International Journal of Terahertz Science and Technology
Vol.3, No.3, September 2010. PP.97-148 (5)
date2010-09-29 22:30:03 Click No.5918

TST, Vol. 3, No. 3, PP. 97-108

Dynamic Characteristics of III-V and IV-IV Semiconductor Based Transit Time Devices in the Terahertz Regime: A Comparative Analysis

1Moumita Mukherjee, 2Soumen Banerjee and 3J. P. Banerjee
1,3Centre of Millimeter-wave Semiconductor Devices & Systems,
Institute of Radio Physics & Electronics, University of Calcutta
1, Girish Vidya Ratna Lane, Kolkata-700009, West Bengal, India.
2Hooghly Engineering & Technology College, West Bengal University of Technology,
Vivekananda Road, Pipulpati, Hooghly, West Bengal, India.

(Received July 29, 2010; accepted September 20, 2010)

Abstract: High-frequency characteristics of IMPATT Oscillators based on III-V Indium Phosphide (InP), III-V Wurtzite Gallium Nitride (Wz-GaN) and IV-IV Silicon Carbide (3C, 4H and 6H polytypes) are studied and their performances are compared at a frequency of 0.3THz. A double drift p+ p n n+ structure is chosen for all the materials and investigation has been carried out to obtain maximum conversion efficiency and device negative resistance by optimizing bias current density through modeling and simulation technique. A double iterative computer method based on modified drift-diffusion model has been used to study their performance. The simulation studies reveal that these devices are potential sources for generating high power in the Terahertz regime. The avalanche response time for each of the IMPATT diodes are simulated by a newly developed simulation technique and it has been observed that avalanche response time in InP, GaN and SiC IMPATTs are less than the corresponding transit time of the diodes- which is an essential condition to generate THz oscillation in these devices. The conversion efficiency of III-V InP IMPATT diode is found to be 18.4% at 0.3THz with an output power of 2.81W, whereas, III-V Wz-GaN IMPATT is found to generate much higher output power of 6.23W with a conversion efficiency of 15.47% at 0.3THz. On the other side, IMPATTs based on IV-IV SiC generate output power of 11.5W (3C-SiC), 20W (4H-SiC) and 7.5W (6H-SiC) with corresponding conversion efficiencies of 12.5% (3C-SiC), 15% (4H-SiC) and 12% (6H-SiC) at 0.3THz. The extensive simulation results reveal that though IMPATT diode based on SiC gives better performance in terms of higher output power in the Terahertz domain compared to other materials, GaN IMPATT has the advantage of higher efficiency. The design data and the proposed fabrication methodology, presented in this paper, will be helpful to realize InP, SiC and GaN based IMPATT oscillators for Terahertz communication.

Keywords: Terahertz frequency, Indium Phosphide, Silicon Carbide, Gallium Nitride, Double Drift Impatt Diode, Wide band gap semiconductors, Avalanche response time, Transit Time limitation, Bias current optimization.

doi: 10.11906/TST.097-108.2010.09.10

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TST, Vol. 3, No. 3, PP. 109-116

Superlattice Electronic Devices as Submillimeter-Wave Sources

Heribert Eisele*, Suraj P. Khanna, and Edmund H. Linfield
Institute of Microwaves and Photonics, School of Electronic and Electrical Engineering
University of Leeds, Leeds LS2 9JT, United Kingdom
(Received July 03, 2010; accepted September 27, 2010)

Abstract: Negative differential resistance devices were fabricated from four epitaxial wafers with different GaAs/AlAs superlattices and evaluated in resonant-cap full-height WR-15 and WR-10 waveguide cavities. These devices on integral heat sinks generated output powers in the fundamental mode between 62C108GHz. The best RF powers (and their corresponding dc-to-RF conversion efficiencies) were 58mW (3.5%) at 66GHz, 42mW (2.6%) at 78 GHz, and 28 mW (1.8%) at 94GHz. The RF power of 15mW at 101GHz constitutes a 30-fold improvement over previous results; the highest fundamental oscillation frequency was 108GHz. In a second-harmonic mode, one device yielded 2.0mW at 216GHz, the highest second-harmonic frequency to date for a GaAs/AlAs superlattice.

