Properties Of Polarization And Transmittance For Two-Dimensional Metallic Gratings

One-dimensional subwavelength metallic gratings have been considered as one of the most promising ways to improve and replace the traditional grating components for their advantages of high integration, excellent polarization properties, wide spectrum, and high transmittance and extinction. Thus, the plasmonic gratings have received great deal of interest from the researchers. Surface plasmon polaritons (SPPs) polarization control based on the subwavelength nanometer process technology is a breakthrough of the traditional polarization state research. However, the one-dimensional subwavelength metallic gratings are limited in the polarization application because of its birefringence effect. In this paper, based on the equivalent medium theory (EMT) of two-dimensional subwavelength grating which is promoted from that of one-dimensional structure, a plasmonic metallic polarization control structure is proposed. The arbitrary polarized state between TE and TM input waves are achieved with high transmittance.First of all, we introduce two numerical methods for the metallic grating structure. These methods are based on the Maxwell equations:finite difference time domain method (FDTD) and rigorous coupled-wave method (RCWA). Through the theoretical analysis and simulation, the influence of the design parameters on the polarization and the transmittance are discussed in detail, including the groove depth, the period and the duty cycle of the square and rectangular metallic grating along x-axis and y-axis directions, as well as the incident wavelength.Secondly, a proposed two-dimensional metallic rectangular grating is simulated based on the FDTD method, and the polarization transformation and the transmittance are obtained. With the optimized structure parameters, the simulation results show that the conversion of linear input to circular polarization state output is realized, when the linearly polarized light (wavelength of550nm) normally incidents on the proposed two-dimensional nano-structure with the polarization angle of30°.Thirdly, reference to the parameters of the grating with circular polarization state output, the characteristics of polarization and transmittance are achieved by space angle modulation. Simulation results show that the full Poincare sphere coverage could be obtained theoretically, and a high transmission is achieved as well.Finally, based on the equivalent medium theory and the SPP Bloch model, two kinds of novel metallic waveplates are analyzed with optimized parameters:two-dimensional rectangular metallic gratings and elliptical pillars metallic gratings. Firstly, for these two structures, the relationships of the transmittance and phase retardation to the incident light and the polarization angle are compared and analyzed. Especially, the transmission properties are studied in detail, when both of the gratings are performed as quarter-wave plates. The simulation results show that both structures can act as quarter-wave plates when the input polarization angle is75°. The corresponding transmittances of rectangular and elliptical cylinder grating are0.77and0.67, respectively. In addition, both gratings exhibit relatively flat transmission spectra within the wavelength range from550nm to800nm in the presence of different input polarization angles.Two-dimensional metallic grating structures proposed in this paper have extensive application prospects in polarization and high transmittance of photonic integrated devices. They can be used in the field of optical fiber communication, optical inter-connects, data storage and imaging.