Frontiers of Optoelectronics

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Generation and detection of pulsed terahertz waves in gas: from elongated plasmas to microplasmas
Fabrizio BUCCHERI, Pingjie HUANG, Xi-Cheng ZHANG
Front. Optoelectron.    2018, 11 (3): 209-244.
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The past two decades have seen an exponential growth of interest in one of the least explored region of the electromagnetic spectrum, the terahertz (THz) frequency band, ranging from to 0.1 to 10 THz. Once only the realm of astrophysicists studying the background radiation of the universe, THz waves have become little by little relevant in the most diverse fields, such as medical imaging, industrial inspection, remote sensing, fundamental science, and so on. Remarkably, THz wave radiation can be generated and detected by using ambient air as the source and the sensor. This is accomplished by creating plasma under the illumination of intense femtosecond laser fields. The integration of such a plasma source and sensor in THz time-domain techniques allows spectral measurements covering the whole THz gap (0.1 to 10 THz), further increasing the impact of this scientific tool in the study of the four states of matter.

In this review, the authors introduce a new paradigm for implementing THz plasma techniques. Specifically, we replaced the use of elongated plasmas, ranging from few mm to several cm, with sub-mm plasmas, which will be referred to as microplasmas, obtained by focusing ultrafast laser pulses with high numerical aperture optics (NA from 0.1 to 0.9).

The experimental study of the THz emission and detection from laser-induced plasmas of submillimeter size are presented. Regarding the microplasma source, one of the interesting phenomena is that the main direction of THz wave emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. Perhaps the most important achievement is the demonstration that laser pulse energies lower than 1 mJ are sufficient to generate measurable THz pulses from ambient air, thus reducing the required laser energy requirement of two orders of magnitude compared to the state of art. This significant decrease in the required laser energy will make plasma-based THz techniques more accessible to the scientific community, as well as opening new potential industrial applications.

Finally, experimental observations of THz radiation detection with microplasmas are also presented. As fully coherent detection was not achieved in this work, the results presented herein are to be considered a first step to understand the peculiarities involved in using the microplasma as a THz sensor.

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A special issue on Photonics Research in Canada
Jianping YAO, Lawrence R. CHEN
Front. Optoelectron.    2018, 11 (2): 105-106.
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Inkjet printing for electroluminescent devices: emissive materials, film formation, and display prototypes
Luhua LAN, Jianhua ZOU, Congbiao JIANG, Benchang LIU, Lei WANG, Junbiao PENG
Front. Optoelectron.    2017, 10 (4): 329-352.
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Inkjet printing (IJP) is a versatile technique for realizing high-accuracy patterns in a cost-effective manner. It is considered to be one of the most promising candidates to replace the expensive thermal evaporation technique, which is hindered by the difficulty of fabricating low-cost, large electroluminescent devices, such as organic light-emitting diodes (OLEDs) and quantum dot light-emitting diodes (QLEDs). In this invited review, we first introduce the recent progress of some printable emissive materials, including polymers, small molecules, and inorganic colloidal quantum dot emitters in OLEDs and QLEDs. Subsequently, we focus on the key factors that influence film formation. By exploring stable ink formulation, selecting print parameters, and implementing droplet deposition control, a uniform film can be obtained, which in turn improves the device performance. Finally, a series of impressive inkjet-printed OLEDs and QLEDs prototype display panels are summarized, suggesting a promising future for IJP in the fabrication of large and high-resolution flat panel displays.

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A special issue on Biophotonics in Europe
Valery V. TUCHIN, Ekaterina BORISOVA, Małgorzata JĘDRZEJEWSKA-SZCZERSKA, Martin J. LEAHY, Francesco S. PAVONE, Jürgen POPP, Jose POZO
Front. Optoelectron.    2017, 10 (3): 203-210.
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Hole-transporting layer-free inverted planar mixed lead-tin perovskite-based solar cells
Yuqin LIAO, Xianyuan JIANG, Wenjia ZHOU, Zhifang SHI, Binghan LI, Qixi MI, Zhijun NING
Front. Optoelectron.    2017, 10 (2): 103-110.
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Mixed lead-tin (Pb-Sn) perovskites present a promising strategy to extend the light-harvesting range of perovskite-based solar cells (PSCs). The use of electron-transporting layer or hole-transporting layer (HTL) is critical to achieve high device efficiency. This strategy, however, requires tedious layer-by-layer fabrication as well as high-temperature annealing for certain oxides. In this work, we fabricated HTL-free planar FAPb0.5Sn0.5I3 PSCs with the highest efficiency of 7.94%. High short-circuit current density of 23.13 mA/cm2 was attained, indicating effective charge extraction at the ITO/FAPb0.5Sn0.5I3 interface. This finding provides an alternative strategy to simplify the manufacture of single-junction or tandem PSCs.

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Retinal projection head-mounted display
Junguo LIN,Dewen CHENG,Cheng YAO,Yongtian WANG
Front. Optoelectron.    2017, 10 (1): 1-8.
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Retinal projection displays (RPDs) are an important development direction for head-mounted displays (HMDs). This paper reviews the literature on optical engineering aspects based on the data on advanced technology in RPD design and development. The review includes the principles and applications of four theories, e.g., the Maxwellian view and its modified modality and the monocular and binocular depth cues of stereoscopic objects in the physiology of the human visual system. To support the Maxwellian view and achieve retinal projection systems with depth cues, results of previous design works were summarized using different methods and their advantages and disadvantages are analyzed. With an extremely long focal depth, a prototype of a full-color stereoscopic see-through RPD system was discussed. Finally, a brief outlook of the future development trends and applications of the RPDs was presented

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A tutorial introduction to graphene-microfiber waveguide and its applications
Xiaoying HE,Min XU,Xiangchao ZHANG,Hao ZHANG
Front. Optoelectron.    2016, 9 (4): 535-543.
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Graphene-microfiber with the advantage of graphene material and the microfiber has been hailed as a wonderful waveguide in optics. A tutorial introduction to the graphene-microfiber (GMF) waveguides including the effect of graphene on waveguide, fabrication and applications has been presented. Here, we reviewed recent progress in the graphene waveguides from mode-locking and Q-switching in fiber laser to gas sensing and optical modulation. A brief outlook for opportunities and challenges of GMF in the future has been presented. With the novel nanotechnology emerging, GMF could offer new possibilities for future-optic circuits, systems and networks.

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