Time-resolved signatures across the intramolecular response in substituted cyanine dyes.

The optically populated excited state wave packet propagates along multidimensional intramolecular coordinates soon after photoexcitation. This action occurs alongside an intermolecular response from the surrounding solvent. Disentangling the multidimensional convoluted signal enables the possibility to separate and understand the initial intramolecular relaxation pathways over the excited state potential energy surface. Here we track the initial excited state dynamics by measuring the fluorescence yield from the first excited state as a function of time delay between two color femtosecond pulses for several cyanine dyes having different substituents. We find that when the high frequency pulse precedes the low frequency one and for timescales up to 200 fs, the excited state population can be depleted through stimulated emission with efficiency that is dependent on the molecular electronic structure. A similar observation at even shorter times was made by scanning the chirp (frequencies ordering) of a femtosecond pulse. The changes in depletion reflect the rate at which the nuclear coordinates of the excited state leave the Franck-Condon (FC) region and progress towards achieving equilibrium. Through functional group substitution, we explore these dynamic changes as a function of dipolar change following photoexcitation. Density functional theory calculations were performed to provide greater insight into the experimental spectroscopic observations. Complete active space (CAS) self-consistent field and CAS second order perturbation theory calculated potential energy surfaces tracking twisting and pyramidalization confirm that the steeper potential at the FC region leads to the observation of faster wave packet dynamics.

Order of Magnitude Dissociative Ionization Enhancement Observed for Pulses with High Order Dispersion.

While the interaction of atoms in strong fields is well understood, the same cannot be said about molecules. We consider how dissociative ionization of molecules depends on the quality of the femtosecond laser pulses, in particular, the presence of third- and fourth-order dispersion. We find that high-order dispersion (HOD) unexpectedly results in order-of-magnitude enhanced ion yields, along with the factor of 3 greater kinetic energy release compared to transform-limited pulses with equal peak intensities. The magnitude of these effects is not caused by increased pulse duration. We evaluate the role of pulse pedestals produced by HOD and other pulse shaping approaches, for a number of molecules including acetylene, methanol, methylene chloride, acetonitrile, toluene, and o-nitrotoluene, and discuss our findings in terms of processes such as prealignment, preionization, and bond softening. We conclude, based on the quasi-symmetric temporal dependence of the observed enhancements that cascade ionization is likely responsible for the large accumulation of charge prior to the ejection of energetic fragments along the laser polarization axis.

Stimulated Emission Enhancement Using Shaped Pulses.

Controlling stimulated emission is of importance because it competes with absorption and fluorescence under intense laser excitation. We performed resonant nonlinear optical spectroscopy measurements using femtosecond pulses shaped by π- or π/2-step phase functions and carried out calculations based on density matrix representation to elucidate the experimental results. In addition, we compared enhancements obtained when using other pulse shaping functions (chirp, third-order dispersion, and a time-delayed probe). The light transmitted through the high optical density solution was dominated by an intense stimulated emission feature that was 14 times greater for shaped pulses than for transform limited pulses. Coherent enhancement depending on the frequency, temporal, and phase characteristics of the shaped pulse is responsible for the experimental observations.

Controlling S2 Population in Cyanine Dyes Using Shaped Femtosecond Pulses.

Fast population transfer from higher to lower excited states occurs via internal conversion (IC) and is the basis of Kasha's rule, which states that spontaneous emission takes place from the lowest excited state of the same multiplicity. Photonic control over IC is of interest because it would allow direct influence over intramolecular nonradiative decay processes occurring in condensed phase. Here we tracked the S2 and S1 fluorescence yield for different cyanine dyes in solution as a function of linear chirp. For the cyanine dyes with polar solvation response IR144 and meso-piperidine substituted IR806, increased S2 emission was observed when using transform limited pulses, whereas chirped pulses led to increased S1 emission. The nonpolar solvated cyanine IR806, on the other hand, did not show S2 emission. A theoretical model, based on a nonperturbative solution of the equation of motion for the density matrix, is offered to explain and simulate the anomalous chirp dependence. Our findings, which depend on pulse properties beyond peak intensity, offer a photonic method to control S2 population thereby opening the door for the exploration of photochemical processes initiated from higher excited states.

Mitigating self-action processes with chirp or binary phase shaping.

