Optical and photoluminescence 3D imaging of small fused silica laser-induced damage sites allows us to understand the damage growth mechanisms. The laser damage growth process is driven by local absorption centers and its location and depth are the key factors. To quantitatively extract the factors from the 3D multi-modal image data set, various metrics are obtained by image analysis techniques and evaluated. We believe that our measurement and analysis approach can allow rapid identification of growth-prone damage sites, providing a pathway to fast, non-destructive predictions of laser-induced damage growth and enable selective damage site mitigation. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-863515
Quantum ghost imaging can be an important tool in making optical measurements. One of the most useful aspects of ghost imaging is the unique ability to correlate two sets of independently collected information. We aim to use the principles of ghost imaging to build out a 3-dimensional microscope which utilizes detection from two imaging detectors that simultaneously capture entangled light. Further advancements and application of this relatively new imaging method depends on understanding the limits of the optical system. What quality should we expect? Can we image out-of-focus objects? How long do we need to expose? For ghost imaging, these answers are not so obvious. This is because entangled light sources are atypical: the light profile, frequency distribution, and intensity, for instance, all depend on an assortment of parameters associated with how the entangled light was generated. While we cannot practically explore the extent of this configuration space, we present here an exploration of a very accessible range. We show in which ways a commonly used bulk non-linear crystal can alter the imaging capabilities. In this study, we utilize a pair of state-of-the-art, single-photon avalanche diode (SPAD) array detectors. Thus, we also use this study as an opportunity to demonstrate the capabilities of these detectors in their use for ghost imaging applications.
Fluorescence microscopy has become integral to biological studies for the technique’s ability to elucidate structures of biomolecules for in-situ studies with high selectivity and specificity. Imaging of intrinsic indicators, such as fluorescent amino acids in proteins, provides important information, but can be challenging to accomplish. Current microscopy techniques that measure native fluorescence without the use of exogenous labels involve either direct UV excitation which is commonly non-localized and can be detrimental to the system, or multiphoton absorption which must be conducted at high intensities, therefore posing high risks of photodamage. As such, we seek to investigate an efficient way to gently excite native fluorescence in biological systems in a way that overcomes these limitations. Quantum entangled photon pairs generated via spontaneous parametric downconversion (SPDC), may be an alternative to conducting two-photon absorption (TPA) to excite fluorescence in amino acids without the high fluences currently used. These photon pairs are highly correlated in time. Thus, the arrival of one photon is simultaneously followed by the arrival of its sister photon. As a result, a molecule interacting with the photon pair should simultaneously absorb both photons, leading to a linear two-photon absorption rate, and the linearity of the two-photon process should dramatically reduce the light intensity necessary for TPA. Therefore, quantum entangled photon pairs offer the possibility of performing low intensity UV excitation using photons in the visible wavelength range. With this work, we generated and characterized entangled photons generated via SPDC, and investigated whether fluorescent amino acids can be excited, and the subsequent fluorescence induced with entangled two-photon absorption. Results show that much higher entangled two-photon rates than what are currently available are needed to measure significant signals with entangled two-photon excitation.
For pulse lengths between 1 and 60 ps, laser-induced modifications of optical materials undergo a transition from mechanisms intrinsic to the materials to defect-dominated mechanisms. Elucidating the location, size, and identity of these defects will greatly help efforts to reduce, mitigate, or eliminate these defects. We discuss our work that detailed the role of defects in the ps laser-modifications of SiO2 and HfO2 1/2-wave coatings. For HfO2 coatings, we included a study of environmental effects on the damage process. We found that the response of defects very near the surface are dependent on the environment, leading to worse damage in vacuum than in air. One or more constituents of air, most likely oxygen and/or water, suppress or lessen the effects of these defects during laser exposure. We discuss the implications of these findings for defect-driven laser-induced damage for ps to ns laser pulses and for mechanisms for laser-induced damage initiation.
