Abstract-Broadband terahertz antireflective microstructures on quartz crystal surface by CO2 laser micro-processing

Du Wang, Yaguo Li, Chuanchao Zhang, Wei Liao, Zeyu Li, Qinghua Zhang, and Qiao Xu

Fig. 1 (a) Experimental set-up of CO2 laser processing system. (b) Effective refractive index of SWS layer along the depth direction. For rectangular arrays the index step from the material to the air is from 1.35 to 1 and for hexagonal arrays the index step is from 1.16 to 1. (c) AR-SWSs of rectangular crater arrays.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-13-18351

Anti-reflection (AR) coating is a critical technology and an ongoing challenge for terahertz systems. The subwavelength structure (SWS) is an effective AR method, whereas the current manufacturing techniques, such as chemical etching and ultrafast laser processing, are low-efficient and low-quality for processing structures at the hundred-micron scale on hard brittle materials. We present a study of broadband SWSs directly ablated on the surface of quartz crystal by precisely controlled CO2 laser pulses, instead of commonly used ultra-fast lasers. The processing time of SWS can be shortened by two orders of magnitude compared with that by ultra-fast laser pulses. The SWS samples exhibit excellent AR properties with maximum transmittance of 97% at 0.71 THz, peak transmittance improvement of 13.5%, and optimal efficiency spectrum of 0.28–1.21 THz with transmittance >90%. The AR properties of SWS samples are in agreement with the simulated expectation and exist over a wide range of incidence angles up to ∼40°. The imaging of an object using SWS as the substrate shows an obvious improvement in imaging quality. We present an efficient and practical way to improve the transmission of optical components of materials, such as quartz crystal, alumina, and sapphire, in the terahertz band.

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

Abstract-Sparsity-based continuous wave terahertz lens-free on-chip holography with sub-wavelength resolution

Zeyu Li, Qiang Yan, Yu Qin, Weipeng Kong, Guangbin Li, Mingrui Zou, Du Wang, Zhisheng You, and Xun Zhou

Fig. 1 Schematic layout (a) and real picture (b) of the experiment setup. PM1 and PM2 are gold-coated confocal off-axis parabolic mirrors with the focal length of 50.8 mm and 101.6 mm, respectively. An output THz laser beam of ~10 mm in diameter is expanded and collimated to ~20 mm so that the detector array can be completely covered by THz wave. The wave scattered by the sample forms the object wave while the unscattered part of the illumination forms the reference wave. The resulting interference pattern is called an in-line hologram.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-2-702

We demonstrate terahertz (THz) lens-free in-line holography on a chip in order to achieve 40 μm spatial resolution corresponding to ~0.7λ with a numerical aperture of ~0.87. We believe that this is the first time that sub-wavelength resolution in THz holography and the 40 μm resolution were both far better than what was already reported. The setup is based on a self-developed high-power continuous wave THz laser at 5.24 THz (λ = 57.25 μm) and a high-resolution microbolometer detector array (640 × 512 pixels) with a pitch of 17 μm. This on-chip in-line holography, however, suffers from the twin-image artifacts which obfuscate the reconstruction. To address this problem, we propose an iterative optimization framework, where the conventional object constraint and the L1 sparsity constraint can be combined to efficiently reconstruct the complex amplitude distribution of the sample. Note that the proposed framework and the sparsity-based algorithm can be applied to holography in other wavebands without limitation of wavelength. We demonstrate the success of this sparsity-based on-chip holography by imaging biological samples (i.e., a dragonfly wing and a bauhinia leaf).

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

Abstract-Resolution and quality enhancement in terahertz in-line holography by sub-pixel sampling with double-distance reconstruction

Zeyu Li, Lei Li, Yu Qin, Guangbin Li, Du Wang, and Xun Zhou
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-18-21134

We demonstrate the enhancement of resolution and image quality in terahertz (THz) lens-free in-line digital holography by sub-pixel sampling with double-distance reconstruction. Multiple sub-pixel shifted low-resolution (LR) holograms recorded by a pyroelectric array detector (100 μm × 100 μm pixel pitch, 124 × 124 pixels) are aligned precisely to synthesize a high-resolution (HR) hologram. By this method, the lateral resolution is no more limited by the pixel pitch, and lateral resolution of 150 μm is obtained, which corresponds to 1.26λ with respect to the illuminating wavelength of 118.8 μm (2.52 THz). Compared with other published works, to date, this is the highest resolution in THz digital holography when considering the illuminating wavelength. In addition, to suppress the twin-image and zero-order artifacts, the complex amplitude distributions of both object and illuminaing background wave fields are reconstructed simultaneously. This is achieved by iterative phase retrieval between the double HR holograms and background images at two recording planes, which does not require any constraints on object plane or a priori knowledge of the sample.

© 2016 Optical Society of America

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