2, NRC Research Associate Program, Washington, District of Columbia, United States
Surface-phonon polaritons (SPhPs) have emerged as attractive alternatives to plasmon polaritons because of the extremely high quality factors of their localized and propagating resonances (SPhPRs). The high quality arises from their dependence on concerted nuclear motion rather than electronic motion. These resonances can only be supported when the resonance frequency lies in the Reststrahlen band of a material, the region between the longitudinal and transverse optical phonons and characterized by metal-like optical constants. To date, most SPhPs have been observed in polar semiconductors, where the Reststrahlen band tends to occur at wavelengths longer than 6 micrometers. Shorter wavelength, higher frequency SPhPRs could potentially be useful for a variety of chemical applications like sensing or energy-transfer modulation. Here, we present infrared reflection spectroscopy of a number of molecular crystals that possess Reststrahlen bands in chemically relevant frequency ranges. We identify and assign resonances that appear within the Reststrahlen bands of some of these materials, specifically W(CO)6, and comment on the range of resonances that can be supported on this class of SPhP materials.