It is a great challenge to create a needle-like field with properties of long beam length, narrow lateral width, uniformity, and high optical efficiency. Here we show a method that can realize these properties all at once. The key element is a 90° apex-angle concave conical mirror. By using this condenser along with a radially polarized incident beam of a specific field distribution, we numerically created a super slim, uniform, pure needle-like axially polarized field. This axially polarized field has a length of 50,000λ along the optical axis, and its lateral width still maintains a minimum 0.36λ size.
An achromatic six-region phase mask coronagraph, used for the detection of exoplanets, is proposed. The mask has six regions in angular direction and could work in wideband. Furthermore, a six-level phase mask, as an example of the six-region phase mask, is theoretically investigated. According to numerical simulations, this specific mask has a deep elimination of starlight, good performance of achromatism and small inner working angle. As a single phase mask, the ratio of the remaining starlight of the six-level phase mask to the total incident starlight is less than 0.001 when the wavelength is between 500 nm and 600 nm.
An tapered hyperbolic metal waveguide is suggested for the nanofocusing of terahertz waves. We numerically show that, at the frequency of 1 THz, the focal spot can be as small as only 5 nm, which is smaller than that of a plate waveguide by 2 orders of magnitude. Correspondingly, the longitudinal component of the energy flow density is stronger than that of a plate waveguide by 3 orders of magnitude for the same input. It is shown that these significant improvements come from the small imaginary part of the effective index of the hyperbolic metal waveguide.
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