Using double pulse laser ablation in air to enhance the strength of laser-driven shocks.

In the process of multi-pulse laser ablation, inter-pulse delay time, Δt, is known to be an important parameter for maximizing ablation efficiency as well as impulse imparted to the target. In this work, using photon Doppler velocimetry, we show that for single pairs of colinear pulses (1064 nm, 8 ns, ∼ 60 J cm-2 per pulse) in air, the peak free surface velocity of the back surface of an aluminum target (125 µm thick) is increased, by a factor of nearly 3, when Δt = 10 microseconds, compared with both pulses arriving simultaneously (Δt = 0). Fast imaging of the ablation process suggests this enhancement is due to rarefaction of the contiguous air in the passage of the leading shock produced by ablation, which then in turn allows a larger fraction of the energy of the second pulse to reach the target surface. This interpretation is strengthened by additional experiments in which the two pulses do not overlap on the target surface, but the shock strength is nevertheless enhanced. Given a fixed energy budget this work suggests a prescription for maximizing laser-driven shock strength by judicious choice of inter-pulse delay.

Tamper performance for confined laser drive applications.

The shock imparted by a laser beam striking a metal surface can be increased by the presence of an optically transparent tamper plate bonded to the surface. We explore the shock produced in an aluminum slab, for a selection of tamper materials and drive conditions. The experiments are conducted with a single-pulse laser of maximum fluence up to 100 J/cm2. The pressure and impulse are measured by photon doppler velocimetry, while plasma imaging is used to provide evidence of nonlinear tamper absorption. We demonstrate a pressure enhancement of 50x using simple commercially available optics. We compare results from hard dielectric glasses such as fused silica to soft plastics such as teflon tape. We discuss the mechanism of pressure saturation observed at high pulse fluence, along with some implications regarding applications. Below saturation, overall dependencies on pulse intensity and material parameters such as mechanical impedances are shown to correlate with a model by Fabbro et al.

Label-Free Multiphoton Imaging of Microbes in Root, Mineral, and Soil Matrices with Time-Gated Coherent Raman and Fluorescence Lifetime Imaging.

Imaging biogeochemical interactions in complex microbial systems─such as those at the soil-root interface─is crucial to studies of climate, agriculture, and environmental health but complicated by the three-dimensional (3D) juxtaposition of materials with a wide range of optical properties. We developed a label-free multiphoton nonlinear imaging approach to provide contrast and chemical information for soil microorganisms in roots and minerals with epi-illumination by simultaneously imaging two-photon excitation fluorescence (TPEF), coherent anti-Stokes Raman scattering (CARS), second-harmonic generation (SHG), and sum-frequency mixing (SFM). We used fluorescence lifetime imaging (FLIM) and time gating to correct CARS for the autofluorescence background native to soil particles and fungal hyphae (TG-CARS) using time-correlated single-photon counting (TCSPC). We combined TPEF, TG-CARS, and FLIM to maximize image contrast for live fungi and bacteria in roots and soil matrices without fluorescence labeling. Using this instrument, we imaged symbiotic arbuscular mycorrhizal fungi (AMF) structures within unstained plant roots in 3D to 60 µm depth. High-quality imaging was possible at up to 30 µm depth in a clay particle matrix and at 15 µm in complex soil preparation. TG-CARS allowed us to identify previously unknown lipid droplets in the symbiotic fungus, Serendipita bescii. We also visualized unstained putative bacteria associated with the roots of Brachypodium distachyon in a soil microcosm. Our results show that this multimodal approach holds significant promise for rhizosphere and soil science research.

Wavefront shaping with a Hadamard basis for scattering soil imaging.

