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Description:
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This work is devoted to formulation and development of a laser spectroscopic technique
for rapid detection of biohazards , such as Bacillus anthracis spores . Coherent anti -Stokes
Raman scattering (CARS ) is used as an underlying process for active retrieval of
species -specific characteristics of an analyte . Vibrational modes of constituent molecules
are Raman -excited by a pair of ultrashort , femtosecond laser pulses , and then probed
through inelastic scattering of a third , time -delayed laser field .
We first employ the already known time -resolved CARS technique . We apply it
to the spectroscopy of easy -to -handle methanol -water mixtures , and then continue
building our expertise on solutions of dipicolinic acid (DPA ) and its salts , which happen
to be marker molecules for bacterial spores . Various acquisition schemes are evaluated ,
and the preference is given to multi -channel frequency -resolved detection , when the
whole CARS spectrum is recorded as a function of the probe pulse delay . We
demonstrate a simple detection algorithm that manages to differentiate DPA solution
from common interferents . We investigate experimentally the advantages and
disadvantages of near -resonant probing of the excited molecular coherence , and finally
observe the indicative backscattered CARS signal from DPA and NaDPA powders . The possibility of selective Raman excitation via pulse shaping of the preparation pulses is
also demonstrated .
The analysis of time -resolved CARS experiments on powders and B . subtilis
spores , a harmless surrogate for B . anthracis , facilitates the formulation of a new
approach , where we take full advantage of the multi -channel frequency -resolved
acquisition and spectrally discriminate the Raman -resonant CARS signal from the
background due to other instantaneous four -wave mixing (FWM ) processes . Using
narrowband probing , we decrease the magnitude of the nonresonant FWM , which is
further suppressed by the timing of the laser pulses . The devised technique , referred to as
hybrid CARS , leads to a single -shot detection of as few as 104 bacterial spores , bringing
CARS spectroscopy to the forefront of potential candidates for real -time biohazard
detection . It also gives promise to many other applications of CARS , hindered so far by
the presence of the overwhelming nonresonant FWM background , mentioned above . |