Ultra-High-Speed Wireless Communication Systems
Terahertz Tomography Systems
近年來太赫茲科技(terahertz technology)在國際上受到相當廣泛的關注，因為其可針對化學藥劑、藥物、空氣品質、有毒氣體、爆烈物及生化機構提供相當完整的識別，而對於醫療透視影像及軍事探測上更有極大的影響。然而，現今太赫茲系統(terahertz system)大多僅可見於實驗室。原因在於系統最重要的部份：太赫茲發射源(terahertz emitter)普遍存在有低太赫茲輸出功率、極低轉換效率、以及高成本且結構複雜等問題。這使得高效能太赫茲系統難以普及量化實現。利用光電導體轉換機制(photoconduction)產生寬頻及極窄頻太赫茲波已成為未來可攜式高效能太赫茲系統的重要實現關鍵之一，因光電導太赫茲源(photoconductive terahertz emitter)可在室溫下操作且可以提供相對其他技術高的太赫茲功率。不過現今光電導體太赫茲源的效能尚受限於其超快光電導體(ultrafast photoconductor)的極低量子效益(quantum efficiency)。
Yang Research Group 致力於將光電導體的量子效益大幅提昇，進而產生高能太赫茲輻射。透過整合現有雷射(laser)、超快光學(ultrafast optics)、電漿子(plasmonics)、固態物理(solid-state physics)、天線(antenna)以及光纖通訊(fiber-optic communication)科技，我們成功將光電導太赫茲源與電漿子電極(Plasmonic Contact Electrode)整合使得光源可在奈米尺度下有效集中光於電極側，進而大幅提升量子效益。相較於傳統光電導體太赫茲源，我們所研發的超高功率寬頻電漿子光導太赫茲源(broadband plasmonic photoconductive terahertz emitter)已達到超過三個量級(>1,000倍)的太赫茲功率提昇。透過整合光電混頻器(photomixer)與電漿子電極，我們更成功實現室溫操作、超寬可調頻率(>3 THz) 、窄頻率線寬(<< 1MHz)電漿子光電混頻器(plasmonic photomixer)。在1THz頻段，此窄頻電漿子光電混頻器的輻射功率為前世界紀錄的三倍。
Yang Research Group 另一個研究主軸為太赫茲集成系統。我們將高效能光電導太赫茲光電元件應用至超高速無線通訊以及生醫影像系統。現今無線通訊系統的通訊速率主要被兩大因素受限：(1) 發射源可調頻率範圍過窄及 (2) 通道頻譜滿載至60GHz。透過窄頻電漿子光電混頻器，我們可將通訊通道設置在在尚未被利用之次太赫茲(sub-terahertz)/太赫茲通道頻譜。因為信號載波頻率(signal carrier frequency)大幅提昇，可加載的訊號量也得以顯著的躍進 。我們預計可傳輸速率提高至百倍於現今無線傳輸系統。此超高速無線傳輸系統對於下一代的超高速手機通訊、室內無線通訊、雲端運算和可穿戴式裝置實時分析有極大的助益。利用高功率光電導太赫茲發射器及高敏光導太赫茲探測器，我們可架構出太赫茲斷層掃描系統並在非破壞性的平台上直接擷取癌症生物標記資訊。我們預計其三維影像解析度可達到亞毫米(sub-mm)等級且影像深度可達到0.3 – 1 公分。藉由整合光纖系統與微米雷射技術，此太赫茲斷層掃描系統之探測端可微縮至毫米等級大小，這使得其探測端可深入人體進而取得亞毫米解析度活體三維影像。因此，本實驗室之太赫茲斷層掃描系統可透過及時生成高解析影像來幫助精確的初期癌症診斷。
Terahertz technology has attracted extensive attention because of its unique applications in environmental monitoring, space explorations, chemical identification, material characterization, security screening, medical imaging, and biological sensing. In the meantime, the practical feasibility of many terahertz systems is still limited by the low power, low efficiency, and bulky nature of existing terahertz sources. Among various techniques for terahertz generation, photoconduction has demonstrated very promising performance for generating both pulsed and continuous-wave (CW) terahertz radiation. Compared to other optically driven terahertz emitters based on nonlinear optical processes, performance of photoconductive terahertz emitters is not constrained by the Manley–Rowe limit and, therefore, can offer significantly higher optical-to-terahertz conversion efficiencies. In spite of their great promise, the performance of existing photoconductive terahertz emitters is severely limited by poor quantum efficiency of ultrafast photoconductors. This limitation is mainly caused by inefficient collection of the majority of the photocarriers in a sub-picosecond time scale. To address this limitation, we introduce novel photoconductive terahertz source designs that incorporate plasmonic contact electrodes to offer significantly higher efficiencies compared to conventional designs. By utilizing plasmonic contact electrodes, a large portion of the incident optical pump beam is concentrated and absorbed in close proximity to the plasmonic contact electrodes. Therefore, the average transport path length of photo-generated carriers to the contact electrodes is greatly reduced. As a result, higher photocurrent levels are fed to the terahertz antenna within the oscillation cycle of the terahertz radiation and higher optical-to-terahertz conversion efficiencies are achieved. To further increase both conversion efficiency and terahertz radiation power, we are exploring new types of nanostructures/substrates that can efficiently harvest light and forming an array of plasmonic photoconductive terahertz emitters, enabling a highly directional and high intensity terahertz output with ultra-high optical-to-terahertz conversion efficiencies.
