基于鐵磁薄膜可調(diào)諧太赫茲微結(jié)構(gòu)的研究

章 強，張曉渝，邢園園，趙 磊

基于鐵磁薄膜可調(diào)諧太赫茲微結(jié)構(gòu)的研究

章 強，張曉渝*，邢園園，趙 磊

蘇州科技大學(xué)數(shù)理學(xué)院，江蘇省微納熱流技術(shù)與能源應(yīng)用重點實驗室，江蘇 蘇州 215009

通常的太赫茲微結(jié)構(gòu)主要采用Au薄膜制備金屬結(jié)構(gòu)，很難利用微結(jié)構(gòu)中Au薄膜性能對太赫茲波進行實時調(diào)控。本文設(shè)計并制備了基于高磁導(dǎo)率軟磁FeNHf薄膜的太赫茲開口三角形結(jié)構(gòu)，通過外磁場調(diào)控微結(jié)構(gòu)中軟磁薄膜磁化強度方向，系統(tǒng)研究了外磁場調(diào)控下微結(jié)構(gòu)中的太赫茲波傳輸特性和電磁共振模式。軟磁FeNHf薄膜具有磁各向異性的特點，外磁場可以調(diào)控磁化強度方向分別垂直和平行于太赫茲波磁場的方向，采用太赫茲時域光譜系統(tǒng)測試微結(jié)構(gòu)的太赫茲透射特性，通過時域有限差分的方法，分析了基于軟磁薄膜微結(jié)構(gòu)的太赫茲場電磁場分布和調(diào)制機理。實驗結(jié)果表明，外磁場可調(diào)控開口三角形太赫茲微結(jié)構(gòu)的諧振頻率，在1.3 THz頻段，調(diào)諧率約為5.7%，調(diào)制深度約為15%。

太赫茲波；軟磁薄膜；磁導(dǎo)率；磁各向異性

1 引 言

超材料具有天然材料所不具備的電磁響應(yīng)特性，基于超材料可以研制太赫茲濾波器、調(diào)制器、諧振器等[1-3]，從而引起了國際科研工作者的廣泛研究[4-5]。目前研究的金屬基太赫茲超材料主要由Au金屬和低損耗基板材料組成，形成的太赫茲器件難以利用Au金屬特性對太赫茲波進行調(diào)控[6-7]。如果能夠動態(tài)調(diào)控太赫茲器件中超材料的性能，將進一步拓展器件的實用性。現(xiàn)階段，調(diào)控超材料性能方式主要有通過旋轉(zhuǎn)諧振單元角度或位置改變器件工作頻率[8-9]，通過改變溫度調(diào)制超材料性能、對石墨烯結(jié)構(gòu)外加磁場等[10-11]，實現(xiàn)對太赫茲波的調(diào)控。上述的調(diào)制方式很可能會限制調(diào)制速度的提高。還有基于GaN或GaAs異質(zhì)結(jié)電調(diào)制太赫茲器件[12]，該類調(diào)制器件在調(diào)制深度(>50%)和調(diào)制速度(~GHz)上都取得了較好的進展，對太赫茲器件微納加工提出了較高的要求。然而，采用磁性金屬薄膜制備太赫茲超材料的文獻報道很少，因此，本文在原有研究太赫茲調(diào)制器基礎(chǔ)上提出基于FeNHf軟磁薄膜的太赫茲微結(jié)構(gòu)，軟磁薄膜具有飽和磁化強度高，矯頑場低和各向異性場大等特點，磁化強度方向可在外磁場約8000 A/m下翻轉(zhuǎn)，可調(diào)控性較高。同時，與傳統(tǒng)的開口環(huán)和圓環(huán)結(jié)構(gòu)相比，開口三角形結(jié)構(gòu)在相同空間內(nèi)器件密度大，有利于系統(tǒng)的穩(wěn)定發(fā)揮，在器件中有部分微結(jié)構(gòu)破損的情況下也能發(fā)揮器件的大部分功能。實驗表明，開口三角形結(jié)構(gòu)中開口部分和邊長部分相互耦合形成的亮模和暗模形式同樣能激發(fā)器件的電磁誘導(dǎo)透明機制，為器件設(shè)計提供依據(jù)[7]。通過外磁場調(diào)控太赫茲微結(jié)構(gòu)中軟磁薄膜磁化強度與太赫茲磁場的方向，對透射的太赫茲波產(chǎn)生不同程度的微擾，實現(xiàn)對太赫茲微結(jié)構(gòu)工作頻率或太赫茲波透射率的調(diào)控。該磁性薄膜太赫茲微結(jié)構(gòu)無需在結(jié)構(gòu)上制備電極等調(diào)制單元，以非接觸方式對太赫茲波進行調(diào)控，為太赫茲無源器件的研究提供一種新的途徑。

