Abstract-By discussing the basic schemes of the terahertz generation methods based on the 1550-nm ultrafast lasers briefly, GaAs and ZnGeP 2 are likely to be promising nonlinear optical crystals for terahertz waves generation by using optical rectification process. However, the mismatches of velocities between the terahertz waves and optical pulses are so large that the phase-matching coherent lengths are quite short, for example, the coherent length of 0.7 mm for GaAs and 0.5 mm for ZnGeP 2 at 2 THz around, respectively. That limited extremely the applications of these bulk excellent nonlinear optical crystals in terahertz regime. In this paper, we demonstrated theoretically that the dielectric planar waveguide could be used to enhance the coherent length of optical rectification process in THz regime. And for the first time, a dielectric planar THz waveguide that has potential applicable value in THz generation by optical rectification method was proposed. We predicted that the effective coherent length could be extended to 4 mm at 2 THz in a GaAs/ZnGeP 2 dielectric planar waveguide during optical rectification process pumped by ultrafast optical pulses at wavelength of 1550 nm.

We propose a mechanism for subwavelength focusing at optical frequencies based on the use of a tapered microtube fabricated from a glass capillary tube. Using coherent illumination at 671nm and a near-field scanning optical microscope probe which was mounted on a 3-axis piezo nanopositioning stage, a sequence of 2-D intensity profiles were obtained. Our experimental results reveal the smallest focal spot with a near diffraction-limited full width at half-maximum of ~435nm （0.65λ） at a distance of ~1.47μm (2.2λ) from the output endface of microtube. The experimental results are in excellent agreement with our numerical simulation.In recent years, there has been an explosion of interest in obtaining subwavelength focusing that can reach beyond the Abbe-Rayleigh diffraction limit, which remains a dominant barrier to achieving features smaller than approximately half a wavelength with optical instruments. Higher optical resolution will be beneficial to such fields as semiconductor lithography, optical trapping 1 and high density optical data storage.2 Several approaches have been proposed and experimentally realized to meet these challenges, and the most successful ones are: 1. Plasmonic lenses based on surface plasmon polaritons (SPP), such as nanoslits, nanoholes and surface corrugations in a metallic film; 3-6 2. So-called superlens, a perfect lens first proposed by Pendry, making use of materials or structures exhibiting negative refraction; 7-10 3. Non-plasmonic lenses via Talbot effect, 11 in the form of nanohole quasi-periodic array in a metal screen, 12 a circle of planar nanoholes in a dielectric film 13 and a micro/nanofiber (MNF) array. 14 However, the focusing effects are still severely influenced by the metallic absorption for plasmonic lenses, and the superlenses are only capable of projecting an image with perfect resolution in the near field in experiments so far, due to the intrinsic losses.15 While non-plasmonic lenses made from dielectric materials have considerably lower loss than the previous two types.The concept of direct synthesis of the angular spectrum has been proposed asserting that a focused beam can be synthesized directly from its angular spectrum of plane waves. 16 And，in Ref. 16, the feasibility of this approach for focusing of light with subwavelength resolution has also been experimentally verified by using a designed apparatus converting 15 laser beams to constitute a converging circular cone. Inspired by this intriguing approach, in this letter, we propose a mechanism for subwavelength focusing waveguide of the tapered microtube where the cone part at optical frequencies by using a tapered microtube. As shown in Fig. 1, a laser beam propagates through sidewall converts the input laser beam into a circular cone. With this rather simple structure, we demonstrate that the laser beam focal spot size is close to diffraction limit, which agree well with our numerical simulation results.The microtube investigated in this work was fabricated by flame-heated drawing method. A glass capill...

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