Soil is a scattering medium that inhibits imaging of plant-microbial-mineral interactions that are essential to plant health and soil carbon sequestration. However, optical imaging in the complex medium of soil has been stymied by the seemingly intractable problems of scattering and contrast. Here, we develop a wavefront shaping method based on adaptive stochastic parallel gradient descent optimization with a Hadamard basis to focus light through soil mineral samples. Our approach allows a sparse representation of the wavefront with reduced dimensionality for the optimization. We further divide the used Hadamard basis set into subsets and optimize a certain subset at once. Simulation and experimental optimization results demonstrate our method has an approximately seven times higher convergence rate and overall better performance compared to that with optimizing all pixels at once. The proposed method can benefit other high-dimensional optimization problems in adaptive optics and wavefront shaping.

Enhancement of laser material drilling using high-impulse multi-laser melt ejection.

Laser drilling and cutting of materials is well established commercially, although its throughput and efficiency limit applications. This work describes a novel approach to improve laser drilling rates and reduce laser system energy demands by using a gated continuous wave (CW) laser to create a shallow melt pool and a UV ps-pulsed laser to impulsively expel the melt efficiency and effectively. Here, we provide a broad parametric study of this approach applied to common metals, describing the role of fluence, power, spot size, pulse-length, sample thickness, and material properties. One to two order-of-magnitude increases in the average removal rate and efficiency over the CW laser or pulsed-laser alone are demonstrated for samples of Al and stainless steel for samples as thick as 3 mm and for holes with aspect ratios greater than 10:1. Similar enhancements were also seen with carbon fiber composites. The efficiency of this approach exceeds published values for the drilling of these materials in terms of energy to remove a given volume of material. Multi-laser material removal rates, high-speed imaging of ejecta, and multi-physics hydrodynamic simulations of the melt ejection process are used to help clarify the physics of melt ejection leading to these enhancements. Our study suggests that these high-impulse multi-laser enhancements are due to both laser-induced surface wave instabilities and cavitation of the melt for shallow holes and melt cavitation and ejection for deeper channels.

Protective spin-labeled fluorenes maintain amyloid beta peptide in small oligomers and limit transitions in secondary structure.

Alzheimer's disease is characterized by the presence of extracellular plaques comprised of amyloid beta (Aß) peptides. Soluble oligomers of the Aß peptide underlie a cascade of neuronal loss and dysfunction associated with Alzheimer's disease. Single particle analyses of Aß oligomers in solution by fluorescence correlation spectroscopy (FCS) were used to provide real-time descriptions of how spin-labeled fluorenes (SLFs; bi-functional small molecules that block the toxicity of Aß) prevent and disrupt oligomeric assemblies of Aß in solution. Furthermore, the circular dichroism (CD) spectrum of untreated Aß shows a continuous, progressive change over a 24-hour period, while the spectrum of Aß treated with SLF remains relatively constant following initial incubation. These findings suggest the conformation of Aß within the oligomer provides a complementary determinant of Aß toxicity in addition to oligomer growth and size. Although SLF does not produce a dominant state of secondary structure in Aß, it does induce a net reduction in beta secondary content compared to untreated samples of Aß. The FCS results, combined with electron paramagnetic resonance spectroscopy and CD spectroscopy, demonstrate SLFs can inhibit the growth of Aß oligomers and disrupt existing oligomers, while retaining Aß as a population of smaller, yet largely disordered oligomers.

Utilizing dynamic laser speckle to probe nanoscale morphology evolution in nanoporous gold thin films.

This paper demonstrates the use of dynamic laser speckle autocorrelation spectroscopy in conjunction with the photothermal treatment of nanoporous gold (np-Au) thin films to probe nanoscale morphology changes during the photothermal treatment. Utilizing this spectroscopy method, backscattered speckle from the incident laser is tracked during photothermal treatment and both the characteristic feature size and annealing time of the film are determined. These results demonstrate that this method can successfully be used to monitor laser-based surface modification processes without the use of ex-situ characterization.

Advances in superresolution optical fluctuation imaging (SOFI).

