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摘要
通常的太赫兹微结构主要采用Au薄膜制备金属结构，很难利用微结构中Au薄膜性能对太赫兹波进行实时调控。本文设计并制备了基于高磁导率软磁FeNHf薄膜的太赫兹开口三角形结构，通过外磁场调控微结构中软磁薄膜磁化强度方向，系统研究了外磁场调控下微结构中的太赫兹波传输特性和电磁共振模式。软磁FeNHf薄膜具有磁各向异性的特点，外磁场可以调控磁化强度M方向分别垂直和平行于太赫兹波磁场的方向，采用太赫兹时域光谱系统测试微结构的太赫兹透射特性，通过时域有限差分的方法，分析了基于软磁薄膜微结构的太赫兹场电磁场分布和调制机理。实验结果表明，外磁场可调控开口三角形太赫兹微结构的谐振频率，在1.3 THz频段，调谐率约为5.7%，调制深度约为15%。
Abstract
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 H. The THz transmission characteristics and electromagnetic resonance mode of the microstructure under the control of H in split triangular structure are systematically studied. The soft magnetic FeNHf film has the characteristic of magnetic anisotropy. Therefore, the direction of the magnetization M in FeNHf film can be controlled by the external magnetic field H to 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.
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Key words:
- terahertz waves /
- soft magnetic film /
- magnetic permeability /
- magnetic anisotropy
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Overview
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 H~ 50 Oe can change the direction of the magnetization M in 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 H. 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.
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图 1 (a) 超材料结构的实验照片；(b) TDS测试和非对称三角形结构尺寸示意图
Figure 1. (a) Experimental photographs of metamaterial structures; (b) Schematic of THz TDS measurement and geometry of the asymmetric triangular structures
图 3 FeNHf薄膜。(a)磁滞回线；(b)复数磁导率与频率曲线
Figure 3. Characterizations of FeNHf film. (a) Hysteresis loop of FeNHf film; (b) Frequency dependence of complex permeability
图 4 样品的THz透射率。(a) FeNHf薄膜和Au薄膜结构的实验结果；(b) FeNHf薄膜结构的模拟结果
Figure 4. THz transmissivity of samples based on FeNHf film and Au film. (a) Experiments; (b) Simulations
图 5 d峰位在磁化强度M分别平行和垂直H时，磁性薄膜结构在(a) fr=1.26 THz，(b) fr=1.33 THz的电场分布；(c) fr=1.26 THz，(d) fr=1.33 THz的磁场分布
Figure 5. (a), (b) Distributions of the electric field in the structure at (a) fr=1.26 THz, (b) fr=1.33 THz; (c), (d) Distribution of the magnetic field in the structure at (c) fr=1.26 THz, (d) fr=1.33 THz. Peak d is when magnetization M is parallel and perpendicular to terahertz magnetic field H, respectively
表 1 太赫兹微结构各谐振峰数据
Table 1. The resonance frequency of THz microstructures
Resonance points FeNHf microstructures Au microstructures Experiments/THz Simulations/THz Resonance points Experiments/THz M//H M⊥H Δfr M//H M⊥H Δfr a1, 2 0.175 0.177 0.002 0.250 0.253 0.003 a3 0.218 b1, 2 0.632 0.641 0.009 0.751 0.804 0.053 b3 0.668 c1, 2 0.902 0.947 0.045 0.974 1.071 0.097 c3 0.982 d1, 2 1.103 1.166 0.063 1.259 1.325 0.066 d3 1.209 
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