Abstract-Printing special surface components for THz 2D and 3D imaging

                                                                 

Bo Yan, Zhigang Wang, Xing Zhao, Lie Lin, Xiaolei Wang, Cheng Gong,  Weiwei Liu

https://www.nature.com/articles/s41598-020-77998-9

The paper reports an off-axis large focal depth THz imaging system which consists of three 3D printed special surface components (two aspherical mirrors and an axicon). Firstly, the optical design software is used to design and optimize the aspherical parabolic mirror. Secondly, the optimized mirror is prepared by a 3D printing and metal cladding method. Thirdly, a THz axicon is designed for generation of quasi-Bessel Beam and a new geometric theoretical model of oblique incident light for axicon is established. Finally, the imaging system based on the special surface components is constructed. Its maximum diffraction-free distance is about 60 mm, which is 6 times higher than the traditional system. To verify the effectiveness, THz two-dimensional imaging experiments and three-dimensional computed tomography experiment are carried out. The results are consistent with the design and calculations.

Abstract-Tightly focusing terahertz wave using gradient-type slotted grating based on spoof surface plasmons

Xiaolei Wang, Jiehui Zhao, Fang Bo, Guangke Jiang, Xiaoxue Hu, Zhuqing Zhu, and Weiwei Liu

Structural design of tight-focusing array devices: (a) lateral arrangement of single device; (b) longitudinal arrangement of single device; (c) square arrangement of single device.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-11-16298

The d1-d2-d3-d4-d5 gradient-type spoof surface plasmons (SSP) grating was designed and found to exert an obvious effect on electric field localization. Two gradient-shaped planar ports were added to the bottom of this grating to form a gradient-type slotted SSP grating and achieve tight focusing and local electric field enhancement for a terahertz wave. The size of the focal spot was optimized to 0.01λ. The single-gradient-type slotted SSP grating was considered as a unit and arranged in one and two dimensions to generate a longitudinal focal line and square focal spots array. This did not only improve the resolution of terahertz imaging, but also simultaneously scan multiple focal spots to increase the speed of terahertz imaging. This work makes the manipulation of terahertz wave more flexible and efficient which has great potential in terahertz high-resolution near-field scanning imaging.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-A combined study on the skeletal vibration of aminopyrine by terahertz time-domain spectroscopy and DFT simulation

Fan Huang, Mingming Liu, Maoquiang Xie, Weiwei Liu, Lu Sun,

https://www.sciencedirect.com/science/article/pii/S003040261931811X

Aminopyrine is an inexpensive drug that is highly effective on analgesic and anti-inflammatory. However, it is found adulterated to traditional Chinese medicine by unscrupulous merchants though side effects, such as neutropenia, myelosuppression and formation of nitrosamine carcinogens, make it banned in many countries. In this work, terahertz time-domain spectroscopy has been proposed to perform characterization of aminopyrine in the range of 0.2∼2.4 THz. Characteristic absorption peaks of aminopyrine are found at 1.54 THz and 2.15 THz. To gain further insight, DFT studies have been carried out to simulate the THz spectrum and analyze molecular vibration mode of aminopyrine. The spectrum simulated is in good agreement with the experiments and the calcuated absorption peaks at 1.48 THz and 2.29 THz matches with the experimental result of 1.54 THz and 2.15 THz. The main vibrational modes of the characteristic peaks are the skeletal vibrations of the molecule, which is represented by the rotation and the swing of dimethylamino group, pyrazolone and benzene rings. This study shows that terahertz spectroscopy has potential application prospects for the detection of adulteration of aminopyrine in rheumatic Chinese patent medicines and the examination of counterfeit drugs.

