Interferometric quantum control (IQC) by fs/ns rotational coherent anti-Stokes Raman spectroscopy (RCARS).

A new rotational coherent anti-Stokes Raman spectroscopy (RCARS) concept based on interferometric quantum control (IQC) is demonstrated. Two wavepackets originating from pure rotational states are created by a femtosecond stimulated rotational Raman interaction. The two Raman responses are instantly probed by a single-mode ns pulse generating two interfering RCARS polarizations. The resulting signal is an IQC-RCARS spectrum detected by a streak camera. Here we demonstrate IQC-interferograms of N2 by varying the temporal separation between the two fs pulses within a full rotational revival period, as well as signal amplification and selective detection of nuclear-spin isomers at room conditions and inside a flame.

Assessment and application of wavelet-based optical flow velocimetry (wOFV) to wall-bounded turbulent flows.

The performance of a wavelet-based optical flow velocimetry (wOFV) algorithm in extracting high accuracy and high-resolution velocity fields from tracer particle images in wall-bounded turbulent flows is assessed. wOFV is first evaluated using synthetic particle images generated from a channel flow DNS of a turbulent boundary layer. The sensitivity of wOFV to the regularization parameter ( λ ) is quantified and results are compared to cross-correlation-based PIV. Results on synthetic particle images indicated different sensitivity to under-regularization or over-regularization depending on which region of the boundary layer is being analyzed. Nonetheless, tests on synthetic data revealed that wOFV can modestly outperform PIV in vector accuracy across a broad λ range. wOFV showed clear advantages over PIV in resolving the viscous sublayer and obtaining highly accurate estimates of the wall shear stress and thus normalizing boundary layer variables. wOFV was also applied to experimental data of a developing turbulent boundary layer. Overall, wOFV revealed good agreement with both PIV and a combined PIV + PTV method. However, wOFV was able to successfully resolve the wall shear stress and correctly normalize the boundary layer streamwise velocity to wall units where PIV and PIV + PTV showed larger deviations. Analysis of the turbulent velocity fluctuations revealed spurious results for PIV in close proximity to the wall, leading to significantly exaggerated and non-physical turbulence intensity in the viscous sublayer region. PIV + PTV showed only a minor improvement in this aspect. wOFV did not exhibit this same effect, revealing that it is more accurate in capturing small-scale turbulent motion in the vicinity of boundaries. The enhanced vector resolution of wOFV enabled improved estimation of instantaneous derivative quantities and intricate flow structure both closer to the wall and more accurately than the other velocimetry methods. These aspects show that, within a reasonable λ range that can be verified using physical principles, wOFV can provide improvements in diagnostics capability in resolving turbulent motion occurring in the vicinity of physical boundaries.

Single-shot coherent control of molecular rotation by fs/ns rotational coherent anti-Stokes Raman spectroscopy.

We present a novel method, to our knowledge, to control the shape of the spectra using 2-beam hybrid femtosecond (fs)/nanosecond (ns) coherent anti-Stokes Raman scattering (RCARS). The method is demonstrated experimentally and theoretically by utilizing a species-selective excitation approach via a field-free molecular alignment as an illustrative example. Two non-resonant fs laser pulses with proper delay selectively create and then annihilate N2 resonances in a binary mixture with O2 molecules. The RCARS signal is simultaneously resolved in spectral and temporal domains within a single-shot acquisition. The method requires very low pulse energies for excitation, hence minimizing multiphoton ionization probability, allowing for coherent control at various temperatures and pressures, with spectroscopic applications in non-stationary and unpredictable reacting flows.

Dual-probe 1D hybrid fs/ps rotational CARS for simultaneous single-shot temperature, pressure, and O2/N2 measurements.

We employ dual-probe one-dimensional (1D) femtosecond (fs)/picosecond (ps) hybrid rotational coherent anti-Stokes Raman spectroscopy (HRCARS) to investigate simultaneous temperature, pressure, and O2/N2 measurements for gas-phase diagnostics. The dual-probe HRCARS technique allows for simultaneous measurements from the time and frequency-domain. A novel approach for measuring pressure, which offers high accuracy (<1%) and precision (0.42%), is presented. The technique is first demonstrated in a chamber for a range of pressures (1-1.5 bar). This technique shows an impressive capability of resolving 1D pressure gradients arising from a N2 jet impinging on a surface, both in laminar and turbulent conditions. The technique is shown to be capable of resolving single-shot pressure gradients (0.04 bar/mm) originating from kinetic energy conversion to pressure and resolves characteristic O2/N2 structures from laminar and turbulent mixing.

