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We investigated molecular alignment wakes of femtosecond laser pulses. Evolution of nonadiabatic molecular alignment in nitrogen gas has been measured via its nonlinear interaction effects with a variably delayed probe pulse. The induced rotational wave packet was mapped as a function of the angular difference between polarization directions of femtosecond pump and probe pulses as well as their relative delay and the plot of the variations of the rotational wave packet, i.e. "quantum carpet", was found to be in good agreement with the calculated angular and temporal dependencies of molecular alignment parameter.

We investigated the variations in generated white-light when crossing two femtosecond laser beams in a Kerr medium. By changing the relative delay of two interacting intense femtosecond laser pulses, we show that white-light generation can be enhanced or suppressed. With a decrease of the relative delay an enhancement of the white-light output was observed, which at even smaller delays was reverted to a suppression of white-light generation. Under choosen conditions, the level of suppression resulted in a white-light output lower than the initial level corresponding to large delays, when the pulses do not overlap in time. The enhancement of the white-light generation takes place in the pulse that is lagging. We found that the effect of the interaction of the beams depends on their relative orientation of polarization and increases when the polarizations are changed from perpendicular to parallel. The observed effects are explained by noting that at intermediate delays, the perturbations introduced in the path of the lagging beam lead to a shortening of the length of filament formation and enhancement of the white-light generation, whereas at small delays the stronger interaction and mutual rescattering reduces the intensity in the central part of the beams, suppressing filamentation and white-light generation.

The surface perturbations related to sound bulk waves are excited in a solid simultaneously with surface acoustic waves and propagate away from the source. These waves arrive before the arrival of the surface wave that gives the main contribution to the observed signal and therefore are referred to as precursors. In the wave zone the precursors can be registered at the surface as separate signals providing information on the elastic moduli of the material. We have observed the precursors for fused silica and stainless steel at relatively large distances due to extremely high pressure amplitudes created in the generation region. The parameters of the precursors were also calculated using a model of a normal force acting on a free solid surface. Relatively higher amplitudes observed in the experiments as compared to theoretical predictions are attributed to the effect of liquid loading during the generation process.

The generation of surface acoustic wave (SAW) pulses in Si single crystal by femtosecond laser pulses with high energy of up to 30 mJ is studied. The generation is rather efficient even at a free silicon surface. In this case the shape of the observed SAW pulses corresponds to the thermoelastic mechanism despite the fact that a well developed ablation takes place. Excitation through a liquid layer is also studied and provides longer SAW pulses. SAW gratings are generated by focusing the laser beam through a mask and the changes in the spectrum of the pulse due to nonlinear interactions are observed.

We use two femtosecond Erbium-doped fiber lasers with slightly different repetition rates to perform a modern type of Fourier transform spectroscopy without moving parts. The measurements are done in real time, and it takes less than 50 microseconds. We work with somewhat different spectral outputs from two Erbium-doped fiber lasers and employ spectral filtering based on a 2f-2f grating setup to select the common spectral region of interest, thereby increasing the signal-to-noise ratio. The interferogram is taken with a 20 cm long gas cell, containing a mixture of acetylene and air at atmospheric pressure, and is fast-Fourier-transformed to obtain the broadband spectral fingerprint of the gas., 4 pages, 4 figures

We studied white-light generation in water using spatially- structured beams of femtosecond radiation. By changing the transverse spatial phase of an initial Gaussian beam with a 1D spatial light modulator to that of an Hermite-Gaussian (HGn,m) mode, we were able to generate beams exhibiting phase discontinuities and steeper intensity gradients. When the spatial phase of an initial Gaussian beam (showing no significant white-light generation) was changed to that of a HG01, or HG11 mode, significant amounts of white-light were produced. Because self-focusing is known to play an important role in white-light generation, the self-focusing lengths of the resulting transverse intensity profiles were used to qualitatively explain this production. Distributions of the laser intensity for beams having step-wise spatial phase variations were modeled using the Fresnel-Kirchhoff integral in the Fresnel approximation and found to be in good agreement with experiment.

We demonstrate energy exchange between two filament-forming femtosecond laser beams in liquid methanol. Our results are consistent with those of previous works documenting coupling between filaments in air; in addition, we identify an unreported phenomenon in which the direction of energy exchange oscillates at increments in the relative pulse delay equal to an optical period (2.6 fs). Energy transfer from one filament to another may be used in remote sensing and spectroscopic applications utilizing femtosecond laser filaments in water and air.