2 實 驗

磁性薄膜太赫茲微結(jié)構(gòu)是基于半導(dǎo)體微納加工技術(shù)制備得到，選用電阻率為8000 Ω?cm的雙面拋光高阻硅為基片，裂片后尺寸為10 mm×10 mm×0.5 mm。首先對硅基片進行清洗，接著涂覆RZJ-306光刻膠，曝光顯影后對應(yīng)的太赫茲微結(jié)構(gòu)光刻膠部分被去除。采用高真空磁控濺射設(shè)備沉積厚度約為50 nm的FeNHf薄膜，采用Bruker臺階儀測量薄膜的厚度。靶材為純度99.99% Fe靶，金屬Hf片放置在靶上，面積約為Fe靶的1%。射頻功率80 W，沉積氣體為Ar和N2，沉積氣壓為0.5 Pa。將沉積FeNHf薄膜樣品浸泡在丙酮溶液中進行剝離工藝，剝離后微結(jié)構(gòu)顯微照片如圖1(a)。太赫茲傳輸特性采用THz Photonics TP15K太赫茲時域光譜(terahertz time domain spectroscopy，THz-TDS)系統(tǒng)，樣品的磁性能由綜合物性測量系統(tǒng)(PPMS)表征。FeNHf薄膜磁導(dǎo)率采用基于微擾法的諧振腔測試得到。器件的工作頻率由微結(jié)構(gòu)的尺寸和介質(zhì)材料共同決定，如圖1(b)，樣品微結(jié)構(gòu)參數(shù)=100 μm，=5 μm，=8 μm，=5 μm，開口位于三角形邊長中心處。

3 結(jié)果與討論

圖2是沉積10 min的FeNHf薄膜的輪廓圖。FeNHf薄膜厚度約為50 nm，插圖為FeNHf薄膜的原子力表面形貌圖，掃描范圍為500 nm×500 nm，薄膜的平均顆粒尺寸約為6.7 nm。同時，通過范德堡法測試得到FeNHf薄膜(5 mm×10 mm)的面電阻率約為2×10-6Ω?m。

圖3(a)為FeNHf薄膜樣品的磁滯回線，可以看到薄膜具有顯著的軟磁特性和磁各向異性。FeNHf薄膜易軸方向矯頑場為3.5 Oe，難軸方向矯頑場為10.4 Oe，飽和磁化強度4π為15.1 kG，在8000 A/m外磁場下可實現(xiàn)絕大部分的磁化強度翻轉(zhuǎn)。圖3(b)為采用基于微擾法測試FeNHf薄膜磁導(dǎo)率隨頻率的曲線。圖中可以看到復(fù)數(shù)磁導(dǎo)率='-i''的實部'和虛部''與頻率的關(guān)系，在0.3 GHz下，薄膜難軸方向的磁導(dǎo)率約為408，共振頻率為2.02 GHz，易軸方向無法測試到鐵磁共振信號，說明可以通過外磁場調(diào)控磁導(dǎo)率在難軸和易軸的方向。根據(jù)Landau-Lifshitz-Gilbert對磁化進動的理論闡述，對于具有面內(nèi)單軸各向異性的軟磁薄膜，鐵磁共振頻率可以近似表達為[13]

