Subwavelength Metallic Microstructures For Terahertz Wave Polarization Manipulation

Terahertz (THz) radiation, referred to as the electromagenetic wave in a frequency range from 0.1 THz to 10 THz, is very important in the research area of electronics and information technology, and behaves as a bridge connecting the electronics and photonics. As a research hotspot, THz technology has intensively potential applications in the fields of communication, healthcare, military and aerospace. The phase and polarization manipulation should involve in the application of THz technology. However, traditional polarizer and waveplate used to manipulate the polarization states of electromagnetic wave have weakness of material, structures, efficiency and energy loss. With the development of micro-and nano-manufacturing and photonics, subwavelength metallic structures with different shapes are applied to manipulate the THz wave polarization.In this thesis, the optical properties of metallic structures are investigated using FDTD method and related theories. After analyzing their optical polarization response and phase-changing properties, we designed different structures working as devices for polarization manipulation of THz wave. The works in the thesis are as follows:1, After investigating the refractive index of metals in THz domain and confirming the reliability of FDTD simulation with computers, transmission properties of subwavelength rectangle holes array for different polarization incidence are analyzed. The fundamental TEi,o mode of rectangle waveguide is proved to contribute to the extraordinary optical transmission (EOT) and the so-called cut-off peak.2, Two perpendicular rectangle holes are combined as a unit cell of T-shaped holes array. The two rectangle holes can support waveguide mode and FP resonance respectively, with certain phase difference at proper geometrical parameters. A superposition of the waveguide mode and the FP resonance enables the T-shaped holes array working as a half-wave plate to change the polarization direction of incident wave from 45° to-45°. Another superposition of two rectangle holes results in L-shaped holes array. The overlay of the TEi,o modes in the two arms of L-shaped holes leads to the symmetric and antisymmetric mode, the cut-off frequency and effective index of which are quite different. A joint of this two modes converses the linear polarization wave to elliptical wave.3, Adding an air spacer between two same L-shaped holes array imports FP resonance between the two metallic layers. Consequently, the coincidences of symmetric mode, antisymmeteric mode and FP resonance contribute to the multi-peak of the transmission spectrum of the L-shaped holes array. Particularly, overlay of symmetric mode and FP resonance changes the linear polarization direction from x-axis to y-axis at 3 THz with nearly 100% conversion rate and high energy transmission. Besides, the split of the FP resonance peak because of the coupling with the waveguide mode is observed, which is called anticrossing behavior.4, The symmetric and antisymmetric modes have different effective index and cut-off frequencies fc1 and fc2, respectively. When the thickness of the single layered L-shaped holes array is large enough, the transmission of the ±45° incidence is cut off at fc1 and fc2, respectively. As a result, when the incident frequency f is between fc1 and fc2, the thick metallic structure with L-shaped holes array works as high efficiency polarizer with a polarizing angle at 45° direction, which tranmits the 45° polarized component but restrains the -45° polarized componetns. When f is bigger than fc2, the structure has the artificial birefringence. The effective index difference between its two optical axes is about ten times bigger than that of traditional quartz crystal.Therefore, the thickness of waveplate mading of this artificial structures is only one tenth of the quartz birefringence crystal in THz domain. By changing the layer thickness and side length, this artificial birefringent structures can be used to work at different frequency and generate different phase delay to get the 90° polarization degree conversion or cirulary polarized wave. The thinner layer thickness than traditional quartz and the tunable properties of those designed metallic structures are quite useful for applications of polarization manipulation in integrated optics and micro- and nano-photonics.