Beam quality is a core issue in the field of terahertz full-field imaging. In this paper, we present a terahertz spatial filter consisting of two mounted ellipsoidal silicon lenses, which have the same eccentricity but unequal size, and an opening aperture on a thin gold layer between the lenses. At the frequency of 2.52 THz, the beam transmissivity and Gaussicity of the filtering system is investigated by simulation. COMSOL Multiphysics is used to conduct two-dimensional simulation experiments, aiming to find the appropriate aperture size and gold layer thickness, depending on the wavelength of incident light wave and numerical aperture of the ellipsoidal silicon lens. The filtering system is capable of filtering the non-Gaussian beam to a nearly fundamental Gaussian beam and achieves a high transmissivity. In terahertz full-field imaging, the system can not only obtain an imaging beam with uniform intensity distribution, but also reduce the beam energy loss caused by multiple surfaces. Besides, the lens system is applicable for a wide terahertz frequency range if the wavelength dependent part is properly scaled.
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.
Fourier ptychographic microscopy (FPM) is an effective computational imaging method that incorporates many advantages such as large field of view, high resolution, label-free, and quantitative phase-contrast imaging. Typically, an LED array is used as the illumination source. Due to the small size and narrow spectral width of the LED unit, FPM is usually treated as a coherent imaging system. However, practically, each LED unit is a spatially-extended light source with a certain emitting area, rather than an ideal point source. The ideal point source approximation will decrease the quality of the reconstructed image, to a certain extent. In this paper, the spatial coherence characteristics of FPM system based on LED illumination is analyzed, and it is found that the optical field on the object plane is partially coherent. Thus, the artifacts exist when the coherent transfer function is used as the frequency domain constraint. To address this problem, a Butterworth-weighted transfer function is proposed as the frequency domain constraint in the iterative reconstruction process. The simulation and experimental results demonstrate that this new constraint approach can not only reduce the ringing effect in low-resolution images, but also improve the reconstruction quality due to its following better the actual physical mechanism inherent in FPM. In addition, it may reduce the requirement of the spatial coherence of the illumination source, which has great significance for promoting the application of FPM.
In the THz band, the spoof surface plasmon polariton (SSP) can limit the light to the designed specific spatial frequency by constructing a sub-wavelength structure on the metal surface. This ability makes it possible to break through the diffraction limit of THz imaging. In this paper, a one-dimensional metal groove structure is used to generate the SSP, and the frequency shift characteristics of the object's spatial spectrum under the excitation of a 2.52 THz beam are analyzed. On this basis, a grating-coupling-based SSP excitation structure is designed, which consists of a combined array of subwavelength metal grooves and subwavelength slits with different periods. This structure is used to generate strong intensity and make full use of the incident beam. It is promising for generating illumination beams for terahertz far-field super-resolution imaging of subwavelength-spacing samples.
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.
Optical synthetic aperture (OSA) imaging system is an effective method for astronomical telescopes to realize superresolution imaging, which has important applications in astronomical observation and space remote sensing. The timedivision multiplexing sub-aperture array is a potentially powerful way to construct the larger array configuration by changing the basic array configuration several times with less apertures. In this paper, according to the theory of incoherent optical imaging, the physical mechanism of space/frequency domain response of OSA system is discussed. From the basic pinhole-aperture array to the circular-aperture array, the evolution of the point spread function (PSF) and the modulation transfer function (MTF) is analyzed. The inherent laws and imaging differences between the time-division multiplexing sub-aperture array and the traditional synthetic aperture array are revealed. Also, it is shown that under the specific array condition, the reconstructed image obtained by the time-division multiplexing synthetic aperture system can approach the imaging quality and resolution given by the traditional synthetic aperture array. It provides a new perspective for the array configuration design of OSA imaging system.
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