Abstract-Theoretical study on 2.52 terahertz beam shaping and polarization conversion based on the transmissive all-dielectric metasurface

Author links open overlay panelZewen Wang,  Qi Li, Fei Yan



https://www.sciencedirect.com/science/article/abs/pii/S0030401819311320

Based on the physical optics vector diffraction integral formula and the commercial electromagnetic software EastWave, a transmissive all-dielectric square metasurface with a side length of 12.4 mm is designed in this paper. It can simultaneously modulate the phase and polarization of the terahertz wave. The 2.52 terahertz x-polarized normal incident Gaussian beam can be shaped into a radially polarized narrow-width annular Bessel-Gaussian beam (radii of the inner and outer circles are 4.125 mm and 5.5 mm, respectively) at a propagation distance of 23.8 mm away from the metasurface. The metasurface is composed of the poly (4-methyl-1-pentene) substrate and the rectangular silicon resonators with different dimensions and rotation angles (counterclockwise rotation angle of the rectangle long axis as the reference of positive x axis). The relative square error between the amplitude distribution of the shaped beam and the target beam is 7.36%, and the fitting coefficient of them is 96.6%. The diffraction efficiency (defined as the ratio of the total light field energy in the target region to that in the rear plane of the metasurface along the incident beam propagation direction) of the shaped annular region is 75.5%. By focusing the narrow-width annular Bessel-Gaussian beam obtained in this paper, we can get smaller focal spot and longer focal depth, which is of great significance for using terahertz wave in particle acceleration and material processing. In addition, the emergent polarization can be flexibly switched between radially and azimuthally polarized beams by changing the polarization direction of the linearly polarized incident beam.

Abstract-Theoretical study on 2.52 terahertz Gaussian beam shaping based on an off-axis transmissive all-dielectric metasurface

Zewen Wang and Qi Li

Fig. 1. Schematic diagram of beam propagation process.

https://www.osapublishing.org/osac/abstract.cfm?uri=osac-2-1-201

An off-axis transmissive all-dielectric square metasurface with a side length of 55 mm is designed and studied in this paper. It can shape the 2.52 terahertz Gaussian beam into a rigorous plane wave (we define the required field-quality of the rigorous plane wave region: peak-to-peak amplitude ripple less than 1 dB and phase ripple less than 10°). The metasurface is composed of silicon cylindrical resonators whose phase modulation can almost span over the entire 2π range and the poly (4-methyl-1-pentene) substrate. The diameters of the resonators are from 11 to 35 μm and the heights of them are all 60 μm. The working range of the obtained plane wave is calculated by the physical optics integral formula when the off-axis angle is 22°. The results show that when the off-axis propagation distance is 1050 mm, the area satisfying the plane wave condition is 17×20 mm2, and the diffraction efficiency (we define the diffraction efficiency: the ratio of the total light field energy on the target plane to the total energy of light field that is just after the metasurface along the propagation of incident light) of the rigorous plane wave region cross-section is 23.4%. In addition, a random silicon cylinder diameter fabrication error within ± 4.5 μm is taken into account for the actual fabrication condition. The error satisfies the normal distribution with a mean of 0 μm and a standard deviation of 3 μm. The influence of the fabrication error on the shaping result is studied. We anticipate that the whole study will have certain pragmatic applications in some fields, such as enlarging the capture area of terahertz optical tweezers.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Creation of 3D terahertz photonic crystals by the direct writing technique with a TiO2 sol-gel ink

Rong Wang, Weiyi Yang, Pengfei Zhu, Shuang Gao, Bo Li, Qi Li

http://onlinelibrary.wiley.com/doi/10.1111/jace.15347/abstract

Three-dimensional TiO2 woodpile terahertz photonic crystals with feature size less than 100 μm were created by the direct writing technique with a TiO2 sol-gel ink. With proper heat treatment, the rheological properties of the TiO2 sol-gel ink were modulated to meet the requirement to construct 3D-TPCs with feature size less than 100 μm by the direct writing technique. Well-crystallized rutile TiO2 3D-TPCs were obtained by the calcination process to provide the required high refractive indices, while the lattice periods of these TiO2 3D-TPCs could be changed to tune their terahertz properties. With the increase of the lattice period, a shift of their THz photonic band gap peaks towards the lower frequency was observed in both the simulated and experimental investigations.

This article is protected by copyright. All rights reserved.

Abstract-Direct Writing of Flexible Barium Titanate/Polydimethylsiloxane 3D Photonic Crystals with Mechanically Tunable Terahertz Properties

http://onlinelibrary.wiley.com/doi/10.1002/adom.201600977/full

Mechanically flexible 3D terahertz photonic crystals (3D-TPCs) are created by the direct-writing technology with a composite ink system composed of polydimethylsiloxane (PDMS) and barium titanate (BaTiO3) nanoparticles. The direct-writing technology allows an easy creation of complex 3D structures with designed geometry, while the refractive indices of the composite ink can be modulated by varying the content of BaTiO3 nanoparticles. Thus, 3D-TPCs with different terahertz properties are obtained by the direct-writing technology. More interestingly, these 3D-TPCs demonstrate a unique tunable terahertz property under external force field due to their mechanical flexibility from the PDMS matrix of the composite ink. Thus, their terahertz property is responsive to external force fields reversibly, which can find novel applications in terahertz technology and other related technological applications

