A method of acquiring a dark hollow beam of nanometer level

By using the finite difference time-domain (FDTD) method, the distribution of the modulus of the electric field intensity is obtained in the near field of an optical fiber head. We found that different dimensions of the cylindrical core in a hollow optical fiber had an influence on the electric field intensity. It is concluded that the size of the cylindrical core must be increased to make the hollow beam to cover a larger area. At the same time, after discussing the two situations that the size of the hollow optical fiber either rather large or rather small, we obtained a dark spot in the near field of the fiber that was almost of the same size as the hollow region (10 nanometer level). However, the background light in the dark hollow beam was brighter, and so the hollow region was designed to be a metal (such as silver). We found that the background light in the dark spot became evidently weaker and a cleaner dark hollow beam was acquired in the near field. When the hollow region was made smaller, the dark spot in the near field would become smaller, even reaching the nanometer level, which offered a method of obtaining a dark hollow beam of nanometer level.