| In this paper, we investigate the polarization properties and slow-lighteffects of subwavelength metallic gratings in terahertz （THz） region. Usingholographic lithography, a subwavelength aluminum-grid polarizer issuccessfully fabricated and its polarization property is characterized. Themajority of my work is as follows:First, we investigate the polarization properties of subwavelength metallicgratings in THz region. Using equivalent medium theory, the polarizationprosperities of the grating can be qualitatively analyzed. Grating is equivalent to adielectric layer when illuminated by a TM-polarized laser and a metal layer forTE-polarized. Using the RCWA, we investigated the dependence of thepolarization prosperities on the following factors: structural parameters of thegrating, the materials of grating, substrate and the filling dielectric layer andfinally got the optimal parameters for the system. A aluminum grating depositedon a SiO2substrate with a2μm period, a2μm depth and a filling factor of0.4canrealize a transmission of82%under the illumination of TM waves with thefrequency ranges from1to10THz and a high extinction ratio of62dB can bereached which indicate the polarization prosperities of our designed grating aresignificantly improved from the traditional polarization devices. Base on the abovedesigned grating, we further designed a double-layer Al grating which shows agood transmutation for a TM wave and a extinction ratio of132dB which isstrongly improved compared with the single grating case.For experiment part, we set up a system for dual-light interferencelithography and then subwavelength Al grating are successfully fabricated byholographic lithography. Characterized by atomic force microscope and opticalmicroscope, the structural parameters of the gratings are as follows: a period of1.85μm, a thickness of120nm and a filling factor of0.5. The transmission of thegrating is measured under the illumination of THz laser o f different polarizationangles. The experimental results agree well with the simulated results whichindicate the polarization prosperities of the grating designed subwavelength Algrating are satisfactory in THz region.At last, we investigated the surface plasmon assisted slow-light effects of themetal gratings in THz region. The dispersion of the Spoof SPPs can be modifiedby filling dielectric into the grooves of the grating. The simulation results of FDTD shows that a group velocity of0.035c can be reached by fully filling siliconinto the grooves of the grating. An increase of the amount of the filling dielectricmaterial will give rise to a decline of the dispersion curve of Spoof SPP s.Consequently, its cutoff frequency of will shift to the lower frequency which willbe minimum for the full-filled case. The decay length and the propagation lengthsfor gratings filled with Si of different thickness are calculated. A stronger fieldconfinement and shorter decay length will arise from the increase of the fillingfactor of the dielectric layer especially for the high lossy aluminum grating. Basedon the dependence of the dispersion on the filling factor of the dielectric layer, wedesigned a grating of which the grooves are filled with Si of gradually c hangedthickness to realize a slow-light effect for broad spectrum band. Moreover, bydepositing dielectric layer of gradient thickness on a full-filled grating, thedispersion of the Spoof SPPs can be also modified and the similar effect of fillingdielectric material into the grooves of the gating can be obtained. Based on theabove studies, we finally designed a grating with a gradient-thick dialectic layeron top of it to realize “rainbow trapping†in THz region. Finally, a briefintroduction of slow-light effect within a grating cosmists of bare metal ridges isgiven. Based on this idea, a multilayer metal grating is proposed for a THzdetector. |
