Optical critical coupling into highly confining metal-insulator-metal resonators

We demonstrate controlled optical critical coupling into highly confining metal-insulator-metal grating-based resonators. We achieve the coupling—and hence the absorption—of more than 95% of the incoming photons in a gallium arsenide based system confined between a metallic ground plane and a metallic grating. The demonstration is given in the terahertz range of the electromagnetic spectrum, at 75 μm ≤ λ ≤ 120 μm, for a semiconductor core thickness of only 10 μm. It is valid, however, at any wavelength, upon linear scaling. The critical coupling regime is judiciously tuned by precise etching of the semiconductor material in between the metallic fingers. The experimental results are in accordance with the universal behaviour predicted by temporal coupled mode theory.