While the concentration of airborne particulate matter is well-known to be correlated with people's health, a chemical evaluation must also be important. Noting that airborne dust particles in the 3À5 μm size regime are among the largest that get into people's lungs, such particles were collected by pumping air through plasmonic metal films with a 12.6 μm square lattice of 5 μm square holes. Capture of a dust particle in a metallic hole enables the recording of "scatter-free" infrared absorption spectra whose peaks reveal the infrared active components. The study of the spectra of individual particles allows minority components to be characterized in a way that is quite incisive which is difficult with bulk samples. A library of 63 spectra of individual particles captured from our laboratory air is presented along with a preliminary analysis of the contributing components.

The scattering effects in the infrared (IR) spectra of single, isolated bread yeast cells (Saccharomyces cerevisiae) on a ZnSe substrate and in metal microchannels have been probed by Fourier transform infrared imaging microspectroscopy. Absolute extinction [(3.4±0.6)×10(-7) cm(2) at 3178 cm(-1)], scattering, and absorption cross sections for a single yeast cell and a vibrational absorption spectrum have been determined by comparing it to the scattering properties of single, isolated, latex microspheres (polystyrene, 5.0 μm in diameter) on ZnSe, which are well modeled by the Mie scattering theory. Single yeast cells were then placed into the holes of the IR plasmonic mesh, i.e., metal films with arrays of subwavelength holes, yielding "scatter-free" IR absorption spectra, which have undistorted vibrational lineshapes and a rising generic IR absorption baseline. Absolute extinction, scattering, and absorption spectral profiles were determined for a single, ellipsoidal yeast cell to characterize the interplay of these effects.

Both experiments and finite difference time domain (FDTD) calculations show that infrared (IR) absorption spectroscopy and IR transmission resonances of metal films with arrays of subwavelength holes (meshes) are more sensitive to material in the holes of the mesh than to material on the front or back surfaces of the mesh. IR extinction spectra of isolated, individual latex (polystyrene) spheres inside or outside of a hole on plasmonic Ni mesh are compared to similar isolated microspheres on a ZnSe substrate. Although vibrational spectra of single microspheres on ZnSe are dominated by scattering, vibrational spectra of microspheres in microholes of metal mesh are dominated by absorption. The IR absorption spectrum of a latex microsphere with mesh is much stronger when the microsphere is inside a mesh hole rather than on the front or back surface, which pertains to the nature of spectroscopic enhancements on the plasmonic mesh. These results are supported and enhanced by FDTD simulations showing changes in the primary infrared transmission resonance of the mesh when latex spheres are placed in different positions. The simulations demonstrate how interaction with a plasmonic resonance changes the intensity and line shape of a vibration in the dielectric material.

Metal films with arrays of subwavelength holes (mesh) exhibit extraordinary transmission resonances to which many attribute a role for surface plasmon polaritons (SPPs); others debated this point. Experimental measurements of propagation lengths are presented under conditions that pertain to the use of SPPs for surface spectroscopy. The lateral extent of electromagnetic propagation along the mesh surface is measured by recording absorption spectra of a line of latex microspheres as a function of distance away from the line along the mesh. Measurements reveal an exponential functional form for decay of absorption signal laterally from the absorption source. Results at 697 cm−1, which are closest to the strongest transmission resonance of the mesh, reveal a 1/e propagation distance along the surface of 17.8±2.9 μm. This is 40% larger than the lattice spacing implicating the holes as the SPP damping mechanism, however, this is significantly shorter than smooth metal expectations.