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9. Joint Hardware Design and Capacity Analysis for Intelligent Reflecting Surface Enabled Terahertz MIMO Communications

Author

Ma, Xinying, Chen, Zhi, Yan, Longfei, Han, Chong, and Wen, Qiye

Subjects
Computer Science - Information Theory, Computer Science - Emerging Technologies, Electrical Engineering and Systems Science - Signal Processing
Abstract

Terahertz (THz) communications have been envisioned as a promising enabler to provide ultra-high data transmission for sixth generation (6G) wireless networks. To tackle the blockage vulnerability brought by severe path attenuation and poor diffraction of THz waves, an intelligent reflecting surface (IRS) is put forward to smartly control the incident THz waves by adjusting the phase shifts. In this paper, we firstly design an efficient hardware structure of graphene-based IRS with phase response up to 306.82 degrees. Subsequently, to characterize the capacity of the IRS-enabled THz multiple-input multiple-output (MIMO) system, an adaptive gradient descent (A-GD) algorithm is developed by dynamically updating the step size during the iterative process, which is determined by the second-order Taylor expansion formulation. In contrast with conventional gradient descent (C-GD) algorithm with fixed step size, the A-GD algorithm evidently improves the achievable rate performance. However, both A-GD algorithm and C-GD algorithm inherit the unacceptable complexity. Then a low complexity alternating optimization (AO) algorithm is proposed by alternately optimizing the precoding matrix by a column-by-column (CBC) algorithm and the phase shift matrix of the IRS by a linear search algorithm. Ultimately, the numerical results demonstrate the effectiveness of the designed hardware structure and the considered algorithms.

Published
2020

31. Nanoscale Ga/Al substituted yttrium iron garnet films by liquid phase epitaxy.

Author

Zhang, Yuanjing, Xue, Qiang, Wang, Feng, Li, Han, Li, Zhuo, Yang, Shuting, Liu, Yingli, Hu, Aimin, Jin, Lichuan, Wen, Qiye, Zhang, Huaiwu, and Yang, Qinghui

Subjects
YTTRIUM iron garnet, LIQUID phase epitaxy, LIQUID films, ENERGY levels (Quantum mechanics), FERROMAGNETIC resonance
Abstract

Yttrium iron garnet (YIG) has minimum damping factor and low ferromagnetic resonance (FMR) linewidth, making it a preferred material for low loss microwave and spintronic devices. The saturation magnetization of YIG is 1750 Gauss, and for low-frequency devices, a lower saturation magnetization is more suitable. Ga3+ and Al3+ are with smaller radii and non-magnetic moment, so the substitution of Ga3+ and Al3+ can decrease saturation magnetization. Here, 4.8–193.7 nm ultra-thin Y3(GaAlFe)5O12 garnet (GaAl-YIG) monocrystalline films are prepared on gadolinium gallium garnet (GGG) substrates by using the liquid-phase epitaxy (LPE) method. As expected, these films exhibit a low saturation magnetization of almost less than 100 Gauss, while their FMR linewidth remains at levels close to that of YIG. The films show a (111) orientation and in a state of tension, and the diffraction intensity of the films get stronger as films thickness increases. The free energy and density of states are calculated for different Ga/Al substitution position by density functional theory simulations. The elements show a different diffusion distance in the GaAl-YIG/GGG interface, and the variation of magnetization properties with interface width are analyzed. The surface roughness of the films is only a few angstroms. The damping factor of these ultra-thin films are on an order of 10−4 except the 4.8 nm film, which suggests that the minimum thickness of the garnet film with good performance by using the LPE method is about 10 nm. According to the analysis of structure and magnetization properties, it demonstrates that the LPE method has potential to provide nanoscale garnet films for low loss microwave and spintronic devices. [ABSTRACT FROM AUTHOR]

Published
2024
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32. Terahertz Magnetic Moiré Grating for Enhanced Chirality Manipulation and Nonreciprocal Spin Isolation.

