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Crystals
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Nonlinear Crystals for Terahertz Generation
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Nonlinear Crystals for Terahertz Generation

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Inorganic Crystalline Materials".

Deadline for manuscript submissions: closed (20 June 2023) | Viewed by 12983

Special Issue Editors

Dr. Kai Zhong

E-Mail Website
Guest Editor
School of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China
Interests: diode-pumped solid-state lasers; nonlinear optical frequency conversion; terahertz photonics and applications.
Dr. Xinyuan Zhang

E-Mail Website
Guest Editor
Institute of Functional Crystals, Tianjin University of Technology, Tianjin 300384, China
Interests: nonlinear optical crystals; structure design; crystal growth.
Dr. Pengxiang Liu

E-Mail Website
Guest Editor
Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110169, China
Interests: nonlinear optics; coherent terahertz source; terahertz and mid-infrared up-conversion detection.

Special Issue Information

Dear Colleagues,

Terahertz technology has attracted considerable interest and made significant progress in the past two decades due to its wide applications, ranging from basic research to industry uses. As one of the most important parts, terahertz sources used to be challenging for satisfying various requirements. Nowadays, there is a selection of good options for coherent terahertz generation, owing to the developments in both electronics and optics. Among these methods, those based on nonlinear optics are particularly valuable because they cover the entire terahertz range from 0.1 to 10 THz.

Nonlinear optical (NLO) methods rely on nonlinear crystals to convert laser to terahertz waves via difference frequency generation (DFG), stimulated polariton scattering (SPS) and optical rectification (OR). To achieve efficient and high-power terahertz sources, nonlinear crystals for terahertz generation should exhibit superior growth, optical and also mechanical properties. Moreover, the laser damage threshold, transparency and capability of phase-matching are especially concerned. Available nonlinear materials for this purpose include inorganic crystals, such as traditional infrared (GaSe and ZnGeP2), ferroelectric (LiNbO3 and LiTaO3), and zinc-blende crystals (GaAs, GaP and ZnTe), as well as KTiOPO4-type and organic crystals, such as salts (DAST, DSTMS) and hydrogen-bonded crystals (OH1, BNA). New materials and novel crystalline morphologies (microstructures, waveguides, thin films, etc.) keep emerging and improving the performance of NLO-based terahertz sources.

The present Special Issue on “Nonlinear Crystals for Terahertz Generation” is dedicated to collecting reviews and original research papers in this field. This scope includes, but is not limited to, the design, synthesis, growth and characterization of nonlinear crystals, as well as their applications in terahertz generation.

Dr. Kai Zhong
Dr. Xinyuan Zhang
Dr. Pengxiang Liu
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Crystals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • nonlinear crystals
  • crystal growth
  • crystal characterization
  • inorganic crystals
  • organic crystals
  • difference frequency generation
  • stimulated polariton scattering
  • optical rectification
  • terahertz photonics

Published Papers (7 papers)