Abstract-2.52 terahertz dual-axis reflection confocal scanning microscope

Qi Li, Yi Zhou, Yong-Fa Yang, and Guang-Hao Chen
https://www.osapublishing.org/josaa/abstract.cfm?uri=josaa-33-4-637

We present a dual-axis reflection confocal scanning microscope operating at 2.52 terahertz with axial resolution of 0.67 mm. The spatial resolution of the system was evaluated by utilizing the resolution test chart. Lateral resolution exceeded 0.314 mm, and the lengthwise resolution was over 0.353 mm. We introduced a 0.3 mm pinhole to improve the resolution. Targets such as the Chinese character “TAI” written on paper with a pencil and the metal letter “G” were scanned to test the imaging quality. To verify the imaging ability of the axial sections, two pairs of metal straps and a combinatorial metal ring were scanned, further revealing the satisfying 3D imaging capability.

© 2016 Optical Society of America

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Abstract-Support-domain constrained phase retrieval algorithms in terahertz in-line digital holography reconstruction of a nonisolated amplitude object

Jiaqi Hu, Qi Li, and Yi Zhou
https://www.osapublishing.org/ao/abstract.cfm?uri=ao-55-2-379

Phase retrieval algorithms applied to in-line digital holography reconstruction can weaken interference from the region outside the study target and an unstable light source, etc., by adopting the object-plane support domain constraint. Based on threshold segmentation and morphological filtering, a method to directly calculate the object-plane support domain is proposed in this paper. Combined with the above method, an improved support-domain constrained phase retrieval algorithm is presented. Then, imaging simulations and experiments on terahertz in-line digital holography reconstruction of nonisolated objects are conducted. The simulations study the influence of transmittance of the background plate, structural element of morphological filtering, etc., on the reconstruction effect of the improved algorithm without noise interference. Simulation and experiment results suggest that good reconstructed images can be obtained by this algorithm when transmittance of the background plate is greater than 0.90.

© 2016 Optical Society of America

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Abstract-Terahertz digital holography image denoising using stationary wavelet transform

Shan-Shan Cui ; Qi Li ; Guanghao Chen

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2275304

Shan-Shan Cui, Qi Li, Guanghao Chen

Harbin Institute of Technology (China)

Proc. SPIE 9522, Selected Papers from Conferences of the Photoelectronic Technology Committee of the Chinese Society of Astronautics 2014, Part II, 952230 (April 13, 2015); doi:10.1117/12.2182877

Terahertz (THz) holography is a frontier technology in terahertz imaging field. However, reconstructed images of holograms are inherently affected by speckle noise, on account of the coherent nature of light scattering. Stationary wavelet transform (SWT) is an effective tool in speckle noise removal. In this paper, two algorithms for despeckling SAR images are implemented to THz images based on SWT, which are threshold estimation and smoothing operation respectively. Denoised images are then quantitatively assessed by speckle index. Experimental results show that the stationary wavelet transform has superior denoising performance and image detail preservation to discrete wavelet transform. In terms of the threshold estimation, high levels of decomposing are needed for better denoising result. The smoothing operation combined with stationary wavelet transform manifests the optimal denoising effect at single decomposition level, with 5×5 average filtering. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Continuous-wave 2.52 Terahertz Gabor inline compressive holographic tomography

Qi Li,  

Yun-Da Li

http://link.springer.com/article/10.1007%2Fs00340-014-5871-5

Continuous-wave (CW) 2.52 Terahertz (THz) 3D tomographic images were obtained by numerically reconstructing a single Gabor inline digital hologram based on modified compressive sensing reconstruction algorithm. Three metallic copper samples which are separately adhered to three Teflon plate were used as the targets. The actual axial resolution achieved was higher than 6 mm, and the lateral resolution was higher than 0.4 mm. Similarly, a paper clip and a handwritten character sample on a white paper were also imaged. Numerical simulation and experimental results verified the preferable reconstruction characteristics of the proposed modified algorithm. The feasibility of CW THz Gabor inline compressive holographic tomography is confirmed by adding barriers such as Teflon boards and thermal paper to block the samples.

Abstract-Research on object-plane constraints and hologram expansion in phase retrieval algorithms for CW terahertz inline digital holography reconstruction

Jiaqi Hu, Qi Li, and Shanshan Cui
http://www.opticsinfobase.org/ao/upcoming_pdf.cfm?id=217516

In terahertz inline digital holography, the zero-order diffraction light and conjugate image cause reconstructed image misty. In this paper, three phase retrieval algorithms with different object-plane constrain are applied to conduct reconstruction under the same near-field diffraction propagation conditions and image-plane constraint. The impacts of different object-plane constraints on terahertz inline digital holographic reconstruction are studied. The results show that it is not suitable for phase retrieval algorithm to impose restriction on the phase when the object is a little complicated in the terahertz inline digital holography. In addition, the effects of zero-padding expansion, boundary replication expansion and apodization operation on reconstructed images are studied. The results indicate that the conjugate image can be eliminated and a better reconstructed image can be obtained by adopting appropriate phase retrieval algorithm after the normalized hologram extending to the minimum area which meets the applicable range of angular spectrum reconstruction algorithm by means of boundary replication.