Third-Order Nonlinear Optical Properties of ALD Grown TiO2 Films by Thermally Managed Z-scan Method
Third-Order Nonlinear Optical Properties of ALD Grown TiO2 Films by Thermally Managed Z-scan Method
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通过热管理 Z 扫描方法 ALD 生长的 TiO2 薄膜的三阶非线性光学特性
DOI：
10.1364/fio.2018.jw4a.37
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发表时间：
2018
期刊：
影响因子：
3.6
通讯作者：
A. Johnson
中科院分区：
文献类型：
--
作者：
Isaac Basaldua;Paul Burkins;R. Kuis;J. A. Kropp;T. Gougousi;A. Johnson
在ALD生长的TiO 2膜上进行的热管理Z扫描表明，在100°C和250°C下生长的膜的n2值分别为1.7x10-11和1.94x10 - 10 cm 2/W-大于其他生长方法的1000倍。OCIS代码：（190.0190）非线性光学;（320.0320）超快光学下一代高速光子器件的发展，如超快集成调制器，需要新的材料将大的光学非线性[1]-[2]。从大块晶体或液体切割的已建立的非线性材料不适合与CMOS技术集成。除了全光片上器件应用外，表现出高非线性吸收和快速响应时间的材料在光限幅应用中也很有用[3]，用于保护光学传感器和人眼免受高强度光（如激光）的影响[4]。过去提出的用于光限幅的典型材料是半导体、富勒烯、碳纳米管、纳米结构材料如纳米颗粒、石墨烯、掺杂在干凝胶和溶胶凝胶膜中的非线性吸收体、玻璃、滤光器、有机/无机簇以及2D原子晶体和有机染料分子。对于这些材料中的大多数，在它们的光限幅能力和损伤阈值以及响应时间之间存在折衷。这些材料中的绝大多数不适合覆盖大面积区域，而敏感应用（如红外对抗措施传感器）需要一致的可重复性。因此，需要具有相当大的非线性光学性质的CMOS兼容材料。使用原子层沉积（ALD）生长的薄膜和纳米层压材料具有满足这一需求的潜力。据报道[5]，通过ALD生长的ZnO/Al 2 O3薄膜的三阶非线性分别比硼硅酸锌玻璃和普通ZnO薄膜大200倍和13倍。非线性的增强是由于通过沉积非晶Al 2 O3来控制氧化锌层的结晶度。氧化钛是一种在可见光和红外波长下用于非线性纳米光子器件的有前途的材料，使用这种技术制造[6]。有几个条件可以影响TiO 2薄膜的类型，进而影响非线性特性，例如生长和退火温度。我们已经证明[7]，无定形ALD生长的二氧化钛膜具有比通过其他方法生长的类似膜（1x 10 -15[8] -5.9x10 - 13 [9] cm 2/W）大得多的非线性系数n2（1.2x10-11-7.8x10-10 cm 2/W）。因此，我们报告了在100°C（样品1）和250°C（样品2）的温度下通过ALD生长的60 nm厚的TiO 2膜上进行的实验。对样品1和2进行了热管理Z扫描实验[10]，得到的n2值分别为1.7x10-11和1.94 x10- 10 cm 2/W。每个样品的退火（在空气中450°C的温度持续3小时）版本不产生可辨别的Z扫描迹线，即，低于我们的检测极限样品的XRD表征表明，沉积态样品为非晶态，而热处理样品为部分晶化。样品的晶化对非线性光学性能有一定的影响，但根据实验结果，ALD法制备的TiO 2薄膜是一种很有前途的非线性光学材料。2.实验和结果实验装置包括标准Z扫描[11]装置（见图1），该装置采用机械斩波器，该斩波器经过修改，可有效降低激光的重复频率，以最大限度地减少热效应。采用闭孔Z扫描技术测量了样品的非线性折射率。为了生成Z扫描轨迹，光束（在我们的情况下为76 MHz 100 fs 800 nm激光器）最初被分裂，产生两条路径，图1;一条路径将未受干扰的光束直接发送到检测器D1，而其余光束穿过样品。测试中的样品被定位在平移台上，使得其可以移动通过束腰（光束的最聚焦区域），通常从靠近聚焦透镜开始并且在正z方向上移动离开。光束离开样品并一分为二;反射的扩展光束直接聚焦到检测器D3中，称为开放孔径（OA）Z扫描，而透射部分穿过远场中的针孔孔径，最终撞击检测器D2，称为闭合孔径（CA）Z扫描。图2示出了从样品2生成的Z扫描迹线。样品1和2的n2分别为1.7x10-11和1.94x10-10 cm 2/W，比其他方法生长的TiO 2大3 - 4个数量级[8]-[9]。样品的热处理导致痕迹变得不可辨别（图2b）。这表明，退火后的n2值已降低到低于我们的系统的灵敏度的水平。Fig. 1.基本CA和OA Z扫描实验装置。样品的X射线衍射分析表明，所沉积的样品是无定形的，并且热处理的样品是部分结晶的（样品1：金红石和样品2：金红石）。因此，由于热处理，样品中微晶的形成可以显著改变n2的大小。图二.样品2a）在250 ℃下沉积和B）在450 ℃下退火3小时后的Z扫描迹线。3.结论在100 oC和250 oC温度下用ALD法制备的TiO 2薄膜的非线性折射率n2得到了测量。测得的非线性折射率值远大于用其他方法沉积的薄膜。退火处理使薄膜部分晶化，降低了薄膜的有效非线性折射率。然而，通过ALD生长的非晶TiO 2薄膜显示出作为非线性光学材料的前景。
Thermally managed Z-scan performed on ALD grown TiO2 films demonstrated n2 values of 1.7x10-11 and 1.94 x10-10cm2/W for films grown at 100°C and 250°C, respectively – greater than 1000X that of other growth methods. OCIS codes: (190.0190) Nonlinear optics; (320.0320) Ultrafast optics Development of next-generation high-speed photonics devices, such as ultrafast integrated modulators, require novel materials will large optical nonlinearities [1]-[2]. Established nonlinear materials cut from bulk crystals or liquids are not suitable for integration with CMOS technology. In addition to all-optical on-a-chip device applications, materials that exhibit high nonlinear absorption and a fast response time are useful in optical limiting applications [3] for the protection of optical sensors and the human eye from high intensity light such as lasers [4]. Typical materials proposed in the past for optical limiting have been semiconductors, fullerenes, carbon nanotubes, nanostructured materials such as nanoparticles, graphene, nonlinear absorbers doped in xerogels and sol gel films, glasses, filters, organic/inorganic clusters, as well as 2D atomic crystals and organic dye molecules. For most of these materials, there is a tradeoff between their optical limiting ability and damage thresholds, and response time. The vast majority of these materials are not suitable for covering large-scale areas with consistent reproducibly required for sensitive applications such as infrared counter measures sensors. Therefore, there is a need for CMOScompatible materials with sizable nonlinear optical properties. Thin-films and nanolaminates, grown using atomic layer deposition (ALD), have the potential to meet this need. ZnO/Al2O3 