Based on the diffraction propagation mechanism of terahertz waves and the basic theory of binary optics, transmission terahertz Dammann gratings is designed, fabricated, and characterized for the working wavelength of 2.52THz. The simulated annealing algorithm is used to design the phase structure of a 2D terahertz Dammann gratings which can generate 2×2 and 6×6 diffraction beams. MATLAB is used to conduct diffraction beam splitting simulation of the designed Dammann grating. The simulation results show that the 2D diffraction efficiency of the grating reaches around 60 %, and the unevenness of the split sub-spots is controlled below 1%. During the grating fabrication process, N-type high-resistivity silicon (n = 3.4175 @ 2.52 THz) is used as the grating material, and silicon nitride (Si3N4) with 200nm thickness is used as the hard mask layer. To achieve extremely high etching depths, the wet deep silicon etching process of potassium hydroxide (KOH) solution is used as the fabrication process. Various pivotal process parameters, such as the initial concentration of each component, are optimized through many experiments. The etching depth of the order of 50 μm is achieved. The grating surface flatness and step sidewall steepness satisfy the needs of use. The characterization experiment results show that the 2D diffraction efficiency of the grating is about 50 %, and the unevenness is about 2.5 %, which basically verifies the reliability of the design theory and fabrication process.
Ptychography is a lensless, wide-field, diffraction imaging method that can reconstruct the object transmittance function and the probe distribution simultaneously. Traditionally, lateral mechanical movement is necessary in the time-consuming data acquisition process, resulting in disability to investigate dynamic phenomena. Single-shot ptychography is therefore proposed which divides the incident light into multiple tilted sub-probes through a beam splitting element, e.g., pinhole array and Dammann grating, meanwhile the diffraction patterns are recorded simultaneously in spatial multiplexing mode. In this study, we propose a single-shot ptychography approach using a phase-only spatial light modulator. In order to realize multi-angle modulation, four different grey images are loaded in different quadrants of the modulator, which generates a specific phase delay. Considering the inconsistent wavefront of the probe beams, multi-mode ptychographic algorithm is also adopted which treats probe beams differently. The proposed method is expected to promote single-shot ptychography for piratical applications.
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