Using Subwavelength Diffraction Gratings to Design Open Electromagnetic Cavities

In this Letter we propose to use subwavelength diffraction gratings as very good semitransparent mirrors for electromagnetic waves to design open cavities. To do so, we replace part of the walls of a cavity by such a grating. We numerically and analytically link the grating characteristics to the spectral properties of the realized open cavity. Then we demonstrate that the eigenmodes of the cavity can be transmitted perfectly through the grating to the exterior, thereby turning a point source inside the cavity into a very directive source. We investigate the effect of disorder, which leads to isotropic radiation patterns, and perform experiments in the microwave domain in order to support our claims. Finally, we present an example of application of the concept in fundamental physics, by measuring from outside the eigenmodes of a disordered microwave cavity.