High harmonic generation (HHG) enables the production of coherent light in the extreme-ultraviolet (XUV) spectral region. However, the radiation conversion efficiency from infrared to the XUV is low. We observed enhancement of HHG in gas mixtures of H2 and Ne compared to pure gases H2 or Ne. Our experiments show that at an intensity of ∼1.5×1014 W/cm2 and in mixtures of gases with significantly different ionization potentials (IPs), the initial HHG takes place in the component with lower IP (H2), inducing excited states and facilitating ionization and HHG in the component with higher IP (Ne). We show that the combination of the two gas components results in an enhancement or suppression of the output of high harmonics as compared to pure gases, which is also confirmed by the employed phenomenological model.

The concentration of the acoustic field along preferred directions in different anisotropic single crystals, as well as in a layered medium consisting of a single crystal covered with an amorphous film was studied.

We present experimental results for the ionization of aniline and benzene molecules subjected to intense ultrashort laser pulses. Measured parent molecular ions yields were obtained using a recently developed technique capable of three-dimensional imaging of ion distributions within the focus of a laser beam. By selecting ions originating from the central region of the focus, where the spatial intensity distribution is nearly uniform, volumetric-free intensity-dependent ionization yields were obtained. The measured data revealed a previously unseen resonance-enhanced multiphoton ionization (REMPI)-like process. Comparison of benzene, aniline, and Xe ion yields demonstrates that the observed intensity-dependent structures are not due to geometric artifacts in the focus. Finally for intensities greater than {approx}3x10{sup 13} W/cm{sup 2}, we attribute the ionization of aniline to a stepwise process going through the {pi}{sigma}{sup *} state which sits three photons above the ground state and two photons below the continuum.

We present an in situ beam characterization technique to analyze femtosecond optical beams in a folded version of a 2f-2f setup. This technique makes use of a two-dimensional spatial light modulator (SLM) to holographically redirect radiation between different diffraction orders. This manipulation of light between diffraction orders is carried out locally within the beam. Because SLMs can withstand intensities of up to I ∼ 10(11) W/cm2, this makes them suitable for amplified femtosecond radiation. The flexibility of the SLM was demonstrated by producing a diverse assortment of "soft apertures" that are mechanically difficult or impossible to reproduce. We test our method by holographically knife-edging and tomographically reconstructing both continuous wave and broadband radiation in transverse optical modes.

The transmission of ultrashort (7 fs) broadband laser pulses through periodic gold nano-structures is studied. The distribution of the transmitted light intensity over wavelength and angle shows an efficient coupling of the incident p-polarized light to two counter-propagating surface plasmon (SP) modes. As a result of the mode interaction, the avoided crossing patterns exhibit energy and momentum gaps, which depend on the configuration of the nano-structure and the wavelength. Variations of the widths of the SP resonances and an abrupt change of the mode interaction in the vicinity of the avoided crossing region are observed. These features are explained by the model of two coupled modes and a coupling change due to switching from the higher frequency dark mode to the lower frequency bright mode for increasing wavelength of the excitation light.

We optimized the two-photon fluorescence (TPF) of a Coumarin 30 dye by using a feedback-controlled femtosecond pulse shaping technique. For optimization we implemented an evolutionary algorithm with a liquid crystal phase-only pulse shaper in a folded 4f setup. The optimization procedure applied to the second harmonic generation, and TPF noticeably improved the output signals and demonstrated good convergence. In addition, signal ratios involving TPF and second harmonic generation (SHG) were successfully optimized. The correlation between TPF and SHG was studied, and it was found to decrease when the pulse shape was close to the optimum. These experimental results are of interest for potential applications of coherent control to complex molecular systems as well as in biomedical imaging.

We present an analytical inversion technique which can be used to recover ionization probabilities from spatially averaged data in an N-dimensional detection scheme. The solution is given as a power series in intensity. For this reason, we call this technique a multiphoton expansion (MPE). The MPE formalism was verified with an exactly solvable inversion problem in 2D, and probabilities in the postsaturation region, where the intensity-selective scanning approach breaks down, were recovered. In 3D, ionization probabilities of Xe were successfully recovered with MPE from simulated (using the ADK tunneling theory) ion yields. Finally, we tested our approach with intensity-resolved benzene ion yields showing a resonant multiphoton ionization process. By applying MPE to this data (which was artificially averaged) the resonant structure was recovered-suggesting that the resonance in benzene may have been observable in spatially averaged data taken elsewhere., 19 pages and 3 figures

The propagation of laser-excited surface plasmons along a gold film with surface roughness is directly observed via scattered light. The attenuation length of surface plasmons in a broad wavelength interval is calculated for smooth gold and silver films. The surface roughness, which was characterized with an AFM, introduces corrections to the attenuation length, angular dependence of the surface plasmon resonance, and the effective dielectric constant of the metal film. These corrections are also taken into account and discussed.

