A new paper by Laaya Sabri, Qinglan Huang, Jui-Nung Liu, and Brian T. Cunningham from the Nanosensors Group at the University of Illinois at Urbana-Champaign introduces a fundamentally new class of all-dielectric nanoantenna field-enhancement structures for biosensing applications that, due to destructive farfield interference of resonant modes, functions as a nonradiating “anapole” in the visible spectrum. Unlike other dielectric and plasmonic resonator structures for electromagnetic field enhancement, anapole modes are strictly confined within their cylindrical disk structure and thus can produce a single point electromagnetic hotspot by providing a nanometer-scale opening at the anapole mode, located in the center of the disk.  Utilizing a strategy for coupling additional electromagnetic energy from an underlying mirror-backed substrate to an anapole nanoantenna in an aqueous environment, we utilize Finite Difference Time Domain simulations to demonstrate the potential for generating high-intensity, highly localized electromagnetic hotspots, for purposes of amplifying the excitation of photon emitters, such as fluorophores, that are used as tags for observing biomolecular interactions. Our simulations show that an anapolar nanoantenna comprised of a simple silicon nanodisk with a central slot fabricated on a dielectric substrate will provide a 3.5x electromagnetic field enhancement of |E|, and that integration of the nanoantenna with an underlying mirror-backed substrate will be capable of 11.5x enhancement of |E| in the 630-650 nm wavelength range that is compatible with the excitation of commonly used fluorescent dyes. As fluorophore photon output scales with excitation power, and thus |E|², our results suggest the potential to obtain ~130x enhanced excitation factor.  In this work, we characterize the effects of the substrate design and slot dimensions on the field enhancement magnitude, for devices operating in a water medium.  By applying additional mechanisms for enhanced photon capture and reduced fluorescence lifetime, our models show the potential to obtain ~700,000x amplification of detectable signal from photon emitters confined to the 0.001 – 0.065 femtoliter volume of the electromagnetic hotspot in the anapole device.

 

For full details, please see the publication:  “Design of anapole mode electromagnetic field enhancement structures for biosensing applications,” L. Sabri, Q. Huang, J.-N. Liu, and B.T. Cunningham, Optics Express, Vol. 27, No. 5, p. 7196-7212, 2019. https://doi.org/10.1364/OE.27.007196