| The clamworm and the peacock tails of caterpillar show the colours in the sun.Because of their special arrangement of fine wool can make the light beam through the complex reflection and refraction to show the phenomenon on the macro.A kind of artificial crystal formed by the periodic arrangement of different materials,which successfully reproduced the magic of nature.This new kind of artificial nano material called photonic crystal.Another name of the photonic crystal is the semiconductor in optics,because photonic crystals have photonic band gap,which can compare with semiconductor.Because of this unique characteristic,photonic crystals has the possibility of controlling the beam.This new type of artificial metamaterials since birth has won the attention of a large number of researchers,which assessed as one of the world's nine scientific and technological achievements by Science magazine in 1999.With the development of photonic crystals,many branches have been developed,and the graded index photonic crystal is the most important one.By modifying some parameters of ordinary photonic crystals,the gradient refractive index can be gained,and the ordinary photonic crystal can be changed into gradient refractive index photonic crystals.The results show that the graded index photonic crystal has better focusing performance than the ordinary one.Due to the complicated structure of the graded photonic crystals,the finite difference time domain(FDTD)method is used as the simulation method.In this paper,the theory of finite difference time domain(FDTD)method and its three elements are introduced.Based on the investigation of several unique optical properties of photonic crystals,the properties of graded index photonic crystals are compared and analyzed.The main work of this thesis is to design a new focusing structure using the graded index photonic crystal with a slot,which has the ability of the local and the guided photons.The polarized plane wave with a wavelength of 1.5 m is used as the light source,and a very narrow focal spot can be obtained on the sub surface of the photonic crystal.The extremely small focusing spot sited at full-width and half maximum(FWHM)as fine as 1/75?.This new type of photonic crystal structure can be used to achieve the sub surface super resolution focus.In order to investigate the influence of each parameter on the focusing structure,the simulation results are further analyzed,following conclusions are reached:1.The width and length of the rectangular slot have an influence on the focusing performance of the graded photonic crystals.The graded photonic crystal has the best focusing performance with the width of the rectangle is 0.02?m and the length is 0.5?m.2.This novel focusing structure is very sensitive to the substrate material.Our further study reveal that the same ultra-fine focusing phenomenon can be observed as long as the refractive index of the substrate material is set ranging from 3.0 to 3.5.3.The graded photonic crystal focusing structure is suitable for working in the near infrared band,and is not suitable for working in ultraviolet and visible light bands.Possessing with such a significant advantage of the focusing performance,the designed PCs lenses may find extensive applications in fluorescent imaging/analysis,biosensing with ultra-high resolution and sub-surface nanometrology. |
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