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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.
https://doi.org/10.1007/s12200-018-0819-8
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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Plasma characteristics of energetic liquid polymer ablated by nanosecond laser pulses
Jing QI, Siqi ZHANG, Tian LIANG, Weichong TANG, Ke XIAO, Lu GAO, Hua GAO, Zili ZHANG, Zhiyuan ZHENG
Front. Optoelectron.. 2018, 11 (3): 261-266.
https://doi.org/10.1007/s12200-018-0752-x
The plasma characteristics of carbon-doped glycidyl azide polymer (GAP) are investigated ablation by nanosecond laser pulses. For the GAP energetic liquid, a specific impulse of 840 s and an ablation efficiency up to 98% are obtained, which can be attributed to the low mass loss owing to the carbon doping. A comparison between the chemical energies shows that the carbon-doped GAP provides better propulsion than pure GAP. This indicates that even for an energetic liquid, an efficient approach to enhance the thrust performance is to reduce the splashing. High ablation thrust could be achieved at a low laser fluence and high carbon content.
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Detection of small ship targets from an optical remote sensing image
Mingzhu SONG, Hongsong QU, Guixiang ZHANG, Guang JIN
Front. Optoelectron.. 2018, 11 (3): 275-284.
https://doi.org/10.1007/s12200-018-0744-x
Detection of small ships from an optical remote sensing image plays an essential role in military and civilian fields. However, it becomes more difficult if noise dominates. To solve this issue, a method based on a low-level vision model is proposed in this paper. A global channel, high-frequency channel, and low-frequency channel are introduced before applying discrete wavelet transform, and the improved extended contrast sensitivity function is constructed by self-adaptive center-surround contrast energy and a proposed function. The saliency image is achieved by the three-channel process after inverse discrete wavelet transform, whose coefficients are weighted by the improved extended contrast sensitivity function. Experimental results show that the proposed method outperforms all competing methods with higher precision, higher recall, and higher F-score, which are 100.00%, 90.59%, and 97.96%, respectively. Furthermore, our method is robust against noise and has great potential for providing more accurate target detection in engineering applications.
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Discrete combination method based on equidistant wavelength screening and its application to near-infrared analysis of hemoglobin
Tao Pan, Bingren Yan, Jiemei Chen, Lijun Yao
Front. Optoelectron.. 2018, 11 (3): 296-305.
https://doi.org/10.1007/s12200-018-0804-2
A wavelength selection method for discrete wavelength combinations was developed based on equidistant combination-partial least squares (EC-PLS) and applied to a near-infrared (NIR) spectroscopic analysis of hemoglobin (Hb) in human peripheral blood samples. An allowable model set was established through EC-PLS on the basis of the sequence of the predicted error values. Then, the wavelengths that appeared in the allowable models were sorted, combined, and utilized for modeling, and the optimal number of wavelengths in the combinations was determined. The ideal discrete combination models were obtained by traversing the number of allowable models. The obtained optimal EC-PLS and discrete wavelength models contained 71 and 42 wavelengths, respectively. A simple and high-performance discrete model with 35 wavelengths was also established. The validation samples excluded from modeling were used to validate the three models. The root-mean-square errors for the NIR-predicted and clinically measured Hb values were 3.29, 2.86, and 2.90 g·L−1, respectively; the correlation coefficients, relative RMSEP, and ratios of performance to deviation were 0.980, 0.983, and 0.981; 2.7%, 2.3%, and 2.4%; and 4.6, 5.3, and 5.2, respectively. The three models achieved high prediction accuracy. Among them, the optimal discrete combination model performed the best and was the most effective in enhancing prediction performance and removing redundant wavelengths. The proposed optimization method for discrete wavelength combinations is applicable to NIR spectroscopic analyses of complex samples and can improve prediction performance. The proposed wavelength models can be utilized to design dedicated spectrometers for Hb and can provide a valuable reference for non-invasive Hb detection.
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9 articles
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