Research On Terahertz Functional Devices Based On The Vector Theory Of Binary Optics

Terahertz functional device is indispensable in the construction of terahertz application system, which has become a hotspot in the international research. There are series of methods such as photonic crystal, metal micro structure and liquid crystal etc. to design the terahertz filters, polarization splitters wavelength division multiplexers and switches. However, they mainly focus on photonic crystal and lack of other effective methods to the design theory and manufacture to terahertz fuctional devices. At the same time, the problems of single design method, difficult manufacture process, and high cost existing in terahertz functional devices have limited the extensive application of the terahertz technology. To solve the problems above, in this paper, the vector theory of binary optics combining global optimal algorithms are introduced to design terahertz functional devices, and the subwavelength binary simple periodic structure is presented to realize even beam splitting. These research results indicate that, under the suitable material and structural parameters configulation, the structure can achieve even beam splitting with effective zero-order depressing, high efficiency performance and uniformity. These results break the limitation of even beam splitting based on the traditional scalar theory and reduce difficulty and costs in the manufacture process of the device and can be expanded to the design of terahertz functional devices such as terahertz filters, absorbers, modulators and wave plates. The main contents of this paper are summarized as follows.1) The characteristic of beam splitting of the monolayer subwavelength binary simple period structure is researched and it is presented to realize terahertz waveguide coupling even splitters. Take the terahertz region for example, a scientific evaluation function is established and vector diffraction theory and global optimization algorithm is combined to optimize the structure parameters of the grating and a series of terahertz waveguide coupling even splitters are obtained to realize zero-order depression, high efficiency and uniform beam splitting. The research results indicate that, under the optimal structure parameters, one of the silicon-based beam splitters designed to separate one incident beam into four emergent beams has total efficiency up to 91.83%. Zero-order diffraction efficiency is reduced to less than 0.211% and the error of uniformity decreased to 1.52×10-4, which represent good performance of even beam splitting. Furthermore, the physical mechanism of the method to break the limitation that even beam splitting based on the traditional scalar theory is investigated. Researches show that zero-order diffraction order resonates in the subwavelength structure and causes interference cancellation, which results in its inability to transmission. On the other hand, the non-zero-order diffraction orders satisfy the condition of constructive interference, which results in more energy dispatching to other non-zero orders. The rearch can be referred to Chapter 3 in the paper.2) The vector theory of binary optics is also introduced to design terahertz free spatial even splitters based on Multilayer dielectric film grating. Resonance effect and the evanescent wave propagation effects are applied to depress zero-order diffraction and dispatch the efficiency evenly to non-zero-order diffraction. Then zero-order depression, high efficiency and uniform beam splitting are effectively realized. These results break the limitation o based on the traditional scalar theory and reduce the difficulty and the cost in manufacturing. The best optimal total diffraction efficiency can be up to 99.50% and the zero-order diffraction efficiency can be reduce to 2.01×10-12. The silicon-based four-splitter has total efficiency up to 92.23%. Zero-order diffraction efficiency is reduced to less than 0.192% and the error of uniformity decreased to 6.51×10-6, which indicates that the uniform distribution of energy for each beam is implemented. The rearch can be referred to Chapter 5 in the paper.3) Tolerances to manufacture and test the terahertz waveguide coupling even splitters are analyzed and tolerances of the incident parameters, structure parameters and the aperture are obtained. Lithography and plasma etching is applied to manufacture the silicon-based four-beam splitter. Surface topography of the element is tested and the influences of the manufacture errors to the performance are discussed. Taking the silicon-based four-beam splitter for example, the effects of incident conditions(i.e. the incident angle, wavelength, and the polarizing angle) as well as the structure parameters(i.e. the ridge width, the groove width, the groove depth, the steepness) on the diffraction efficiency and uniformity are investigated. The allowable deviation of the incident parameters and the structure parameters are obtained. A relatively good uniformity and high diffraction efficiency of the element can be maintained with a deviation of wavelength less than 4 μm, the polarizing angle less than 8°, the incident angle less than 15°, ridge width and groove width less than 2 μm, ridge depth less than 6 μm and the steepness less than 1.3°. Tolerance of the aperture is also analyzed. The influences of the aperture of the element as well as the beam width of the incident wave to the performance of the element are investigated. These results have guiding significance in designing, manufacturing and testing the element. The rearch can be referred to Chapter 4 in the paper.4) Tolerances to manufacture and test the terahertz free spatial even splitters are analyzed and tolerances of the incident parameters, structure parameters and the aperture are obtained. Taking the silicon-based four-beam splitter for example, the effects of incident conditions(i.e. the incident angle, wavelength, and the polarizing angle) as well as the structure parameters(i.e. the ridge width, the groove width, the groove depth, substrate thickness, the steepness) on the diffraction efficiency and uniformity are investigated. The allowable deviation of the incident parameters and the structure parameters are obtained. A relatively good uniformity and high diffraction efficiency of the element can be maintained with a deviation of wavelength less than 1 μm, the polarizing angle less than 25°, the incident angle less than 1°, ridge width, groove width, ridge depth and substrate thickness less than 1 μm, and the steepness less than 0.4°. Tolerance of the aperture is also analyzed by using FDTD. The rearch can be referred to Chapter 5 in the paper.In this paper, a novel method of a subwavelength binary simple periodic rectangular structure is presented to realize even beam splitting by combining the vector theory of binary optics with global optimization algorithms, breaking the limitation based on the traditional scalar theory. It can be said that our method can solve the problems of zero-order elimination, diffraction efficiency and uniformity improvements of even beam splitting by applying the binary simple periodical rectangular structure. Due to the simple structure, extensible function and convenient manufacture process, it has potential values to be applied in the terahertz communication and imaging systems.