Direct Monitoring of Thickness and Refractive Index of Optical Thin Film Deposited on Fiber End-face

We propose a system for depositing thin films on waveguides which enables low-temperature deposition and precise control of the refractive index and film thickness. It is composed of a conventional ion-beam sputtering (IBS) system and a new system for directly monitoring film characteristics during deposition. We controlled refractive indices over a wide range from 1.52 to 1.97 by moving the sputtering targets (SiO₂ and Si₃N₄) in the IBS system. The refractive index or film thickness was in-situ monitored by observing the optical power reflected from the end-face of a monitoring fiber set in the deposition chamber. Antireflection coating films were successfully deposited on a fiber end-face and a laser diode chip facet with low reflectivity from 0.05 to 0.07%. This deposition system is attractive for constructing highly functional optical devices for future photonic networks.     

光学材料光学均匀性检测方法分析

光学均匀性是光学材料的重要指标,直接影响到透射光学系统的波面质量,改变系统的波相差。惯性约束聚变（Inertial Confine Fusion,ICF）激光驱动器的研制要求对材料的光学均匀性进行高精度的检测,同时兼顾洁净度要求。实验中利用斐索干涉仪实现了大口径光学材料光学均匀性的检测,并与国外检测数据进行了对比,对检测过程中的影响因素主要包括样品的厚度测量偏差及折射系数偏差进行了分析。结果表明,样品的厚度测量偏差及折射系数偏差对结果的影响较小,可以忽略。同时用两种干涉仪专用软件对大量样品测量数据进行处理,对比了不同干涉仪光学均匀性的计算结果,表明这两种情况下对光学均匀性的处理结果相符,解决了大口径光学坯件光学均匀性的检测问题。     

Optical parameters of epitaxial GaN thin film on Si substrate from the reflection spectrum

A method has been proposed for determining the optical properties of a thin film layer on absorbing substrates. The film optical parameters such as thickness, refractive index, absorption coefficient, extinction coefficient and the optical energy gap of an absorbing film are retrieved from the interference fringes of the reflection spectrum at normal incidence. The envelopes of the maxima of the spectrum E_M and of the minima E_m are introduced in analytical forms to find the reflectance amplitudes at the interfaces and approximate values of the thin film refractive index. Then, the interference orders and film thickness are calculated to get accurate values of the needed optical parameters. There are no complex fitting procedures or assumed theoretical refractive index dispersion relations. The method is applied to calculate the optical properties of an epitaxial gallium nitride thin film on a silicon (1 1 1) substrate. Good agreement between our results and the published data are obtained.     

Optical properties of hexagonal boron nitride thin films deposited by radio frequency bias magnetron sputtering

The optical properties of hexagonal boron nitride (h-BN) thin films were studied in this paper. The films were characterized by Fourier transform infrared spectroscopy, UV--visible transmittance and reflection spectra. h-BN thin films with a wide optical band gap E_g (5.86 eV for the as-deposited film and 5.97 eV for the annealed film) approaching h-BN single crystal were successfully prepared by radio frequency (RF) bias magnetron sputtering and post-deposition annealing at 970~K. The optical absorption behaviour of h-BN films accords with the typical optical absorption characteristics of amorphous materials when fitting is made by the Urbach tail model. The annealed film shows satisfactory structure stability. However, high temperature still has a significant effect on the optical absorption properties, refractive index n, and optical conductivity σ of h-BN thin films. The blue-shift of the optical absorption edge and the increase of E_g probably result from stress relaxation in the film under high temperatures. In addition, it is found that the refractive index clearly exhibits different trends in the visible and ultraviolet regions. Previous calculational results of optical conductivity of h-BN films are confirmed in our experimental results.     

SiO2和HfO2薄膜光学性能的椭偏光谱测量

结合XRD和原子力显微镜等方法,利用椭圆偏振光谱仪测试了单层SiO2薄膜（K9基片）和单层HfO2薄膜（K9基片）的椭偏参数,并用Sellmeier模型和Cauchy模型对两种薄膜进行拟合,获得了SiO2薄膜和HfO2薄膜在300-800nm波段内的色散关系。用X射线衍射仪确定薄膜结构,并用原子力显微镜观察薄膜的微观形貌,分析表明：SiO2薄膜晶相结构呈现无定型结构,HfO2薄膜的晶相结构呈现单斜相结构;薄膜光学常数的大小和薄膜的表面形貌有关;Sellmeier和Cauchy模型较好地描述了该波段内薄膜的光学性能,并得到薄膜的折射率和消光系数等光学常数随波长的变化规律。     

