Optical detection of acoustic waves with surface plasmons.

For broadband and sensitive detection of acoustic waves, the surface plasmon resonance (SPR) can be used, which responds to variations of dielectric properties in close proximity to a metal film supporting surface plasmon waves. When an acoustic wave is incident onto a receiving plate positioned within the penetration depth of the surface plasmons, it creates displacements of the surface of the plate and thus modulates the dielectric properties, affecting the SPR and the reflection of the incident light. Here we study characteristics and determine the optimal configuration of such an acousto-optical transducer with surface plasmons for efficient conversion of an acoustic signal into an optical one. We simulate the properties of this transducer and present estimates showing that it can have a large frequency bandwidth and high sensitivity.

Time slicing in 3D momentum imaging of the hydrogen molecular ion photo-fragmentation.

Photo-fragmentation of the hydrogen molecular ion was investigated with 800 nm, 50 fs laser pulses by employing a time slicing 3D imaging technique that enables the simultaneous measurement of all three momentum components which are linearly related with the pixel position and slicing time. This is done for each individual product particle arriving at the detector. This mode of detection allows us to directly measure the three-dimensional fragment momentum vector distribution without having to rely on mathematical reconstruction methods, which additionally require the investigated system to be cylindrically symmetric. We experimentally reconstruct the laser-induced photo-fragmentation of the hydrogen molecular ion. In previous experiments, neutral molecules were used as a target, but in this work, performed with molecular ions, the initial vibrational level populations are well-defined after electron bombardment, which facilitates the interpretation. We show that the employed time-slicing technique allows us to register the fragment momentum distribution that reflects the initial molecular states with greater detail, revealing features that were concealed in the full time-integrated distribution on the detector.

Probing nonadiabatic molecular alignment by spectral modulation.

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.

White-light generation control with crossing beams of femtosecond laser pulses.

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.

Cascade Raman sideband generation and orbital angular momentum relations for paraxial beam modes.

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.

Pressure optimization of high harmonic generation in a differentially pumped Ar or H2 gas jet.

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 × 10(14) W/cm(2). To enable measurements over a wide range of pressures, we employed differential pumping with an additional cell (∼20 cm(3) 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.

Multipass cell based on confocal mirrors for sensitive broadband laser spectroscopy in the near infrared.

We report on broadband absorption spectroscopy in the near IR using a multipass cell design based on highly reflecting mirrors in a confocal arrangement having the particular aim of achieving long optical paths. We demonstrate a path length of 314 m in a cell consisting of two sets of highly reflecting mirrors with identical focal length, spaced 0.5 m apart. The multipass cell covers this path length in a relatively small volume of 1.25 l with the light beam sampling the whole volume. In a first application, the absorption spectra of the greenhouse gases CO(2), CH(4), and CO were measured. In these measurements we used a femtosecond fiber laser with a broadband spectral range spanning the near IR from 1.5 to 1.7 µm. The absorption spectra show a high signal-to-noise ratio, from which we derive a sensitivity limit of 6 ppmv for methane observed in a mixture with air.

Femtosecond-laser-induced shockwaves in water generated at an air-water interface.

We report generation of femtosecond-laser-induced shockwaves at an air-water interface by millijoule femtosecond laser pulses. We document and discuss the main processes accompanying this phenomenon, including light emission, development of the ablation plume in the air, formation of an ablation cavity, and, subsequently, a bubble developing in water. We also discuss the possibility of remotely controlling the characteristics of laser-induced sound waves in water through linear acoustic superposition of sound waves that results from millijoule femtosecond laser-pulse interaction with an air-water interface, thus opening up the possibility of remote acoustic applications in oceanic and riverine environments.

White-light generation using spatially-structured beams of femtosecond radiation.

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.

Energy transfer between laser filaments in liquid methanol.

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.

Coherent transfer of optical orbital angular momentum in multi-order Raman sideband generation.

Experimental results from the generation of Raman sidebands using optical vortices are presented. By generating two sets of sidebands originating from different locations in a Raman-active crystal, one set containing optical orbital angular momentum and the other serving as a reference, Young's double slit experiment was simultaneously realized for each sideband. The interference between the two sets of sidebands was used to determine the helicity and topological charge in each order. Topological charges in all orders were found to be discrete and follow selection rules predicted by a cascaded Raman process.

In situ tomography of femtosecond optical beams with a holographic knife-edge.

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.

[Dose loads on and radiation risk values for cosmonauts on a mission to Mars estimated from actual Martian vehicle engineering development].

The current design philosophy of a Mars orbiting vehicle, takeoff and landing systems and the transport return vehicle was taken into consideration for calculating the equivalent doses imparted to cosmonaut's organs and tissues by galactic cosmic rays, solar rays and the Earth's radiation belts, values of the total radiation risk over the lifespan following the mission and over the whole career period, and possible shortening of life expectancy. There are a number of uncertainties that should be evaluated, and radiation limits specified before setting off to Mars.

