Polarization-multiplexed Devices Based On Terahertz Surface Plasmon Polaritons

Surface plasmon polaritons(SPPs)are collective electromagnetic excitations propagating along the metal/dielectric interface.With the properties of sub-wavelength and field confinement and enhancement,SPPs have vital potential applications in areas including micro/nano integrated optics,sensing,super-resolution lithography and imaging,enhancing the efficiency of solar cell,etc.SPPs can be generated when light is incident on the subwavelength metallic structure.The amplitude and phase of SPPs can be modulated by designing the subwavelength structures.SPPs devices with all kinds of functionalities(focusing,vortex,reflection,non-diffraction SPPs beams,etc.)can be realized.Polarization,one of the basic freedoms of light,plays an important role in enhancing the capacity of radio communications,3D display and biological molecule spectroscopy and sensing and so on.However,most optoelectrical detectors as well as human eyes are insensitive to the polarization of light,which hinders the further application of polarization.In this thesis,taking advantage of the trait that the amplitude and phase of SPPs are closely related to the polarization of incident light,three kinds of polarization-multiplexed devices(semicircular terahertz SPPs lens,terahertz SPPs lens based on slit arrays and metasurface lens)are designed and studied in the terahertz(THz)band.With the illumination of linearly polarized(LP),left circularly polarized(LCP)and right circularly polarized(RCP)THz wave,different focusing field distributions can be obtained in the 2D plane or 3D free space with the designed devices.The designed devices consist of sub-wavelength metallic structures and thus are small in size and integratable.Potential application fields of the designed devices include circular polarization analyzer,polarization based information processing and transmission,biological molecule spectroscopy and sensing,etc.Different devices can be chosen according to the actual demand and the parameters of the devices can also be adjusted accordingly.The proposed devices can be extended to other wavebands(visible,infrared,millimeter wave,etc.)by scaling the parameters of the structure.The contents of this study are summarized as follows:A terahertz surface plasmon polaritons(THz-SPPs)imaging system with features of larger imaging area and higher speed is built up.Combining the raster scanning THz-SPPs detection system and THz holographic imaging system,in the built system,the probe beam is expanded and a CCD camera is used to replace the double-eye detector for THz-SPPs imaging.Moreover,both the amplitude and phase of THz-SPPs can be obtained with the imaging system,which is an advantage over SPPs detection techniques in the visible frequency range.Utilizing the imaging system,THz-SPPs generated by the semicircular slit is measured for horizontally polarized incident THz.The amplitude images reveal the focusing process and the Gouy phase shift during the focusing can be clearly seen from the phase images.Focusing of THz-SPPs with different frequencies are also compared and analyzed.The THz imaging system is a powerful tool to fully characterize the functionality of THz-SPPs devices and can also be helpful for the design and understanding of THz-SPPs devices.Polarization-multiplexed focusing of semicircular THz-SPPs lens is experimentally studied and is theoretically analyzed with THz-SPPs Huygens Fresnel principle and Fourier transform.The experimental results show that the focuses of THz-SPPs will shift transversely upward and downward for LCP and RCP incident THz wave,respectively.And the distance between the two focuses in the 2D plane is smaller than the wavelength of incident THz wave.The spiral phase which is the origin of the transversal shift of THz-SPPs focus can be seen from the phase images and this observation can provide solid foundation for the related theoretical explanation.Different from the previous analyses based on Coriolis effect and spin Hall effect,an explanation based on THz-SPPs Huygens Fresnel principle and Fourier transform is proposed and an equation which can evaluate the transversal shift of the focus is given.The circumstance that the equation applied is also pointed out,which is clearer and more comprehensive.A THz-SPPs lens based on metallic slit arrays is designed and in the 2D plane,with the illumination of LCP and RCP THz,the distance between the THz-SPPs focuses can be dozens times larger than the wavelength of incident THz.Polarization-controlled directional THz-SPPs excitation based on slit arrays is theoretically analyzed and numerically simulated.The results show that THz-SPPs excited by LCP THz propagates to the left and THz-SPPs excited by RCP THz propagates to the right.Further optimizing the distribution of the slits with simulated annealing algorithm,polarization-multiplexed focusing is realized.The simulated and experimental results show that THz-SPPs excited by LCP and RCP THz can be focused on the left and right side of the lens,respectively.Two focuses of THz-SPPs on the left and right of the lens can be obtained with the illumination of LP THz.A polarization-multiplexed metasurface lens is proposed and polarization-multiplexed focusing and polarization-multiplexed imaging is achieved in the 3D free space.With the incidence of LCP and RCP THz,the amplitude and phase of the far field radiated by the rectangular slit antenna is studied.Utilizing the principle of spatial multiplexing,the distribution of the slits is designed to realize polarization-multiplexed focusing.The focus is on the left of the focal plane for LCP incident THz and is on the right for RCP incident THz,two focuses can be obtained with the illumination of LP THz.The separation between the two focuses is several times larger than the wavelength of incident THz wave and can be adjusted.Polarization-multiplexed imaging can also be realized with designed lens,which can be used for polarization based information processing and transmission.