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 λ = 2.84 μm 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 λ/55 and height λ/77) and the box (width λ/7 and height λ/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.