We are utilizing control of molecular processes at the quantum level via the best capabilities of recent laser
technology and recent discoveries in optimal shaping of laser pulses to significantly enhance the detection of
explosives. Optimal dynamic detection of explosives (ODD-Ex) is a methodology whereby laser pulses are
optimally shaped to simultaneously enhance the sensitivity and selectivity of any of a wide variety of
spectroscopic methods for explosives signatures while reducing the influence of noise and environmental
perturbations. We discuss here recent results using the Gerchberg-Saxton algorithm to provide an optimal shaped
laser pulse for selective coherent anti-Stokes Raman signal generation of a single component in a mixture.
We are utilizing control of molecular processes at the quantum level via the best capabilities of recent laser
technology and recent discoveries in optimal shaping of laser pulses to significantly enhance the standoff
detection of explosives. Optimal dynamic detection of explosives (ODD-Ex) is a methodology whereby laser pulses
are optimally shaped to simultaneously enhance the sensitivity and selectivity of any of a wide variety of
spectroscopic methods for explosives signatures while reducing the influence of noise and environmental
perturbations. We discuss here recent results using complementary ODD-Ex methods.
The detection of explosives is a notoriously difficult problem, especially at stand-off distances, due to their (generally)
low vapor pressure, environmental and matrix interferences, and packaging. We are exploring optimal dynamic
detection to exploit the best capabilities of recent advances in laser technology and recent discoveries in optimal shaping
of laser pulses for control of molecular processes to significantly enhance the standoff detection of explosives. The core
of the ODD-Ex technique is the introduction of optimally shaped laser pulses to simultaneously enhance sensitivity of
explosives signatures while reducing the influence of noise and the signals from background interferents in the field
(increase selectivity). These goals are being addressed by operating in an optimal nonlinear fashion, typically with a
single shaped laser pulse inherently containing within it coherently locked control and probe sub-pulses. With sufficient
bandwidth, the technique is capable of intrinsically providing orthogonal broad spectral information for data fusion, all
from a single optimal pulse.
Ultrafast dynamic ellipsometry, a technique that probes a sample with chirped laser pulses at two angles and with two
orthogonal polarizations, was used to measure the effective refractive index across the ablation region of a Si(111) wafer
exposed to a 100 fs ablation pulse. The resulting refractive index data show a significant increase in the extinction
coefficient, indicative of the melting of silicon.
Ellipsometry is a very useful optical technique to probe the complex index of refraction of a material. We perform
dynamic ellipsometry using ultrafast lasers to probe the complex index dynamics during passage of compressional
shock waves through materials of interest. When used to measure passage of a shock wave through dielectric materials,
dynamic ellipsometry provides a direct measure of the equation of state (except temperature). In addition, the changes in
complex index can be used to measure phase transformations and their kinetics. Using two CCD cameras and two
Wollaston prisms, ellipsometric data at two incidence angles and two polarizations can be obtained simultaneously.
Adding a spectrograph in front of each CCD camera and using chirped probe pulses and frequency domain
interferometry provides a few hundred ps of ellipsometric data from a single shock event.
We have used two-dimensional ultrafast microscopic interferometry to observe the hydrodynamic motion and the time dependent changes in the optical properties of two metals (6061-T6 aluminum and ASTM 336 1018 steel) illuminated with 130 fs 800 nm laser pulses. We hve also observed the electron dynamics in optically excited aluminum, gallium, and gold at a metal-glass interface using the same technique. The interferometric technique allows construction of the two-dimensional spatial profile for laser pumped materials with a temporal resolution of < 300 fs and out-of-plane spatial resolution of 0.5 nm using 130 fs probe pulses. Best fits to the diamond turned aluminum data were obtained by assuming physically motivated functional forms for the expected hydrodynamic motion and the time-dependent complex index of refraction. Extraction of changes in the index of refraction provides evidence for melting in the gold targets. These experiments offer a new path for the observation of phase changes and/or for temperature measurements in shocked or laser excited materials, by allowing a determination of the complex index under dynamic conditions and comparing the measured values to those obtained under static conditions.
Shaped femtosecond laser driven shock waves fulfill the requisite time resolution required for following shock induced chemical kinetics, and enable very small scale shock physics experimentation. However, comparison to large scale experiments requires detailed characterization of the generated pressure profile. Spectroscopic studies at pressure and temperature are hindered by fluctuations in shock strength as functions of space and time. Representative time resolved broadband infrared absorption data on shocked polyvinylnitrate thin films are presented, followed by current efforts to alleviate spatial shock strength variations with spatial pulse shaping, and calculations of the interferometric phase shifts expected at long times, during the rarefaction.
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