Abstract-0.1THz super-resolution imaging based on 3D printed confocal waveguides

Ting Yu, Xu Zuo, Weiwei Liu, Cheng Gong,


https://www.sciencedirect.com/science/article/abs/pii/S0030401819310065

The paper reports a waveguide-based lens-free terahertz (THz) imaging method. It not only inherits the advantages of traditional confocal imaging, but also realizes super-resolution in THz band. The waveguides prepared by a 3D printing and metal-cladding technology can replace the traditional lens to transmit and focus THz wave effectively. For verification, two hollow waveguides (8 mm inner diameter, 60 mm length) were fabricated and a 0.1 THz confocal waveguides imaging system was built. High quality THz images with a minimum resolution of 1.41 mm (less than 1/2 of the wavelength) were obtained by placing the imaging targets at the waveguide’s focus and performing two-dimensional scanning. The focusing mechanism and transmission characteristics of THz in the waveguide are simulated and analyzed. The simulations are in agreement with the experiments.

Abstract-Control of terahertz pulse polarization by two crossing DC fields during femtosecond laser filamentation in air

Qiang Su, Qiang Xu, Nan Zhang, Yang Zhang, and Weiwei Liu

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-36-10-G1

Terahertz (THz) radiation induced by the femtosecond laser filament in air can be strongly enhanced through applying an external DC field. In the present work, this technique has been extended to two-dimensional biasing; namely, two pairs of electrodes, which are placed perpendicular to each other, are applied to provide two crossing DC fields. The experimental results have demonstrated that the amplitudes of two orthogonal components of the polarized THz pulse could be modulated by the electric field parallel to it, in an in situ and real-time manner, while the phase delay between them remains constant. Hence, the polarization of the output THz pulse could be freely controlled by adjusting the voltages of the two crossing DC fields, achieving arbitrary ellipticity. It might be the simplest method to manipulate the THz polarization during femtosecond laser filamentation in air.

© 2019 Optical Society of America

Abstract-Numerical simulation on terahertz wave propagation in plasma channels

panel

Dan Lua, Hui Gao, Jiayu Zhao, Bo Yang, Weiwei 
Liu, https://www.sciencedirect.com/science/article/pii/S0030402619300075

Terahertz (THz) wave generated by femtosecond laser filamentation can spatially constrained inside the plasma channels. Our simulation results show that this constraint to THz wave will gradually weaken with the increasing THz frequency. At the same time, results also indicate that the higher plasma density possesses the stronger constraint. In addition, the model field radius and width of THz wave will become larger with the increasing plasma channel radius. Based on the simulation results we find the changes of refractive index at THz wave play an important role on THz wave propagation in plasma channels.

Scientists 3D Print an Effective Terahertz Waveguide

(a) Segments of the hollow waveguide; (b) experimental results of the loss coefficient; (c) mechanical spliced 90 cm hollow waveguide.

https://3dprint.com/229232/scientists-3d-print-effective-terahertz-waveguide/

The terahertz (THz) wave is the electromagnetic radiation at frequencies from 0.1 to 10 THz, which is located between the millimeter wave and the far infrared wave. It has not been fully studied because of a lack of effective means of generation, detection and transmission, so it is referred to as the “Terahertz Gap.” The terahertz wave has a lot of potential in non-destructive imaging, biomedicine and national security and defense, because it has penetrability for most of non-polar materials and does not cause ionization damage while covering the vibration and rotational energy levels of biological macromolecules.

In a paper entitled “A 0.1 THz low-loss 3D printed hollow waveguide,” a group of researchers discusses the use of 3D printing to create THz functional devices, such as terahertz lenses, phase plates, waveguides and more. 3D printing is a low-cost, simple and effective way to create these devices, they point out.

“Therefore, the combination of low-loss dielectric waveguide and low-cost 3D printing will help to break through the bottlenecks and realize THz remote applications,” the researchers state. “The paper focuses on the design, fabrication, and characterization of a novel 0.1 THz low-loss hollow waveguide. Its theoretical loss is as low as 0.009 cm−1 and the measured loss is 0.015 cm−1. The experimental results show that the proposed hollow waveguide not only reduces the transmission loss of the terahertz wave, but also can effectively localize the terahertz field and confine the divergence angle of the terahertz beam.”