We review the concept of superresolution optical fluctuation imaging (SOFI), discuss its attributes and trade-offs (in comparison with other superresolution methods), and present superresolved images taken on samples stained with quantum dots, organic dyes, and plasmonic metal nanoparticles. We also discuss the prospects of SOFI for live cell superresolution imaging and for imaging with other (non-fluorescent) contrasts.

Gigashot optical degradation in silica optics at 351 nm.

As applications of lasers demand higher average powers, higher repetition rates, and longer operation times, optics will need to perform well under unprecedented conditions. We investigate the optical degradation of fused silica surfaces at 351 nm for up to 10(9) pulses with pulse fluences up to 12 J/cm(2). The central result is that the transmission loss from defect generation is a function of the pulse intensity, I(p), and total integrated fluence, φ(T), and is influenced by oxygen partial pressure. In 10(-6) Torr vacuum, at low I(p), a transmission loss is observed that increases monotonically as a function of number of pulses. As the pulse intensity increases above 13 MW/cm(2), the observed transmission losses decrease, and are not measureable for 130 MW/cm(2). A physical model which supports the experimental data is presented to describe the suppression of transmission loss at high pulse intensity. Similar phenomena are observed in anti-reflective sol-gel coated optics. Absorption, not scattering, is the primary mechanism leading to transmission loss. In 2.5 Torr air, no transmission loss was detected under any pulse intensity used. We find that the absorption layer that leads to transmission loss is less than 1 nm in thickness, and results from a laser-activated chemical process involving photo-reduction of silica within a few monolayers of the surface. The competition between photo-reduction and photo-oxidation explains the measured data: transmission loss is reduced when either the light intensity or the O(2) concentration is high. We expect processes similar to these to occur in other optical materials for high average power applications.

On-demand and location selective particle assembly via electrophoretic deposition for fabricating structures with particle-to-particle precision.

Programmable positioning of 2 µm polystyrene (PS) beads with single particle precision and location selective, "on-demand", particle deposition was demonstrated by utilizing patterned electrodes and electrophoretic deposition (EPD). An electrode with differently sized hole patterns, from 0.5 to 5 µm, was used to illustrate the discriminatory particle deposition events based on the voltage and particle-to-hole size ratio. With decreasing patterned hole size, a larger electric field was required for a particle deposition event to occur in that hole. For the 5 µm hole, particle deposition began to occur at 10 V/cm where as an electric field of 15 V/cm was required for particles to begin depositing in the 2 µm holes. The likelihood of particle depositions continued to increase for smaller sized holes as the electric field increased. Eventually, a monolayer of particles began to form at approximately 20 V/cm. In essence, a voltage threshold was found for each hole pattern of different sizes, allowing fine adjustments in pattern hole size and voltage to control when a particle deposition event took place, even with the patterns on the same electrode. This phenomenon opens a route toward controlled, multimaterial deposition and assembly onto substrates without repatterning of the electrode or complicated surface modification of the particles. An analytical approach using the theories for electrophoresis and dielectrophoresis found the former to be the dominating force for depositing a particle into a patterned hole. Ebeam lithography was used to pattern spherical holes in precise configurations onto electrode surfaces, where each hole accompanied a polystyrene (PS) particle placement and attachment during EPD. The versatility of e-beam lithography was utilized to create arbitrary pattern configurations to fabricate particle assemblies of limitless configurations, enabling fabrication of unique materials assemblies and interfaces.

Quantifying interactions of a membrane protein embedded in a lipid nanodisc using fluorescence correlation spectroscopy.

Using fluorescence correlation spectroscopy, we measured a dissociation constant of 20 nM between EGFP-labeled LcrV from Yersinia pestis and its cognate membrane-bound protein YopB inserted into a lipid nanodisc. The combination of fluorescence correlation spectroscopy and nanodisc technologies provides a powerful approach to accurately measure binding constants of interactions between membrane bound and soluble proteins in solution. Straightforward sample preparation, acquisition, and analysis procedures make this combined technology attractive for accurately measuring binding kinetics for this important class of protein-protein interactions.

