Accomplishments and Advancements

Raman Spectra of Individual Airborne Particles Several optics projects in BED are aimed at single-particle detection for biohazard threat applications. BED has made significant contributions in the area of laser-induced fluorescence (LIF) detection methods that have been developed and now deployed, but these LIF methods do not provide adequate differentiation to reliably identify specific pathogens. Definitive characterization is more straightforward for chemical detection using mass spectrometry techniques, for example, but pathogen-specific biosensors in atmospheric sensing require techniques such as Raman spectroscopy or infrared (IR) absorption/emission spectroscopy. Both of these techniques suffer from signal-to-noise constraints at the single-particle level, and BED is engaged in basic early-stage research aimed at addressing this challenge. For Raman spectroscopy, current work provides a novel method for trapping individual particles using a new cylindrical (Bessel) beam-focusing geometry that provides a hollow optical ellipsoidal intensity chamber containing particles through photophoretic forces. This provides sufficient residence time (approximately 1 second) yielding the signal-to-noise ratio required for reliable pathogen identification. This technique has already demonstrated the ability to trap a significant range of particle sizes using readily available milliwatt-level beam powers along with practical optics, and it has also demonstrated well-resolved Raman signatures of trapped particles using the same trapping beam. This technique also has the flexibility of allowing for additional specific Raman probe beams. The work is in the very early stages, but it has demonstrated that in airborne particle applications the photophoretic forces are much stronger than the usual optical tweezer radiation pressure techniques that are used routinely in aqueous environments. There is opportunity for obtaining a wealth of data and for data analysis, combined with theoretical modeling, to fully map out the efficacy of the approach. Once this is known, the group has competence in the hardware implementations necessary to demonstrate a combined system that uses preliminary scattering and LIF-based particle sorting, followed by the Raman technique for specific pathogen identification.

Turbulence Propagation Theory and Effects This study has matured very nicely. The principal investigators have published results and have used reviewer feedback to develop a more complete theory for the modulation transfer function (MTF). The present study has two objectives: (1) quantification of optical turbulence effects in passive and active imagers and (2) development of prototype methods for mitigating turbulence effects on imagers. With regard to the first objective, the research team should proceed with publication of the new (more complete) theoretical findings. With regard to the second objective, the researchers have begun to develop concepts for operationalizing the theoretical results. The research team should develop a prototype system to demonstrate that the theory correctly describes what happens in practice, and it should use the results to design and implement a full demonstration program to present to a funding agency within the Army. The most recent research is unique and fundamental; unlike the adaptive optics approach also developed within BED, this approach does not require a cooperative target to enhance the target image.

Climatological Assessment Utilizing Airborne Acoustic Sensors The problem addressed in this activity is locating a source of acoustic emission on Earth’s surface using detectors on an elevated/airborne platform. Once the location of the emission is identified, a camera can be directed toward the source for full identification. The path of the propagating acoustic signal has to be corrected for refraction or bending resulting from temperature variations in the atmospheric density and shifts due to horizontal wind. The researchers lucidly defined the problems, and they tested the corrections for a case of heated ground that has a monotonically changing temperature profile concave with respect to Earth’s surface.

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