The researchers used PLA to create the hollow waveguide. First they needed to 3D print a PLA disk in order to obtain the elecrtromagnetic parameters of the material. The disk was printed on an Ultimaker 3D printer and characterized by terahertz time-domain spectroscopy (THz-TDS).

“After that the design for the hollow waveguide could be started,” the researchers continue. The first step is designing the cross section of the waveguide based on the anti-resonant waveguide model and drawing the cross section’s two-dimensional graph. Secondly, the graph is imported into the finite element simulation software (Comsol Multiphysics) and a larger circle around the cross section is drawn as the perfect matching layer. Thirdly, the different materials and the corresponding refractive index are selected and the design model is meshed. Finally, according to the simulation, the effective refractive index of different modes transmitted in the center air hole of the hollow waveguide can be acquired.”

(a) Cross section of the hollow waveguide; (b) HE11 fundamental mode field distribution

The 90-cm-long hollow waveguide was then 3D printed and characterized. To verify the localization effect of the hollow waveguide on THz wave, the researchers measured the THz divergence angle at the end of the waveguide. The measured loss was 0.015 cm−1. The experimental results showed that the hollow waveguide can not only reduce the transmission loss of the terahertz wave in the air, but also effectively localize the terahertz wave. The researchers conclude that remote low-cost THz sensing and imaging can be achieved in the future by the development of flexible and longer hollow waveguides.

Authors of the paper include Pengfei Qi, Weiwei Liu and Cheng Gong.

Abstract-An broadband terahertz metamaterial filter based on multiplexed metallic bar resonators

Zijie Dai, Jing Yang, Qiang Su, Pengfei Qi, Dan Lu, Cheng Gong, Lu Sun, and Weiwei Liu

https://www.osapublishing.org/abstract.cfm?uri=CLEO_QELS-2018-JTh2A.178

An ultrabroad terahertz metamaterial filter based on multiplexed metallic bar resonators is designed and fabricated. The bandwidth of terahertz filter can be significantly broadened by the multiplexed configurations.

© 2018 The Author(s)

Abstract-Clue to a thorough understanding of terahertz pulse generation by femtosecond laser filamentation

Jiayu Zhao, Weiwei Liu, Shichang Li, Dan Lu, Yizhu Zhang, Yan Peng, Yiming Zhu, and Songlin Zhuang

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-6-4-296

In this work, it has been demonstrated that in order to fully understand the terahertz (THz) pulse generation process during femtosecond laser filamentation, the interaction between THz wave and air plasma has to be taken into account. This interaction is mainly associated with the spatial confinement of the THz pulse by the plasma column, which could be described by the one-dimensional negative dielectric (1DND) waveguide model. By combining the 1DND model with the conventional four-wave mixing (4WM) and photocurrent (PC) models, the variation of THz spectral amplitude and width obtained in experiments could be better understood. Finally, a three-step procedure, with 1DND bridging 4WM and PC processes, has been established for the first time to describe the underlying mechanism of THz radiation from plasma sources.

© 2018 Chinese Laser Press

Abstract-Optical programmable metamaterials

Cheng Gong,  Nan Zhang,  Zijie Dai, Weiwei Liu,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10697/106974M/Optical-programmable-metamaterials/10.1117/12.2309751.short

We suggest and demonstrate the concept of optical programmable metamaterials which can configure the device’s electromagnetic parameters by the programmable optical stimuli. In such metamaterials, the optical stimuli produced by a FPGA controlled light emitting diode array can switch or combine the resonance modes which are coupled in. As an example, an optical programmable metamaterial terahertz absorber is proposed. Each cell of the absorber integrates four meta-rings (asymmetric 1/4 rings) with photo-resistors connecting the critical gaps. The principle and design of the metamaterials are illustrated and the simulation results demonstrate the functionalities for programming the metamaterial absorber to change its bandwidth and resonance frequency.

© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Giant Two-Photon Absorption and Its Saturation in 2D Organic–Inorganic Perovskite

Weiwei Liu, Jun Xing, Jiaxin Zhao, Xinglin Wen,Kai Wang, Peixiang Lu, Qihua Xiong

http://onlinelibrary.wiley.com/doi/10.1002/adom.201601045/abstract

Organic–inorganic perovskites have attracted great attentions driven by exceptional progress in photovoltaics, photonics, and optoelectronics. Different from the corner sharing framework of 3D perovskite, 2D organic–inorganic perovskites possess a layered staking structure composed of alternative organic and inorganic components. Due to the inherent multi-quantum-well-like structure, it is intriguing to explore the optical properties of 2D perovskites enabled by spatial and dielectric confinement. Herein, the two-photon absorption (TPA) properties of 2D perovskite phenylethylamine lead iodide ((PEA)2PbI4) are systematically studied. The 2D perovskite exhibits a giant TPA and saturation effect under excitation of 800 nm femtosecond laser. The TPA coefficient of a (PEA)2PbI4 flake is measured to be about 211.5 cm MW−1, which is at least one order of magnitude larger than those of 3D perovskite films and some typical semiconductor nanostructures. The giant TPA can be attributed to the enhanced quantum and dielectric confinement in the organic–inorganic multi-quantum-well structure. In addition, a highly thickness-dependent TPA is observed for the 2D perovskite flakes. The result advocates a great promise of 2D organic–inorganic perovskites for nonlinear optical absorption related optoelectronic devices.

Abstract-Femtosecond-laser-driven wire-guided helical undulator for intense terahertz radiation

• Ye Tian,
• Jiansheng Liu,
• Yafeng Bai,
• Shiyi Zhou,
• Haiyi Sun,
• Weiwei Liu,
• Jiayu Zhao,
• Ruxin Li
• & Zhizhan Xu
• • 
    
    
  • http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2017.16.html?WT.feed_name=subjects_materials-science

The capability of synchrotron radiation to produce ultrabright emission has attracted considerable interest over the last half a century. To date, magnetic undulators with a period of several centimetres are commonly used for wiggling relativistic electrons in a modulated field. Here, we propose a novel compact undulator with a period down to the submillimetre level based on a spontaneous electric field that is driven by a femtosecond laser. Both the guided energetic electrons and the gyrotron-like undulator are spontaneously produced by irradiating a thin metallic wire with an intense laser pulse. An intense radial electric field instantaneously created on the wire can guide the electrons' helical motion along the wire and induce periodic THz emission. We have demonstrated that this scheme can produce intense THz sources with a conversion efficiency of 1% that are frequency-tunable by adjusting the diameter of the wire. Amplified emission of THz radiation by more than tenfold has been observed.

Abstract-Tunable reflecting terahertz filter based on chirped metamaterial structure

• Jing Yang
• , Cheng Gong
• , Lu Sun
• , Ping Chen
• , Lie Lin
•  & Weiwei Liu

http://www.nature.com/articles/srep38732
Tunable reflecting terahertz bandstop filter based on chirped metamaterial structure is demonstrated by numerical simulation. In the metamaterial, the metal bars are concatenated to silicon bars with different lengths. By varying the conductivity of the silicon bars, the reflectivity, central frequency and bandwidth of the metamaterial could be tuned. Light illumination could be introduced to change the conductivity of the silicon bars. Numerical simulations also show that the chirped metamaterial structure is insensitive to the incident angle and polarization-dependent. The proposed chirped metamaterial structure can be operated as a tunable bandstop filter whose modulation depth, bandwidth, shape factor and center frequency can be controlled by light pumping.

