High-efficiency square-hole single-mode waveguide photon sieves for THz waves

The photon sieve has had the problem of low diffraction efficiency since it was proposed. Dispersion from different waveguide modes in the pinholes also reduces the quality of focusing. To overcome the above drawbacks, we propose a kind of photon sieve working in the terahertz band. In a metal square-hole waveguide, the effective index is determined by the side length of the pinhole. We adjust the optical path difference by changing the effective indices of those pinholes. When the thickness of the photon sieve is fixed, the optical path in a zone is set to be a multilevel distribution from 0 to λ. In this way, the optical path differences caused by the waveguide effect of pinholes are used to compensate for those caused by the positions of pinholes. We also derive the focusing contribution of an individual square pinhole. The simulated example shows a 60 times increase in intensity than that of the equal-side-length single-mode waveguide photon sieve.