Investigation On The Optical Properties Of Nano-waveguides

In the 1959 annual meeting of the American Physical Society, the famous physicist and Nobel laureate Richard Feynman at the California Institute of Technology made the speech:"There is plenty of room at the bottom", which mentioned that humanity can use the macro-machine to make a smaller one, which could be used to make an even smaller one, and step by step to achieve molecular degrees so that we could slowly reduce the production facilities and finally arrange the atoms and manufacture products as our wishes. He predicted that the chemistry would become an issue of the accurate one by one atom placement according to the wishes of the people. In recent years, with the rapid development of the nano-technology in optoelectronic functional materials, photochemical, bio-medical photonics and so on, nano-optics research field has greatly expanded, and are constantly broadening and deepening as time goes by.Recently, as one of the basic apparatuses, the silica nano-fiber, can be as small as several tens of nanometers in the diameter. The surface roughness is close to 0.2nm order of magnitude, reaching the intrinsic roughness of the molten glass surface. The transmission loss of the silica single-mode nano-fiber with the diameter of 360nm is less than 0.01dB/cm, which is far less than other types of waveguide of the same width. The nano-fiber optical sensor, with the using of nano-fiber small size and strong features such as evanescent field and through measuring the optical output intensity, phase or other physical quantities, has been achieved for the measurement on liquid refractive index changes, hydrogen concentration and analysis of surface molecular adsorption. It has shown its superior characteristics in small size, high sensitivity, and fast response speed and so on. In this thesis, based on the previous research, we systematically studied and discussed the basic properties and the applications of nano-optical waveguide and the devices based on it. We provided a certain theoretical basis and numerical reference for the experimental study of nano-optical waveguide. The chapters are listed as follows:In chapter one, a brief basic background of nano-optics, its development and the basic theory of nano-optics are present. Then we discuss the basic model and processing technology of nano-waveguide. Finally an overview of the main research contents of this thesis is given.In chapter two, we use the FDTD method to investigate the focusing behavior of the dielectric nano-waveguide array. Numerical simulation results show that a beam can effectively be focused, when it transfers through an array which composes of nano-waveguide with length of 1.0～2.0μm, Both the TM and TE mode can be focused. The results also show that, for the same nano-waveguide array, the focal length of TE mode is shorter than the one of TM mode. The focal length of the nano-waveguide array can be altered by adjusting the width of the nano-waveguide, the spacing between nano-waveguides, waveguide length, the number of nano-waveguide and the FWHM of the incident light. It is worthy to mention that the change in the width of the nano-waveguide or the separation between them is altering the width of the nano-waveguide array. This proves once again that focal length of lens and the lens aperture are related to a large extent. We attribute this behavior of the nano-waveguide array to a longer length radiation mode and strong evanescent field. The wavelength order focus length is also achieved by using the nano-waveguide array with linearly variable separations.In chapter three, we work out the two-beam interference equation in the nano-fiber from the basic equation. With the equations and the electric dipole model, we study and discuss the optical trapping properties of the small particles in non-absorptive and absorptive nano-fiber. The results show that, comparing with non-absorptive nano-fiber, the absorptive nano-fiber is more applicable for the small particles optical trapping. In Z-axis, which is also the direction of the fiber extension, scattering force is determinant for trapping. If the intensity of two beams is equal, the particles can be trapped in the mid-point of the nano-fiber. Adjusting the intensity of two beams can make particles move along the z-axis. In the horizontal direction, the gradient force is determinant for trapping. In theφ-direction, there are two stable positions for the trapping, locating in the beam polarization plane. Changing the polarization of the incident light can achieve the modulation of the stable trapping position in theφ-direction; in r direction, the gradient force can be used to achieve unstable trapping to a certain extent. It is more suitable for particles transport. Therefore, we call it quasi-three-dimensional trapping. Meanwhile, we also have discussed the issue that the incident light is polychromatic. Results are the same as the one of monochromatic light.In chapter four, we study the energy attenuation in the nano-fiber with material absorption. The results show that energy attenuation in the nano-fiber can still be described by the exponential function. By theoretical calculation and FDTD method, we have obtained the experiential formula of absorption coefficient in nano-fiber, and verified it.In Chapter five, the thesis is summarized and an outlook of the further studies on the nano-waveguide and the devices based on it are represented.