Abstract-Strong Spatial Confinement of Terahertz Wave inside Femtosecond Laser Filament

Jiayu Zhao, Wei Chu, Zhi Wang, Yan Peng, Cheng Gong, Lie Lin, Yiming Zhu, Weiwei Liu, Ya Cheng, Songlin Zhuang, and Zhizhan Xu

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.6b00512

Publication Date (Web): November 8, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.6b00512

In this paper, a new experimental phenomenon is demonstrated. During the femtosecond laser filamentation, the generated terahertz (THz) pulse has been found to be strongly confined inside the plasma channel, reaching a spatial diameter of a few tens of micrometres. It has been attributed to the formation of a plasma negative dielectric waveguide induced by the transverse inhomogeneous plasma density distribution. The new experimental phenomenon will renew the understanding of the THz wave generation and propagation dynamics during the femtosecond laser and air plasma interaction. Due to this strong spatial confinement, THz electric field strength could be enhanced by orders of magnitude, potentially providing a new approach to perform THz nonlinear optics with low laser energy.

Abstract-3D printed low-loss THz waveguide based on Kagome photonic crystal structure

Jing Yang, Jiayu Zhao, Cheng Gong, Haolin Tian, Lu Sun, Ping Chen, Lie Lin, and Weiwei Liu

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-20-22454

A low-loss hollow core terahertz waveguide based on Kagome photonic crystal structure has been designed and fabricated by 3D printing. The 3D printed waveguide has been characterized by using THz time-domain spectroscopy. The results demonstrate that the obtained waveguide features average power propagation loss of 0.02 cm−1 for 0.2-1.0 THz (the minimum is about 0.002 cm−1 at 0.75 THz). More interesting, it could be simply mechanically spliced without any additional alignment, while maintaining the excellent performance. The 3D printing technique will be a promising solution to fabricate Kagome THz waveguide with well controllable characteristics and low cost.

© 2016 Optical Society of America

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Abstract-Broadband terahertz metamaterial absorber based on sectional asymmetric structures

Cheng Gong, Mingzhou Zhan, Jing Yang, Zhigang Wang, Haitao Liu, Yuejin Zhao & Weiwei Liu

http://www.nature.com/articles/srep32466

We suggest and demonstrate the concept and design of sectional asymmetric structures which can manipulate the metamaterial absorber’s working bandwidth with maintaining the other inherent advantages. As an example, a broadband terahertz perfect absorber is designed to confirm its effectiveness. The absorber’s each cell integrates four sectional asymmetric rings, and the entire structure composed of Au and Si3N4 is only 1.9 μm thick. The simulation results show the bandwidth with absorptivity being larger than 90% is extended by about 2.8 times comparing with the conventional square ring absorber. The composable small cell, ultra-thin, and broadband absorption with polarization and incident angle insensitivity will make the absorber suitable for the applications of focal plane array terahertz imaging.

Abstract-Enhanced plasmonic nanofocusing of terahertz waves in tapered graphene multilayers

Weiwei Liu, Bing Wang, Shaolin Ke, Chengzhi Qin, Hua Long, Kai Wang, and Peixiang Lu
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-13-14765

We investigate the plasmonic nanofocusing of terahertz waves in tapered graphene multilayers separated by dielectrics. The nanofocusing effect is significantly enhanced in the graphene multilayer taper compared with that in a single layer graphene taper due to interlayer coupling between surface plasmon polaritons. The results are optimized by choosing an appropriate layer number of graphene and the field amplitude has been enhanced by 620 folds at λ = 50 μm. Additionally, the structure can slow light to a group velocity ~1/2815 of the light speed in vacuum. Our study provides a unique approach to compress terahertz waves into deep subwavelength scale and may find great applications in terahertz nanodevices for imaging, detecting and spectroscopy.

