Research Of Subwavelength Focusing Effect Of Light By A Tapered Microtube

In recent years, there has been an explosion of interest in obtaining subwavelength focusing and imaging. The traditional Abbe-Rayleigh diffraction limit of traditional optics has been the dominant barrier to achieving features smaller than approximately half a wavelength with optical instruments. Subwavelength focusing spot and higher resolution that can reach beyond the diffraction limit will be beneficial to a wide range of applications including semiconductor lithography, optical trapping, high density optical data storage, and confocal microscopy, etc.Based on the concept of direct synthesis of the angular spectrum, we propose a mechanism for subwavelength focusing at optical frequencies based on the use of a tapered microtube.The tapered microtube was fabricated from a glass capillary tube by flame-heated drawing method. Using coherent illumination at 671 nm and a near-field scanning optical microscope probe which was mounted on a three-axis piezonanopositioning stage, a sequence of two-dimensional 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 end face of microtube. The experimental results are in agreement with our numerical simulation.At the beginning of the thesis, the significance and development status of the subwavelength focusing is presented in five categories, including superlens based on negative refractive materials, planar plasmonic lens based on surface plasmon polaritons (SPPs), nanohole quasiperiodic array, micro/nano fiber array and multiple concentric air nanorings.The concept of direct synthesis of the angular spectrum is firstly discussed in the second chapter. Inspired by the experimental apparatus which is used for verifying the concept above, we propose a "tapered microtube" for subwavelength focusing.In the third chapter, by using 3D-BPM, we calculated the focusing effect of tapered microtube and investigated the influences of structure parameters and input optical field properties on focusing effect.In the forth chapter, we firstly introduce the flame-heated drawing method for manufacturing the tapered microtube. And we deliberately illustrate the experiment system for mapping the subwavelength focusing spot produced by the tapered microtube with a SNOM probe.At the end of the thesis, conclusion for previous work is drawn and research directions in the future are given.