We present a temporally and spatially resolved photoluminescence (PL) measurement technique developed to rapidly characterize fused silica damage sites and determine their propensity to grow under subsequent laser irradiation. A diffusional model is used to describe the observed PL dynamics and correlation to the local damage morphologies. We believe that our measurement and analysis approach can allow rapid identification of growth-prone damage sites, providing a pathway to fast, non-destructive predictions of laser-induced damage growth and enable selective damage site mitigation which will greatly reduce the time required to recycle NIF’s optics.
Soil is a highly scattering media that inhibits imaging of plant-microbial-mineral interactions that are essential to plant health and soil carbon sequestration. However, wavefront shaping can be used to focus light through or even deep inside highly scattering objects. In this work, we seek to overcome the fundamental challenges of imaging through soil minerals by developing a custom wavefront shaping method for a multiphoton microscope. We use the adaptive stochastic parallel gradient descent optimization algorithm combined with Hadamard basis to correct the aberration and the scattering in order to focus through the soil.
Soil is a highly scattering media that inhibits imaging of plant-microbial-mineral interactions that are essential to plant health and soil carbon sequestration. In this work, we seek to overcome the fundamental challenges of imaging through soil minerals by developing a custom wavefront sensor-less adaptive optics (AO) system for a multiphoton microscope. We are using a combined experimental and modeling approach, characterizing mineral optical characteristics with scatterometry, modeling the wavefront distortion and the image quality degradation after imaging through the soil medium, simulating the image quality improvement with AO correction, and experimentally testing our models with a stand-alone AO testbed.
We have developed a novel multiphoton nonlinear microscopy with a highly integrated optical imaging system that offers numerous label-free techniques including two-photon excited fluorescence, second-harmonic generation, third-harmonic generation, fluorescence lifetime imaging, and spectral focusing coherent anti-Stokes Raman scattering in one platform. We have applied our system to investigate plant-microbe-mineral interactions in the rhizosphere. The system provides time efficient monitoring of the rhizosphere, offering an array of simultaneous biomolecular information without staining, three-dimensional sub-micron resolution with deeper penetration , and less photodamage. We believe that multiphoton nonlinear optical microscopy will become a valuable imaging tool in the rhizosphere and soil mineral sciences.
We present an approach for wafer-level rapid multi-modal defect non-destructive imaging of device-relevant GaN defects with high resolution and high sensitivity. The scanning GaN defects detection system is based on laser pump-and-probe photoluminescence and photothermal measurements that are compared to diode device reliability data from accelerated lifetime testing. This work hypothesis is that defects probed at optical frequencies can reliably predict reliability or performance issues of power electronic devices at near DC frequencies. Imaging, growth, and device data are correlated to validate the proposed multi-modal defect detection approach for detection of GaN defects relevant to power electronic devices.
Absorbing defects such as fractures and contaminants are a leading cause of surface damage in nanosecond pulsed lasers. Etching such defects has proven to be a powerful technique for increasing the laser damage threshold of fused silica, but to date no etching process has been reported for potassium dihydrogen phosphate (KH2PO4 or KDP) or its deuterated analog (DKDP). We show that physical dissolution in water is a viable strategy for etching DKDP surfaces but surface-redeposited byproducts can serve as laser damage precursors. We use a water-in-oil microemulsion to etch engineered surface fractures in DKDP. Etching widens surface fractures laterally and decreases their optical activity, as measured by photoluminescence. The removal of 1 μm of the surface of a DKDP crystal increases the laser damage threshold (λ = 355 nm, 7 ns) of the engineered surface fractures by 2-4 J/cm2 (15-30%).
The utility and accuracy of computational modeling often requires direct validation against experimental measurements. The work presented here is motivated by taking a combined experimental and computational approach to determine the ability of large-scale computational fluid dynamics (CFD) simulations to understand and predict the dynamics of circulating tumor cells in clinically relevant environments. We use stroboscopic light sheet fluorescence imaging to track the paths and measure the velocities of fluorescent microspheres throughout a human aorta model. Performed over complex physiologicallyrealistic 3D geometries, large data sets are acquired with microscopic resolution over macroscopic distances.
