Qing Cao

Theory for terahertz plasmons of metallic nanowires with sub-skin-depth diameters

We investigate the propagation properties of terahertz plasmon of a metallic nanowire with sub-skin-depth diameter. By taking the small radius and the huge relative permittivity into account, we establish an approximate analytical description for this kind of surface plasmon. It is shown that the main propagation properties are closely related to the product of the radius of the metallic nanowire and the complex wave number of the metal. In addition, when the radius of the metal wire is smaller than the skin-depth, the size of the modal field is simply proportional to the radius of the metal wire. We also carefully verify these analytical predictions with rigorous numerical simulations.

Creation of a 50,000λ long needle-like field with 036λ width

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.

Tapered dual elliptical plasmon waveguides as highly efficient terahertz connectors between approximate plate waveguides and two-wire waveguides

We present a tapered dual elliptical plasmon waveguide for terahertz waves. This element is composed of a pair of tapered elliptical metal structures and is especially suitable for the coupling of terahertz waves from an approximate plate waveguide to a two-wire waveguide. The long axes of the two ellipses gradually reduce to the same sizes as the short axes, and thus the two-ellipse structure is now a two-wire waveguide. The slowly tapered structure eliminates the reflection and scattering during the coupling process according to WKB approximation. The numerical result shows that the coupling efficiency of this connector can reach as high as 94%.

Analytical recurrence formula for the zeroth-order metal wire plasmon of terahertz waves

An analytical recurrence formula for the zeroth-order metal wire plasmon of terahertz waves is presented. In the derivation, the property that the relative permittivity of a metal in the spectral region of terahertz wave is huge, the property that the effective index is about 1, some properties of modified Bessel functions, and a suitable Taylor expansion are employed. The recurrence formula is numerically tested for 11 nonmagnetic metals for the whole spectral region of terahertz waves and for the wide radius range from 10 mum to infinity. We find that the relative deviation for the effective index always becomes smaller than 2.1x10(-6) after only four recurrences if a suitable initial input is used. Some related problems, such as the connection to the Newton method and the dependence of the accuracy on frequency, are also discussed.

Nanofocusing of terahertz wave in a tapered hyperbolic metal waveguide.

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.

Wide-band six-region phase mask coronagraph

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.

Wide-band coronagraph with sinusoidal phase in the angular direction

We suggest a new phase mask coronagraph that can work in a wide band of wavelengths. The phase mask has alternatively sinusoidal and uniform functions in the angular direction. We compare it with the four-quadrant phase mask coronagraph and vortex phase mask coronagraph. Through numerical tests, we find that the new mask gives a deep extinction of star light and has a small inner working angle. It is also shown that this mask has a better performance in chromatism than the others for a wide band of wavelengths.

Individual far-field model for photon sieves composed of square pinholes

A photon sieve can be composed of a large number of square pinholes. By taking the related coordinate transform into account, we present here an individual far-field model for a photon sieve composed of many square pinholes whose edges are symmetrically vertical or parallel to the polar coordinate. In particular, a simple analytical expression for the diffracted far field of an individual square pinhole is given, and the focusing contribution from an individual square pinhole is further discussed. The obtained results can be used for the analysis, design, and simulation of a high numerical aperture photon sieve composed of the above-mentioned square pinholes.

Creation of a 50,000λ long needle-like field with 0.36λ width: reply

This is the reply to the comment by Chavez-Cerda and Pu [J. Opt. Soc. Am. A32, 1209 (2015)JOAOD61084-752910.1364/JOSAA.32.001209] on our recent work about the 50,000λ long needle-like field [J. Opt. Soc. Am. A31, 500 (2014)JOAOD60740-323210.1364/JOSAA.31.000500]. First, they employed an incorrect boundary condition as the fundament of their argument. In fact, it is not the electric field but its tangential component that must be zero at the surface of the perfect metal. Our result is completely consistent with the correct boundary condition. Second, a constant phase factor in the incident radially polarized beam, exp(jπ/4), for instance, has no influence on the result. Accordingly, our initial condition is proper.

Three-wave approximation for the modal field inside high-index dielectric rods of hybrid plasmonic waveguides

Abstract An approximate three-wave model is suggested for describing the modal field inside the high-index dielectric rod of a hybrid plasmonic waveguide. An evanescent wave, an uniform wave and a propagating wave are considered along the direction perpendicular to the metal surface. The superposition of these three waves forms the modal field inside the high-index rod. Through numerical tests, we find that this model is highly valid for a large range of waveguide sizes.