Understanding gas-phase combustion reactions in turbulent flows and energetic materials requires high-speed multiparameter imaging at kHz-MHz frame rates. Such measurements require energies per pulse in the range of 10s to 100s mJ at MHz rates, which led to development of burst-mode nanosecond lasers with burst durations up to 100 ms and energies per burst of up to 400 J. Burst-mode laser technology have enabled significant advances in measurement capabilities, for accessing multiple species, temperature, and velocity. However, the number of simultaneously measured parameters are limited by number of lasers and interference between modalities having similar output spectral characteristics. Here we report the laser system with three outputs with up to 1 J/pulse at 10 kHz and variable time delays between outputs to avoid interference between modalities. The laser is based on the common all-fiber oscillator and free-space Nd:YAG flashlamppumped preamplifier to reduce the cost and size of the overall system. The oscillator/preamplifier produces up to four pulses with user-selected time delays between pulses. This four-pulse sequence is repeated at user selected repetition rate up to 500 kHz. The pulse sequence is split between three free-space Nd:YAG amplifiers by two Pockels cells with time resolution of 10 ns. Special precaution is taken to minimize the crosstalk between amplifiers with less than 5% prepulse/afterpulse in the fundamental output and less than 1% in the harmonics. The system is designed for measurements of species (via planar laser induced fluorescence, PLIF) and velocity (via particle image velocimetry, PIV) at rates up to 500 kHz and total burst duration of 10 ms, with the feasibility of achieving MHz rates with further developments. The simultaneous measurements of velocity using PIV and PLIF imaging of hydroxyl and formaldehyde in a turbulent jet flame is demonstrated.
Zeolite catalysis has been exploited by the petrochemical industry since the 1940’s for catalytic cracking reactions of long chain hydrocarbons. The selectivity of zeolites strongly depends on a pore size, which is controlled by the chosen structure-directing agent (SDA) and by the SDA decomposition/removal process. Although zeolites are composed of micron-sized crystals, studies of zeolite materials typically focus on bulk (i.e., ensemble) measurements to elucidate structure-function information or to optimize catalysts and/or process parameters. To examine these phenomena on the microscale, non-linear optical microscopy is used to provide real-time imaging of chemical reactions in zeolites at temperatures exceeding 400°C. The template decomposition mechanism is studied, as elucidation of the mechanism is critical to understanding the relationship between the decomposition chemistry and the nanoscale features of the zeolite (topology, Si/Al ratio, added dopants). Forward stimulated Raman scattering (SRS), forward coherent anti-Stokes Raman scattering (CARS) and epi two-photon fluorescence (TPF) modalities are acquired simultaneously providing video-rate structural and chemical information. A high-temperature cell with gas inlet system is used for the study of reactions under various temperatures and gas environments. Examining the decomposition process with single-particle resolution enables access to ensemble-level and spatially-resolved behavior. Parallel experiments on bulk zeolite powders are conducted to enable comparison of ensemble and single-particle behavior during template decomposition. Our multi-technique approach has high potential for gaining insight into the link between nanoscale structure and catalytic activity and selectivity of zeolitic materials.
KEYWORDS: Axons, In vivo imaging, Microscopy, Nerve, Signal processing, Imaging systems, Image segmentation, Real time imaging, Raman scattering, Visualization
The myelin sheath plays an important role as the axon in the functioning of the neural system, and myelin degradation is a hallmark pathology of multiple sclerosis and spinal cord injury. Electron microscopy, fluorescent microscopy, and magnetic resonance imaging are three major techniques used for myelin visualization. However, microscopic observation of myelin in living organisms remains a challenge. Using a newly developed stimulated Raman scattering microscopy approach, we report noninvasive, label-free, real-time in vivo imaging of myelination by a single-Schwann cell, maturation of a single node of Ranvier, and myelin degradation in the transparent body of the Xenopus laevis tadpole.
Photoacoustic imaging employing molecular overtone vibration as a contrast mechanism opens a new avenue for bond-selective imaging of deep tissues. Broad use of this modality is, however, hampered by the extremely low conversion efficiency of optical parametric oscillators at the overtone transition wavelengths. To overcome such a barrier, we demonstrate the construction and use of a compact, barium nitrite crystal-based Raman laser for photoacoustic imaging of C–H overtone vibrations. Using a 5-ns Nd∶YAG laser as the pumping source, up to 21.4 mJ pulse energy at 1197 nm was generated, corresponding to a conversion efficiency of 34.8%. Using the 1197 nm pulses, three-dimensional photoacoustic imaging of intramuscular fat was demonstrated.