Binding of apolipoprotein E inhibits the oligomer growth of amyloid-ß peptide in solution as determined by fluorescence cross-correlation spectroscopy.

One of the primary neuropathological hallmarks of Alzheimer disease is the presence of extracellular amyloid plaques resulting from the aggregation of amyloid-ß (Aß) peptides. The intrinsic disorder of the Aß peptide drives self-association and progressive reordering of the conformation in solution, and this dynamic distribution of Aß complicates biophysical studies. This property poses a challenge for understanding the interaction of Aß with apolipoprotein E (apoE). ApoE plays a pivotal role in the aggregation and clearance of Aß peptides in the brain, and the ε4 allele of APOE is the most significant known genetic modulator of Alzheimer risk. Understanding the interaction between apoE and Aß will provide insight into the mechanism by which different apoE isoforms determine Alzheimer disease risk. Here we applied alternating laser excitation fluorescence cross-correlation spectroscopy to observe the single molecule interaction of Aß with apoE in the hydrated state. The diffusion time of freely diffusing Aß in the absence of apoE shows significant self-aggregation, whereas in the presence of apoE, binding of the protein results in a more stable complex. These results show that apoE slows down the oligomerization of Aß in solution and provide direct insight into the process by which apoE influences the deposition and clearance of Aß peptides in the brain. Furthermore, by developing an approach to remove signals arising from very large Aß aggregates, we show that real-time single particle observations provide access to information regarding the fraction of apoE bound and the stoichiometry of apoE and Aß in the complex.

Matching HIV, tuberculosis, viral hepatitis, and sexually transmitted diseases surveillance data, 2000-2010: identification of infectious disease syndemics in New York City.

CONTEXT: In 2012, the New York City Department of Health and Mental Hygiene matched HIV, tuberculosis, viral hepatitis, and sexually transmitted disease surveillance data to identify the burden of infection with multiple diseases. METHODS: HIV, tuberculosis, hepatitis B, hepatitis C, chlamydia, gonorrhea, and syphilis surveillance data from 2000 to 2010 were matched using a deterministic method. Data on deaths from the Department of Health and Mental Hygiene's Office of Vital Statistics were also matched. RESULTS: The final data set contained 840,248 people; 13% had 2 or more diseases. People with a report of syphilis had the highest proportion of matches with other diseases (64%), followed by gonorrhea (52%), HIV (31%), tuberculosis (23%), hepatitis C (20%), chlamydia (16%), and hepatitis B (11%). CONCLUSIONS: The findings indicate several possible infectious disease syndemics in New York City and highlight the need to integrate surveillance data from different infectious disease programs. Conducting the match brought surveillance programs together to work collaboratively and has resulted in ongoing partnerships on programmatic activities that address multiple diseases.

Extracting the distribution of laser damage precursors on fused silica surfaces for 351 nm, 3 ns laser pulses at high fluences (20-150 J/cm2).

Surface laser damage limits the lifetime of optics for systems guiding high fluence pulses, particularly damage in silica optics used for inertial confinement fusion-class lasers (nanosecond-scale high energy pulses at 355 nm/3.5 eV). The density of damage precursors at low fluence has been measured using large beams (1-3 cm); higher fluences cannot be measured easily since the high density of resulting damage initiation sites results in clustering. We developed automated experiments and analysis that allow us to damage test thousands of sites with small beams (10-30 µm), and automatically image the test sites to determine if laser damage occurred. We developed an analysis method that provides a rigorous connection between these small beam damage test results of damage probability versus laser pulse energy and the large beam damage results of damage precursor densities versus fluence. We find that for uncoated and coated fused silica samples, the distribution of precursors nearly flattens at very high fluences, up to 150 J/cm2, providing important constraints on the physical distribution and nature of these precursors.

Stoichiometry of reconstituted high-density lipoproteins in the hydrated state determined by photon antibunching.

