Sonny Ly

Extracting the distribution of laser damage precursors on fused silica surfaces for 351 nm, 3 ns laser pulses at high fluences (20-150 J/cm 2 )

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.

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.

Binding of Apolipoprotein E Inhibits the Oligomer Growth of Amyloid-β Peptide in Solution as Determined by Fluorescence Cross-correlation Spectroscopy

Background: ApoE is the most significant risk factor for Alzheimer disease, with known effects on Aβ deposition in the brain. Results: ApoE binds to aggregating Aβ peptides and maintains a faster diffusion rate for the Aβ peptide over time. Conclusion: Binding of apoE to Aβ slows the oligomerization of Aβ. Significance: FCCS measurements quantify isoform-dependent differences in apoE binding to Aβ in solution. One of the primary neuropathological hallmarks of Alzheimer disease is the presence of extracellular amyloid plaques resulting from the aggregation of amyloid-β (Aβ) peptides. The intrinsic disorder of the Aβ peptide drives self-association and progressive reordering of the conformation in solution, and this dynamic distribution of Aβ complicates biophysical studies. This property poses a challenge for understanding the interaction of Aβ with apolipoprotein E (apoE). ApoE plays a pivotal role in the aggregation and clearance of Aβ 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 Aβ 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 Aβ with apoE in the hydrated state. The diffusion time of freely diffusing Aβ 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 Aβ in solution and provide direct insight into the process by which apoE influences the deposition and clearance of Aβ peptides in the brain. Furthermore, by developing an approach to remove signals arising from very large Aβ 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 Aβ in the complex.

Characterizing diffusion dynamics of a membrane protein associated with nanolipoproteins using fluorescence correlation spectroscopy.

Nanolipoprotein particles (NLPs) represent a unique nanometer‐sized scaffold for supporting membrane proteins (MP). Characterization of their dynamic shape and association with MP in solution remains a challenge. Here, we present a rapid method of analysis by fluorescence correlation spectroscopy (FCS) to characterize bacteriorhodopsin (bR), a membrane protein capable of forming a NLP complex. By selectively labeling individual components of NLPs during cell‐free synthesis, FCS enabled us to measure specific NLP diffusion times and infer size information for different NLP species. The resulting bR‐loaded NLPs were shown to be dynamically discoidal in solution with a mean diameter of 7.8 nm. The insertion rate of bR in the complex was ∼55% based on a fit model incorporating two separate diffusion properties to best approximate the FCS data. More importantly, based on these data, we infer that membrane protein associated NLPs are thermodynamically constrained as discs in solution, while empty NLPs appear to be less constrained and dynamically spherical.

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.

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 systems 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.

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.

Quasi-continuum photoluminescence: Unusual broad spectral and temporal characteristics found in defective surfaces of silica and other materials

We previously reported a novel photoluminescence (PL) with a distribution of fast decay times in fused silica surface flaws that is correlated with damage propensity by high fluence lasers. The source of the PL was not attributable to any known silica point defect. Due to its broad spectral and temporal features, we here give this PL the name quasi-continuum PL (QC-PL) and describe the features of QC-PL in more detail. The primary features of QC-PL include broad excitation and emission spectra, a broad distribution of PL lifetimes from 20 ps to 5 ns, continuous shifts in PL lifetime distributions with respect to emission wavelength, and a propensity to photo-bleach and photo-brighten. We found similar PL characteristics in surface flaws of other optical materials, including CaF2, DKDP, and quartz. Based on the commonality of the features in different optical materials and the proximity of QC-PL to surfaces, we suggest that these properties arise from interactions associated with high densities of defects,...

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

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

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.