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一维光子晶体表面波及其传感应用研究
One-dimensional Photonic Crystal Surface Waves and Their Sensing Applications
【作者】 李延辉;
【导师】 韩圣浩;
【作者基本信息】 山东大学 , 微电子学与固体电子学, 2014, 博士
【摘要】 光子晶体在表面处被截断,其平移对称性被打破,会在禁带内引入表面态,这种表面态对应的电磁波沿着光子晶体与外界媒质的界面传播,被称为布洛赫表面波(Bloch surface waves, BSWs)。表面态的存在可以改变电磁场的空间分布,使得表面处的局域电磁场增强,这种表面增强的电磁场可用来加强相关的光学效应,如表面荧光效应、表面拉曼散射效应等。一维光子晶体布洛赫表面波与被广泛研究的表面等离子共振非常相似,被激发时反射谱上会出现很窄的共振dip,这个共振dip出现的位置及强度会随着外界折射率的变化而变化。因此可以开发新型的布洛赫表面波传感器。本论文用平面波展开法系统研究了一维光子晶体表面态的色散关系、场分布等基本性质,讨论了一维光子晶体表面态的起源,表面态的存在规律,给出了一维光子晶体布洛赫表面波器件的理论设计方法。用传输矩阵方法研究了布洛赫表面波的传输特性。我们首次在理论及实验上研究了一维光子晶体布洛赫表面波的相位特性及传感应用。我们也首次提出了基于相位检测的新型布洛赫表面波传感技术,有望应用在免标记的生物化学传感领域。本论文共由六章组成。第一章,首先简单介绍了光子晶体的概念和一般性质,回顾了近几年光子晶体及相关领域的一些重要进展,并综述了一维光子晶体布洛赫表面波的研究历史和研究现状。第二章,介绍了本论文中所用的理论研究方法。简要介绍了光子晶体能带计算中常用的平面波展开法;详细推导了传输矩阵方法,高斯光束角谱分解法。简要介绍了我们编写的基于传输矩阵法的计算程序,该程序可以实现包括多层薄膜的反射率和透过率,反射相位,椭偏参数,场分布等参数的计算;并且结合高斯光束角谱分解方法,本程序还可以实现高斯光束在多层薄膜中传输的可视化,也能够计算高斯光束的反射率等参数。第三章,用平面波展开法系统地研究了一维光子晶体表面态的基本性质。研究表明TM和TE偏振都可以存在表面态;表面态的场强局域在表面,而且对于越大的平行波矢,局域性也越强。讨论了表面态的起源,我们确认了光子晶体表面态起源于有限光子晶体的带边态。由于表面截断效应,一维光子晶体最末层所处的环境与内层所处的环境不同,当条件合适时,带边态从通带中分离出来进入禁带,形成表面束缚模式。研究了最末层对一维光子晶体表面态的影响。为此我们引入了截断参数t。考虑厚度为dH的高折射率层及厚度为dL的低折射率层交替组成的一维光子晶体,光子晶体晶格常数为a,不同的ta值就表示了光子晶体表面的截断情况。当ta<1/2dH和ta>1/2dH+dL时,对应的最末层是高折射率层。当取1/2dH<ta <1/2dH+dL时,对应的最末层是低折射率层。研究表明表面态不是在任意结构的光子晶体表面都能够存在。对于第一个禁带来说,我们总结了如下规律:1.平行波矢很大时。当1/2dH<ta<1/2dH+dL(?)寸,光子晶体结构不存在表面态;当ta<1/2dH时,TM偏振和TE偏振都存在表面态,且随着最外层厚度的减小,表面态的频率也越高。ta>1/2dH+dL时, TM偏振不存在表面态,TE偏振也只在ta≈1a时才能存在表面态,且表面态频率位于第二个通带底部的带边。当ta趋近dH/2时,表面态的频率越靠近第一通带顶部的带边。2.平行波矢较小时。表面态存在的规律与上述平行波矢大的情况类似,只是不那么严格遵循上述规律。当ta<1/2dH时,且在ta≈0或1a时,TE偏振的表面态近似位于禁带中间,而TM偏振的表面态位于第二个通带的底部附近。ta>1/2dH+dL时,TM偏振仍然无表面态,而TE偏振在ta较大范围内都存在表面态。ta趋近1/2dH时,表面态频率越靠近第一通带顶部的带边。而当1/2dH<ta<1/2dH+dL时,TE偏振和TM偏振仍然无表面态存在。在ta大于dH/2+dL+dH/4时TE偏振可以存在表面态。第四章中,给出了一维光子晶体布洛赫表面波的器件理论设计方法,并在理论上研究了一维光子晶体布洛赫表面波的传输性质。当布洛赫表面波被激发时,相应地反射谱中会出现共振dip,由于光子晶体是由损耗很小的全介质组成的结构,布洛赫表面波的共振dip非常尖锐,理论上半高宽一般小于0.1。。我们的计算表明,光子晶体的周期以及组成光子晶体的各介质层的损耗对dip的半高宽和深度都有很大影响。为了得到性能良好的器件,需要根据各介质层的损耗及其周期数进行优化。我们也研究了一维光子晶体的反射相位特性,结果表明在发生布洛赫表面波共振时,相应的反射相位会发生迅速改变,而且共振位置附近反射相位曲线的陡度与构成布洛赫表面波器件的一维光子晶体周期数及损耗有很大关系。当他们发生改变时,共振dip的深度达到最大,且反射相位曲线的斜率接近无穷大,此时斜率的符号也会发生改变。特别地当介质损耗很小甚至无损耗时,反射率dip深度会变小甚至消失,而此时反射相位仍然急剧变化。在这种情况下,布洛赫表面波的反射相位比反射强度更为敏感,对于布洛赫表面波器件的传感应用来说,采用检测相位的方式更为合适。我们联合了传输矩阵和高斯光束角谱分解方法,分析了高斯光束发散角对布洛赫表面波反射率的影响,给出了这一影响的定量结果。我们的研究表明,高斯光束的发散角会展宽布洛赫表面波的共振dip,并使共振dip变浅。当组成布洛赫表面波器件的各介质层的消光系数很小时,被展宽的dip的宽度接近高斯光束的发散角。这种dip的展宽效应会劣化布洛赫表面波器件的响应和灵敏度。为了提高BSW检测限及灵敏度,实验中可以使用具有准直性更好的激光光束。利用本方法,我们也实现了一维光子晶体布洛赫表面波的可视化工作,这一方法可以模拟实际光束激发布洛赫表面波的情形,可以更直观地研究布洛赫表面波的传输特性,如传输距离、布洛赫表面波引起的巨古斯汉森效应等。理论模拟表明,对于我们设计的器件,布洛赫表面波的传播距离可达300μ m。第五章中,我们用离子辅助电子束蒸发法在以半圆柱型棱镜上交替生长了Ti02和Si02周期性多层薄膜结构,成功制备了一维光子晶体布洛赫表面波器件。利用传输矩阵法理论计算了所制备器件的椭偏参数,并用全椭圆偏振光谱技术表征了该器件的特性。实验结果表明,我们制备的器件可以支持布洛赫表面波的传输,椭圆偏振光谱技术可以有效地表征布洛赫表面波器件进行。通过对椭偏参数ψ和△的测量,能够同时得到布洛赫表面波器件的反射光的幅值及相位信息,能够有效对布洛赫表面波器件的反射相位特性进行研究。实验结果也验证了理论上预期的布洛赫表面波激发引起的相位突变这一重要特性。我们用所制备的布洛赫表面波器件进行了传感应用的研究,提出了基于相位检测的新型布洛赫表面波传感器。我们分别采用角度扫描以及波长扫描两种方法,用不同浓度的溶液作为检测物质,在实验上证明了布洛赫表面波的反射相位对一维光子晶体外部折射率的微小变化非常敏感.我们制备的角度扫描型布洛赫表面波器件的角度灵敏度约为42.94°/RIU,其相位灵敏度高达6.57×103。/RIU,基于相位探测的布洛赫表面波器件的折射率检测限可达6.09×10-6RIU;我们制备的波长扫描型布洛赫表面波器件的灵敏度为631nm/RIU,我们也通过引入品质因子,评价了相位灵敏度,结果表明基于相位检测的布洛赫表面波传感器的灵敏度比传统的基于强度检测的布洛赫表面波传感器的灵敏度高近一个数量级。总之,在本论文中,我们系统地研究了一维光子晶体表面态的基本性质,总结了表面态存在的规律,为设计一维光子晶体布洛赫表面波器件提供了理论依据。总结了有限一维光子晶体周期数和介质损耗对布洛赫表面波的反射率和反射相位的影响。我们也利用椭圆偏振光谱技术成功表征了布洛赫表面波器件的传输特性,首次在实验上验证了布洛赫表面波引起的反射相位突变,我们的研究表明可以利用反射相及传感应用。并首次提出了基于相位检测的新型布洛赫表面波传感技术,有望应用在免标记的生物化学传感领域。
