Friday, November 24, 2017

Abstract-Wavelength-swept fiber laser based on bidirectional used linear chirped fiber Bragg grating




Lin Wang, Minggui Wan, Zhenkun Shen, Xudong Wang, Yuan Cao, Xinhuan Feng, and Bai-ou Guan

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-5-3-219&origin=search

A wavelength-swept fiber laser is proposed and successfully demonstrated based on a bidirectional used linear chirped fiber Bragg grating (LC-FBG). The wavelength-swept operation principle is based on intracavity pulse stretching and compression. The LC-FBG can introduce equivalent positive and negative dispersion simultaneously, which enables a perfect dispersion matching to obtain wide-bandwidth mode-locking. Experimental results demonstrate a wavelength-swept fiber laser that exhibits a sweep rate of about 5.4 MHz over a 2.1 nm range at a center wavelength of 1550 nm. It has the advantages of simple configuration and perfect dispersion matching in the laser cavity.
© 2017 Chinese Laser Press

Abstract-Applications of terahertz frequency technologies in biology



R.H. Giles, Fallavi Doradla,  Jillian Martin,  C.S. Joseph

http://ieeexplore.ieee.org/document/8067014/

Over the past few decades, researchers have made significant progress on exploring the potential biomedical applications of terahertz frequency spectroscopic and imaging instrumentation as summarized by Yang et.al.1 Advances in terahertz source/receiver hardware have facilitated these developments, promising new operative screening options for meeting the standard care challenges in areas such as breast cancer.2 Since the terahertz radiation is a non-invasive and non-ionizing illumination source proven for characterizing biomolecules, terahertz imaging techniques are being broadly investigated for both in vitro and in vivo evaluations. Spectroscopic methodologies have also enabled demonstrative studies involving the chemical analysis of human breath3

Abstract-Evanescent-Wave Filtering in Images Using Remote Terahertz Structured Illumination


M. Flammini, E. Pontecorvo, V. Giliberti, C. Rizza, A. Ciattoni, M. Ortolani, and E. DelRe


Imaging with structured illumination allows for the retrieval of subwavelength features of an object by conversion of evanescent waves into propagating waves. In conditions in which the object plane and the structured-illumination plane do not coincide, this conversion process is subject to progressive filtering of the components with high spatial frequency when the distance between the two planes increases, until the diffraction-limited lateral resolution is restored when the distance exceeds the extension of evanescent waves. We study the progressive filtering of evanescent waves by developing a remote super-resolution terahertz imaging system operating at a wavelength λ=1.00mm, based on a freestanding knife edge and a reflective confocal terahertz microscope. In the images recorded with increasing knife-edge-to-object-plane distance, we observe the transition from a super-resolution of λ/1760μm to the diffraction-limited lateral resolution of Δxλ expected for our confocal microscope. The extreme nonparaxial conditions are analyzed in detail, exploiting the fact that, in the terahertz frequency range, the knife edge can be positioned at a variable subwavelength distance from the object plane. Electromagnetic simulations of radiation scattering by the knife edge reproduce the experimental super-resolution achieved.
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Absrtact-Helicity-dependent terahertz emission spectroscopy of topological insulator S b 2 T e 3 thin films



Chien-Ming Tu, Yi-Cheng Chen, Ping Huang, Pei-Yu Chuang, Ming-Yu Lin, Cheng-Maw Cheng, Jiunn-Yuan Lin, Jenh-Yih Juang, Kaung-Hsiung Wu, Jung-Chun A. Huang, Way-Faung Pong, Takayoshi Kobayashi, and Chih-Wei Luo


We report on helicity-dependent terahertz emissions that originate from the helicity-dependent photocurrents in topological insulator Sb2Te3 thin films due to ultrafast optical excitation. The polarity of the emitted terahertz radiation is controlled by both the incident angle and the helicity of optical pulses. Using an unprecedented decomposition-recombination procedure in the time domain, the signals of the Dirac fermions are fully separated from bulk contributions. These results provide insights into the optical coupling of topological surface states and open up opportunities for applying helicity-dependent terahertz emission spectroscopy in spintronics.
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Thursday, November 23, 2017

Abstract-Self-Mixing Spectra of Terahertz Emitters Based on Bi 2 Sr 2 CaCu 2 O 8 + δ Intrinsic Josephson-Junction Stacks


Ya Huang, Hancong Sun, Deyue An, Xianjing Zhou, Min Ji, Fabian Rudau, Raphael Wieland, Johannes S. Hampp, Olcay Kizilaslan, Jie Yuan, Nickolay Kinev, Oleg Kiselev, Valery P. Koshelets, Jun Li, Dieter Koelle, Reinhold Kleiner, Biaobing Jin, Jian Chen, Lin Kang, Weiwei Xu, Huabing Wang, and Peiheng Wu

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.8.054023

Josephson junctions can serve as mixers for electromagnetic radiation, producing difference frequencies |mfsnfLO| of the signal frequency fs and the local oscillator frequency fLO, where the latter can be provided by ac Josephson currents, and m and n are natural numbers. In order to obtain a better understanding of the purity of the terahertz radiation generated by stacks of intrinsic Josephson junctions (IJJs), we study self-mixing—i.e., fs is also produced by Josephson currents inside the stacks—in the difference-frequency range between 0.1 and 3.0 GHz. Simultaneously, we perform off-chip terahertz emission detection and transport measurements. We find that at high-bias currents, when a hot spot has formed in the stack, the power level of self-mixing can be low and sometimes is even absent at the terahertz emission peak, pointing to a good phase locking among all IJJs. By contrast, at low-bias currents where no hot spot exists, the self-mixing products are pronounced even if the terahertz emission peaks are strong. The mixing products at high operation temperature, at which the temperature variation within the stack is moderate, are minor, indicating that the low junction resistance, perhaps in combination with the lowered Josephson critical current density, may play a similar role for synchronization as the hot spot does at low temperature. While these observations are helpful for the task to synchronize thousands of IJJs, the observation of self-mixing in general may offer a simple method in evaluating the coherence of terahertz radiation produced by the IJJ stacks.