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Research

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10 pages, 2526 KiB  
Article
Luminescence Properties of Fe2+:ZnSe Single Crystals Grown via a Traveling Heater Method
by Weina Nan, Da Yang, Boru Zhou, Liang Zhang, Jing Xiao, Hongwei Yu, Zhanggui Hu and Yicheng Wu
Crystals 2023, 13(3), 411; https://doi.org/10.3390/cryst13030411 - 27 Feb 2023
Viewed by 1341
Abstract
The luminescence properties of iron-doped ZnSe (Fe2+:ZnSe) single crystals grown via a traveling heater method have been studied via photoluminescence (PL). Nine emission bands were identified in the PL spectra of Fe2+:ZnSe single crystals and their origins were also [...] Read more.
The luminescence properties of iron-doped ZnSe (Fe2+:ZnSe) single crystals grown via a traveling heater method have been studied via photoluminescence (PL). Nine emission bands were identified in the PL spectra of Fe2+:ZnSe single crystals and their origins were also discussed. The near-infrared emission bands seen at 820 nm and 978 nm can be attributed to the emission bands formed by the background Fe or other impurity-related defect complexes in Fe2+:ZnSe single crystals, rather than by doped transition-metal-related defects. With the increase in temperature, the PL intensity increased slightly and reached a maximum near room temperature for bound excitons (430–490 nm), but the PL intensity decreased significantly for impurity-defect emission bands (500–720 nm), indicating the occurrence of a thermal quenching effect. The excitation wavelength-dependent PL spectra showed that PL intensity first increased and then decreased with an increase in the excitation wavelengths, and the maximum PL intensity of the bound excitons was obtained at 364 nm. In addition, the X-ray photoelectron spectroscopy (XPS) results showed that both bivalent and trivalent iron ions were found, but bivalence was the dominant charge state for iron atoms in the iron-doped ZnSe single crystals, meaning that they are suitable for developing mid-infrared gain medium applications. Full article
(This article belongs to the Special Issue Nonlinear Crystals for Terahertz Generation)
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8 pages, 3255 KiB  
Article
Dual Optical Frequency Comb Generation with Dual Cascaded Difference Frequency Generation
by Yanli Yin, Kaiwu Wang, Gege Zhang, Zhongyang Li, Pibin Bing, Sheng Yuan, Anfu Zhu, Degang Xu and Jianquan Yao
Crystals 2022, 12(10), 1392; https://doi.org/10.3390/cryst12101392 - 01 Oct 2022
Cited by 1 | Viewed by 1204
Abstract
In this work, we propose a novel dual optical frequency comb (DOFC) generation scheme based on dual cascaded difference frequency generation (DCDFG). Feasible designs are introduced that enable the two sets of cascaded optical waves, initially generated by DCDFG in an aperiodically periodically [...] Read more.
In this work, we propose a novel dual optical frequency comb (DOFC) generation scheme based on dual cascaded difference frequency generation (DCDFG). Feasible designs are introduced that enable the two sets of cascaded optical waves, initially generated by DCDFG in an aperiodically periodically poled lithium niobate (APPLN) crystal with a pump wave and two signal waves, then transferred to high-order Stokes waves by oscillations of cascaded Stokes waves and the optimization of phase mismatching of each-order DCDFG; finally, a DOFC was constructed. We demonstrate a high-performance DOFC with characteristics of high repetition frequency difference, tunable repetition frequency difference, high flatness, and a tunable spectral distribution range by providing a theoretical framework. We argue that the scheme proposed in this work is promising for achieving a high-quality DOFC. Full article
(This article belongs to the Special Issue Nonlinear Crystals for Terahertz Generation)
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12 pages, 3673 KiB  
Article
Study of Optical Rectification in Polycrystalline Materials Based on Random Quasi-Phase Matching
by Sijia Wang, Kai Zhong, Hongzhan Qiao, Fangjie Li, Jining Li, Degang Xu and Jianquan Yao
Crystals 2022, 12(9), 1188; https://doi.org/10.3390/cryst12091188 - 24 Aug 2022
Viewed by 1407
Abstract
Random quasi-phase matching (RQPM) in polycrystalline materials has been considered as an important technique for nonlinear optical frequency conversion, especially in wideband laser generation involving femtosecond lasers. In this paper, optical rectification (OR) based on RQPM in ZnSe ceramics was investigated for the [...] Read more.
Random quasi-phase matching (RQPM) in polycrystalline materials has been considered as an important technique for nonlinear optical frequency conversion, especially in wideband laser generation involving femtosecond lasers. In this paper, optical rectification (OR) based on RQPM in ZnSe ceramics was investigated for the first time, which could convert ultrashort optical pulses to broadband terahertz waves. A theoretical model of RQPM OR was built, with which the effects of material grain size, thickness, pump wavelength and pump pulse duration on terahertz generation, were simulated and discussed. It was found that RQPM OR in ZnSe is indeed effective in terahertz generation but insensitive to the material grain size and pump wavelength RQPM OR is even better than traditional single crystals such as ZnTe in bandwidth and robustness. The results can be a reference to optimize the RQPM OR nonlinear process, and hopefully, this work will promote the application of costless polycrystalline materials in the hot field of terahertz generation. Full article
(This article belongs to the Special Issue Nonlinear Crystals for Terahertz Generation)
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10 pages, 4737 KiB  
Article
Wideband Collinear Phase Matching in Cubic Semiconductors via the Linear Electro-Optic Effect: A Theoretical Study
by Kai Zhong, Fangjie Li, Hongzhan Qiao, Xianzhong Zhang, Degang Xu and Jianquan Yao
Crystals 2022, 12(6), 764; https://doi.org/10.3390/cryst12060764 - 26 May 2022
Cited by 1 | Viewed by 1399
Abstract
In order to achieve collinear phase-matched nonlinear optical frequency conversion in cubic crystals, a novel method to induce and modulate the birefringence based on the linear electro-optic effect was studied. Taking terahertz generation with ZnTe and CdTe crystals of the [...] Read more.
In order to achieve collinear phase-matched nonlinear optical frequency conversion in cubic crystals, a novel method to induce and modulate the birefringence based on the linear electro-optic effect was studied. Taking terahertz generation with ZnTe and CdTe crystals of the 4¯3m point group as an example, an external electric field provided the freedom to realize perfect phase matching (PM) in a wide bandwidth up to 2 THz for difference frequency generation, with monochromatic optical waves at around 800 nm and 1010 nm, respectively. Theoretical simulations showed that such a method helps to extend the terahertz frequency, enhance the conversion efficiency, and alleviate the limitation on the pump wavelength, which is highly favorable for nonlinear optical uses of cubic crystals. Simultaneous wideband terahertz generation pumped by ultrashort laser pulses via optical rectification or difference frequency mixing was also discussed, indicating that an electric field can be used to modulate the PM characteristics without changing the group velocity-matching condition. Tuning the nonlinear interaction by an applied voltage is fast, robust, and convenient compared to other approaches. In addition, the linear electro-optic effect can make the nonlinear crystal a compact and high-speed terahertz amplitude modulator, which has great potential in radar, communication, imaging, etc. Full article
(This article belongs to the Special Issue Nonlinear Crystals for Terahertz Generation)
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Review