圖1 (a) 超材料結(jié)構(gòu)的實驗照片；(b) TDS測試和非對稱三角形結(jié)構(gòu)尺寸示意圖

圖2 FeNHf薄膜輪廓圖和表面形貌圖

圖3 FeNHf 薄膜。(a) 磁滯回線；(b) 復(fù)數(shù)磁導(dǎo)率與頻率曲線

式中：、k和分別表示旋磁比、各向異性場和飽和磁化強度，通過PPMS測量，F(xiàn)eNHf薄膜樣品的k為38 Oe，通過計算可以得到2π約為2.67 MHz/Oe。

其中：和分別為金屬線的長度和寬度，為有效的磁導(dǎo)率。因此，由磁化強度變化引起的磁導(dǎo)率變化導(dǎo)致了有效電感值的變化，最終使得太赫茲微結(jié)構(gòu)諧振頻率發(fā)生變化。實驗和模擬結(jié)果統(tǒng)計在表1中，調(diào)控磁化強度方向，諧振頻率峰位移動了?r2 GHz，諧振頻率峰位移動了?r9 GHz，電磁誘導(dǎo)透射峰峰位移動了?r45 GHz，諧振頻率峰位移動了?r63 GHz，四個諧振頻率調(diào)諧率約為1.1%~5.7%。不同諧振峰來自于不同諧振模式，峰位諧振來源于三角形邊的電磁耦合[7]，磁化方向變化對邊長有效磁導(dǎo)率的改變較為明顯，因此在四個峰位中由邊長引起的諧振峰位?r變化最為明顯。由此，外磁場調(diào)控鐵磁薄膜的磁化強度方向能有效調(diào)控太赫茲微結(jié)構(gòu)的諧振頻率。

同時，圖4(a)中給出了Au薄膜制備相同尺寸的太赫茲微結(jié)構(gòu)透射曲線，磁性金屬薄膜與Au金屬薄膜制備的太赫茲微結(jié)構(gòu)雖然表現(xiàn)出一致的電磁諧振行為，但磁性太赫茲微結(jié)構(gòu)具有其顯著的特點，一是磁性微結(jié)構(gòu)諧振頻率比Au薄膜微結(jié)構(gòu)諧振頻率低，這主要是因為磁性微結(jié)構(gòu)具有較高的等效電感值。二是磁性微結(jié)構(gòu)透射深度比Au薄膜微結(jié)構(gòu)的低，這主要是磁性薄膜電阻率高于Au薄膜所引起的太赫茲歐姆損耗較大而導(dǎo)致。圖4(b)為基于時域有限差分方法(FDTD)模擬不同磁化方向的鐵磁薄膜基太赫茲濾波結(jié)構(gòu)的透射譜，模擬時磁導(dǎo)率設(shè)置為各向異性值=(20,1,1)?？梢钥闯?，當(dāng)磁化強度平行于太赫茲波磁場時，微結(jié)構(gòu)諧振頻率低于磁化強度垂直于太赫茲波磁場時的諧振頻率，模擬結(jié)果與實驗測試是一致的。同時，微結(jié)構(gòu)調(diào)制深度在實驗測試與模擬之間數(shù)值上有一定的差異，其中調(diào)制深度差異約65%，諧振頻率存在約10%的差異。分析認為，有以下兩個因素導(dǎo)致了實驗和模擬之間的差異。第一，經(jīng)過光刻工藝后，圖形化FeNHf薄膜的電阻率很可能遠高于2×10-6Ω?m，這會導(dǎo)致微結(jié)構(gòu)中電子濃度的降低，從而減小了微結(jié)構(gòu)的調(diào)制深度。第二，實驗中制備的超材料結(jié)構(gòu)的完整性與模擬結(jié)構(gòu)有一定的差距，這會導(dǎo)致微結(jié)構(gòu)諧振頻率的偏移。

圖4 樣品的THz透射率。(a) FeNHf薄膜和Au薄膜結(jié)構(gòu)的實驗結(jié)果；(b) FeNHf薄膜結(jié)構(gòu)的模擬結(jié)果