Archival of experimental data in public databases has increasingly become a requirement for most funding agencies and journals. These data-sharing policies have the potential to maximize data reuse, and to enable confirmatory as well as novel studies. However, the lack of standard data formats can severely hinder data reuse.
In photon-counting-based single-molecule fluorescence experiments, data is stored in a variety of vendor-specific or even setup-specific (custom) file formats, making data interchange prohibitively laborious, unless the same hardware-software combination is used. Moreover, the number of available techniques and setup configurations make it difficult to find a common standard.
To address this problem, we developed Photon-HDF5 (www.photon-hdf5.org), an open data format for timestamp-based single-molecule fluorescence experiments. Building on the solid foundation of HDF5, Photon- HDF5 provides a platform- and language-independent, easy-to-use file format that is self-describing and supports rich metadata. Photon-HDF5 supports different types of measurements by separating raw data (e.g. photon-timestamps, detectors, etc) from measurement metadata. This approach allows representing several measurement types and setup configurations within the same core structure and makes possible extending the format in backward-compatible way.
Complementing the format specifications, we provide open source software to create and convert Photon- HDF5 files, together with code examples in multiple languages showing how to read Photon-HDF5 files. Photon- HDF5 allows sharing data in a format suitable for long term archival, avoiding the effort to document custom binary formats and increasing interoperability with different analysis software. We encourage participation of the single-molecule community to extend interoperability and to help defining future versions of Photon-HDF5.
We investigate the optical damage performance of multi-layer dielectric (MLD) coatings suitable for use in high energy, large-aperture petawatt-class lasers. We employ small-area damage test methodologies to evaluate the damage resistance of various coatings as a function of deposition methods and coating materials under simulated use conditions. In addition, we demonstrate that damage initiation by raster scanning at lower fluences and growth threshold testing are required to estimate large-aperture optics’ performance.
We investigate the multipulse degradation of fused silica surfaces exposed at 351 nm for up to 109 pulses at pulse fluences greater than 10 J/cm2. In vacuum, the transmission loss increases as a function of the number of shots at low pulse intensity. However, as the pulse intensity increases, the transmission loss decreases and is not measureable above a certain intensity. Transmission loss is highest when measured at shorter wavelengths, and decreases towards the IR. Absorption is the primary mechanism that leads to transmission loss and is from photo-reduction of the silica surface.
Controlling laser damage is essential for reliable and cost-effective operation of high energy laser systems. We will
review important optical damage precursors in silica up to UV fluences as high as 45J/cm2 (3ns) along with studies of
the damage mechanisms involved and processes to mitigate damage precursors. We have found that silica surface
damage is initiated by nano-scale precursor absorption followed by thermal coupling to the silica lattice and formation of
a laser-supported absorption front. Residual polishing compound and defect layers on fracture surfaces are primarily
responsible for optic damage below about 10J/cm2; they can be mitigated by an optimized oxide etch processes. At
fluences above about 10J/cm2, precipitates of trace impurities are responsible for damage; they can be mitigated by
eliminating the chances of impurity precipitation following wet chemical processing. Using these approaches, silica
damage densities can be reduced by many orders of magnitude allowing large increases in the maximum operating
fluences these optics see.
Chemical vapor deposition (CVD) has been used for the production of fused silica optics in high power laser
applications. However, relatively little is known about the ultraviolet (UV) laser damage threshold of CVD films
and how they relate to intrinsic defects produced during deposition. We present a study relating structural and
electronic defects in CVD films to the 355 nm pulsed laser damage threshold as a function of post-deposition
annealing temperature (THT). Plasma-enhanced CVD, based on SiH4/N2O under oxygen-rich conditions, was used
to deposit 1.5, 3.1 and 6.4 μm thick films on etched SiO2 substrates. Rapid annealing was performed using a
scanned CO2 laser beam up to THT~2100 K. The films were then characterized using X-ray photoemission
spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and photoluminescence (PL). A gradual
transition in the damage threshold of annealed films was observed at THT up to 1600 K, correlating with a decrease
in NB silanol and broadband PL emission. An additional sharp transition in damage threshold also occurs at ~1850
K indicating substrate annealing. Based on our results, a mechanism for damage-related defect annealing is
proposed, and the potential of using high-THT CVD SiO2 to mitigate optical damage is also discussed.