Apolipoprotein A-I plays a central role in the solution structure of high-density lipoproteins. Determining the stoichiometry of lipid-bound apo A-I in the hydrated state is therefore fundamental to understanding how high-density lipoproteins form and function. Here, we use the quantum optical phenomenon of photon antibunching to determine the number of apo A-I molecules bound to discoidal lipoproteins and compare this with values obtained by photon-counting histogram analysis. Both the photon antibunching and photon-counting analyses show that reconstituted high-density lipoprotein particles contain two apo A-I molecules, which is in agreement with the commonly accepted double-belt model.

Resonance excitation of surface capillary waves to enhance material removal for laser material processing.

The results of detailed experiments and high fidelity modeling of melt pool dynamics, droplet ejections and hole drilling produced by periodic modulation of laser intensity are presented. Ultra-high speed imaging revealed that melt pool oscillations can drive large removal of material when excited at the natural oscillation frequency. The physics of capillary surface wave excitation is discussed and simulation is provided to elucidate the experimental results. The removal rates and drill through times as a function of driving frequency is investigated. The resonant removal mechanism is driven by both recoil momentum and thermocapillary force but the key observation is the latter effect does not require evaporation of material, which can significantly enhance the efficiency for laser drilling process. We compared the drilling of holes through a 2 mm-thick Al plate at modulation frequencies up to 20 kHz. At the optimal frequency of 8 kHz, near the resonant response of the melt pool, the drilling efficiency is greater than 10x with aspect ratio of 12:1, and without the collateral damage that is observed in unmodulated CW drilling.

Time-gated single photon counting enables separation of CARS microscopy data from multiphoton-excited tissue autofluorescence.

We demonstrate time-gated confocal imaging as a means to separate coherent anti-Stokes Raman scattering (CARS) microscopy data from multi-photon excited endogenous fluorescence in tissue. CARS is a quasi-instantaneous process and its signal decay time is only limited by the system's instrument response function (IRF). Signals due to two-photon-excited (TPE) tissue autofluorescence with excited state lifetimes on the nanosecond scale can be identified and separated from the CARS signal by employing time-gating techniques. We demonstrate this improved contrast on the example of CARS microscopy of intact roots of plant seedlings as well as on rat arterial tissue.

Metal vapor micro-jet controls material redistribution in laser powder bed fusion additive manufacturing.

The results of detailed experiments and finite element modeling of metal micro-droplet motion associated with metal additive manufacturing (AM) processes are presented. Ultra high speed imaging of melt pool dynamics reveals that the dominant mechanism leading to micro-droplet ejection in a laser powder bed fusion AM is not from laser induced recoil pressure as is widely believed and found in laser welding processes, but rather from vapor driven entrainment of micro-particles by an ambient gas flow. The physics of droplet ejection under strong evaporative flow is described using simulations of the laser powder bed interactions to elucidate the experimental results. Hydrodynamic drag analysis is used to augment the single phase flow model and explain the entrainment phenomenon for 316 L stainless steel and Ti-6Al-4V powder layers. The relevance of vapor driven entrainment of metal micro-particles to similar fluid dynamic studies in other fields of science will be discussed.

Fluorescence correlation spectroscopy at micromolar concentrations without optical nanoconfinement.

Fluorescence correlation spectroscopy (FCS) is an important technique for studying biochemical interactions dynamically that may be used in vitro and in cell-based studies. It is generally claimed that FCS may only be used at nM concentrations. We show that this general consensus is incorrect and that the limitation to nM concentrations is not fundamental but due to detector limits as well as laser fluctuations. With a high count rate detector system and applying laser fluctuation corrections, we demonstrate FCS measurements up to 38 µM with the same signal-to-noise as at lower concentrations. Optical nanoconfinement approaches previously used to increase the concentration range of FCS are not necessary, and further increases above 38 µM may be expected using detectors and detector arrays with higher saturation rates and better laser fluctuation corrections. This approach greatly widens the possibilities of dynamic measurements of biochemical interactions using FCS at physiological concentrations.