Abstract-A Model of Terahertz Parametric Process Including Spontaneous Parametric Down-conversion

 

Yu Qin, Zeyu Li, Qiang Yan, Xun Zhou, Mingrui Zou,  Weipeng Kong, 

https://link.springer.com/article/10.1007/s10762-021-00778-7

A new numerical model of terahertz parametric process is demonstrated, which takes the spontaneous parametric down-conversion into consideration by adding tens of frequency components in the coupled-wave equations. To set the values of some parameters in this model, the residual pump energy and idler spectra during spontaneous parametric down-conversion are measured experimentally for different pump energy and crystal lengths. Compared with previous models which only involve three waves, the new model is more accurate, especially when the seed power is low or the generated terahertz-wave is in the low frequency region.

Abstract-Theoretical and experimental study on the enhancement of seed injection in terahertz-wave generation

Weipeng Kong, Zeyu Li, Qiang Yan, Mingrui Zou, Xun Zhou, and Yu Qin

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-37-8-2479

We have studied the seed enhancement of an injection-seeded terahertz (THz)-wave parametric generator (is-TPG) with the help of a theoretical model. During simulation, the power of the seed beam varied from 1 W to 1 MW as the initial condition, while the pump power is set to 18.5 MW. When the peak power of the seed beam reaches megawatt level, the output power of THz wave increases rapidly and the peak position for the THz output dramatically moves towards the front of the crystal. The calculated output power of the injection-pulse-seeded TPG (ips-TPG) with 0.3 MW pulsed seed is 3.4 kW in the case of a surface-emitting configuration and 14 kW in the case of a Si-prism coupled output configuration. The enhancement ratios are 3.5 and 1.4 times, respectively, compared with the watt level CW-seeded is-TPG. Guided by the simulation, we experimentally demonstrated an 11 kW peak power Si-prism output coupled ips-TPG. The enhancement ratio is 1.6 times. The energy of the pulsed seed is 30 µJ (0.3 MW). The pump energy is 7.2 mJ, which is about half that of the typical is-TPG with the same output power. Compared with a typical is-TPG, most of the THz power in ips-TPG accumulates in the front of the crystal and thus is easier to collect. One obvious advantage of ips-TPG is that the output THz energy can be increased while keeping the pump power at a safe level.

© 2020 Optical Society of America

Abstract-Terahertz synthetic aperture in-line holography with intensity correction and sparsity autofocusing reconstruction

Zeyu Li, Ruijiao Zou, Weipeng Kong, Xuemin Wang, Qinghua Deng, Qiang Yan, Yu Qin, Weidong Wu, and Xun Zhou

Synthetic aperture hologram with intensity correction for a dragonfly forewing. (a) Nine normalized sub-holograms with intensity correction. (b) Synthetic aperture hologram composed of (a). (c) Synthetic aperture hologram without intensity correction. (d) Optical image of the dragonfly forewing sample. (e) Amplitude distribution reconstructed from (b) with 20 iterations. (f) Amplitude distribution reconstructed from (c) with 20 iterations. The effect of non-uniform intensity on reconstruction can be seen from the parts marked by the white and blue dotted circles.

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-7-12-1391

We demonstrate high-resolution and high-quality terahertz (THz) in-line digital holography based on the synthetic aperture method. The setup is built on a self-developed THz quantum cascade laser, and a lateral resolution better than 70 μm (∼𝜆$\sim \lambda$) is achieved at 4.3 THz. To correct intensity differences between sub-holograms before aperture stitching, a practical algorithm with global optimization is proposed. To address the twin-image problem for in-line holography, a sparsity-based phase retrieval algorithm is applied to perform the high-quality reconstruction. Furthermore, a new autofocusing criterion termed “reconstruction objective function” is introduced to obtain the best in-focus reconstruction distance, so the autofocusing procedure and the reconstruction are unified within the same framework. Both simulation and experiment prove its accuracy and robustness. Note that all the methods proposed here can be applied to other wavebands as well. We demonstrate the success of this THz synthetic aperture in-line holography on biological and semiconductor samples, showing its potential applications in bioimaging and materials analysis.

© 2019 Chinese Laser Press

Abstract-Subdiffraction focusing of total electric fields of terahertz wave

Mengyu Yang, Desheng Ruan, Lianghui Du, Chunyan Qin, Zeyu Li, Cuiping Lin, Gang Chen, Zhong Quan Wen,



https://www.sciencedirect.com/science/article/abs/pii/S0030401819309307

Terahertz lens is an essential component in terahertz application. We propose a focusing of total electric fields planar lens based on super-oscillation. This planar lens is designed for a wavelength ($\lambda$) of 118.$8\mathrm{\mu }\mathrm{m}$ with a radius of 160$\lambda$, a focal length of 210 $\lambda$ and a numerical aperture of 0.606. Our experiment demonstrates a subdiffraction and subwavelength focusing of total electric fields. The full width at half-maximum of the focal spot is 0.67 $\lambda$, which is smaller than the diffraction limit of 0.825$\lambda$. The results shows that it has powerful application for terahertz imaging, especially in the fields of biomedical science.