Generation of narrowband pulses from chirped broadband pulse frequency mixing.

We extend an approach based upon sum-frequency generation of oppositely chirped pulses to narrow the bandwidths of broadband femtosecond pulses. We efficiently generate near-transform-limited pulses with durations of several picoseconds, while reducing the pulse bandwidth by a factor of 120, which is more than twice the reduction reported in previous literature. Such extreme bandwidth narrowing of a broadband pulse enhances the effects of dispersion nonlinearities. Precise chirp control enables us to characterize the efficacy of frequency mixing broadband pulses with nonlinear temporal chirps. We demonstrate the use of these narrowband pulses as probes in coherent anti-Stokes Raman spectroscopy.

Visualization of acceleration in multiphase fluid interactions.

Probing the dynamics of structures in turbid media is important for understanding the internal forces that drive the time evolution of many fluid systems; the breakup of fuel injection sprays is a prime example. We demonstrate a three-pulse configuration for time-gated ballistic imaging, applied to a turbulent, steady spray allowing the acquisition of time-correlated image data. Coupled with targeted region-matching analysis, the detected image triplets are used to generate time-resolved velocity and acceleration vectors representing motion and forces involved in spray development.

Optical sectioning for measurements in transient sprays.

We describe a practical arrangement for optical sectioning by means of time-gated backscatter imaging using ultrafast illumination and a CS2-based optical Kerr effect shutter. This arrangement can reveal additional information when probing transient turbid media such as fuel injection sprays or complex multiphase flows which require single-shot imaging with sufficient time resolution to freeze the dynamics of the flow.

Evaluation of optical arrangements for ballistic imaging in sprays.

This work investigates the imaging performance, in terms of contrast and resolution, of two different time-gated ballistic imaging setups commonly used in spray research. It is shown that the two setups generate similar spatial resolution in the presence of scattering media. The simpler (2f) setup, however, is less sensitive to component misalignments and time-gate induced aberrations than the commonly used (4f) system. Measurements comparing both arrangements indicated slightly higher contrast for the 2f system under the densest conditions for small scatterers. Subsequent computational modeling confirmed the observed tolerance of the 2f system to misalignment and gate effects. The best performing setup was also compared experimentally to its non-time-gated shadow-imaging equivalent, to establish when the time-gate enhances imaging performance. It is shown that the time-gated setup generates higher contrast under almost all of the scattering conditions tested, while the non-time-gated setup generates higher spatial resolution only in the lower scatterer size range at the lowest scatterer concentrations.

Quantitative image contrast enhancement in time-gated transillumination of scattering media.

Experimental work in turbid media has shown that trans-illumination images can be significantly improved by limiting light collection to a subset of photons which are minimally distorted by scattering. The literature details numerous schemes (commonly termed ballistic imaging), most often based on time-gating and/or spatially filtering the detected light. However, due to the complex nature of the detected signal, analysis of this optical filtering process has been heretofore limited to qualitative comparisons of image results. In this article we present the implementation of a complete system model for the simulation of light propagation, including both the scattering medium and all stages of the optical train. Validation data from ballistic imaging (BI) measurements of monodisperse scatterers with diameter, d = 0.7 µm, at optical depths 5, 10, and 14, are compared with model results, showing excellent agreement. In addition, the validated model is subsequently applied to a modified time-gated optical system to probe the comparative performance of the BI system used in validation and the modified BI system. This instrument comparison examines scatterers with diameters of 0.7 and 15 µm at optical depths 10 and 14, and highlights the benefits of each system design for these specific scattering conditions. These results show that the modified optics configuration is more suitable for particles which are much larger than the incident wavelength, d >> λ, while the configuration employed in the validation system provides a better contrast for particle diameters on the order of the wavelength, d ~λ, where the scattering process exhibits a more homogeneous phase function. The insights and predictions made available by the full numerical model are important for the design of optimized imaging systems suited to specific turbid media, and make possible the quantitative understanding of both the effects of light propagation in the measurement and the performance of the complete imaging system.