Calculation of optical constant of amorphous germanium arsenoselenide wedge-shaped thin films from their shrunk transmittance and reflectance spectra

The optical transmittance spectrum is influenced by inhomogeneities in germanium arsenoselenide thin films. The non-uniformity of thickness, found under the present deposition conditions, gives rise to a clear shrinking of the interference fringes of the transmittance spectrum at normal incidence. Inaccuracies and even serious errors occur if the refractive index and film thickness are calculated from such a shrunken transmittance spectrum, under the unrealistic assumption that the film is uniform. The analytical expressions proposed by Swanepoel [J. Phys. E. Sci. Instrum. 17 (1984) 896] enabled derivation of the refractive index and film thickness of a wedge-shaped thin film from its shrunk transmittance spectrum. This method was applied in this study making it possible to derive the refractive index and average thickness to an accuracy better than 1%. Dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model [Phys. Rev. B 3 (1971) 1338]. The absorption coefficient and, thus the extinction coefficient, can be calculated from transmittance and reflectance spectra in the strong absorption region. The optical energy gap is derived from Tauc's extrapolation [Amorphous and Liquid Semiconductor (Plenum Press, New York, 1974)]. The relationship between the optical gap and chemical composition in the Ge _x As_{30– x} Se₇₀ (with 0 ≤ x ≤ 30) amorphous system is discussed in terms of the chemical bond approach and cohesive energy.     

自适应模拟退火遗传算法在薄膜厚度及光学常数反演中的应用

准确的测量薄膜的厚度和光学常数,在薄膜的制备、研究和应用中都是十分重要的。借助Cauchy色散模型,通过薄膜透过率测量曲线,用改进的自适应模拟退火遗传算法对透过率曲线进行全光谱拟合,从而反演得到薄膜的厚度和光学常数。对由电子束蒸发制备的TiO2单层膜和SiO2/TiO2双层膜的厚度和光学常数进行了测量计算。实验结果表明,计算得到的光学参数与实测结果相一致,厚度误差小于2nm,在560nm波长处折射率误差小于0.03。     

An Approach to Optical Reflection Tomography along the Geometrical Thickness

We propose and demonstrate a novel optical reflection tomography along the geometrical thickness. This technique is based on simultaneous measurement of refractive index n and thickness t of a sample using the combination of a low coherence interferometer and confocal optics. The interferometer provides optical coherence tomography (OCT) of the dimension of the optical thickness (=n × t) along the optical axis, while the confocal optics gives us another type of reflection tomography, having the thickness dimension of nearly t/n along the optical axis. This sort of tomography can be called confocal reflection tomography (CRT) and has not yet been demonstrated, to our knowledge. Simple image processing of OCT and CRT results in the desired reflection tomographic image, showing two-dimensional refractive index distribution along the geometrical thickness.     

基底温度对电子束蒸发制备氧化铝薄膜的影响

为了考察基底温度对氧化铝薄膜折射率以及沉积厚度的影响情况,在不同基底温度环境下,通过离子辅助电子束蒸发方式,在玻璃基底上制备了同一Tooling因子条件下所监测到相同厚度的Al2O3薄膜,利用分光光度计测量光谱透过率,依据光学薄膜相关理论,计算了基底温度在25℃～300℃范围内获得的膜层实际物理厚度为275.611 nm～348.447 nm,以及膜层折射率的变化。通过对实验结果的数值计算和曲线模拟,给出了基底温度对于薄膜的折射率和实际厚度的影响情况。     

沿膜厚方向的簿膜非均匀折射率及其色散测量

以传统的光学监控法与石英晶体振荡法为基础,在薄膜沉积过程中,同时测量膜片的透过率及膜层的厚度,以及与它们对应的控制波长,并根据三者与折射率之间的关系,由计算机反解出不同膜厚处的折射率——非均匀折射率及不同厚度、不同波长处的薄膜折射率——非均匀色散。同时给出了硫化锌薄膜在水晶石薄膜的测量实例,并进行了分析与讨论。     

Maxima of the spectral reflectance ratio of polarized waves used to measure the thickness of a nonabsorbing thin film