Absolute measurements of spectral lines of Ar+ ions using collinear and anticollinear geometries were performed. To provide a precise reference for the laser wavelength, iodine saturation spectroscopy was applied. The precision of this reference is effected by observing the beat node between the spectroscopy laser and the corresponding mode of a femtosecond laser frequency comb. Laser‐induced fluorescence allowed to perform precision frequency measurements of an Ar+ transition in collinear and anticollinear geometries simultaneously; then an exact relativistic formula for the absolute transition frequency V0 = VcVa was used. In this geometry the influence of ion source instabilities due to pressure and anode voltage fluctuations was minimized. The result is v0 = 485,573,619.7(3) MHz, which corresponds to Δv/v = 6*10−10. This represents an improvement of two orders of magnitude over the previous NIST published value.

A novel scheme of collinear fast beam laser spectroscopy for the detection of the long lived rare isotope 85Kr by observing the optical hyperfine structure spectrum is presented. The technique utilizes cascade two‐step excitation to pump metastable krypton atoms to a high‐lying Rydberg level. The present work on krypton was motivated by the fact that 85Kr is a major tracer gas for exploring the reservoir structure of large oil fields. 85Kr detection in ambient air is also of importance for monitoring nuclear activities on a world wide scale. The technique has been successfully applied to stable krypton isotopes and to 85Kr. The selectivity is at the one part in 1010 level and the sensitivity at a few hundred ions/s.

In this work, the nonlinear parametric interaction of optical radiation in various transverse modes in a Raman-active medium is investigated both experimentally and theoretically. Verification of the orbital angular momentum algebra (OAM-algebra) [Strohaber et al.,Opt. Lett.37,3411 (2012)] was performed for high-order Laguerre Gaussian modes l>1. It was found that this same algebra also describes the coherent transfer of OAM when Ince-Gaussian modes were used. New theoretical considerations extend the OAM-algebra to even and odd Laguerre Gaussian, and Hermite Gaussian beam modes through a change of basis. The results of this work provide details in the spatiotemporal synthesis of custom broadband pulses of radiation from Raman sideband generation.

We utilize mid-infrared dual frequency comb spectroscopy for the detection of methane in ambient air. Two mid-infrared frequency comb sources based on femtosecond Er:fiber oscillators are produced through difference frequency generation with periodically poled MgO-doped lithium niobate crystals and stabilized at slightly different repetition rates at about 250 MHz. We performed dual frequency comb spectroscopy in the spectral range between 2900 cm−1 and 3150 cm−1 with 0.07 cm−1 resolution using a multipass cell of ~580 m path length, and achieved the sensitivity about 7.6 × 10−7 cm−1 with 80 ms data acquisition time. We determined the methane concentration as ~1.5 ppmv in the ambient air of the laboratory, and the detection limit as ~60 ppbv for the current setup.

We experimentally studied the dependence of high harmonic generation in argon and molecular hydrogen on pressure changes in a gas jet that cause variations of the phase matching conditions and absorption. The study was performed at a peak laser intensity of ∼1.5 × 1014 W/cm2. To enable measurements over a wide range of pressures, we employed differential pumping with an additional cell (∼20 cm3 volume) enclosing the gas jet. By increasing the pressure in the gas jet up to a maximum of 1.5 bars with argon or 0.5 bars with hydrogen, we observed an increase in the high harmonic (HH) yield until an optimum pressure of 0.2 bars was reached for Ar, beyond which the output began decreasing. For H2, we observed an increase of the HH output up to the maximum pressure of 0.5 bars. This pressure-dependence study allowed us to achieve a tenfold enhancement in the high harmonic yield at the optimum pressure.

It has become increasingly evident that fabrication of novel biomaterials through molecular self-assembly is going to play a significant role in material science and possibly the information technology of the future. Tubulin, microtubules (MTs) and the cytoskeleton are dynamic, self-assembling systems and we asked whether their structure and function contain the clues on how to fabricate biomolecular information processing devices. Here we review our neurobiological studies of transgenic Drosophila that strongly suggest the microtubular cytoskeleton is near the 'front lines' of intracellular information manipulation and storage. We also establish that spectroscopic techniques such as refractometry, surface plasmon resonance sensing and dielectric spectroscopy, coupled with molecular dynamic simulations and (quantum) electrodynamic analytical theory are useful tools in the study of the electrodynamic and possible quantum effects in cytoskeletal proteins. Implicit in our driving question is the possibility that if tubulin and MTs can indeed be cast as the basis of a classical or quantum computer, then perhaps nature has already done so and tubulin and MTs already play such a role in living neural and other cells. If quantum mechanics is found to be important in cellular function (through its involvement in proteins) it is natural to ask whether there are indeed quantum effects pertinent to consciousness. We conclude this work by suggesting a number of ways towards experimentally testing this "Quantum Consciousness Idea" (QCI), directly at the protein level.

