Infrared Propagating Electromagnetic Surface Waves Excited by Induction
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.472
Infrared Propagating Electromagnetic Surface Waves Excited by Induction Jonathan R. Brescia1,3 , Justin W. Cleary2 , Evan M. Smith2,3 , Robert E. Peale1 1
Physics, University of Central Florida, Orlando FL 32816 USA
2
Air Force Research Laboratory, Sensors Directorate, RYDH, Wright-Patterson AFB OH 45433 USA
3
KBRwyle, Beavercreek, OH, 45431
Abstract
Propagating inhomogeneous electromagnetic waves called surface plasmon polaritons (SPPs) can be excited by free-space beams on corrugated conducting surfaces at resonance angles determined by corrugation period, permittivity, and optical frequency. SPPs are coupled to and co-propagate with surface charge displacements. Complete electrical isolation of individual conducting corrugations prevents the charge displacement necessary to sustain an SPP, such that excitation resonances of traveling SPPs are absent. However, SPPs can be excited via electric induction if a smooth conducting surface exists below and nearby the isolated conducting corrugations. The dependence of SPP excitation resonances on that separation is experimentally investigated here at long-wave infrared wavelengths. We find that excitation resonances for traveling SPPs broaden and disappear as the dielectric’s physical thickness is increased beyond ~1% of the free-space wavelength. The resonance line width increases with refractive index and optical thickness of the dielectric.
INTRODUCTION Concentration and confinement of electromagnetic fields via excitation of surface plasmon polaritons (SPP) has well known commercial applications in bio-sensing [1]. Generally, a coupler is required for free space optical beams to excite SPPs on conducting surfaces. The coupler used in commercial systems is the Kretschmann prism [1]. The sensing mechanism is the angular shift of an angularly sharp SPP excitation
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resonance when analyte molecules bind to the surface and change the local refractive index. Most commercial systems operate at visible or near-IR optical frequencies, but there are potential advantages to operating farther into the infrared [2,3]. However, prisms make poor long-wave-infrared couplers due in part to high indices and dispersion of suitably-transparent non-hygroscopic materials [3]. Grating couplers comprising corrugated conducting surfaces are well known for excitation of infrared traveling SPPs [4,5]. Such have been used to excite SPPs for molecular sensing by out-coupling the traveling SPP at a second grating into a free-space homogeneous wave, which can be collected by a conventional infrared detector [4]. A sensor based on changes in SPP-excitation resonance angle, as in commercial biosensors [1], on a single grating faces the challenge that any surface analyte or flow-confinem
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