A simple method to determine the thickness of a nonabsorbing thin film on an absorbing substrate from maxima of the ratio between the spectral reflectances of p- and s-polarized components reflected from the thin-film structure is presented. The spectral reflectance ratio, which can be measured in a simple polarimetry configuration at a fixed angle of incidence, consists of maxima whose number and positions depend on the thickness of a thin film. An approximative linear relation between the thin-film thickness and a wavelength of the maximum of the reflectance ratio for a specific angle of incidence is revealed, provided that the wavelength-dependent refractive index of the thin film is known and the substrate is weakly absorbing. The relation permits the calculation of the thickness from the measured spectral reflectance ratio by using one maximum only, as is demonstrated theoretically for a SiO₂ thin film on a Si substrate. The application of this method is demonstrated experimentally for the same thin-film structure with different thicknesses of the SiO₂ thin film. The results are compared with those given by the algebraic fitting method, and very good agreement is confirmed.     

薄膜厚度对AgInSbTe相变薄膜的光学性质的影响

采用射频溅射法制备了Ag8In14Sb55Te23相变薄膜,对深积态薄膜在300℃时进行了热处理,测量了不同厚度薄膜的反射、吸收谱及光学常数。研究了薄膜的光学常数与薄膜厚度的关系。结果表明在一定的厚度范围其光学常数随膜层厚度的不同有较大的变化,尤其在短波长范围内更为明显,这对于短波长记录相变光盘有重要意义。     

基底微缺陷对介质薄膜光学性能影响的理论研究

基于非均匀膜理论提出一种存在微缺陷的介质基底的折射率分层模型，将基底依次分为表面层、亚表面层和体材料层，其中表面层和亚面层分别等效为折射率服从统计分布的非均匀膜，将它们分别再次细分为N₁和N₂个子层，每一子层均视为均匀介质 膜.应用光学薄膜特征矩阵法对其进行理论分析，并对单层介质膜的光学性能进行数值计算. 研究结果表明：基底的表面和亚表面微缺陷改变了薄膜和基底的等效折射率，导致了准Brew ster角和组合反射率与理想情形的偏离.同时这些微缺陷也改变了光在薄膜和基底中的传播 特性，因此反射相移和相位差均偏离理想情形.在研究基底的微缺陷对多层介质膜光学性能 影响的分析和计算时，该模型同样适用. 关键词： 微缺陷 介质薄膜 非均匀膜 光学性能     

The relationship between refractive index-energy gap and the film thickness effect on the characteristic parameters of CdSe thin films

CdSe thin films were deposited on glass substrates using Successive Ionic Layer Adsorption and Reaction (SILAR) method at room temperature and ambient pressure. The relationship between refractive index and energy bandgap was investigated. The film thickness effect on the structural, morphological, optical and electrical properties of CdSe thin films was investigated. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies showed that all the films exhibit polycrystalline nature with hexagonal structure and are covered well with glass substrates. The crystalline and surface properties of the films improved with increasing film thickness. The optical absorption studies revealed that the films are found to be a direct allowed transition. The energy bandgap values were changed from 1.93 to 1.87 eV depending on the film thickness. The electron effective mass (m_e^?/m_o), refractive index (n), optical static and high frequency dielectric constant (ε_o, ε_∞) values were calculated by using the energy bandgap values as a function of the film thickness. The resistivity of the films changed between 10⁶ and 10² Ω-cm with increasing film thickness at room temperature.     

Preparation and optical constants of the nano-crystal and polymer composite Bi4Ti3O12/PMMA thin films

Bismuth titanate, Bi₄Ti₃O₁₂ (BTO), is a typical ferroelectric material with useful properties for optical memory, piezoelectric and electro-optic devices. Its nano-crystals were compounded by the chemical solution decomposition technique. Its structure and size were analyzed by X-ray diffraction and transmissive electron microscopy. The composite thin film of BTO nano-crystals and high transparency polymethylmethacrylate (PMMA) polymer was prepared by spin coating. The transmitted spectrum of BTO/PMMA composite thin film in 300–1500 nm was measured. The film thickness d and the optical constants of the film, such as the refractive index n, the absorption coefficient α, and the extinction coefficient κ were obtained using the data from the transmitted spectrum.     

