Appl. Phys. Lett. 106, 021101 (2015)

Credit: AIP

Bessel beams — non-diffracting beams with a self-healing wavefront — at terahertz frequencies could prove useful for applications such as deep focal imaging and communications. To date, such beams have mostly been generated by bulky axicon lenses. Now, Yasuaki Monnai and colleagues in Japan, Germany and Australia experimentally demonstrate a means that is more amenable to integration. The approach relies on a planar metal structure featuring a tightly packed series of concentric grooves that couple light to surface waves, known as spoof surface plasmon polaritons. Furthermore, a secondary series of more widely spaced concentric deeper grooves act as scatterers that convert these radial surface waves into plane waves propagating in free space. The period of the scatterers is designed to be shorter than the wavelength of the surface waves so that the free-space plane waves can be tilted towards the central axis, and the interference of these waves creates a Bessel beam. The structure is excited through a small central slit. Using structure, the team generate a Bessel beam with a frequency of 0.29 THz with non-diffractive behaviour maintained over a distance of 20 wavelengths from the aperture. As the excitation slit can be replaced with a solid-state terahertz oscillator, the beamformer could be part of an integrated terahertz system.