Designing nanoantenna that could strongly and efficiently concentrate incident light into deep subwavelength volumes is a key issue to locally enhance the electric field and thus produce strong light-matter interactions. Many existing designs are inspired by structures widely used in the radiofrequency domain such as bowtie or Yagi-Uda antennas.
Here, we rather use an analogy between acoustics and electromagnetism wave equations, in order to adapt the acoustic Helmholtz resonator to optics. This structure is made of a tiny slit above a larger cavity and exhibits several appealing features: total absorption at resonance, absence of harmonic resonance, giant field intensity enhancement in the whole slit volume, angular independence of the Helmholtz resonance [1-3]. We demonstrate experimentally various structures with Helmholtz-like resonances, and we take advantage of the huge field enhancement in the resonator for sensing applications. In particular, we show how this resonator can be used for both surface plasmon resonance sensing (SPR) and surface enhanced infrared absorption (SEIRA), in order to detect and identify molecules. We demonstrate experimentally that the SEIRA signature of 2,4-dinitrotoluene (DNT) is enhanced by several orders of magnitude, leading to reflectivity variations up to 15%. Similar experiments have been done on various nitrosamines molecules, each having its own infrared fingerprint. These results are promising for the use of these Helmholtz-like resonators as specific and sensitive sensor of molecules.
 P. Chevalier, P. Bouchon, R. Haïdar, and F. Pardo, Appl. Phys. Lett. 105, 071110(2014)
 P. Chevalier et al, P. Bouchon, J.J Greffet, J.L. Pelouard, R. Haïdar, and F. Pardo, Phys. Rev. B 90, 195412 (2014)
 P. Chevalier et al., Appl. Phys. Lett. 112, 171110 (2018)
Hasnaa El Ouazzani, Alice Fabas, Patrick Bouchon, Riad Haïdar, Jean-Jacques Greffet, and Jean Paul Hugonin, "Experimental demonstration of Helmholtz-like resonators for application to infrared sensing (Conference Presentation)," Proc. SPIE 10927, Photonic and Phononic Properties of Engineered Nanostructures IX, 109271U (Presented at SPIE OPTO: February 08, 2019; Published: 8 March 2019); https://doi.org/10.1117/12.2510548.6011668107001.
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