Optical cavities are used in many things including lasers, LEDs, and optical sensors. These structures are specifically made to create standing light waves, by reflecting the wave back and forth between its sides. Natural optical cavities are limited in their capabilities though, because some light will leak out, but with metamaterials not only can the leakage be reduced, but the entire thing can be taken to subwavelength sizes.
Researchers at Berkeley Lab have built the world's smallest 3D optical cavity using indefinite metamaterials. This class of metamaterials is capable of having light refract backwards, which is impossible with normal materials. It is this property which allowed the researchers to make the cavity much smaller than the wavelength of the light it holds, and potentially much smaller. The physics of the design will actually cause the cavity to leak less as it gets smaller.
The small size and quality the cavity opens up many possibilities from lasers to optical sensors. Smaller cavities have a higher photon density which means there is more light to interact with matter.