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-All-dielectric metalens for terahertz wave imaging

Xue Jiang, Hao Chen, Zeyu Li, Hongkuan Yuan, Luyao Cao, Zhenfei Luo, Kun Zhang, Zhihai Zhang, Zhongquan Wen, Li-guo Zhu, Xun Zhou, Gaofeng Liang, Desheng Ruan, Lianghui Du, Lingfang Wang, Gang Chen,

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-11-14132

Terahertz wave imaging offers promising properties for non-destructive testing applications in the areas of homeland security, medicine, and industrial inspection. However, conventional optical lenses are heavy and bulky and difficult to integrate. An all-dielectric metasurface provides an attractive way to realize a planar lens of light weight that is ultrathin and offers ease of integration. Terahertz lenses based on various metasurfaces have been studied, especially for the application of wave focusing, while there are few experimental demonstrations of terahertz wave imaging lenses based on an all-dielectric metasurface. In the present work, we propose a metalens based on an all-dielectric metasurface with a sub-wavelength unit size of 0.39λ for terahertz wave imaging and experimentally demonstrate its performance in focusing and imaging. A large numerical aperture metalens was fabricated with a focal length of 300λ, radius of 300λ, and numerical aperture of 0.707. The experimental results show that the lens can focus THz waves with an incident angle up to 48°. More importantly, clear terahertz wave images of different objects were obtained for both different cases of forward- and inverse-incident directions, which demonstrate the reversibility of the metalens for imaging. Such a metalens provides a way for realization of all-planar-lens THz imaging system, and might find application in terahertz wave imaging, information processing, microscopy, and others.

© 2018 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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Abstract-Terahertz in-line digital holography of human hepatocellular carcinoma tissue

http://www.nature.com/srep/2015/150213/srep08445/full/srep08445.html

• Lu Rong,
• Tatiana Latychevskaia,
• Chunhai Chen,
• Dayong Wang,
• Zhengping Yu,
• Xun Zhou,
• Zeyu Li,
• Haochong Huang,
• Yunxin Wang
• & Zhou Zhou

• Affiliations
• Contributions
• Corresponding authors

Scientific Reports

 

5,

 

Article number:

 

8445

 

doi:10.1038/srep08445

Received

 

16 July 2014 

Accepted

 

20 January 2015 

Published

 

13 February 2015

Terahertz waves provide a better contrast in imaging soft biomedical tissues than X-rays, and unlike X-rays, they cause no ionisation damage, making them a good option for biomedical imaging. Terahertz absorption imaging has conventionally been used for cancer diagnosis. However, the absorption properties of a cancerous sample are influenced by two opposing factors: an increase in absorption due to a higher degree of hydration and a decrease in absorption due to structural changes. It is therefore difficult to diagnose cancer from an absorption image. Phase imaging can thus be critical for diagnostics. We demonstrate imaging of the absorption and phase-shift distributions of 3.2 mm × 2.3 mm × 30-μm-thick human hepatocellular carcinoma tissue by continuous-wave terahertz digital in-line holography. The acquisition time of a few seconds for a single in-line hologram is much shorter than that of other terahertz diagnostic techniques, and future detectors will allow acquisition of meaningful holograms without sample dehydration. The resolution of the reconstructions was enhanced by sub-pixel shifting and extrapolation. Another advantage of this technique is its relaxed minimal sample size limitation. The fibrosis indicated in the phase distribution demonstrates the potential of terahertz holographic imaging to obtain a more objective, early diagnosis of cancer.

Abstract-Terahertz in-line digital holography of dragonfly hindwing: amplitude and phase reconstruction at enhanced resolution by extrapolation

Lu Rong, Tatiana Latychevskaia, Dayong Wang, Xun Zhou, Haochong Huang, Zeyu Li, and Yunxin Wang  »View Author Affiliations http://8.18.37.105/oe/abstract.cfm?uri=oe-22-14-17236

Optics Express, Vol. 22, Issue 14, pp. 17236-17245 (2014)
http://dx.doi.org/10.1364/OE.22.017236

We report here on terahertz (THz) digital holography on a biological specimen. A continuous-wave (CW) THz in-line holographic setup was built based on a 2.52 THz CO₂ pumped THz laser and a pyroelectric array detector. We introduced novel statistical method of obtaining true intensity values for the pyroelectric array detector’s pixels. Absorption and phase-shifting images of a dragonfly’s hindwing were reconstructed simultaneously from single in-line hologram. Furthermore, we applied phase retrieval routines to eliminate twin image and enhanced the resolution of the reconstructions by hologram extrapolation beyond the detector area. The finest observed features are 35 μm width cross veins.

© 2014 Optical Society of America

Abstract-Terahertz in-line digital holography of dragonfly hindwing: amplitude and phase reconstruction at enhanced resolution by extrapolation

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-14-17236

Lu Rong, Tatiana Latychevskaia, Dayong Wang, Xun Zhou, Haochong Huang, Zeyu Li, and Yunxin Wang  »View Author Affiliations

Optics Express, Vol. 22, Issue 14, pp. 17236-17245 (2014)
http://dx.doi.org/10.1364/OE.22.017236

We report here on terahertz (THz) digital holography on a biological specimen. A continuous-wave (CW) THz in-line holographic setup was built based on a 2.52 THz CO₂ pumped THz laser and a pyroelectric array detector. We introduced novel statistical method of obtaining true intensity values for the pyroelectric array detector’s pixels. Absorption and phase-shifting images of a dragonfly’s hindwing were reconstructed simultaneously from single in-line hologram. Furthermore, we applied phase retrieval routines to eliminate twin image and enhanced the resolution of the reconstructions by hologram extrapolation beyond the detector area. The finest observed features are 35 μm width cross veins.

© 2014 Optical Society of America