為了更好地理解磁化強度方向分別平行和垂直THz波磁場時對FeNHf磁性微結(jié)構(gòu)在諧振模式下的影響，圖5給出了峰位在分別平行和垂直時的磁性薄膜微結(jié)構(gòu)太赫茲電場強度和磁場強度的分布圖，電磁諧振峰位主要來源于三角形邊長的電磁耦合。圖5(a)和5(b)為微結(jié)構(gòu)的太赫茲電場強度分布，可以看到，磁化方向的變化對太赫茲電場強度的改變不足1%。圖5(c)和5(d)為微結(jié)構(gòu)的太赫茲磁場強度分布，方向的變化引起的太赫茲磁場強度變化約為14%。所以，磁化強度對峰位頻率的改變主要是太赫茲磁場強度的變化，磁性薄膜磁化強度通過對太赫茲磁場的影響可以實現(xiàn)對太赫茲波調(diào)控。

表1 太赫茲微結(jié)構(gòu)各諧振峰數(shù)據(jù)

圖5 d峰位在磁化強度M分別平行和垂直H時，磁性薄膜結(jié)構(gòu)在(a) fr=1.26 THz，(b) fr=1.33 THz的電場分布；(c) fr=1.26 THz，(d) fr=1.33 THz的磁場分布

4 結(jié) 論

本文在基于半導(dǎo)體微納工藝技術(shù)上制備FeNHf軟磁薄膜太赫茲微結(jié)構(gòu)，軟磁薄膜具有良好的磁各向異性，外磁場約8000 A/m可較好調(diào)控微結(jié)構(gòu)中磁化強度的方向。通過外磁場調(diào)控微結(jié)構(gòu)磁化強度方向能有效調(diào)控太赫茲微結(jié)構(gòu)諧振頻率，在1.3 THz頻段最大調(diào)諧率約5.7%。基于時域有限差分法的模擬結(jié)果較好地與實驗測試結(jié)果一致。利用磁性薄膜磁化強度對太赫茲波的調(diào)控為探索太赫茲波器件提供了新的途徑。

致 謝

本文工作得到了南京大學(xué)金飚兵教授和張彩虹老師在THz-TDS測量上的幫助，蘇州大學(xué)湯如俊老師在PPMS磁性能測試上的幫助，以及蘇州科技大學(xué)姜昱丞老師在光刻工藝上的指導(dǎo)，在此一并表示感謝。

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Tunable terahertz structure based on the ferromagnetic film

Zhang Qiang, Zhang Xiaoyu*, Xing Yuanyuan, Zhao Lei

Jiangsu Key Laboratory of Micro and Nano Heat Fluid Flow Technology and Energy Application, School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou, Jiangsu 215009, China

Schematic of THz TDS measurement and geometry of the asymmetric triangular structures

Overview:The terahertz (THz) microstructure is generally fabricated by Au film. It is difficult to control the THz wave by using the physical properties of Au film when the dimension of Au structures are fixed. It is suggested that combination of the tunable materials with the microstructure can improve the performance of terahertz microstructure and simplify the fabrication process. In this paper, the THz microstructure based on the magnetic FeNHf film is fabricated by using the high vacuum RF magnetron sputtering on the high resistivity silicon substrate. A complete terahertz microstructure of FeNHf magnetic thin film was prepared by the semiconductor micro-nano processing technology. The transmission characteristics of magnetic microstructure were characterized by the terahertz time-domain spectroscopy (THz-TDS). The THz transmission of magnetic microstructures were measured under the different external magnetic field. The soft magnetic FeNHf film has the high magnetization of ~16000 kG and the low coercivity of 3 Oe. The magnetic field~ 50 Oe can change the direction of the magnetizationin FeNHf film perpendicular and parallel to the terahertz magnetic field, respectively. The THz transmission and electromagnetic resonance of the magnetic THz microstructure are systematically studied with the change of the external field. The distribution of terahertz electromagnetic field and the surface current distribution based on the FeNHf film microstructure are discussed by the finite difference time domain method. The mechanism of the modulation of THz transmittance and resonance frequency of the magnetic microstructures is clarified with the change of the magnetic field. At the same time, for the comparison, the THz transmission characteristics of the microstructures with the same dimensional Au film are also discussed. The experimental results show that the resonance frequency of the split triangular THz microstructure can be modulated under magnetic field. At the frequency of 1.3 THz, the tunability and modulation depth are about 5.7% and 15%, respectively. The change of magnetization of FeNHf film which results in the perturbation of the magnetic field of terahertz wave. Furthermore, the distribution of electrons in FeNHf film will be changed under the external field, and the effective inductor is varied in the terahertz region. Therefore, it is found that the resonance frequency of FeNHf microstructure shifts to the lower frequency when the magnetization is perpendicular to the magnetic field of terahertz. Experimental and theoretical research on the THz transmission of the magnetic microstructure can further improve the understanding of the THz modulation mechanism for the active devices. At the same time, our efforts provide more experimental data for the development of passive THz devices.

