Multi-frequency focusing of microjets generated by polygonal prisms

We systematically investigate the power distribution characteristics of microjets generated by prismatic scatterers with different shapes at sub-THz region (λ = 8.57 mm). Among these prismatic scatterers, the hexagonal-type one shows better focusing feature than the others. Aiming at the hexagonal-type one, we propose a double-layer scatterer composed of a Teflon hexagonal prism as an outer layer and a semiconductor cuboid as an inner layer. Aiming at the double-layer scatterer, we further study the effects of refractive index, size, and shape of the inner cuboid on microjet's features. The study allows us to present an optimized double-layer scatterer, which has a side length λ /2 (λ) and a refractive index 2.0 (1.4) for the inner (outer) layer. We show that the optimized scatterer can produce an ultra-strong, ultra-narrow microjet with a power enhancement of ~30 and a full width at half maximum (FWHM) of ~0.26λ, and the microjet is just located at the output face. The microjet keeps compact within the distance range of λ from the output face. These features and effects are explained from the viewpoint of ray optics theory. According to the optimized double-layer scatterer, we further study the multi-frequency focusing features of the microjets, and find that the microjet remains good features at harmonic frequencies 2f₀ and 3f₀. In addition, we investigate the effect of an Au sphere presence in the center of the microjet on the power distribution. The results show that a spherical dark spot with a size similar to that of the Au sphere emerges in the area where the Au sphere is placed. The feature can be used to measure the size of a metallic particle.