Photoacoustic imaging employing molecular overtone vibration as contrast mechanism opens a new avenue for deep tissue imaging with chemical bond selectivity. Here, we demonstrate vibration-based photoacoustic tomography with an imaging depth on the centimeter scale. To provide sufficient pulse energy at the overtone transition wavelengths, we constructed a compact, barium nitrite crystal-based Raman laser for excitation of 2nd overtone of C-H bond. Using a 5-ns Nd:YAG laser as pumping source, up to 105 mJ pulse energy at 1197 nm was generated. Vibrational photoacoutic spectroscopy and tomography of phantom (polyethylene tube) immersed in whole milk was performed. With a pulse energy of 47 mJ on the milk surface, up to 2.5 cm penetration depth was reached with a signal-to-noise ratio of 12.
Though single-color coherent Raman microscopy has been widely used for vibrational imaging of isolated Raman bands, it is still challenging to visualize molecules having overlapping Raman bands. We address this issue by developing a spectroscopic SRS microscope with a time-lens laser source synchronized to a femtosecond laser. The time-lens source provides 2-ps pulse at the wavelength of 1064 nm. A pulse shaper is installed for intra-pulse spectral scanning of the femtosecond laser output. By electronically modulating the time-lens source at MHz frequency, spectroscopic stimulated Raman loss (SRL) images were obtained on a laser-scanning microscope. Using this microscope, we have been able to detect 0.2% DMSO in aqueous solution. Spectroscopic SRL images of prostate cancer cells were obtained. Multivariate curve resolution analysis was further applied to decompose the SRL images into concentration maps of proteins and lipids. With high sensitivity and high spectral resolution, this method offers exciting potential in label-free imaging of live cells using fingerprint Raman bands.
We demonstrate nonlinear vibrational imaging of isolated Raman bands by detecting femtosecond pulse stimulated
Raman loss. Femtosecond pulse excitation produces a stimulated Raman loss signal that is 12 times larger than what
picosecond pulse excitation produces. The strong signal allowed real-time, bond-selective imaging of deuterated palmitic
acid-d31 inside live cells, and 3D sectioning of fat storage in live C. elegans. With the high peak power provided by
femtosecond pulses, this system is highly compatible with other nonlinear optical modalities such as two-photon excited
fluorescence. With most of the excitation power contributed by the Stokes beam in the 1.0 - 1.2 μm wavelength range,
photodamage of biological samples was not observed.
Multimodal nonlinear optical imaging has opened new opportunities and becomes a powerful tool for imaging complex
tissue samples with inherent 3D spatial resolution.. We present a robust and easy-to-operate approach to add the coherent
anti-stokes Raman scattering (CARS) imaging modality to a widely used multiphoton microscope. The laser source
composed of a Mai Tai femtosecond laser and an optical parametric oscillator (OPO) offers one-beam, two-beam and
three-beam modalities. The Mai Tai output at 790 nm is split into two beams, with 80% of the power being used to pump
the OPO. The idler output at 2036 nm from OPO is doubled using a periodically poled lithium niobate (PPLN) crystal.
This frequency-doubled idler beam at 1018 nm is sent through a delay line and collinearly combined with the other Mai
Tai beam for CARS imaging on a laser-scanning microscope. This Mai Tai beam is also used for multiphoton
fluorescence and second harmonic generation (SHG) imaging. The signal output at 1290 nm from OPO is used for SHG
and third-harmonic generation (THG) imaging. External detectors are installed for both forward and backward detection,
whereas two internal lamda-scan detectors are employed for microspectroscopy analysis. This new system allows
vibrationally resonant CARS imaging of lipid bodies, SHG imaging of collagen fibers, and multiphoton fluorescence
analysis in fresh tissues. As a preliminary application, the effect of diacylglycerol acyltransferase 1 (DGAT1) deficiency
on liver lipid metabolism in mice was investigated.
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