【Abstract】 As photonic crystals are truncated at the surface, surface states will be introduced into the forbidden band due to the broken translational symmetry. Electromagnetic waves corresponding to surface states propagate along the interface of photonic crystals and external medium, known as Bloch surface waves (BSWs). Surface states can change the spatial distribution of the electromagnetic field, leading to the enhancement of the local electromagnetic field on the surface, which can be used to enhance relevant optical effects, such as surface fluorescence effect, surface Raman effect etc. BSWs of one-dimensional photonic crystals (1D-PC) are quite similar to the widely studied surface plasmon resonance:when they are excited, a narrow resonance dip will appear on the corresponding reflection spectrum; the location and intensity of this resonance dip vary with the external refractive index. Therefore, we can employ the properties of BSWs to develop new optical sensors based on detecting the refractive index.This thesis systematically studies properties (such as dispersion relation, field distribution etc) of surface states of ID-PCs with the plane wave expansion method, discusses the origin of surface states of1D-PCs and rules of surface states’existing, and also offers the theoretical approach to designing1D-PC BSW devices. The transmission properties of BSWs are studied with the transfer matrix method. We are the first to study the reflection phase properties and its sensing applications of1D-PC BSWs in both theory and experiment, and also the first to propose a BSW sensing technique based on phase detection, which promises applications in the label-free biological and chemical sensing.This thesis consists of six chapters.In Chapter1, we briefly introduce the concept of photonic crystals and their general properties, and review some important progress in photonic crystal fields in recent years, the research history and research status of1D-PC BSWs.In Chapter2, we introduce the theoretical research methods used in this thesis. The plane wave expansion method commonly used in the calculation of photonic crystal band is introduced briefly, the transfer matrix method and angular spectrum decomposition for a Gaussian beam are deduced in detail. Besides, calculation program written by the author based on the transfer matrix method is also introduced, which can calculate reflectivity and transmittance, reflection phase, ellipsometric parameters and field distribution of multilayer films, and, with the aid of angular spectrum decomposition for a Gaussian input beam, also can visualize transmission of Gauss beam in multilayer films and calculate reflectivity of Gauss beam.In Chapter3, the basic properties of ID-PC surface states are studied systematically with the plane wave expansion method. Our studies show that for TM and TE polarization, surface states can both exist, but their dispersion curves do not coincide and so they will not be excited at the same time; the field of surface states is localized at the surface, and the larger the parallel wave vector is, the higher the locality is.We discuss the origin of surface states, and furthermore confirm that the surface states of photonic crystals originate from band-edge states of finite photonic crystals. The surface termination result in difference in environment between the last layer and the internal of1D-PCs. In suitable conditions, band-edge states separate from the allowed band and enter the forbidden band, forming surface bound mode.Effects of the last