Theoretical Study On Focusing Properties Of Two-layer Spherical Dielectric Microparticles

The transparent dielectric microparticles which size are comparable with wavelength can concentrate light.When a plane-wave illuminates a dielectric microsphere,a high-intensity electromagnetic beam(photonic nanojet)will appear and propagate from the shadow-side surface.Photonic nanojet is a non-evanescent propagating beam with a high intensity,it is available for photolithography on the nanoscale,microscopy,low-loss waveguide,and ultrahigh-density optical storage.We want photonic nanojets to propagate long distance with high intensity and small size in practical applications,so we use four key parameters to characterize the photonic nanojet quantitatively:the peak intensity I_P,the focal distance f,the effective length L,and FWHM w.More and more researches were focused on the design of dielectric microparticles for better photonic nanojets.In this thesis,two kinds of two-layer spherical dielectric microparticles are proposed,and we use CST simulation software to study theoretically the characteristics of photonic nanojets.The main work and results of the thesis are as follows:1.The properties of the photonic nanojet generated by a two-layer dielectric microsphere are studied.Simulation results indicate that this novel structure can generate a photonic nanojet outside its volume when the refractive index contrast relative to the background medium is more than 2:1.Compared with the homogeneous dielectric hemisphere,it has superior performance in jet length and focal distance.The simulate results show that radius contrast ratio plays an important role in the properties of photonic nanojet.When R_s:R_c=10:6,the ultra long photonic nanojets can be obtained.Moreover,the minimum of FWHM w is 167.8nm(n_s=1.6,n_c=1.3),which is smaller than the classical diffraction limitλ/2,that means the photonic nanojet combines the merits of both high spatial localization and long propagating distance,and it has practical value in the far field operation.2.The properties of the photonic nanojet generated from a microsphere(calcium fluoride)encapsulated in a shell are studied.And the design of the microsphere is optimized.Simulation results indicate that the properties of the photonic nanojets can be effectively controlled by changing radius,extent of coating and material of the shell.The optimised structure(R_c=1.5μm,n_c=1.43,R_s=2.8μm,n_s=1.65 and 80%caoting)illuminated by incident light of different wavelengths can generate photonic nanojets with subwavelength beamwidth and the jet length can be up to 8.2μm.