The motivation of this work was to have robust spectroscopic sensors for sensitive detection and chemical analysis of organic and molecular compounds. The solution is to use silica sphere optical resonators to provide surface-enhanced spectroscopic signal.
Whispering-gallery mode (WGM) resonators made from silica microspheres were used for surface-enhanced Raman scattering (SERS) without coupling to a plasmonic mechanism. Large Raman signal enhancement is observed by exclusively using 5.08-micron silica spheres with 785-nm laser excitation. The advantage of this non-plasmonic approach is that the active substrate is chemically inert silica, thermally stable, and relatively simple to fabricate. The Raman signal enhancement is broadly applicable to a wide range of molecular functional groups including aliphatic hydrocarbons, siloxanes, and esters. Applications include trace organic analysis, particularly for in situ planetary instruments that require robust sensors with consistent response.
WGM SERS using microspheres or quartz surface structures provide a chemically robust surface for sensor applications that could be cleaned by resistively heating the sensor element. This is particularly useful for spacecraft instruments used for the detection of organics in planetary soils. The conventional silver-based SERS substrates are limited by reactivity of silver. In the case of gold SERS substrates, high temperatures (<200 ºC) will cause diffusion in the gold that degrades the nanostructure. The use of WGM SERS may also be used for surface analysis in a manner similar to attenuated total reflectance used in infrared spectroscopy. The surface localized field enhancement could be used to probe the upper layers of a sample surface.
WGM resonances (1, 2, X =also termed morphology-dependent resonances) take place when an incident light becomes trapped near the inner surface of a particle resulting from total internal reflection. This results in enhancement of the evanescent electromagnetic field at certain nodes near the surface of the particle. WGM resonance structures have been proposed as chemical sensors and when coupled with conventional Plasmon-based SERS, for single-molecule spectroscopy.
Very large enhancements are feasible by using 5-micron silica microspheres. Enhancement factors comparable to those seen from noble metal spheroids are possible enhancements when the Raman-scattered radiation also overlaps with a WGM resonance. For microspheres with radius of 5 microns, the enhancement factor can exceed an order of magnitude. This is a significant result that indicates that it would be possible to observe SERS with non-resonant scatterers.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, NASA Management Office–JPL. Refer to NPO-47604.