We report the use of binary phase shaping to mitigate pulse degradation and self-focusing in fused silica. The results of simulation and estimated mitigation efficiency are supported by experimental results using both chirped and binary phase-shaped pulses. Possible applications are considered.

Phase-only synthesis of ultrafast stretched square pulses.

We report the generation of square temporal-shape pulses with no loss of spectral bandwidth, using an analytic expression for the spectral phase modulation dependent only on the input spectrum and stretching factor. We demonstrate numerically and experimentally conversion of 40fs pulses into 150 times longer flat top pulses with sharp on and off fronts. Applications in pulse amplification and free electron lasers are considered.

Spectral amplitude and phase noise characterization of titanium-sapphire lasers.

There are few methods capable of characterizing pulse-to-pulse noise in high repetition rate ultrafast lasers. Here we use a recently developed method, termed fidelity, to determine the spectral amplitude and phase noise that leads to lack of pulse repeatability and degrades the performance of laser sources. We present results for a titanium sapphire oscillator and a regenerative amplifier system under different noise conditions. Our experimental results are backed by numerical calculations.

Femtosecond Nanoplasmonic Dephasing of Individual Silver Nanoparticles and Small Clusters.

We present experimental measurements of localized surface plasmon emission from individual silver nanoparticles and small clusters via accurately delayed femtosecond laser pulses. Fourier transform analysis of the nanoplasmonic coherence oscillations reveals different frequency components and dephasing rates for each nanoparticle. We find three different types of behavior: single exponential decay, beating between two frequencies, and beating among three or more frequencies. Our results provide insight into inhomogeneous and homogeneous broadening mechanisms in nanoplasmonic spectroscopy that depend on morphology and nearby neighbors. In addition, we find the optical response of certain pairs of nanoparticles to be at least an order of magnitude more intense than the response of single particles.

Quantifying noise in ultrafast laser sources and its effect on nonlinear applications.

Nonlinear optical applications depend on pulse duration and coherence of the laser pulses. Characterization of high-repetition rate pulsed laser sources can be complicated by their pulse-to-pulse instabilities. Here, we introduce and demonstrate experimentally a quantitative measurement that can be used to determine the pulse-to-pulse fidelity of ultrafast laser sources. Numerical simulations and experiments illustrate the effect of spectral phase and amplitude noise on second and third harmonic generation.

Investigating the role of human serum albumin protein pocket on the excited state dynamics of indocyanine green using shaped femtosecond laser pulses.

Differences in the excited state dynamics of molecules and photo-activated drugs either in solution or confined inside protein pockets or large biological macromolecules occur within the first few hundred femtoseconds. Shaped femtosecond laser pulses are used to probe the behavior of indocyanine green (ICG), the only Food and Drug Administration (FDA) approved near-infrared dye and photodynamic therapy agent, while free in solution and while confined inside the pocket of the human serum albumin (HSA) protein. Experimental findings indicate that the HSA pocket hinders torsional motion and thus mitigates the triplet state formation in ICG. Low frequency vibrational motion of ICG is observed more clearly when it is bound to the HSA protein.

Polyatomic molecules under intense femtosecond laser irradiation.

Interaction of intense laser pulses with atoms and molecules is at the forefront of atomic, molecular, and optical physics. It is the gateway to powerful new tools that include above threshold ionization, high harmonic generation, electron diffraction, molecular tomography, and attosecond pulse generation. Intense laser pulses are ideal for probing and manipulating chemical bonding. Though the behavior of atoms in strong fields has been well studied, molecules under intense fields are not as well understood and current models have failed in certain important aspects. Molecules, as opposed to atoms, present confounding possibilities of nuclear and electronic motion upon excitation. The dynamics and fragmentation patterns in response to the laser field are structure sensitive; therefore, a molecule cannot simply be treated as a "bag of atoms" during field induced ionization. In this article we present a set of experiments and theoretical calculations exploring the behavior of a large collection of aryl alkyl ketones when irradiated with intense femtosecond pulses. Specifically, we consider to what extent molecules retain their molecular identity and properties under strong laser fields. Using time-of-flight mass spectrometry in conjunction with pump-probe techniques we study the dynamical behavior of these molecules, monitoring ion yield modulation caused by intramolecular motions post ionization. The set of molecules studied is further divided into smaller sets, sorted by type and position of functional groups. The pump-probe time-delay scans show that among positional isomers the variations in relative energies, which amount to only a few hundred millielectronvolts, influence the dynamical behavior of the molecules despite their having experienced such high fields (V/Å). High level ab initio quantum chemical calculations were performed to predict molecular dynamics along with single and multiphoton resonances in the neutral and ionic states. We propose the following model of strong-field ionization and subsequent fragmentation for polyatomic molecules: Single electron ionization occurs on a suboptical cycle time scale, and the electron carries away essentially all of the energy, leaving behind little internal energy in the cation. Subsequent fragmentation of the cation takes place as a result of further photon absorption modulated by one- and two-photon resonances, which provide sufficient energy to overcome the dissociation energy. The proposed hypothesis implies the loss of a photoelectron at a rate that is faster than intramolecular vibrational relaxation and is consistent with the observation of nonergodic photofragmentation of polyatomic molecules as well as experimental results from many other research groups on different molecules and with different pulse durations and wavelengths.

