As part of an effort to develop a portable probe for real-time detection of chemical agents using Raman spectroscopy, the kinetics of adsorption of selected volatile organic molecules on high surface area silica supports was studied using Raman spectroscopy with 785-nm excitation. Two aspects of this work are described. First, the affinity of organic molecules in a flowing gas stream to ultraporous mineralized wood substrates was shown to be very high under a variety of representative conditions, suggesting that a sensor based on in-situ Raman measurement of adsorption rates may be feasible. Moreover, the mechanism of adsorption was found to vary for different molecules thereby promoting a high degree of chemical specificity. Second, the possibility of using Surface-Enhanced Raman Spectroscopy (SERS) to enhance Raman signals and thereby improve the sensitivity of an in-situ Raman probe even further was demonstrated. Detection limits were estimated based on the current state of development of the approach.
Methods for predicting the refractive indices of materials are evaluated with respect to the design criteria for composite optical limiters. The efficiency of a nonlinear Christiansen optical limiter is dependent upon refractive index of the individual phases of the composite device and their comparative response characteristics. The overall performance of the structure-property relationships for the linear refractive indices is generally excellent within the training sets; however, the accuracy of an individual case sample is challenged by the low energy case requirements. Outside the general class of materials for which these methods were designed, significant degradation occurs as a result in incomplete parameterizations or compounding errors of imprecise predictions. Accurate predictions of linear and nonlinear refractive indices by TDHF methods are more problematic, yet these techniques demonstrate sensitivity to chemical bonding environments.
A densely packed bed of alkaline earth fluoride particles percolated by a fluid medium has been investigated as potential index-matched optical limiter in the spirit of a Christiansen-Shelyubskii filter. Marked optical limiting was observed through this transparent medium under conditions where the focused second-harmonic output of a Q-Switched Nd:YAG laser was on the order of about 1 J/cm2. An open- aperture Z-scan technique was used to quantify the limiting behavior. In this case, the mechanism of optical limiting is thought to be a nonlinear shift in the fluid index of refraction, resulting in an index mismatch between the disparate phases at high laser fluence. This induced mismatch appears to be promoted by localized electric field enhancement present near the sharp edges at the crystallite/fluid boundaries. Index mismatch between the two phases leads to multiple reflections, loss of coherence, and a significant transmission decrease due to Mie scattering. The presence of many boundaries significantly amplifies the effect. The role of thermally induced changes in refractive index for this system appears to be relatively small in pulsed-laser experiments. However, cw-laser blocking was achieved by a thermal mechanism when an absorber (iodine) was dissolved in the liquid phase. Fundamental studies of such systems are used to verify theoretical predictions of the limiting effect, and aid in the design and development of improved limiters based upon this optical deflection approach.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.