Previous studies have identified two significant precursors of laser damage on fused silica surfaces at fluences <35 J/cm2: photoactive impurities from polishing and surface fractures. We evaluate isothermal heating as a means of remediating the defect structure associated with surface fractures. Vickers indentations are applied to silica surfaces at loads between 0.5 and 10 N, creating fracture networks. The indentations are characterized before and following thermal annealing under various time and temperature conditions using confocal time-resolved photo-luminescence (CTP) imaging, and R/1 damage testing with 3-ns, 355-nm laser pulses. Improvements in the damage thresholds with reductions in CTP intensity are observed at temperatures well below the glass transition temperature (Tg). The damage threshold on 0.5-N indentations improves from <8 to >35 J/cm2 after annealing at approximately 750°C. Larger fracture networks require longer or higher temperature treatment to achieve similar results. At an annealing temperature >1100°C, optical microscopy indicates morphological changes in some of the fractures surrounding the indentations, although remnants of the original fractures are still observed. We demonstrate the potential of using isothermal annealing to improve the laser damage resistance of silica optics, and provide a means of further understanding the physics of optical damage and mitigation.
There is a longstanding, and largely unexplained, correlation between the laser damage susceptibility
of optical components and both the surface quality of the optics, and the presence of near surface
fractures in an optic. In the present work, a combination of acid leaching, acid etching, and confocal
time resolved photoluminescence (CTP) microscopy has been used to study laser damage initiation
at indentation sites. The combination of localized polishing and variations in indentation loads
allows one to isolate and characterize the laser damage susceptibility of densified, plastically flowed
and fractured fused silica. The present results suggest that: 1) laser damage initiation and growth are
strongly correlated with fracture surfaces, while densified and plastically flowed material is
relatively benign, and 2) fracture events result in the formation of an electronically defect rich
surface layer which promotes energy transfer from the optical beam to the glass matrix.
Using high-sensitivity confocal time-resolved photoluminescence (CTP) techniques, we report an ultra-fast
photoluminescence (40ps-5ns) from impurity-free surface flaws on fused silica, including polished, indented or
fractured surfaces of fused silica, and from laser-heated evaporation pits. This fast photoluminescence (PL) is not
associated with slower point defect PL in silica which has characteristic decay times longer than 5ns. Fast PL is
excited by the single photon absorption of sub-band gap light, and is especially bright in fractures. Regions which
exhibit fast PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5eV
ns-scale laser pulses, making CTP a powerful non-destructive diagnostic for laser damage in silica. The use of CTP
to provide insights into the nature of damage precursors and to help develop and evaluate new damage mitigation
strategies will be presented.
We present an alternative approach to optical probes that will ultimately allow us to measure chemical concentrations in microenvironments within cells and tissues. This approach is based on monitoring the surface-enhanced Raman scattering (SERS) response of functionalized metal nanoparticles (50-100 nm in diameter). SERS allows for the sensitive detection of changes in the state of chemical groups attached to individual nanoparticles and small clusters. Here, we present the development of a nanoscale pH meter. The pH response of these nanoprobes is tested in a cell-free medium, measuring the pH of the solution immediately surrounding the nanoparticles. Heterogeneities in the SERS signal, which can result from the formation of small nanoparticle clusters, are characterized using SERS correlation spectroscopy and single particle/cluster SERS spectroscopy. The response of the nanoscale pH meters is tested under a wide range of conditions to approach the complex environment encountered inside living cells and to optimize probe performance.
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