Determination of optical properties in nanostructured thin films using the Swanepoel method

We present the methodological framework of the Swanepoel method for the spectrophotometric determination of optical properties in thin films using transmittance data. As an illustrative case study, we determined the refractive index, thickness, absorption index, and extinction coefficient of a nanostructured 3 mol% Y₂O₃-doped ZrO₂ (yttria stabilized zirconia, 3YSZ) thin film prepared by the sol-gel method and deposited by dipping onto a soda-lime glass substrate. In addition, using the absorption index obtained with the Swanepoel method, we calculated the optical band gap of the film. The refractive index was found to increase, then decrease, and finally stabilize with increasing wavelength of the radiation, while the absorption index and extinction coefficient decreased monotonically to zero. These trends are explained in terms of the location of the absorption bands. We also deduced that this 3YSZ thin film has a direct optical band gap of 4.6 eV. All these results compared well with those given in the literature for similar thin films. This suggests that the Swanepoel method has an important role to play in the optical characterization of ceramic thin films.     

Optical constants for Ge30− x Se70Ag x (0 ≤ x ≤ 30 at%) thin films based only on their reflectance spectra

In this paper, different homogenous compositions of Ge_{30? x} Se₇₀Ag _x (0?≤?x?≤?30 at%) thin films were prepared by thermal evaporation. Reflection spectra, R(λ), for the films were measured in the wavelength range 400–2500?nm. A straightforward analysis proposed by Minkov [J. Phys. D: Appl. Phys. 22 (1989) p.1157], based on the maxima and minima of the reflection spectra, allows us to derive the real and imaginary parts of the complex index of refraction and the film thickness of the studied films. Increasing Ag content at the expense of Ge atoms is found to affect the refractive index and the extinction coefficient of the films. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. Optical absorption measurements were used to obtain the fundamental absorption edge as a function of composition. With increasing Ag content, the refractive index increases while the optical band gap decreases. The compositional dependence of the optical band gap for the Ge_{30? x} Se₇₀Ag _x (0?≤?x?≤?30) thin films is discussed in terms of the chemical bond approach.     

基于反射和透射光谱的氢化非晶硅薄膜厚度及光学常量计算

采用紫外-可见透射光谱仪测量了对靶磁控溅射沉积法制备的氢化非晶硅（a-Si:H）薄膜的透射光谱和反射光谱.利用T/(1-R)方法来确定薄膜的吸收系数,进而得到薄膜的消光系数|通过拟合薄膜透射光谱干涉极大值和极小值的包络线来确定薄膜折射率和厚度的初始值,并利用干涉极值公式进一步优化薄膜的厚度值和折射率|利用柯西公式对得到的薄膜折射率进行拟合,给出了a-Si:H薄膜的色散关系曲线.为了验证该方法确定的薄膜厚度和光学常量的可靠性,将理论计算得到的透射光谱与实验数据进行了比较,结果显示两条曲线基本重合,可见这是确定a-Si:H薄膜厚度及光学常量的一种有效方法.     

Measurement of absolute phase Shift on reflection of thin films using white-light spectral interferometry

A novel method to measure the absolute phase shift on reflection of thin film is presented utilizing a white-light interferometer in spectral domain. By applying Fourier transformation to the recorded spectral interference signal, we retrieve the spectral phase function Ф, which is induced by three parts： the path length difference in air L, the effective thickness of slightly dispersive cube beam splitter Teff and the nonlinear phase function due to multi-reflection of the thin film structure. We utilize the fact that the overall optical path difference （OPD） is linearly dependent on the refractive index of the beam splitter to determine both L and Teff. The spectral phase shift on reflection of thin film structure can be obtained by subtracting these two parts from Ф. We show theoretically and experimentally that our new method can provide a simple and fast solution in calculating the absolute spectral phase function of optical thin films, while still maintaining high accuracy.     

射频磁控反应溅射氮氧化硅薄膜的研究

利用SiOxNy薄膜光学常数随化学计量比连续变化的特性,给出了制备折射率连续可调的SiOxNy薄膜的实验条件。用磁控反应溅射法制备了不同氮氧比的SiOxNy薄膜。研究了不同气流比率条件下薄膜光学常数、化学成分及溅射速率等的变化。用UV-VIS光谱仪测试了透射率曲线,利用改进的单纯型法拟合透射率曲线计算得到了折射率和消光系数。测试了红外傅立叶光谱(FTIR)曲线和X光光电子能谱(XPS)分析了薄膜成分的变化。实验表明薄膜特性与N2／O2流量比率密切相关,通过控制总压和改变气体流量比可控制SiOxNy薄膜的折射率n从1．92到1．46连续变化,应用Wemple-DiDomenico模型计算出光子带隙在6．5eV到5eV之间单调变化。