Tuesday, November 24, 2015

Abstract-Terahertz-bandwidth photonic temporal differentiator based on a silicon-on-isolator directional coupler

Tian Li Huang, Ao Ling Zheng, Jian Ji Dong, Ding Shan Gao, and Xin Liang Zhang


We experimentally demonstrate a terahertz-bandwidth photonic differentiator employing a silicon-on-insulator directional coupler. The integrated waveguide coupler with two identical paralleled strip waveguides achieves a first-order differentiator when full energy coupling is met from one waveguide to another. The integrated waveguide coupler can offer different operation bandwidths by changing the length and gap of the strip waveguides. Due to the large 3 dB bandwidth of the directional coupler, we implement the first differentiator with an operation bandwidth of 1.25 THz. The performance of this photonic differentiator is tested using Gaussian-like pulses with a pulsewidth of 2.8 ps, 4 ps, 6 ps, 8 ps, and 10 ps, respectively. The differentiation processing errors and relative energy efficiency are also discussed. This silicon chip may have potential applications in integrated photonic computing circuits with sub-picosecond pulses.
© 2015 Optical Society of America
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Monday, November 23, 2015

Abstract-Optimization of terahertz generation from LiNbO3 under intense laser excitation with the effect of three-photon absorption

Sen-Cheng Zhong, Zhao-Hui Zhai, Jiang Li, Li-Guo Zhu, Jun Li, Kun Meng, Qiao Liu, Liang-Hui Du, Jian-Heng Zhao, and Ze-Ren Li
We proposed a three-dimensional model to simulate terahertz generation from LiNbO3crystal under intense laser excition (up to ~50 mJ/cm2). The impact of three-photon absorption, which leads to free carrier generation and free carrier saturation (when pump fluence above ~10 mJ/cm2) on terahertz generation was investigated. And further with this model, we stated the optimized experimental conditions (incident postion, beam diameter, and pulse duration, etc) for maximum generation efficiency in commonly-used tilted-pulse-front scheme. Red shift of spectrum, spatial distribution “splitting” effects of emitted THz beam, and primilary experimental verification under intense laser excitation are given.
© 2015 Optical Society of America
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Abstract-Active substrate integrated terahertz waveguide using periodic graphene stack

The transmission properties of a substrate integrated waveguide (SIW) based on periodicgraphene stacks have been theoretically investigated in the terahertz (THz) region. The effects of the dielectric-graphene-dielectric structure of the stack on the propagation properties are shown to be significant and different from the conventional active SIW based on active components. By varying the graphenechemical potential, the cut-off frequency of the proposed waveguide can be dynamically tuned from 3 to 3.7 THz. Moreover, the tunable waveguide displays low leakage loss and single-mode propagation with −120 dB stop-band attenuation.These primary results are very promising for THz integration devices and SIW-based systems.

Abstract-Terahertz response of fractal meta-atoms based on concentric rectangular square resonators


We investigate the terahertz electromagnetic responses of fractal meta-atoms (MAs) induced by different mode coupling mechanisms. Two types of MAs based on concentric rectangular square (CRS) resonators are presented: independent CRS (I-CRS) and junctional-CRS (J-CRS). In I-CRS, each resonator works as an independent dipole so as to result in the multiple resonance modes when the fractal level is above 1. In J-CRS, however, the generated layer is rotated by π/2 radius to the adjacent CRS in one MA. The multiple resonance modes are coupled into a single mode resonance. The fractal level increasing induces resonance modesredshift in I-CRS while blueshift in J-CRS. When the fractal level is below 4, the mode Q factor of J-CRS is in between the two modes of I-CRS; when the fractal level is 4 or above, the mode Q factor of J-CRS exceeds the two modes of I-CRS. Furthermore, the modulation depth (MD) decreases in I-CRS while it increases in J-CRS with the increase in fractal levels. The surfacecurrents analysis reveals that the capacitive coupling of modes in I-CRS results in the modesredshift, while the conductive coupling of modes in J-CRS induces the mode blueshift. A high Q mode with large MD can be achieved via conductive coupling between the resonators of different scales in a fractal MA.

Manipulating transistors at terahertz frequencies


An interdisciplinary team at the Ruhr-Universität Bochum has found a way of accessing the interior of transistors. The researchers have manipulated the electron gas contained within by applying resonators to generate rhythmic oscillation in the terahertz range inside. They shared their findings in the magazine Scientific Reports.

Transistors can be manipulated not only with voltages
Used for switching and amplifying, transistors are fundamental elements of modern electronics. By applying a specific  externally to a transistor, an electric current is controlled inside, which, in turn, generates a new voltage. Compared with the externally applied voltage, the new voltage may be amplified, may oscillate or be logically connected to it. In order to interact with their surroundings via electric current and voltage, transistors contain ultra-thin electron layers, so-called 2D electron gases. The RUB team demonstrated that these gases can be controlled not only via DC and radio-frequency voltages.
Electron gas can be oscillated like jelly
"A 2D electron gas is like jelly," explains Prof Dr Andreas Wieck from the Chair for Applied Solid State Physics. "If pressure is electrically applied to the gas from above with a characteristic frequency, thickness and density oscillations are generated." Accordingly, the gas can be manipulated via electric forces, which oscillates much more rapidly than any radio or microwave frequency. As it has a thickness of just about ten nanometres, the oscillations follow the laws of quantum mechanics. This means: all occurring oscillations have a specific frequency, namely in the terahertz range, i.e. in the range of 1012 Hertz. "Pressure to the electron gas must be applied in that rapid change," elaborates Wieck. Andreas Wieck, Dr Shovon Pal, Dr Nathan Jukam and other colleagues from the workgroup Terahertz Spectroscopy and Technology as well as from the Chair of Electronic Materials and Nanoelectronics have found a way to trigger the required oscillations. Thus, a new method of accessing the interior of a transistor has been created.
Resonators generate thickness oscillations
One hundred nanometres above the electron gas, the RUB researchers evaporated an array of identical metallic resonators which can oscillate with the required fixed frequency. The electron gas was embedded in a semiconductor and could be modified via external DC voltage, namely it could be made a bit thicker or thinner. The thickness determines the frequency which makes the gas oscillate optimally. Deploying external voltage, the researchers were able to fine-tune the  to the resonators, i.e. adjust the gas so that the alternating electric pressure of the resonators excites it optimally to oscillate in the terahertz range.
Sensors for chemical and environmental technology
This method could be of interest for sensors in chemical and environmental applications, as the researchers suggest. This is because molecule oscillations typically happen in the terahertz range. With modified transistors, such  can be recorded and sensors can be developed that react to the frequencies of certain gases or liquids.
More information: Shovon Pal et al. Ultrawide electrical tuning of light matter interaction in a high electron mobility transistor structure, Scientific Reports (2015). DOI: 10.1038/srep16812