Author

Zhao, Dan, Fan, Fei, Xue, Qiang, Wang, Hao, Ji, Yunyun, Yang, Qinghui, Wen, Qiye, and Chang, Shengjiang

Subjects
YTTRIUM iron garnet, OPTICAL rotation, TIME reversal, MIRROR images, CIRCULAR dichroism
Abstract

Magneto‐optical activity is a significant way to achieve asymmetric transmission for spin photons. However, it is severely limited by the weak light‐matter interaction in the terahertz (THz) region. In this work, a magnetic Moiré grating (MMG) is constructed by sandwiching an Yttrium iron garnet (YIG) single crystal in a pair of twisted 2D subwavelength gratings. The twisted angle between two gratings introduces a strong geometric Moiré chirality related to the Moiré pattern. When an external magnetic field is applied to the YIG layer, three different symmetries of spacial mirror image, time reversal, and spin conjugation are broken simultaneously under the chiral coupling between magneto‐optical activity and Moiré chirality, leading to both the enhanced magneto‐optical chiral transmission and the nonreciprocal spin isolation. Under the magnetic field varying from −0.26 to +0.26 T in the experiment, the MMG achieves an active modulation of circular dichroism from 39 to −37.5 dB and optical activity from −35° to 30°. Moreover, the maximum spin isolation over 32.6 dB is also obtained near 0.5 THz. This mechanism provide a new way to realize independent manipulation and active modulation of THz spin photons more flexibly. [ABSTRACT FROM AUTHOR]

Published
2024
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36. Switchable Broadband Terahertz Absorbers Based on Conducting Polymer-Cellulose Aerogels

Author

Kuang, Chaoyang, Chen, Shangzhi, Luo, Min, Zhang, Qilun, Sun, Xiao, Han, Shaobo, Wang, Qingqing, Stanishev, Vallery, Darakchieva, Vanya, Crispin, Reverant, Fahlman, Mats, Zhao, Dan, Wen, Qiye, Jonsson, Magnus, Kuang, Chaoyang, Chen, Shangzhi, Luo, Min, Zhang, Qilun, Sun, Xiao, Han, Shaobo, Wang, Qingqing, Stanishev, Vallery, Darakchieva, Vanya, Crispin, Reverant, Fahlman, Mats, Zhao, Dan, Wen, Qiye, and Jonsson, Magnus

Abstract

Terahertz (THz) technologies provide opportunities ranging from calibration targets for satellites and telescopes to communication devices and biomedical imaging systems. A main component will be broadband THz absorbers with switchability. However, optically switchable materials in THz are scarce and their modulation is mostly available at narrow bandwidths. Realizing materials with large and broadband modulation in absorption or transmission forms a critical challenge. This study demonstrates that conducting polymer-cellulose aerogels can provide modulation of broadband THz light with large modulation range from approximate to 13% to 91% absolute transmission, while maintaining specular reflection loss < -30 dB. The exceptional THz modulation is associated with the anomalous optical conductivity peak of conducting polymers, which enhances the absorption in its oxidized state. The study also demonstrates the possibility to reduce the surface hydrophilicity by simple chemical modifications, and shows that broadband absorption of the aerogels at optical frequencies enables de-frosting by solar-induced heating. These low-cost, aqueous solution-processable, sustainable, and bio-friendly aerogels may find use in next-generation intelligent THz devices., Funding Agencies|Swedish Research Council(VR); Swedish Foundation for Strategic Research (SSF); Stellenbosch Institute for Advanced Study (STIAS); Swedish Foundation for International Cooperation in Research and Higher Education (STINT); Knut and Alice Wallenberg Foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkping University [61831012]; National Natural Science Foundation of China (NFSC) [62131006, 52003202, 2023YFB381130005, 2021JDTD0026, 2023JDGD0012]; National Key Research and Development Program of China [2019-252]; Sichuan Province Science and Technology Support Program [2020-00287, 2022-00211]; Guangdong Science and Technology Major Special Fund [2022-06214]; Wallenberg Wood Science Center; [2009 00971]; [62311530115]; [62235004]

Published
2024
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43. Switchable Broadband Terahertz Absorbers Based on Conducting Polymer‐Cellulose Aerogels

