利用磁流體之兆赫相位調製器

Abstract

兆赫波波段目前還鮮少使用磁光效應的調製器，而調製器對於現在研究技術是一項不可或缺的重要工具，如6G通訊、成像，元素分析……等等，為了發展諸如此類的應用開始研發出相關的元件設備。而磁流體將會是我們研究的主要材料。因磁流體有著一個非常特別的超順磁特性，可以達到精準地被控制，非常適合作為調製器的材料。但由於兆赫波對於水有強烈的吸收，所以本論文中我們選擇的材料主要是以親油性磁流體為主，將利用奈米粒子皆為四氧化三鐵的不同磁流體放置在不同磁力的平行磁場下，觀察磁流體的折射率變化對兆赫波調製的結果。 首先我們先將不同種的磁流體置入兆赫波時域光譜系統，取得其光學特性，包含折射率、穿透率、吸收係數…等，證實了親油性磁流體在兆赫波波段下的優良穿透效果。再藉由磁光效應中的Voigt effect作為基本架構施加不同大小的磁場(178mT、120mT、61mT、20.6mT、9.7mT、4.8mT)改變磁流體的折射率，進而可以調變兆赫波訊號，達到相位的調變效果。實驗中，我們是使用波段為800nm的超快脈衝雷射，利用非線性晶體ZnTe產生兆赫波訊號，施加在樣品上後，分析訊號的色散與振幅變化，推得相關參數。 分析結果顯示，載液為正己烷及煤油的磁流體，因油本身對兆赫波吸收就非常小，所以親油性磁流體吸收也是極微小的。而在施以外加磁場時，除了折射率有著相關規律的變化外，也發現了以煤油為載夜的磁流體發生了一個非常特殊且有趣的現象，就是當磁場施加到約178mT時，在0.5THz處開始有一個極大的吸收峰值，此現象對於未來要研究兆赫波調製器是一項非常優越的表現。預期可行的應用有兆赫波吸收器、兆赫波偏振器及振幅調製器等等。There are little modulators based on magneto-optic effect used in the THz region. A modulator is essential to many research fields like 6G communication, image, elements analysis and so on. To develop such researches, the study of modulator is important. In this work, to create a modulator, it is suitable for us to use ferrofluid that feature the superparamagnetic properties, which can be controlled by magnetic field accurately. And we choose the lipophilic ferrofluid rather than the hydrophilic ferrofluid, because water performs strong absorption to the energy in terahertz region. We applied different amplitude of magnetic field to different kinds of ferrofluids based on Fe3O4 nanoparticles, and observed the change of the refractive indices of the ferrofluids after modulating by terahertz electric field. First, we measured the optical properties of the ferrofluids, including refractive index, transmittance, absorption under the terahertz time-domain spectroscopy (THz-TDS). The results show the great transmittance of the ferrofluid carried by oleic acid in the terahertz region. Then we applied different amplitude of the magnetic field(178mT、120mT、61mT、20.6mT、9.7mT、4.8mT) to change the refractive index of each ferrofluid. The changed refractive index caused a shift to the phase. Here in the system, we pump a 800nm ultrafast pulse laser into ZnTe crystal and generate THz wave transmits through the sample. After analyzing the measured dispersion and changed amplitude of terahertz signal, we can get the related coefficients. The results shows that the ferrofluids in the carrier liquid of Hexane and kerosene perform a weak absorption because of the lipophilic surfactant. And after applying the different magnetic field, the refractive index changed regularly. We also observed a special phenomenon that resonance happened at 0.5 THz when we applied the magnetic field to 178mT at the ferrofluid in the carrier liquid of kerosene. The phenomenon will be a distinguishing feature to the development of amplitude modulator.