We use time-resolved coherent Raman spectroscopy to obtain molecule-specific signals from dipicolinic acid (DPA), which is a marker molecule for bacterial spores. We use femtosecond laser pulses in both visible and UV spectral regions and compare experimental results with theoretical predictions. By exciting vibrational coherence on more than one mode simultaneously, we observe a quantum beat signal that can be used to extract the parameters of molecular motion in DPA. The signal is enhanced when an UV probe pulse is used, because its frequency is near-resonant to the first excited electronic state of the molecule. The capability for unambiguous identification of DPA molecules will lead to a technique for real-time detection of spores.

Raman spectra of dipicolinic acid (DPA) are important for detection of bacterial spores, since DPA and its salts present one of their major components. The implementation of a deeply cooled CCD camera in combination with pulsed excitation at 532 nm allowed measuring well-resolved Raman spectra of the DPA in different forms. Powder preparations, crystals grown from saturated solutions and aqueous solutions of the DPA were studied. The spectral features in different environments and comparison with the spectra obtained by other methods are discussed.

Thin films of the molecular magnet Mn12-acetate, [Mn12 O12(CH3COO)16 (H2O)4] 2CH3COOH 4H2O, have been prepared using a laser ablation technique with a nitrogen laser at low laser energies of 0.8 and 2 mJ. Chemical and magnetic characterizations show that the Mn12-acetate cores remain intact and the films show similar magnetic properties to those of the parent molecular starting material. In addition, the magnetic data exhibit a peak in the magnetization at 27 K indicating the creation of an additional magnetic phase not noted in previous studies of crystalline phases., Comment: 8 pages, 5 figures, In Press - J. Mag. Mag. Mat

Shake‐off of micron‐sized alumina particles by nanosecond laser‐generated surface acoustic wave (SAW) pulses is used to visualize SAW beam propagation on surfaces of semiconductor crystals. Various phenomena in SAW propagation such as reflection from a sample edge, anisotropic diffraction, and beam steering are demonstrated.

The nonlinear evolution (compression and stretching) of the profile of very high-amplitude surface acoustic wave (SAW) pulses with acoustic Mach numbers Mc0.01 and the formation of shock fronts were studied in solid materials. The excitation of such giant SAW pulses was performed with nanosecond laser pulses through a strongly absorbing layer and used for the determination of the nonlinear acoustic constants. The mechanical forces associated with the large surface acceleration in high-amplitude SAWs detach particles from the surface. For nanosecond SAW pulses the limit of the surface acceleration of about 1010 m/s2 is set by the fracture of the material and corresponds to the removal of all particles larger than about 0.1 μm. This technique can also be used for the determination of the Hamaker constant of the adhesion force.

The nonlinear propagation of high-amplitude surface acoustic wave (SAW) pulses in two isotropic materials, polycrystalline aluminum and synthetic fused silica, was studied and exhibited qualitatively different types of nonlinear behavior. A single SAW pulse excited by a nanosecond laser pulse through a strongly absorbing layer was detected at two probe spots along the propagation path with a dual-probe-beam deflection setup. In this way the nonlinear changes of the SAW pulse shape were observed. For aluminum, the compression of the SAW pulse and formation of one negative (inward the solid) narrow peak in the registered normal surface velocity and a shock front in the in-plane velocity were detected. This nonlinear behavior corresponds to a positive value of the nonlinear acoustic constant responsible for the local nonlinearity. For fused silica, the temporal extension of the SAW pulse and formation of two positive sharp peaks in the normal velocity related to two shock fronts in the in-plane velocity were registered. In this case the acoustic constant of the local nonlinearity is negative. The nonlinear acoustic constants for each material were evaluated by fitting the theoretical model based on the nonlinear evolution equation to the registered SAW pulses. The values obtained were found to be consistent with those calculated from nonlinear elastic moduli of the third order, measured previously with different techniques for similar materials. The characterization of solids by their nonlinear acoustic and elastic constants promises to be complimentary and more specific than the characterization based on the linear properties.

The identification of interacting biomolecules in surface plasmon resonance (SPR) measurements can be achieved by attaching small characteristic covalent fluorophores. To implement this technique we studied the SPR response in the vicinity of an absorption band of dye molecules in ethanol and aqueous solutions. It is shown that the indirect determination of the absorption is possible from changes it induces in the SPR characteristics. Another technique studied employs the direct monitoring of the fluorescence signal in the surface plasmon field. The fluorescence light is then collected in the flow cell near the surface of a gold film with an optical fiber and transmitted to the registering photomultiplier. A significant enhancement of the fluorescent signal due to the excitation in the localized surface plasmon field was observed.