Author

Kuang, Chaoyang, primary, Chen, Shangzhi, additional, Luo, Min, additional, Zhang, Qilun, additional, Sun, Xiao, additional, Han, Shaobo, additional, Wang, Qingqing, additional, Stanishev, Vallery, additional, Darakchieva, Vanya, additional, Crispin, Reverant, additional, Fahlman, Mats, additional, Zhao, Dan, additional, Wen, Qiye, additional, and Jonsson, Magnus P., additional

Published
2023
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49. Gradient-Reduced Graphene Oxide Aerogel with Ultrabroadband Absorption from Microwave to Terahertz Bands

Author

Sun, Xiao, Chen, Shangzhi, Kuang, Chaoyang, Fu, Wenjie, Li, Xuesong, Duan, Zhaoyun, Wen, Qiye, Sun, Xiao, Chen, Shangzhi, Kuang, Chaoyang, Fu, Wenjie, Li, Xuesong, Duan, Zhaoyun, and Wen, Qiye

Abstract

Ultrabroadband electromagnetic (EM) absorbers, especially those covering microwave to terahertz (THz) bands, are urgently desired in multispectral applications such as 6G communication, radar stealth, atmospheric remote sensing, and radio astronomy. Here, we demonstrate that chemically reduced graphene oxide aerogels can be designed as an excellent absorber with the features of ultrabroadband, light weight, compressibility, and high-temperature resistance. This magnetic-free pyramidal absorber shows remarkably broad qualified absorption bandwidth from 4.7 GHz to 4 THz, with reflection loss < -20 dB in the microwave and < -40 dB in the THz band. Especially, an unprecedentedly excellent average absorption intensity of -53.9 dB (absorptivity over 99.999%) is obtained in the frequency range from 0.5 to 4 THz. We experimentally clarify that the gradient macrostructure together with the porous microstructure underlies the continuous impedance matching in such a large frequency range spanning about 3 orders of magnitude and leads to the consecutive strong EM absorption from microwave to terahertz. We believe that this absorber will offer multifunctional and multispectral applications in many scientific and technological fields., Funding Agencies|National Natural Science Foundation of China [62235004, 61831012, 62131006]; Sichuan Science and Technology Support Program [2021JDTD0026]; Shenzhen Science and Technology Program [(2021) 105]; Swedish Research Council [2022-00211]

Published
2023
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50. Investigation of nonlinear optical properties in α-A2BB′O6 (A = Li, Na, K; B = Ti, Zr, Hf; B′ = Se, Te) by first-principles calculations.

Author

Fang, Gaojing, Teng, Xiaojun, Yan, Luo, Wu, Yu, Xue, Kui, Zhang, Xiaofeng, Ding, Yi-min, Zhou, Liujiang, and Wen, Qiye

Abstract

Nonlinear optical (NLO) crystals based on oxides typically have wide bandgaps and large laser damage thresholds (LDTs), which are important for generating high-power and continuous terahertz radiation. Recently, a new family of NLO materials α-A2BB′O6 including Li2TiTeO6 (LTTO) with a strong second harmonic generation (SHG) efficiency of 26 × KH2PO4 (KDP) and a large LDT of 550 MW cm−2 were reported. Herein, we systematically study the electronic structures and NLO properties of α-A2BB′O6 (A = Li, Na, K; B = Ti, Zr, Hf; B′ = Se, Te) to explore the relationship between the structure and SHG coefficient. First, 15 members of the A2BB′O6 family are demonstrated to be highly stable and NLO materials, excluding K2TiTeO6, K2TiSeO6 and K2ZrSeO6. Then, the electronic band structure, dipole moment and distortion of BO6/B′O6 octahedrons, SHG coefficient and terahertz absorption spectrum are calculated comprehensively with the element variation of A-site, B-site and B′-site. Finally, the magnitude of the SHG coefficient is found to be directly proportional to the value of total dipole moment and distortion, and inversely proportional to the bandgap value. Most importantly, among the A2BB′O6 materials, K2HfSeO6 shows the smallest direct bandgap of 2.99 eV, the largest SHG coefficient d33 of about 5 × LTTO and low terahertz absorbance from 0.1 to 9 THz. Our results provide new NLO crystals that may have potential application in terahertz radiation sources and other nonlinear electronics. [ABSTRACT FROM AUTHOR]

Published
2024
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