Laser-induced dispersion control.

An intense laser pulse is used to control the spectral phase of a weak probe pulse as they overlap in fused silica. The laser-induced linear chirp is controlled by the delay time between pulses. Dependence from intensity and spectral phase of the pump pulse is also studied. Experimental data is validated by numerical simulation based on optical Kerr effect. Results show that laser-induced pulse shaping is possible and may be useful for intracavity pulse compression and shaping in enhancement cavities.

Solvent Environment Revealed by Positively Chirped Pulses.

The spectroscopy of large organic molecules and biomolecules in solution has been investigated using various time-resolved and frequency-resolved techniques. Of particular interest is the early response of the molecule and the solvent, which is difficult to study due to the ambiguity in assigning and differentiating inter- and intramolecular contributions to the electronic and vibrational populations and coherence. Our measurements compare the yield of fluorescence and stimulated emission for two laser dyes IR144 and IR125 as a function of chirp. While negatively chirped pulses are insensitive to solvent viscosity, positively chirped pulses are found to be uniquely sensitive probes of solvent viscosity. The fluorescence maximum for IR125 is observed near transform-limited pulses; however, for IR144, it is observed for positively chirped pulses once the pulses have been stretched to hundreds of femtoseconds. We conclude that chirped pulse spectroscopy is a simple one-beam method that is sensitive to early solvation dynamics.

Anomalous laser-induced group velocity dispersion in fused silica.

We present 20fs(2) accuracy laser-induced group velocity dispersion (LI-GVD) measurements, resulting from propagation of a femtosecond laser pulse in 1mm of fused silica, as a function of peak intensity. For a 5.5 × 10(11) W/cm(2) peak intensity, LI-GVD values are found to vary from -3 to + 15 times the material GVD. Normal induced dispersion can be explained by the Kerr effect, but anomalous LI-GVD, found when the input pulses have negative pre-chirp, cannot. These findings have significant implications regarding self-compression and the design of femtosecond lasers.

Pulse duration and energy dependence of photodamage and lethality induced by femtosecond near infrared laser pulses in Drosophila melanogaster.

The potential application of nonlinear optical imaging diagnosis and treatment using femtosecond laser pulses in humans accentuates the need for studies carried out in whole organisms instead of single cells or cell cultures. While there is a general consensus that in order to minimize the level of photodamage the excitation power has to be kept as low as possible, it has yet to be determined if shorter pulses have greater benefit than longer pulses. Here we evaluate the rate of death in Drosophila melanogaster as the integral parameter related to photodamage resulting from femtosecond near infrared (NIR) laser irradiation under conditions comparable to those used in two-photon excited fluorescence (TPEF) microscopy. We found that the lethality (resulting from photodamage) as a function of laser energy fluence fits a 3-region dose-response curve. The lethality was accompanied with development of necrosis and apoptosis in irradiated tissues. Quantitative analysis showed that the damage has a mostly linear character on energy fluence per pulse, and for a given TPEF signal, shorter (37 fs) pulse duration results in lower lethality than longer (100 fs) pulse duration. These results have important implications for the use of femtosecond NIR laser pulses in microscopy as well as in vivo medical imaging.

Optical Response of Fluorescent Molecules Studied by Synthetic Femtosecond Laser Pulses.

