【作者】 沈健；

【导师】 张怀武；

【作者基本信息】 电子科技大学 ， 电子科学与技术， 2012， 博士

【摘要】 磁性材料在微波及太赫兹波段的动力学的研究和应用已开展多年。由于近年来光子晶体和负折射系数材料的发展，微波传导的低能耗和谐振器件的小型化逐渐受到重视。在这些领域人们进行了多方面有益的探索，如磁性微波光子晶体和开口谐振环的磁响应。另一方面，随着信息技术的飞速发展，人们对磁性介质数据存储速度的要求越来越高，促使科研工作者不断探索新的磁化态操控技术。由激光脉冲激发的超快磁矩翻转现象的发现和研究，使人们意识到这一技术有可能成为下一代更快的磁存储技术的基础，该领域的工作逐渐成为凝聚态物理的研究热点之一。本工作首先运用调节磁场的方法实现了对微波段磁性光子晶体带隙的调制，该系统由铁磁柱体材料组成的二维磁性光子晶体所构成。除了由光子晶体周期结构导致的布拉格散射带隙，我们在实验中还观测到了另外两种光子带隙：分别来自于磁性表面等离子体共振和自旋波共振。其中，磁性表面等离子体可视为金属表面等离子体在铁磁材料表面的类比物理量，其共振所引起的光子带隙具有外场可调的特殊性质。这一发现对于光子晶体波导理论和应用的研究都具有重要意义。在上述磁性光子晶体带隙理论及实验研究的基础上，我们通过特定的实验方法制备了具有单向导波特性的磁性表面等离子体微波波导。当电磁波的频率处于磁性表面等离子体共振频率区域，由二维磁性光子晶体阵列构成的波导具有不对称的电磁波传输特性：即在由两个二维周期阵列（每个阵列外加方向相反的静磁场）所构成的导波通道中，电磁波只沿着一个方向传播，而在其反方向的传播被显著抑制。实验发现，该波导的单向传输特性不受金属障碍物和光子晶体周期性破坏的影响；更重要的是，其单向导波的工作频率可由外加磁场调制。理论研究表明：这一现象源自于磁性表面等离子体能带结构的时间反演对称破缺，类比于二维电子系统中的量子霍尔效应的手性边缘传输现象。开口谐振环作为一种新型的电磁波谐振系统，其共振频率克服了传统散射波长的限制，尺寸可达到共振频率波长的1/10甚至更小。基于对开口谐振环的磁共振响应特性的研究，本工作运用溅射和光刻方法制备了一种新型的不共面开口谐振环。区别于传统的共面开口谐振环，实验发现不共面开口谐振环的共振频率与环-环间距具有线性关系。环-环间距越小，其共振频率越低。当环-环间距达到微米级别，发生磁共振时其几何尺寸可达到波长的1/120。这一实验结果可为天线的小型化研究提供有益参考。本工作通过实验验证了铁磁薄膜的超快退磁效应可辐射出太赫兹波，并且发现了铁磁薄膜受激光脉冲激发后，其辐射出的太赫兹波的强度与薄膜本身的阻尼系数成明显的线性关系。实验采用磁控溅射方法制备铁磁薄膜，并利用自旋注入效应，改变与铁磁薄膜相邻的金属薄膜的厚度，从而获得具有不同阻尼系数的铁磁薄膜样品。使用抽运-探测技术观测到辐射的太赫兹波的强度与阻尼系数存在显著的正比关系：薄膜的阻尼系数越大，太赫兹波的辐射强度越大，表明磁矩的退磁过程更快。这一发现为进一步研究超快激光操控磁化态的动力学过程开辟了新的思路。实验中通过自旋注入效应改变薄膜的阻尼系数，表明了自旋注入理论也可能适用于太赫兹波范围。更多还原

【Abstract】 The dynamics of magnetic materials in microwave and terahertz range have beenstudied and utilized for dacades. Most of attentions were focused on the insulatingmagnetic materials. Recently, due to the development of magnetic photonic crystals,negative refractive index materials and ultrafast laser pulse techniques, there have beenincreasing interests in the microwave guiding system such as magnetic surface plasmawaveguide and split ring resonator. On the other hand, the demands for theever-increasing speed of data storage in magnetic media have triggered intense searchesfor innovative methods to control magnetization, ultrafast magnetization switching bypulsed laser excitation can be viewed as one of the prospective technique.We experimentally studied magnetically controllable photonic band gaps (PBGs)in two-dimensional magnetic photonic crystals (MPCs) consisting of ferrite rods.Besides the conventional PBG that relates to Bragg scattering, another two types ofPBGs, resulting from magnetic surface plasmon resonance and spin-wave resonance,respectively, are observed. The PBG due to magnetic surface plasmon resonance isparticularly interesting because of its analogy to surface plasmon in metal, furthermore,it is shown to be completely tunable by an external static magnetic field from bothexperimental and theoretical point of view.We also have demonstrated experimentally a one-way magnetic surface plasmonelectromagnetic waveguide in the microwave range based on the MPCs The waveguideexhibits asymmetric transmission of electromagnetic waves in the frequency range nearthe magnetic surface plasmon resonance for an MPC, such that a significant one-waypropagation can be observed in the channel between the two MPC slabs, each in anexternal static magnetic field (ESMF) of opposite directions. The one-way waveguide isnot only immune to interstitial metal defects，but also robust against the disorder of rodposition. Furthermore, its working frequency can be flexibly tuned by an ESMF, whichmakes it more favorable for the design of electromagnetic devices. The physics isrelated to the broken time reversal symmetry of the magnetic surface plasmon bandstates and the excitation of a giant circulation of the energy flow, similar to the case in the quantized Hall Effect.Based on the understanding of the magnetic resonance properties of split ringresonator no longer suffers from half-wavelength requirement for resonance, wedemonstrated that for coplanar split ring resonator, the wavelength of the resonancemode is about10times larger than geometrical size of the ring; for non-coplanar splitring resonator, the resonance frequency depends linearly on the ring-ring separation. Werealized the non-coplanar split ring system using sputtering and etching techniques, it’sgeormetrical size is only1/120of the wavelength at the lowest resonance mode. Thisdemonstration opens a door to miniaturization of resonator.It is believed that ultrafast demagnetization process in ferromagnetic film canproduce terahertz (THz) emission. We present an experimental demonstration that,following ultrafast optical excitation, the magnitude of terahertz electromagnetic pulsesemitted from a ferromagnetic film is proportional to the Gilbert damping constant,which is conventionally used to describe the damping of magnetization precession.Different damping factors are obtained by varying the normal metal film adjacent to themagnetic film via spin pumping. It is shown that the larger the Gilbert damping, thelarger emitted THz signal and therefore, the faster demagnetizing process. On the otherhand, the nonlocally adjusted damping constant via spin pumping in this experimentalso indicates that the spin pumping theory may still be valid in THz regime. We expectthis study would lead to new perspectives of the ultrafast manipulation of magneticorder.更多还原