Citation: Zhang Q, Zhang X Y, Xing Y Y,Tunable terahertz structure based on the ferromagnetic film[J]., 2020, 47(6): 190447

Tunable terahertz structure based on the ferromagnetic film

Zhang Qiang, Zhang Xiaoyu*, Xing Yuanyuan, Zhao Lei

Jiangsu Key Laboratory of Micro and Nano Heat Fluid Flow Technology and Energy Application, School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou, Jiangsu 215009, China

Au film is mainly used to prepare the metal structure of the terahertz (THz) microstructure. When the metal structure is fixed, it is difficult to control the terahertz wave by using the properties of Au film. In this paper, the terahertz microstructure based on the soft magnetic FeNHf film with the high permeability is designed and fabricated on the high resistivity silicon substrate. The magnetization direction of soft magnetic film is controlled by the external magnetic field. The THz transmission characteristics and electromagnetic resonance mode of the microstructure under the control ofin split triangular structure are systematically studied. The soft magnetic FeNHf film has the characteristic of magnetic anisotropy. Therefore, the direction of the magnetizationin FeNHf film can be controlled by the external magnetic fieldto be perpendicular and parallel to the magnetic field of THz wave, respectively. The THz time domain spectroscopy system is used to test the terahertz transmission characteristic of the microstructure. The finite difference time domain method is used to analyze the THz electromagnetic field distribution and modulation mechanism based on the microstructure of the FeNHf film. The experimental results show that the resonance frequency of the split triangular THz microstructure can be modulated under magnetic field. At the frequency of 1.3 THz, the tunability and modulation depth are about 5.7% and 15%, respectively.

terahertz waves; soft magnetic film; magnetic permeability; magnetic anisotropy

TB872

A

10.12086/oee.2020.190447

章強，張曉渝，邢園園，等. 基于鐵磁薄膜可調(diào)諧太赫茲微結(jié)構(gòu)的研究[J]. 光電工程，2020，47(6): 190447

: Zhang Q, Zhang X Y, Xing Y Y,. Tunable terahertz structure based on the ferromagnetic film[J]., 2020,47(6): 190447

Supported by National Natural Science Foundation of China (61107093), Suzhou Key Laboratory for Low Dimensional Optoelectronic Materials and Devices (SZS201611), Jiangsu Key Disciplines of Thirteen Five-Year Plan (20168765), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (19KJA140001), and the Graduate Research and Practice Innovation Project of USTS (SKCX18_Y13).

* E-mail: xyzhang@usts.edu.cn

2019-07-28；

2019-11-04

國家自然科學(xué)基金資助項目(61107093)；蘇州市低維光電材料與器件重點實驗室(SZS201611)；江蘇省十三五重點學(xué)科項目(20168765)；江蘇省高等學(xué)校自然科學(xué)研究項目(19KJA140001)；蘇州科技大學(xué)研究生科研創(chuàng)新計劃項目(SKCX18_Y13)

章強(1994-)，男，碩士研究生，主要從事微納結(jié)構(gòu)遠紅外濾波器的調(diào)控研究。E-mail: 1224052592@qq.com

張曉渝(1978-)，男，博士，副教授，主要從事基于半導(dǎo)體材料和鐵性薄膜材料微波-遠紅外光電器件的調(diào)控機理研究和器件研制。E-mail：xyzhang@usts.edu.cn