Broadband Plasmonic SEIRA Structures for Signal Enhancement in nanoIR Spectroscopy

G. Rutins, L. Tetard
University of Central Florida,
United States


Nanoscale infrared (nanoIR) spectroscopy can be carried out by monitoring the local photothermal expansion of a material resulting from infrared absorption with an atomic force microscopy (AFM) probe. This approach has been used to reveal the chemical makeup of materials with nanoscale lateral resolution. However, using the AFM probe as a detector implies some sensitivity limitations. Plasmonic structures can provide sub-wavelength regions of high field intensity in response to far-field illumination. They have been extensively used for surface enhanced Raman spectroscopy (SERS) and Surface Enhanced Infrared Absorption (SEIRA) structures are now drawing interest for mid-infrared spectroscopy. While enhancing a narrow wavelength range is suitable for Raman spectroscopy, enhancing the signal of the ‘molecular fingerprint’ region in mid-infrared spectroscopy requires enhancement over a broader wavelength range. We present a SEIRA design providing multiband localized field enhancement spanning the mid-infrared range. After evaluating the effect of geometry on the enhancement, we explore their near-field response with nanoIR spectroscopy. We show that placing a functionalized tip in the SEIRA hot spots provides significant enhancement of infrared fingerprints. This approach introduces new opportunities for the investigation of sub-10 nm entities, including with different light polarization.