layer on ID-PC surface states are also studied, for the sake of which we introduce truncation parameter t. For ID-PC composed of alternatively layer of high refractive index with a thickness dH and the layer of low refractive index with a thickness dL, lattice constant is a. with different ta value representing different truncation on the surface of photonic crystals. When ta<1/2dH and ta>1/2dH+dL, the corresponding last layer is the layer of high refractive index; when1/2dH<ta<1/2dH+dL, it is the layer of low refractive index. It is shown that surface states do not exist for all the range of ta values. For the first forbidden band, we conclude rules as follows:1When parallel wave vector is large. In addition to the light cone of the dielectric layer with low refractive index, the band structure, dispersion curves for TE and TM polarizations are very similar. In this area, when1/2dH<ta<1/2dH+dL, there is no surface state. When ta<1/2dH, for TE and TM polarizations surface states can exist. And the thiner the last layer is.the higher frequency of the surface states is. When ta>1/2dH+dL,for TM polarizations surface states can not exist, but for TE polarizations surface states can exist at ta≈1a and are located close to at the bottom of the second allowed band. When ta gradually gets close to dH/2, surface states gets close to the band edge on the top of the first allowed band.2. When parallel wave vector is small. The law is similar to the case in which wave vector is large, but the case follows the rules less strictly. When ta≈0or1a, surface state for TE polarization is located close to the center of the forbidden band, but surface state for TM polarization is located close to at the bottom of the second allowed band. When ta>1/2dH+dL, TM polarization still has no surface states while TE polarization does for the wide range of ta values; the closer ta gets to dH/2, the closer surface states gets to the band edge on the top of the first allowed band. However, when1/2dH<ta<1/2dH+dL, for TE andTM polarization no surface states can exist.Ror TE polarization surface state can exist the when ta>dH/2+dL+dH/4, and the surface state is quite close to the top of the first band.In Chapter4, we propose the theoretical design method of1D-PC BSW devices and study theoretically transmission properties of1D-PC BSWs. When BSW is excited, a resonance dip will appear in the reflection spectrum. As the photonic crystals are composed of all-dielectric with little loss, resonance dips of BSWs are very sharp, their full width at half maximum (FWHM) is less than0.1. Our calculations show that periodicity of photonic crystals and loss of dielectric layer composed of photonic crystals both have a great impact on FWHM and depth of resonance dip. In order to get good devices, periodicity of photonic crystals is optimized according to the loss of dielectric layers.We also study the reflection phase properties of BSWs on1D-PCs. It is shown that when the BSW resonance occurs, the corresponding reflection phase changes rapidly. Gradient of reflection phase curve near resonance location depend on the periodicity and loss of1D-PC composed of the BSW device. In contrast to the reflectivity behavior, when there is little or no loss of dielectric, the depth of resonance dip will dwindle or even disappear, but the phase keep a large steepness. In this case, reflectivity phase has been more sensitive compared to the reflectivity intensity, so detecting phase is more alternative for BSW