Note: Fixture for characterizing electrochemical devices in-operando in traditional vacuum systems.

We describe a fixture that allows electrochemical devices to be studied under electrical bias in the type of vacuum systems commonly used in surface science. Three spring-loaded probes provide independent contacts for device operation and the characterization in vacuum or under in situ conditions with reactive gases. We document the robustness of the electrical contacts over large temperature changes and their reliability for conventional electrochemical measurements such as impedance spectroscopy. The optical access provided to the device enables the analysis by many techniques, as we demonstrate using x-ray photoelectron spectroscopy to measure local electrical potentials on a solid-oxide electrolyte device operating at high temperature in near-ambient pressure.

Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy.

Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO(2-x)/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H(2) and H(2)O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO(2-x) electrodes undergo Ce(3+)/Ce(4+) oxidation-reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H(2) electro-oxidation and H(2)O electrolysis. The active regions extend ~150 µm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce(3+)/Ce(4+) shifts in the ~150 µm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.

Measuring individual overpotentials in an operating solid-oxide electrochemical cell.

We use photo-electrons as a non-contact probe to measure local electrical potentials in a solid-oxide electrochemical cell. We characterize the cell in operando at near-ambient pressure using spatially-resolved X-ray photoemission spectroscopy. The overpotentials at the interfaces between the Ni and Pt electrodes and the yttria-stabilized zirconia (YSZ) electrolyte are directly measured. The method is validated using electrochemical impedance spectroscopy. Using the overpotentials, which characterize the cell's inefficiencies, we compare without ambiguity the electro-catalytic efficiencies of Ni and Pt, finding that on Ni H(2)O splitting proceeds more rapidly than H(2) oxidation, while on Pt, H(2) oxidation proceeds more rapidly than H(2)O splitting.

Fast-framing ballistic imaging of velocity in an aerated spray.

We describe further development of ballistic imaging adapted for the liquid core of an atomizing spray. To fully understand spray breakup dynamics, one must measure the velocity and acceleration vectors that describe the forces active in primary breakup. This information is inaccessible to most optical diagnostics, as the signal is occluded by strong scattering in the medium. Ballistic imaging mitigates this scattering noise, resolving clean shadowgram-type images of structures within the dense spray region. We demonstrate that velocity data can be extracted from ballistic images of a spray relevant to fuel-injection applications, by implementing a simple, targeted correlation method for determining velocity from pairs of spray images. This work presents the first ballistic images of a liquid-fuel injector for scramjet combustion, and the first velocity information from ballistic images relevant to breakup in the near-field of a spray.

Laser light scattering in turbid media Part II: Spatial and temporal analysis of individual scattering orders via Monte Carlo simulation.

In Part I of this study [1], good agreement between experimental measurements and results from Monte Carlo simulations were obtained for the spatial intensity distribution of a laser beam propagating within a turbid environment. In this second part, the validated Monte Carlo model is used to investigate spatial and temporal effects from distinct scattering orders on image formation. The contribution of ballistic photons and the first twelve scattering orders are analyzed individually by filtering the appropriate data from simulation results. Side-scattering and forward-scattering detection geometries are investigated and compared. We demonstrate that the distribution of positions for the final scattering events is independent of particle concentration when considering a given scattering order in forward detection. From this observation, it follows that the normalized intensity distribution of each order, in both space and time, is independent of the number density of particles. As a result, the amount of transmitted information is constant for a given scattering order and is directly related to the phase function in association with the detection acceptance angle. Finally, a contrast analysis is performed in order to quantify this information at the image plane.

Application of structured illumination for multiple scattering suppression in planar laser imaging of dense sprays.

