A Chip-Scale Plasmonic Spectrometer for In Situ Characterization of Solar System Surfaces

We discuss the development of a plasmonic spectrometer for in situ characterization of solar system surface and subsurface environments. The two goals of this effort are to (1) quantitatively demonstrate that a plasmonic spectrometer can be used to rapidly acquire high signal-to-noise spectra between 0.5-1.0 μm at a spectral resolution suitable for unambiguous detection of spectral slopes and features indicative of volatiles and characteristic surface mineralogies, and (2) demonstrate that this class of spectrometer can be used in conjunction with optical fibers to access subsurface materials and vertically map the geochemistry and mineralogy of subsurface layers, thereby demonstrating that a plasmonic spectrometer is feasible in a low-mass, low-power, compact configuration. Our prototype instrument, the Plasmonic Spectrometer for In situ CLassification Of Planetary Surfaces (PSICLOPS), consists of a broadband lamp/source, a fiber optic system to illuminate the sample surface and collect the reflected light, a mosaic filter element based on plasmon resonance, and a focal plane array detector. The critical filter element of the spectrometer is based on the internal plasmon resonance of metallic nanostructures. We developed a membrane-based plasmonic filter that can be directly implemented on an optical fiber. After developing a new fabrication process for nanostructures in thin Au membranes suspended in air, we found that we can control the central wavelength of the filter by changing the index of refraction of the surrounding medium. Therefore we explored several media (e.g. water, glycerol, and glucose) as a means of introducing a range of indices of refraction, and thus central wavelengths of the filter, using microfluidic channels. In addition to the development of the plasmon filter element, we constructed a testbed to explore the use of optical fibers for source illumination and signal transmission to the focal plane array. We discuss our preliminary design studies of the plasmonic nanostructure prototypes and their application to miniaturized instrumentation for in situ characterization of solar system surface and subsurface environments.