Experimental demonstration of the optical Helmholtz resonance.

Optical nanoantennas are studied to manipulate light and enhance light matter interactions. Here, we experimentally demonstrate the optical Helmholtz resonance in a metallic slit-box structure, which is predicted to be harmonic and to enhance the electric field intensity by several orders of magnitude. It is fabricated thanks to a two step electron beam lithography process, between which the box was filled with benzocyclobutene (BCB). Up to 80% of the light is absorbed at a <italic>λ</italic> = 2.84 <italic>μ</italic>m wavelength under a beam focused by a Cassegrain objective (NA = 0.4), even if the dimensions of this resonator are deeply subwavelength for both the slit (width <italic>λ</italic>/55 and height <italic>λ</italic>/77) and the box (width <italic>λ</italic>/7 and height <italic>λ</italic>/37). As expected from the inductance nature of the box, the optical properties of the BCB filling the box have no influence on the resonance behavior. [ABSTRACT FROM AUTHOR]