© 2016 Optical Society of America

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Abstract-Broadband, wide-angle, low-scattering terahertz wave by a flexible 2-bit coding metasurface

Xin Yan, Lanju Liang, Jing Yang, Weiwei Liu, Xin Ding, Degang Xu, Yating Zhang, Tiejun Cui, and Jianquan Yao
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-22-29128

Abstract: Expanding bandwidths and arbitrary control of technology remain key issues in the field of electromagnetic waves, especially in terahertz (THz) wave. In this paper, we propose a novel method to achieve broadband low-scattering THz characteristics with wide-angle and polarization independence by a 2-bit flexible and nonabsorptive coding metasurface. The coding metasurface is composed of four digital elements based on double cross metallic line for “00”, “01”, “10”, and “11.” The reflection phase difference of neighboring elements is about 90° over a broad THz frequency band and wide incident angles. The low scattering coefficients below –10 dB were achieved over a wide frequency band from 0.8 THz to 1.5 THz when the incident angle is less than 50° by coding the four elements sequences. This superior property is maintained when the flexible coding metasurface is wrapped around a metallic cylinder with different dimensions. These results present a novel method to control THz waves freely and demonstrate significant scientific value in practical applications.

© 2015 Optical Society of America

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Abstract-A simple method to enhance terahertz radiation from femtosecond laser filament array with a step phase plate

Jiayu Zhao, Lanjun Guo, Wei Chu, Bin Zeng, Hui Gao, Weiwei Liu, and Ya Chen
https://www.osapublishing.org/ol/upcoming_pdf.cfm?id=236756

• Abstract: In this work, we experimentally demonstrate a 200% enhancement of terahertz (THz) wave amplitude generated by femtosecond laser filamentation in air. The experimental setup simply uses a semicircular phase plate to generate two parallel filaments. Temporally overlapped THz pulses from two filaments coherently add up, giving rise to significant enhancement of the THz pulse amplitude. It has been foreseen that further enhancement would be achieved if the design of phase plates could be optimized to generate filament array. This simple method makes full use of the laser energy and might potentially open a new approach to remotely enhance the THz emission in air.

Abstract-Anomalous Terahertz Reflection and Scattering by Flexible and Conformal Coding Metamaterials

1. Lanju Liang¹, 
2. Meiqing Qi², 
3. Jing Yang³,
4. Xiaopeng Shen², 
5. Jiquan Zhai¹, 
6. Weizong Xu⁴, 
7. Biaobing Jin^1,5,*, 
8. Weiwei Liu^3,5,*,
9. Yijun Feng⁴, 
10. Caihong Zhang¹, 
11. Hai Lu⁴,
12. Hou-Tong Chen⁶, 
13. Lin Kang¹, 
14. Weiwei Xu¹,
15. Jian Chen^1,5, 
16. Tie Jun Cui^2,5,*, 
17. Peiheng Wu¹ and
18. Shenggang Liu^5,7

Article first published online: 30 JUN 2015

http://onlinelibrary.wiley.com/doi/10.1002/adom.201500206/abstract;jsessionid=75D9087BA4464D14DC4C0EA2F158CD85.f03t01?userIsAuthenticated=false&deniedAccessCustomisedMessage=

DOI: 10.1002/adom.201500206

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Arbitrary control of terahertz (THz) waves remains a significant challenge although it promises many important applications. Here, a method to tailor the reflection and scattering of THz waves in an anomalous manner by using 1-bit coding metamaterials is presented. Specific coding sequences result in various THz far-field reflection and scattering patterns, ranging from a single beam to two, three, and numerous beams, which depart obviously from the ordinary Snell's law of reflection. By optimizing the coding sequences, a wideband THz thin film metamaterial with extremely low specular reflection, due to the scattering of the incident wave into various directions, is demonstrated. As a result, the reflection from a flat and flexible metamaterial can be nearly uniformly distributed in the half space with small intensity at each specific direction, manifesting a diffuse reflection from a rough surface. Both simulation and experimental results show that a reflectivity less than −10 dB is achieved over a wide frequency range from 0.8 to 1.4 THz, and it is insensitive to the polarization of the incident wave. This work reveals new opportunities arising from coding metamaterials in effective manipulation of THz wave propagation and may offer widespread applications.