sensing applications.Combining the transfer matrix method with angular spectrum decomposition for a Gaussian input beam, We analyze the influence of Gauss beam divergence angle on the BSW reflectivity and offer quantitative results of this influence. Our studies shows that. Gauss beam divergence angle will widen and shallow resonance dip of BSWs, and the width of the widened dip approximates the Gauss beam angle, thus affecting the sensitivity and detection limit of BSW devices. The laser beam with better collimation can be used to improve the detection liimt and sensitivity of BSW device. With angular spectrum decomposition method for a Gaussian input beam, we also realize the visualization of1D-PC BSWs. This method can simulate the actual situation in which the beam excites the BSW, can help study more intuitively the transmission characteristics, such as propagation distance of BSWs and the giant Goos-Hanchen Effect caused by BSWs. For our device, propagation distance of BSWs is about300u m. Despite a large amount of calculation, it is a good research method.In Chapter5, we alternatively grow TiO2and SiO2multilayer films using the electron beam evaporation method assisted by ion-beam on a semi-cylindrical prism, and successfully fabricate1D-PC BSW devices. The ellipsometric parameters of the fabricated devices are computed with the transfer matrix method, and reflection properties of the designed device are characterized by spectroscopic ellipsometry. Our experimental results show that the ID-PC device we design can sustain the BSWs and spectroscopic ellipsometry can effectively characterize BSW device. By measuring ellipsometric parameters and, we can obtain both the reflection amplitude and phase information of BSWs simultaneously. Spectroscopic ellipsometry can characterize reflection phase properties of BSW devices. The experimental results also verify expected abrupt phase change induced by the BSW resonance in the1D PC.We study sensing applications of BSW devices and propose a BSW sensor based on phase detection. With respectively the angular interrogation and wavelength interrogation methods, we prove experimentally that the reflection phase of BSWs is very sensitive to minor changes in the external refractive index of ID-PC; The angular sensitivity of our BSW devices based on angular interrogation is42.94o/RIU, and the phase sensitivity is as high as6.57X103o/RIU. The sensitivity of our BSW devices based on wavelength interrogation is631nm/RIU. We also evaluate their phase sensitivity by figure of merit (FOM). It is demonstrated that the phase sensitivity of the BSW device is higher by nearly1order of magnitude than its amplitude sensitivity.In this these, we study systematically properties of1D-PC surface states, summerize the existing laws of surface states and supply theoretical foundation for the design of1D-PC BSW devices. We also summarize the effect of finite ID-PC periodicity and dielectric loss on reflectivity and reflection-phase of BSWs and successfully characterize transmission properties of BSW devices using the spectroscopic ellipsometry. We are the first to study the reflection phase properties and their sensing applications of1D-PC Bloch surface waves in both theory and experiment, also the first to present the BSW sensing technique based on phase detection, which promises the application in label-free biological and chemical sensing field.
【Key words】 photonic crystals; Bloch surface waves; reflection phase; opticalsensing;