Donuts make diffractionless electromagnetic waves

This work finds that a diffractionless beam can be obtained using periodically arranged donut (torus) waveguides. The Bessel-like field distribution is observed at the output of the waveguide. The structure may be built for electromagnetic waves of any wavelength, including radiowaves, microwaves, infrared light, visible light and UV light. The diameter of the diffractionless beam is of the order of magnitude of the wavelength. For UV light, the structure can be used in near-field high density storage or photolithography. For high-power visible or infrared laser such as a CO ₂ laser or tera-Watt lasers, the structure can replace collimation lenses to reduce absorption and Fresnel loss. For radiowaves and microwaves, the structure can help directional antenna increase the antenna gain for radar scanning, or highly secure and low-loss communications. The gain media confined in the structure can be adopted to enhance the Purcell effect and thus producing a low-loss and zero-threshold laser.