2, University of Central Florida, Orlando, Florida, United States
3, University of Central Florida, Orlando, Florida, United States
Studying light-matter interactions at the molecular level is critical to accelerate our understanding of life sciences. Chemical speciation has successfully been impeded to Atomic Force Microscopy (AFM) measurements by exciting the material with infrared light and using the AFM cantilever to monitor the photothermal expansion resulting from the vibrational modes excited in the sample. With subwavelength spatial resolution, it is expected that single molecule fingerprints are attainable. Previous work shows that it is possible to design plasmonic substrate to locally enhance the electromagnetic field used to excite the molecules for higher sensitivity.
Here we use plasmonic substrate to polarize the field used to excite the molecular vibration. Our study focuses in circular dichroism at nanoscale for chiral biomolecules. Using cavity-coupled achiral and planer plasmonic structures, we show that it is possible to generate a stronger confined electromagnetic field for our nanoscale infrared spectroscopy platform for wavelengths of 1500-1800cm-1. Chiral biomolecules with absorption and vibrational circular dichroism signature overlapping with the absorption of the cavity-coupled plasmonic structure are identified for the measurements. The sample is excited by both the linearly and polarized tunable IR pulsed laser. For circular polarization we acquire the successive for both the left-handed and right- handed circular polarizations. Upon absorption, energy is transferred to the lattice and heat is generated, leading to thermal expansion. Molecules laying in the confined field at the plasmonic structures show stronger circular dichroism signal. By using this approach, we are able to detect chirality of biomolecules at nanoscale. The results suggest that using the “hot spots” of the cavity coupled plasmonic structures are significant and offer great potential for characterizing the chirality of single molecule.