Here we report on a standoff spectroscopic technique for identifying chemical residues on surfaces. A hand-held infrared
camera was used in conjunction with a wavelength tunable mid-IR quantum cascade laser (QCL) to create hyperspectral
image arrays of a target with an explosive residue on its surface. Spectral signatures of the explosive residue (RDX)
were extracted from the hyperspectral image arrays and compared with a reference spectrum. Identification of RDX was
achieved for residue concentrations of 20 μg per cm2 at a distance of 1.5 m, and for 5 μg per cm2 at a distance of 15 cm.
A micro differential thermal analysis (DTA) system is used for detection of trace explosive particles. The DTA
system consists of two silicon micro chips with integrated heaters and temperature sensors. One chip is used for
reference and one for the measurement sample. The sensor is constructed as a small silicon nitride membrane
incorporating heater elements and a temperature measurement resistor. In this manuscript the DTA system
is described and tested by measuring calorimetric response of 3 different kinds of explosives (TNT, RDX and
PETN). This project is carried out under the framework of the Xsense project at the Technical University of
Denmark (DTU) which combines four independent sensing techniques, these micro DNT sensors will be included
in handheld explosives detectors with applications in homeland security and landmine clearance.
A micro differential thermal analysis (DTA) system is used for detection of trace explosive particles. The DTA
system consists of two silicon micro chips with integrated heaters and temperature sensors. One chip is used
for reference and one for the measurement sample. The sensor is constructed as a small silicon nitride bridge
incorporating heater elements and a temperature measurement resistor. In this manuscript the DTA system is
described and tested by measuring calorimetric response of DNT (2,4-Dinitrotoluene). The design of the senor is
described and the temperature uniformity investigated using finite element modelings and Raman temperature
measurements. The functionality is tested using two different kinds of explosive deposition techniques and
calorimetric responses are obtained. Under the framework of the Xsense project at the Technical University of
Denmark (DTU) which combines four independent sensing techniques, these micro DNT sensors will be included
in handheld explosives detectors with applications in homeland security and landmine clearance.
Nanomechanical response of molecular adsorption has been demonstrated as the basis for a number of extremely
sensitive sensors. Molecular adsorption on microcantilevers results in nanomechanical motion due to adsorption-induced
surface stress variation. Chemical selectivity in nanomechanical sensors is achieved by immobilizing receptors
on the cantilever surface. Although receptor-based detection has high selectivity for biomolecular detection, it fails
when applied to small molecule detection. Nanomechanics, however, offer new possibilities for achieving chemical
selectivity that do not use any receptors. For example, small thermal mass or high temperature sensitivity of a cantilever
beam could be used for detecting molecular adsorption using photothermal effects and physical property variation due
to temperature. Here we describe two such techniques for achieving chemical selectivity without using any receptor
molecules.
We investigated the fabrication of integrated diffractive micro-optical features on MEMS structures for the purpose of motion detection. The process of producing the diffractive features and the MEMS structures by focused ion beam milling is described in detail, as is the ion beam sputtering process used to produce coatings on these structures. The diffractive features of the circular Fresnel zone plate (FZP) and spiral FZP were fabricated on MEMS structures and the relevant diffraction theory is discussed. The spiral FZP diffractive features produced well defined foci whose intensity varies with distance from the FZP. Observation of these intensity variations enable us to detect the motion of the MEMS structure, and the resulting device was used to scan an IR image of a hot object.
We investigated microposition sensing of micro-electro- mechanical systems (MEMS) that is based on optical readout techniques. We determined the parameters that affect or limit the performance of optical readout techniques especially as they apply to detection of infrared radiation. Such microposition sensing schemes are very important as readout mechanisms for large arrays of microstructures which are required for imaging. In addition, we explored the performance of uncooled micromechanical IR sensors using Fresnel zone plates (FZP). This type of diffractive feature diffracts along the optical axis and not perpendicular to that axis. We found that temperature fluctuation noise and background fluctuation noise, are currently the limits to the performance of uncooled micromechanical IR detectors. The noise at the output of the optical readout includes amplified noise from the micromechanical structures and noise added by the optical readout itself. However, the added noise is negligible compared to the amplified temperature fluctuation noise inherent in the microstructures. In this context an optical readout is nearly an ideal, noiseless readout method.
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