[Space radiation doses in the anthropomorphous phantom in space experiment "Matryeshka-R" and spacesuit "Orlan-M" during extravehicular activity].

Russian space experiment "Matryeshka-R" was conducted in 2004-2005 to study dose distribution in the body of anthropomorphous phantom inserted in a spacesuit imitating container mounted on outer surface of the ISS Service module (experiment "Matryeshka"). The objective was to compare doses inside the phantom in the container to human body donned in spacesuit "Orlan-M" during extravehicular activity (EVA). The shielding function was calculated using the geometric model, specification of the phantom shielded by the container, "Orlan-M" description, and results of ground-based estimation of shielding effectiveness by gamma-raying. Doses were calculated from the dose attenuation curves obtained for galactic cosmic rays, and the AE-8/AP-8 models of electron and proton flows in Earth's radiation belt. Calculated ratios of equivalent doses in representative points of the body critical organs to analogous doses in phantom "Matryeshka" H(ORLAN-M)/H(Matryeshka) for identical radiation conditions vary with organs and solar activity in the range from 0.1 to 1.8 with organs and solar activity. These observations should be taken into account when applying Matryeshka data to the EVA conditions.

Single-snapshot and intensity-resolved two-photon fluorescence measurements.

We present a single-snapshot (SSS) method for obtaining intensity-resolved two-photon fluorescence (TPF). This simple method uses a digital camera to image the TPF spot on a liquid dye jet. By making a comparison between the local laser and TPF intensities, TPF probabilities are reconstructed. We compare our intensity-resolved TPF results with those obtained by the more common intensity scanning (IS) and z-scan methods. The dependence of the TPF probability on intensity obtained by the SSS method for coumarin-30 exhibits a clear maximum around I approximately 4 x 10(12) W/cm(2) and a postsaturation decrease, while no such effects were found in the data obtained by the other methods. Additionally, theoretical models are presented to extract the overall probability from within the volume integral. To our knowledge, we present the first reported measurements of such intensity-resolved TPF.

[Radiation conditions and radiation risks for cosmonauts flying to Mars using electrical jet microthrusters].

According to recent workups, the Mars mission spacecraft will be designed with an electrical jet microthrusters rather than a power reactor facility. The article contains analysis of the main sources of radiation hazard during the exploration mission using this cost-efficient, ecological, easy-to-operate propulsion powered by solar arrays. In addition, the authors make predictions of the generalized doses of ionizing radiation for mission durations of 730 and 900 days behind various shielding thicknesses, and on the Martian surface. Calculation algorithms are described and radiation risks are estimated for the crew life span and possible life time reduction in consequence of participation in the mission.

[Radiation safety provisions in a piloted mission to Mars based on calculated risks of overdose behind shielding].

The article deals with the prime sources of radiation hazard in a mission to Mars, compares the radiation risk values in flight and over the life span with consideration for various shielding thicknesses in habitable compartments and radiation shelter, and estimates possible life shortening. Given the stochastic nature of solar cosmic rays effects in a two-year mission and probability of powerful solar proton events, calculated were not only the mean tissue-equivalent doses behind various thickness of the shelter but also probability of their violation, risks of immediate and delayed radiation consequences and conceivable approaches to risk mitigation.

[Radiation protective quality of spacesuit "Orlan-M" during extravehicular activities on the International Space Station].

Sampling irradiation of spacesuit "Orlan-M" allowed construction of a simulation model of the spacesuit shielding function for critical body organs. The critical organs self-shielding model is a Russian standard anthropomorphic phantom. Radiation protective quality of the spacesuit was assessed by calculating the dose attenuation rates for several critical body organs of an ISS crewmember implementing EVA. These calculations are intended for more accurate assessment of radiation risk to the ISS crews donning "Orlan-M" in near-Earth orbits.

Focal transformation and the Gouy phase shift of converging one-cycle surface acoustic waves excited by femtosecond laser pulses.

We studied the changes of the pulse shape and the phase of the spectral components in converging-surface acoustic wave pulses. These pulses were excited with a femtosecond laser by a thermoelastic mechanism. To produce converging acoustic pulses, the laser beam was focused with an axicon in a circle on the surface of an aluminum sample. During propagation through the focus, the shape of the pulses of the normal surface velocity changed from two to three polar. The absolute value of the phase of the spectral components experienced a change close to pi/2 rad (Gouy phase shift) after passage of the focal region. These observations were confirmed by analytical and numerical calculations based on the two-dimensional wave equation for surface acoustic waves.

Raman spectra of dipicolinic acid in crystalline and liquid environments.

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.