In analogy to surface plasmon polaritons, some materials can support surface phonon-polaritons (SPhP) that arise from collective oscillations of phonons rather than electrons. These oscillations can yield narrow, low-loss polaritonic resonances in the infrared. Traditional SPhP materials are polar, wide bandgap semiconductors. We use ultrafast infrared spectroscopy to examine the carrier dynamics and phonon-polariton tuning of bulk and nanostructured SPhP-supporting materials. In another effort, we are working to develop high-frequency SPhPs by moving from semiconductors to novel molecular systems. These high-frequency SPhP materials have great promise for chemical applications like sensing and catalysis.
Vibrational Strong Coupling
When a molecular vibration is resonant with an optical mode in a Fabry Perot cavity or plasmonic system, the two modes can strongly couple to form two new polariton modes. This strong coupling requires that conditions involving the absorption strength and linewidth be satisfied, but is amenable to a wide range of molecules. We study the ultrafast dynamics of these new polariton modes. The combined optical and vibrational character of the modes has important implications on their physics and can lead to optical modification of reactivity.
We are studying the efficiency and kinetics of charge transfer from plasmonic nanoparticles and nanostructures to semiconductor substrates which are important for improving the performance of plasmonic photocatalysis materials. Understanding and optimizing this transfer process is crucial to obtaining functional catalysts and photovoltaic devices. Both idealized model systems and functional materials are being investigated to bridge the gap between optical physics and electrochemistry. In addition, steady-state and transient spectroscopy is used to characterize visible and infrared plasmonic materials and metamaterials.
Optical and Spectroscopic Studies
Various types of optical measurements, spectroscopic studies and computational studies are pursued pertinent to efforts throughout the division and laboratory. These include: remote sensing applications for tagging, tracking, and locating and related efforts; previous work on standoff molecular detection and imaging associated with fire detection; characterization of material optical properties for gases, liquids and solid including films or powders such as in prior studies for obscurant materials
Multi-sensor Towed Array Detection System (MTADS)(maintain page at https://www.nrl.navy.mil/chemistry/research/6110/6111/mtads/)
Methods for efficient classification of buried metal objects are being developed. The MTADS has demonstrated the ability to detect all military unexploded ordnance (UXO) at their maximum penetration depths. The challenge is to reduce the number of scrap and fragment objects identified as UXO. Current work focuses on the shape-dependence of the Electromagnetic Induction response of buried targets for discrimination. Both time-domain and frequency-domain sensors are being investigated as well as a novel, hybrid sensor concept.