films grown by ALD have been reported [5] to have third-order nonlinearity 200 and 13 times larger than zinc borosilicate glass and plain ZnO films, respectively. The enhancements to the nonlinearity are due to the control of the crystallinity of zinc oxide layer by the deposition of amorphous Al2O3. Titanium oxide, a promising material for nonlinear nanophotonic devices at both visible and infrared wavelength, was fabricated using this technique[6]. Several conditions can affect the type of TiO2 film and in turn the nonlinear properties, such as growth and annealing temperature. We have shown[7] that amorphous ALD grown titania films have nonlinear coefficients, n2, much larger (1.2x10-11–7.8x10-10 cm2/W) than similar films grown by other methods (1x10-15[8] –5.9x10-13[9] cm2/W). Thus, we report on experiments conducted on 60 nm thick TiO2 films grown by ALD at a temperatures of 100°C (sample 1) and 250°C (sample 2). Thermally managed Z-scan experiments[10] were conducted for sample 1 and 2, and yielded n2 values of 1.7x10-11 and 1.94 x10-10cm2/W respectively. Annealed (temperature of 450°C for 3 hours in air) versions of each sample produce no discernible Z-scan trace, i.e., below our detection limit. XRD characterization of the samples showed that the as-deposited samples were amorphous while the thermally treated samples were partially crystallized. The sample crystallization affected their nonlinear optical properties, but based on the results the as-deposited ALD TiO2 films are promising nonlinear material. 2. Experiments and Results The experimental setup consists of the standard Z-scan [11] setup (see fig. 1) that employs a mechanical chopper that has been modified to effectively reduce the repetition rate of the laser to minimize thermal effects. The closed aperture Z-scan technique was used to measure the nonlinear index of the samples. To generate a Z-scan trace, the beam, in our case a 76 MHz 100fs 800nm laser, is initially split, creating two paths, fig. 1; one path sending the unperturbed beam directly to detector D1, while the remaining beam travels through the sample. The sample under test is positioned on a translation stage such that it can move through the beam waist, the most focused region of the beam, generally starting close to the focusing lens and moving away in the positive z direction. The beam exits the sample and is split in two; the reflected expanding beam is focused directly into detector D3 called open aperture (OA) Z-scan while the transmitted portion traverses a pinhole aperture in the far field to finally strike detector D2, known as closed aperture (CA) Z-scan. Fig. 2 shows the Z-scan traces generated from sample 2. The n2 for sample 1 and 2 is 1.7x10-11 and 1.94x10-10 cm2/W, which is 3 to 4 orders larger than TiO2 grown in by other methods[8]-[9]. Thermal treatment of the samples resulted in the traces becoming indiscernible (Fig. 2b). This suggests that the n2 values have been reduced after annealing to a level below the sensitivity of our system. Fig. 1. Basic CA and OA Z-scan experimental setup. X-ray diffraction analysis of the samples indicated that the as-deposited samples are amorphous and the thermally treated samples are partially crystallized (sample 1: anatase and sample 2: rutile). Therefore, the formation of crystallites in the samples can significantly alter the magnitude of n2 due to the thermal treatment. Fig. 2. Z-scan traces for sample 2 a) as deposited at 250oC and b) after annealing at 450oC for 3hrs. 3. Conclusion The nonlinear refractive index n2 of TiO2 films deposited using ALD at temperatures of 100oC and 250oC were evaluated. The measured values of the nonlinear refractive index are much larger than those for films deposited by other methods. Annealing of the films reduces the effective nonlinear index due the partial crystallization of the film. However, amorphous TiO2 films grown by ALD show promise as a nonlinear optical material.
具有可调二阶光学非线性的电子超材料。
影响因子：
4.6
作者：
Lin HH;Vallini F;Yang MH;Sharma R;Puckett MW;Montoya S;Wurm CD;Fullerton EE;Fainman Y
通讯作者：
Fainman Y