雷射激發天線輻射

Laser-excited Radiation Antenna

陳家祥(Chia-Hsiang Chen)

指導教授 ： 黃衍介

國立清華大學/電機資訊學院/光電工程研究所/博士(2012年)

摘要

利用不同脈衝寬度的遠紅外光雷射經過不同程度的聚焦提升光能量密度轟擊金屬導線，金屬受光電效應於導線表面產生電子擾動與震盪，電子擾動產生電流進而激發天線輻射，其天線輻射之波長與激發雷射的脈衝寬度相近，本文分別使用脈衝寬度7ns、460ps與160fs的遠紅外光雷射來激發金屬導線，實驗結果分別量測到170 MHz、2.6 GHz與寬度約3 THz的太赫茲天線輻射。同時，將對實驗中的蕭特基能障效應(Schottky effect)與多光子放射(multi-photon emission)現象進行討論。
為了證實金屬導線對於不同脈衝寬度雷射可激發出不同波長的天線輻射。首先我們將遠紅外光聚焦於直徑2 mm拋光金屬導線截面的一端並且於此前端懸空施加電壓，而另一端接地。透過施加正電壓提升光電流至0.8 kA，並且量測到由7 ns脈衝寬度雷射所激發170 MHz無線電波的遠場輻射。我們接著使用自製460 ps脈衝寬度之Nd:Yag閃光燈雷射放大器來激發導線天線輻射，其激發導線跨接於一接地S波段微波波導管，透過環形接收天線分別於波導管的前後量測到1.7 GHz與2.6 GHz的微波訊號。最後用160 fs、800 nm超快脈衝雷射激發導線天線輻射，並透過電光取樣法(EO-sampling)量測到約3THz頻寬的太赫茲訊號，其強度相當於同一實驗環境條件下碲化鋅產生的太赫茲輻射。基於上述我們成功地實驗出多頻段的導線天線輻射(無線電波至太赫波)，我們進而提出一個導線天線的設計應用，利用約100發的雷射脈衝串激發導線天線其輻射於一個微波共振腔中累積，透過雷射挑選器(Pockels cell)於同相位的情況下釋放足以加速電子達相對論光速度的電場梯度(~100 MV/m)，並模擬約5MeV電子能量的電子行為將於清華大學、同步輻射研究中心與北京清華大學設計的S-band光陰極電子槍中，此外，更進一步模擬電子束於一移頻磁鐵中震盪產生百萬瓦(MW)等級的太赫茲自由電子輻射。此設計提供一個小型化、高功率自由電子雷射的概念。此技術有著很大的發展潛力，其中包括控制激發雷射脈衝寬度與調整弦波型導線週期長度的都卜勒位移輻射(Doppler shifted radiation source)， 與利用導線激發輻射源建立同步相位加速場的質子加速器。

關鍵字

天線 ； 輻射 ； 雷射

並列摘要

This thesis studies a laser-excited wire antenna (LEWA) that generates radiation covering radio to THz frequencies. The radio wave was produced from a LEWA with its laser-excited tip biased at an anode voltage of +500 V. When irradiated using a laser pulse with 42 mJ energy in a 7 ns pulse width, the LEWA generated radiation power three to four orders of magnitude higher than that generated by a direct laser-excited antenna without a biased voltage. The radiation power scaled with the square of the laser-induced photocurrent between the antenna tip and anode (800 A in our case). With a higher biased anode field and shorter-wavelength excitation laser, this antenna had a potential to reach MW-GW radiation power in a microwave or even in a millimeter-wave spectrum. The microwave LEWA was driven by a Nd:YAG laser amplifier, producing 40 mJ pulse energy in a 460 ps pulse width. We installed the LEWA in front of an S-band metal waveguide, which functions as a spectral filter to pass and reject the radiation above and below the cutoff frequency of the dominant TE10 waveguide mode with a cutoff frequency of 2.08 GHz. Using a ring antenna, we measured 1.7 and 2.6 GHz microwave signals before and after the waveguide, respectively. When the wire was irradiated by a mode-locked Ti:sapphire laser with a 160 fs pulse width focused to 280 GW/cm2 intensity on the wire, we detected THz radiation with a 3 THz bandwidth, reaching the detection limit of our electro-optic sampling system. The amplitude of the detected THz signal was comparable to that generated by a ZnTe emitter under the same experimental condition.
Based on the successful demonstration of the LEWA, we further propose the use of the compact and highly efficient LEWA to replace the bulky and high-cost RF system for a particle accelerator. With a conventional S-band photoinjector having a quality factor of ~10000, an S-band MW-class LEWA resonantly coupled to the S-band photoinjector can generate ~ 100 MV/m field for particle acceleration in about 35 ns time. We further design an ultra-compact superradiant free-electron laser (FEL) driven by such a LEWA photoinjector. Our simulation study shows that it is possible to generate MW-level radiation power at THz frequencies from the FEL with a ~1% prebunched beam from the proposed LEWA photoinjector.

並列關鍵字

antenna ； radiation ； laser

參考文獻

[2] Z. Y. Chen, J, F. Li, Q. X. Peng, J. Li, J. K. Dan and S. Chen, “Experimental Study of Radio-Frequency Cherenkov Radiation by a Line Focused Laser Pulse Obliquely Incident on a Wire Target,” Chinese Journal of Physics, Vol. 49, 725-731 (2011).

[4] Y. C. Huang, C. H. Chen, A. P. Lee, W. K. Lau, and S. G. Liu, “Laser-beat-wave photoinjector”, Nuclear Instruments and Methods in Physics Rearch A, 637, S1-S6 (2011).

[11] J. Rose, W. Graves, R. Heese, E. Johnson, S. Krinsky and B. Sheehy, “Modeling and measurements of the DUV FEL photoinjector cavity RF properties”, technology report, National Synchrotron Light Source, Brookhaven National Laboratory (2001).

chapter 1

[1] Jack Soohoo, Shi-Kay Yao, James E. MIiller, Richard R. Shurtz, Yuan Taur and R. A. Gudmundsen, “A Laser-Induced Traveling-Wave Device for Generating Millimeter Waves,” IEEE T-MTT, 29 (1981).