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19 pages, 21040 KiB  
Review
Organic Nonlinear Optical Crystals for Highly Efficient Terahertz-Wave Generation
by Ying Yang, Xinyuan Zhang, Zhanggui Hu and Yicheng Wu
Crystals 2023, 13(1), 144; https://doi.org/10.3390/cryst13010144 - 13 Jan 2023
Cited by 7 | Viewed by 2107
Abstract
Terahertz (THz) technology is an emerging field that is promising for future applications. Nonlinear optical (NLO) materials can effectively convert incident light into the THz frequency range using optics methods. Ionic-type organic π-conjugated NLO crystals containing electron donor-π-acceptor motifs have long attracted attention [...] Read more.
Terahertz (THz) technology is an emerging field that is promising for future applications. Nonlinear optical (NLO) materials can effectively convert incident light into the THz frequency range using optics methods. Ionic-type organic π-conjugated NLO crystals containing electron donor-π-acceptor motifs have long attracted attention for their possibility to achieve large nonlinear optical coefficients. In this paper, an overview of the recent progress in ionic-type organic NLO crystals for highly efficient THz wave generation is presented. The substitution design strategies of cations and anions, for increasing optical nonlinearities and reducing absorptions in different structure series, are summarized. In addition, the progress in crystal growth and their THz output performance are also discussed. Full article
(This article belongs to the Special Issue Nonlinear Crystals for Terahertz Generation)
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14 pages, 6509 KiB  
Review
Spintronics Based Terahertz Sources
by Maorong Wang, Yifan Zhang, Leilei Guo, Mengqi Lv, Peng Wang and Xia Wang
Crystals 2022, 12(11), 1661; https://doi.org/10.3390/cryst12111661 - 18 Nov 2022
Cited by 3 | Viewed by 2557
Abstract
Terahertz (THz) sources, covering a range from about 0.1 to 10 THz, are key devices for applying terahertz technology. Spintronics-based THz sources, with the advantages of low cost, ultra-broadband, high efficiency, and tunable polarization, have attracted a great deal of attention recently. This [...] Read more.
Terahertz (THz) sources, covering a range from about 0.1 to 10 THz, are key devices for applying terahertz technology. Spintronics-based THz sources, with the advantages of low cost, ultra-broadband, high efficiency, and tunable polarization, have attracted a great deal of attention recently. This paper reviews the emission mechanism, experimental implementation, performance optimization, manipulation, and applications of spintronic THz sources. The recent advances and existing problems in spintronic THz sources are fully present and discussed. This review is expected to be an introduction of spintronic terahertz sources for novices in this field, as well as a comprehensive reference for experienced researchers. Full article
(This article belongs to the Special Issue Nonlinear Crystals for Terahertz Generation)
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15 pages, 1535 KiB  
Review
Phase-Matching in Nonlinear Crystal-Based Monochromatic Terahertz-Wave Generation
by Pengxiang Liu, Chuncao Niu, Feng Qi, Wei Li, Weifan Li, Qiaoqiao Fu, Liyuan Guo and Zhongyang Li
Crystals 2022, 12(9), 1231; https://doi.org/10.3390/cryst12091231 - 01 Sep 2022
Cited by 3 | Viewed by 2106
Abstract
Optically pumped nonlinear frequency down conversion is a proven approach for monochromatic terahertz (THz)-wave generation that provides superior properties such as continuous and wide tunability as well as laser-like linewidth and beam quality. Phase-matching (PM) is an important connection between the pump sources [...] Read more.
Optically pumped nonlinear frequency down conversion is a proven approach for monochromatic terahertz (THz)-wave generation that provides superior properties such as continuous and wide tunability as well as laser-like linewidth and beam quality. Phase-matching (PM) is an important connection between the pump sources and nonlinear crystals and determines the direction of energy flow (as well as the output power). In past decades, a variety of peculiar PM configurations in the THz region have been invented and are different from the traditional ones in the optical region. We summarize the configurations that have been applied in nonlinear THz-wave generation, which mainly fall in two categories: scalar (collinear) PM and vector PM (including macroscopic noncollinear PM and microscopic vector PM). The development of this technique could relax the matching conditions in a wide range of nonlinear crystals and pump wavelengths and could finally promote the improvement of coherent THz sources. Full article
(This article belongs to the Special Issue Nonlinear Crystals for Terahertz Generation)
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