Keywords: Harmonic operation, Millimeter-wave oscillator, Negative differential resistance device, Spectral purity, Superlattice

doi: 10.11906/TST.109-116.2010.09.11

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TST, Vol. 3, No. 3, PP. 117-129

Wavelet-Based Dimensionality Reduction for Hyperspectral THz Imaging

Henrike Stephani*1 , Michael Herrmann2 , Frank Bauer3 , and Bettina Heise3 
1Fraunhofer ITWM and Technical University, Kaiserslautern, Germany
Address: Fraunhofer Platz 1, 67663 Kaiserslautern, Germany
2Fraunhofer IPM, Kaiserslautern, Germany, 3Johannes Kepler University, Linz, Austria

(Received July 15, 2010; accepted September 20, 2010)

Abstract: With terahertz time-domain spectroscopy, hyperspectral images can be acquired where each pixel contains a full spectrum of the range of several terahertz (THz). An enormous amount of data is generated. Therefore, advanced methods for automated data analysis and image processing are required. We present a wavelet-based approach for channel reduction and feature selection for a subsequent clustering leading to an image segmentation. The main focus of our method is set on the appropriate dimensionality reduction adapted to the THz spectral characteristics of the samples under investigation. A feature reduction to less than 5% is achieved, thereby enabling a channel-wise image processing on the reduced data set. Furthermore, unsupervised classification is chosen for an automatized segmentation including all channel information represented in the wavelet domain. Relevant characteristics of the THz spectra are preserved by our feature selection, in particular the distribution of the peak position and peak depth. The proposed method for channel reduction is verified by extensive simulations at first. Finally, it is demonstrated on various real-world measurements of chemical compounds. The improved performance of the analysis on the reduced feature set could be shown in comparison with the evaluation on the full data set..

Keywords: THz-TDS Imaging, Hyperspectral Image Processing, Feature Selection

doi: 10.11906/TST.117-129.2010.09.12

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TST, Vol. 3, No. 3, PP. 130-142

Study of Physical Mechanism of Two-color Laser Field Pumped THz Wave in Air Plasma

Shengqi Xu, Yizhu Zhang, Yinbo Zheng, Weiwei Liu*
Institute of Modern Optics, Nankai University,
Key Laboratory of Opto-electronic Information Science and Technology,
Education Ministry of China, Tianjin 300071, P.R. China

(Received December 08, 2009; accepted August 03, 2010)

Abstract: THz wave generation during the laser-air interaction is investigated both experimentally and theoretically. The pump laser is a combination of a femtosecond Ti:sapphire laser pulse and its second harmonic. The polarization of the generated THz wave is experimentally measured. Comparison study has been carried out theoretically between the free electron drifting current model and the macroscopic four-wave mixing model. The result indicates that the four-wave mixing model could give better description to the experimental observations rather than the free electron drifting current model.

Keywords: Femtosecond laser, terahertz generation, four-wave mixing

doi: 10.11906/TST.130-142.2010.09.13

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TST, Vol. 3, No. 3, PP. 143-148

The Theoretical Study on THz Frequency Comb Generation via Optical Rectification

Degang Xu*, Pengxiang Liu, Da Lv, Yingjin Lv, Kai Zhong, Yuye Wang, Jianquan Yao
The Institute of Laser & OptoelectronicsThe College of Precision Instruments and Opto-Electronic Engineering ,Tianjin University, Tianjin, 300072, P. R. China
Key Laboratory of Opto-electronics Information and Technical Science (Ministry of Education), Tianjin University, Tianjin 300072, P. R. China

(Received December 05, 2009; accepted August 03, 2010)

Abstract: In this paper, we proposed a method for THz comb generation via surface-emitted optical rectification (OR) of ultra-short laser pulse in periodically poled lithium niobate (PPLN). The mechanism of this phenomenon is spectral interference between early and late THz pulses emitted by one fs-pulse. The generation of THz comb was analyzed both in frequency- and time-domain based on radiating antenna model. The figures of spectrum and waveform of the calculated electric field were presented. Our calculation indicated that THz combs generated by this method cover a large bandwidth and have a wide free spectral range.

Keywords: Optical Rectification, Frequency Comb, Terahertz

doi: 10.11906/TST.143-148.2010.09.14

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