The optical response of the fluorescent molecule IR144 in solution is probed by pairs of collinear pulses with intensity just above the linear dependence using two different pulse shaping methods. The first approach mimics a Michelson interferometer, while the second approach, known as multiple independent comb shaping (MICS), eliminates spectral interference. The comparison of interfering and non-interfering pulses reveals that linear interference between the pulses leads to the loss of experimental information at early delay times. In both cases, the delay between the pulses is controlled with attosecond resolution and the sample fluorescence and stimulated emission are monitored simultaneously. An out-of-phase behavior is observed for fluorescence and stimulated emission, with the fluorescence signal having a minimum at zero time delay. Experimental findings are modeled using a two-level system with relaxation that closely matches the phase difference between fluorescence and stimulated emission and the relative intensities of the measured effects.

Solvation Stokes-Shift Dynamics Studied by Chirped Femtosecond Laser Pulses.

The early optical dynamic response, resulting population, and electronic coherence are investigated experimentally and modeled theoretically for IR144 in solution. The fluorescence and stimulated emission response are studied systematically as a function of chirp. The magnitude of the chirp effect on fluorescence and stimulated emission is found to depend quadratically on pulse energy, even where excitation probabilities range from 0.02 to 5%, in the so-called "linear excitation regime". Interestingly, the shape of the chirp dependence on fluorescence and stimulated emission is found to be independent of pulse energy. The chirp dependence reveals dynamics related to solvent rearrangement following excitation and also depends on electronic relaxation of the chromophore. The experimental results are successfully simulated using a four-level model in the presence of inhomogeneous broadening of the electronic transitions.

Mechanism elucidation for nonstochastic femtosecond laser-induced ionization/dissociation: from amino acids to peptides.

Femtosecond laser-induced ionization/dissociation (fs-LID) has been demonstrated as a novel ion activation method for use in tandem mass spectrometry. The technique opens the door to unique structural information about biomolecular samples that is not easily accessed by traditional means. fs-LID is able to cleave strong bonds while keeping weaker bonds intact. This feature has been found to be particularly useful for the mapping of post-translational modifications such as phosphorylation, which is difficult to achieve by conventional proteomic studies. Here we investigate the laser-ion interaction on a fundamental level through the characterization of fs-LID spectra for the protonated amino acids and two series of derivatized samples. The findings are used to better understand the fs-LID spectra of synthetic peptides. This is accomplished by exploring the effects of several single-residue substitutions.

Group-velocity-dispersion measurements of atmospheric and combustion-related gases using an ultrabroadband-laser source.

The use of femtosecond-laser sources for the diagnostics of combustion and reacting-flow environments requires detailed knowledge of optical dispersive properties of the medium interacting with the laser beams. Here the second- and third-order dispersion values for nitrogen, oxygen, air, carbon dioxide, ethylene, acetylene, and propane within the 700-900 nm range are reported, along with the pressure dependence of the chromatic dispersion. The effect of dispersion on axial resolution when applied to nonlinear spectroscopy with ultrabroadband pulses is also discussed.

Photodissociation dynamics of acetophenone and its derivatives with intense nonresonant femtosecond pulses.

Recent work from our group (Lozovoy, V. V.; Zhu, X.; Gunaratne, T. C.; Harris, D. A.; Shane, J. C.; Dantus, M. J. Phys. Chem. A2008, 112, 3789) using shaped nonresonant femtosecond pulses to ionize and fragment polyatomic molecules indicated that pulse duration is the most important parameter for controlling the relative yield of different fragment ions. Here we explore the time-resolved dynamics that ensue following the interaction of the molecules with a strong 10(15) W/cm(2) nonresonant near-infrared laser field. The data reveal that most of the fragmentation processes occur well after ionization. The molecular dynamics are followed in the 10(-14)-10(-10) s time scale. Studies carried out on acetophenone derivatives are used to assign the observed modulation in the benzoyl product ion yield, which is found to correlate with further ionization and fragmentation through electronic coordination. The resulting experimental data, together with photoelectron spectra and the electron-ionization mass spectra of these compounds, allow us to propose ladder switching processes taking place in this family of compounds which regulate the different fragment ions observed. This analysis sheds light on how pulse duration influences the yield of different fragment ions.