom-wavelength, high-intensity femtosecond pulses needed for revealing the structure of a single protein molecule without crystallization and for observing atomic motion during a chemical reaction via pump–probe experiments. In this paper, we demonstrated X-ray pump–optical probe and optical pump–X-ray probe experiments, creating new possibilities for time-resolved EUV spectroscopy of solid targets including wide-gap semiconductors such as gallium nitride.

Multiple refocusing in BK9 glass generated by a loosely focused high-intensity femtosecond pulse

Multiple refocusing in BK9 glass induced by a loosely focused high-intensity femtosecond pulse is studied experimentally and numerically. Multiple refocusing and small filament are clearly observed in the sample irradiated by loosely focused 120 fs femtosecond laser pulses with high power, up to 500 times the self-focusing critical power. Up to five refocusing peaks are observed along the direction of the pulse propagation. The dependence of the intensity distribution of the plasma luminescence on the pulse energy is investigated.

Femtosecond laser electronic excitation tagging for quantitative velocity imaging in air

Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660 nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.

Electrochemically tunable ultrafast optical response of graphene oxide

We demonstrate reversible and irreversible changes in the ultrafast optical response of multilayer graphene oxide thin films upon electrical and optical stimulus. The reversible effects are due to electrochemical modification of graphene oxide, which allows tuning of the optical response by externally applied bias. Increasing the degree of reduction in graphene oxide causes excited state absorption to gradually switch to saturable absorption for shorter probe wavelengths. Spectral and temporal properties as well as the sign of the ultrafast response can be tuned either by changing the applied bias or exposing to high intensity femtosecond pulses.

Possibility of generating a sequence of attosecond subpulses upon second harmonic generation of high-intensity femtosecond pulses in an optically extended medium

Based on numerical simulation, we demonstrate the possibility of forming a sequence of equalintensity coupled subpulses upon second harmonic generation (SHG) of a high-intensity femtosecond pulse in an optically extended medium whose length is much smaller than the diffraction length of the incident optical radiation. The duration of individual subpulses can be ten (or more) times shorter than the duration of the initial femtosecond pulse. With respect to the spatial coordinate, the breakup of the initial Gaussian or hyper-Gaussian beam into a regular structure of subbeams, whose size can be from two to ten times smaller than the initial beam radius, also takes place. The SHG is shown to result in a limitation of the intensity of axially symmetric beams in a nonlinear focus. The subpulses being formed can propagate with velocities that differ substantially from the velocities of their propagation in the linear medium. The shape of the subpulses is close to a soliton shape, whereas the spatial distribution of the beam has a profile that can be approximated by a set of Gaussian and hyper-Gaussian distributions. The radiation at the double frequency leads to a phase modulation of the optical radiation at the fundamental frequency, which is necessary for the formation of subpulses and subbeams and is the cause of formation of these structures. The formation of subpulses occurs at the fundamental frequency at a small efficiency (≤8%) of energy conversion into the second harmonic.