Convergence of microwave electronics and optoelectronics through emergence of integrated terahertz systems would be crucial for future Tera-bit-per-second (Tbps) wireless communication systems as well as advanced biomedical imaging and security screening systems. However, a fully integrated terahertz system has not yet been realized due to the lack of compact, high performance terahertz components integrated on the same platform. Recently, a significant stride in integrated terahertz systems has been made through the invention of plasmonic photoconductive terahertz sources/detectors by Prof. Mona Jarrahi (UCLA). Plasmonic terahertz sources/detectors are in essence equivalent to terahertz antennas, but with unique additional advantages of higher quantum efficiency and higher power handling capabilities. Not only their room-temperature operation nature and broad frequency tunability offer superior performance compared to all rival terahertz technologies, but they also offer the capability of full system integration on a single chip. This is due to the advances in semiconductor and optical technologies, enabling full integration of plasmonic photoconductive terahertz sources and detectors with semiconductor lasers and optoelectronic components on the same chip. Therefore, a compact, lightweight, and cost effective integrated terahertz system can be realized.
High-speed wireless communication has attracted extensive attention in the past decades because of dramatic changes in the ways people create and share information. The expected data rate to satisfy the needs of customers is projected to be 100 Gbit/s by 2020. However, the speed of today's wireless communication systems is limited by the narrow bandwidth of existing sources and the heavy use of the electromagnetic spectrum up to 60 GHz. This trend has been forcing researchers to push the carrier frequencies to higher frequencies to allow operation at frequency bands which have not been allocated to any specific active service. We have successfully demonstrated high power terahertz sources based on plasmonic photomixers with 3 THz frequency tunability and sub-Hz spectral linewidth. Such high-performance terahertz sources have a revolutionary impact on future high-speed wireless communication systems, enabling > 100 times faster datalinks transmitting in the unused terahertz frequency band. A system with such capabilities can revolutionize the next generation high-speed mobile communication, indoor wireless communication, Kiosk downloads, cloud computing, and real-time data analysis for wearable devices.
In 2012, the International Agency for Research on Cancer (IARC) reported 14.1 million new cancer cases, 8 million deaths from cancer, and 32.6 million five-year cancer survivors worldwide. The mortality rates can be greatly reduced by detecting cancer at early stages (bowel cancer: < 10 %, breast cancer: <10 %, lung cancer: < 30 %). Current biomedical imaging technologies such as X-ray computed tomography (X-ray CT), optical coherence tomography (OCT), magnetic resonance imaging (MRI), confocal microscopy, and high-frequency ultrasound (HFUS) have demonstrated great success with their 3D imaging capabilities. However, none of these techniques can accurately identify millimeter-size cancerous tumors deep inside soft tissues without producing false-positive results that could lead to continuing destructive examinations. A terahertz tomography system offers a promising solution for early-stage cancer diagnosis since it can provide sub-millimeter image resolution with image depths ranging from 0.3-1 cm, depending on the tissue water content. Moreover, a terahertz tomography system allows detecting cancer biomarkers through a non-ionizing, nondestructive imaging platform. Additionally, the size of terahertz tomography systems can be further reduced by incorporating fiber systems and compact lasers, resulting in a breakthrough platform for in vivo biomedical imaging.
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