Rapid identification and source attribution of homemade explosives (HMEs) is vital to national defense and homeland security efforts. Since HMEs can be prepared in a variety of methods with different component ingredients, telltale traces can be left behind in the final structural form of the material. These differences manifest as polymorphs, isomers, conformers or even contaminants that can all impact the low energy vibrational modes of the molecule. Conventional Raman spectroscopy systems confine their measurements to the “chemical fingerprint” region and are unable to detect low frequency Raman signals (<200cm<sup>-1</sup>) where these low energy modes are found. This gap in sensitivity limits the conclusions that can be drawn from a single Raman measurement and creates the need for multiple measurement techniques to confirm any results. We present results from a new rugged, portable approach that is capable of extending the range of Raman to include these low frequency signals down to ~5cm<sup>-1</sup>, plus complementary anti-Stokes spectra, with measurement times on the order of seconds. We demonstrate the diversity of signals that lie in this region that directly correlate to the molecular structure of the material, resulting in a new Raman “structural fingerprint” region. By correlating the measured results with known samples from a spectral library, rapid identification of the specific method of manufacture can be made.
Structural analysis via spectroscopic measurement of rotational and vibrational modes is of increasing interest for many applications, since these spectra can reveal unique and important structural and behavioral information about a wide range of materials. However these modes correspond to very low frequency (~5cm<sup>-1</sup> - 200cm<sup>-1</sup>, or 150 GHz-6 THz) emissions, which have been traditionally difficult and/or expensive to access through conventional Raman and Terahertz spectroscopy techniques. We report on a new, inexpensive, and highly efficient approach to gathering ultra-low-frequency Stokes and anti-Stokes Raman spectra (referred to as “THz-Raman”) on a broad range of materials, opening potential new applications and analytical tools for chemical and trace detection, identification, and forensics analysis. Results are presented on explosives, pharmaceuticals, and common elements that show strong THz-Raman spectra, leading to clear discrimination of polymorphs, and improved sensitivity and reliability for chemical identification.
Raman and Terahertz spectroscopy are both widely used for their ability to safely and remotely identify unknown materials. Each approach has its advantages and disadvantages. Traditional Raman spectroscopy typically measures molecular energy transitions in the 200-5000cm<sup>-1</sup> region corresponding to sub-molecular stretching or bending transitions, while Terahertz spectroscopy measures molecular energy transitions in the 1-200cm<sup>-1</sup> region (30GHz - 6THz) that correspond to low energy rotational modes or vibrational modes of the entire molecule.<p> </p>Many difficult to detect explosives and other hazardous chemicals are known to have multiple relatively strong transitions in this “Terahertz” (<200cm<sup>-1</sup>, <6THz) regime, suggesting this method as a powerful complementary approach for identification. However, THz signal generation is often expensive, many THz spectroscopy systems are limited to just a few THz range, and strong water absorption bands in this region can act to mask certain transitions if great care isn't taken during sample preparation. Alternatively, low-frequency or “THz-Raman” spectroscopy, which covers the ~5cm<sup>-1</sup> to 200cm<sup>-1</sup> (150GHz - 6 THz) regions and beyond, offers a powerful, compact and economical alternative to probe these low energy transitions.<p> </p>We present results from a new approach for extending the range of Raman spectroscopy into the Terahertz regime using an ultra-narrow-band volume holographic grating (VHG) based notch filter system. An integrated, compact Raman system is demonstrated utilizing a single stage spectrometer to show both Stokes and anti-Stokes measurements down to <10cm<sup>-1</sup> on traditionally difficult to detect explosives, as well as other chemical and biological samples.
This course will focus on the key packaging optical assembly and test processes, covering critical process parameters, yield issues, manufacturing and test equipment, design guidelines and validation strategies, and cost models. Examples of different types of packages, including coaxial, butterfly, mini-DIL, and planar waveguide packages, will be used in case studies to compare various assembly methods and their costs. Test methods will be discussed with respect to their suitability for different stages of assembly.