Mode Conversion Of Waveguide Based On Embedded Meta-Unit

Photonic technology is one of the most efficient solutions to satisfy the extremely huge demand to information transmission and processing in big data and artificial intelligence ear.At present time,however,the integration degree of photonic system is relatively low,which prohibits the exertion of the advantages of photonic technology,and the inefficient mode conversion is one of the most important reasons.In order to break this bottleneck,in this dissertation,we propose to use photonic metasurface to increase the mode conversion efficiency in optical waveguide.Metasurfaces are artificial structures composed by periodic subwavelength units,of which the transverse size and thickness are both much less than the wavelength.Using metasurface,the thicknesses of ordinary optical devices have been extremely deceased down to deep-subwavelength scale,and revolutionized the appearance of ordinary optical devices.At the same time,due to the incompatible of the small transverse sizes of photonic waveguide and periodic metasurfaces,we propose to embedded meta-unit in waveguide to improve the mode conversion efficiency.The theoretical analysis methods of two dimensional and three dimensional optical waveguides are discussed firstly,and correct eigenmodes and effective refractive indexes are obtained.Based on those results,the research of mode conversion are carried out.Using the coupled mode analysis method,the momentum conservation condition for perfect mode conversion is obtained,and is realized by long period grating in a two dimensional waveguide.Using the finite difference in time domain method,the conversions between the fundamental mode and the first order mode are investigated,and nearly perfect conversion is achieved.However,the conversion length is not small enough in this case.In order to decease the conversion length further,we propose the reflection type mode conversion based on meta-units.On the end surface of the waveguide,asymmetric meta-units are attached properly,and different phase shifts by the units on the reflection waves are realized.When the difference of the phase shifts reaches to ?,mode conversion between fundamental and first order modes can be realized.The thickness of the meta-units is only 75 nm,which is less than 1/20 of the wavelength,which is the thinnest mode convertor as far as we know.Using the similar method,the mode conversion in three dimensional rectangular waveguides is achieved efficiently also.The polarization rotation in three dimensional waveguides using embedded meta-unit is investigated both in transmission and reflection modes.Using the elliptical nano-rod dielectric meta-unit,two orthogonal linear polarizations get different phase delay.When the phase delay is 90 degrees,the conversion from a linear polarization to a circular polarization is achieved.Using reflection typed metallic reflection meta-unit,larger phase delay up to 1.2? is obtained,and the conversion from incident linear polarization to circular and orthogonal linear polarization are obtained.We extend the investigation of metasurface from the functional devices operated in free space to those in nano-and micro-photonic structures,which greatly broadened the application domain of metasurface.Within a thickness of deep-subwavelength,the conversions between different spatial modes and polarizations are achieved successfully,which is one order of smaller than the conversion thicknesses reported before.The results presented in this thesis is extremely important in the theory and applications of photonic technology.