A novel approach to reduce the multiple light scattering contribution in planar laser images of atomizing sprays is reported. This new technique, named Structured Laser Illumination Planar Imaging (SLIPI), has been demonstrated in the dense region of a hollow-cone water spray generated in ambient air at 50 bars injection pressure. The idea is based on using an incident laser sheet which is spatially modulated along the vertical direction. By properly shifting the spatial phase of the modulation and using post-processing of the successive recorded images, the blurring effects from multiple light scattering can be mitigated. Since hollow-cone sprays have a known inner structure in the central region, the efficiency of the method could be evaluated. We demonstrate, for the case of averaged images, that an unwanted contribution of 44% of the detected light intensity can be removed. The suppression of this diffuse light enables an increase from 55% to 80% in image contrast. Such an improvement allows a more accurate description of the near-field region and of the spray interior. The possibility of extracting instantaneous flow motion is also shown, here, for a dilute flow of water droplets. These results indicate promising applications of the technique to denser two-phase flows such as air-blast atomizer and diesel sprays.

Complications to optical measurements using a laser with an unstable resonator: a case study on laser-induced incandescence of soot.

Temporal behavior of pulses from a Q-switched Nd:YAG laser with an unstable resonator can vary significantly with radial position in the beam. Our laser provides pulses with position-dependent durations spanning 8-11.5 ns at 1064 nm and 7-10 ns at 532 nm. Pulses emerge first and have the longest duration at the center of the beam; they are shorter (by up to 4 ns) and increasingly delayed (by up to 10 ns) with increasing radial distance from the center. This behavior can have a dramatic effect on time-sensitive experiments, such as laser-induced incandescence of soot, if not taken into account.

Simultaneous PIV/OH-PLIF, Rayleigh thermometry/OH-PLIF and stereo PIV measurements in a low-swirl flame.

The diagnostic techniques for simultaneous velocity and relative OH distribution, simultaneous temperature and relative OH distribution, and three component velocity mapping are described. The data extracted from the measurements include statistical moments for inflow fluid dynamics, temperature, conditional velocities, and scalar flux. The work is a first step in the development of a detailed large eddy simulation (LES) validation database for a turbulent, premixed flame. The low-swirl burner used in this investigation has many of the necessary attributes for LES model validation, including a simplified interior geometry; it operates well into the thin reaction zone for turbulent premixed flames, and flame stabilization is based entirely on the flow field and not on hardware or pilot flames.

Laser light scattering in turbid media Part I: Experimental and simulated results for the spatial intensity distribution.

We investigate the scattering and multiple scattering of a typical laser beam (lambda = 800 nm) in the intermediate scattering regime. The turbid media used in this work are homogeneous solutions of monodisperse polystyrene spheres in distilled water. The two-dimensional distribution of light intensity is recorded experimentally, and calculated via Monte Carlo simulation for both forward and side scattering. The contribution of each scattering order to the total detected light intensity is quantified for a range of different scattering phase functions, optical depths, and detection acceptance angles. The Lorentz-Mie scattering phase function for individual particles is varied by using different sphere diameters (D = 1 and 5 mum). The optical depth of the turbid medium is varied (OD = 2, 5, and 10) by employing different concentrations of polystyrene spheres. Detection angles of theta(a) = 1.5 degrees and 8.5 degrees are considered. A novel approach which realistically models the experimental laser source is employed in this paper, and very good agreement between the experimental and simulated results is demonstrated. The data presented here can be of use to validate any other modern Monte Carlo models which generate spatially resolved light intensity distributions. Finally, an effective correction procedure to the Beer-Lambert law is proposed based on the Monte Carlo calculation of the ballistic photon contribution to the total detected light intensity.

Velocity imaging for the liquid-gas interface in the near field of an atomizing spray: proof of concept.

We describe adaptation of ballistic imaging for the liquid core of an atomizing spray. To describe unambiguously the forces that act to break apart the liquid core in a spray, one must directly measure the force vectors themselves. It would be invaluable, therefore, to obtain velocity and acceleration data at the liquid-gas interface. We employ double-image ballistic imaging to extract velocity information through the application of image analysis algorithms. This method is shown to be effective for liquid phase droplet features within the resolution limit of the imaging system. In light of these results, it is clear that a three- or four-image implementation of this technique would allow the determination of acceleration, and by extension, information about the forces active in spray breakup.