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COL Highlight (Vol. 19, Iss. 3): 纳米压印光刻:小芯径中红外硫系光纤端面的抗反射结构

2021-04-02

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纳米压印光刻:小芯径中红外硫系光纤端面的抗反射结构

 

热纳米压印光刻技术,简称为纳米压印,是一种通过将材料加热到特定温度,使之具有延展性、可被塑造定形,从而将纳米或微观结构构造到材料上的过程。由于硫系化合物玻璃的转变温度较低,因此纳米压印技术广泛应用于硫系玻璃的加工中,在常规加热炉可达到的温度范围内即可对玻璃进行构造。因此,热纳米压印已被应用于平面波导、衍射光栅和环形谐振器等各类光子器件的制造中。

然而,由于硫系玻璃折射率较高,光在硫系波导和光纤中传输时,会在光纤与空气或其他低折射率材料的界面处发生强烈的菲涅耳反射。目前已有的抗反射(anti-reflective, AR)方法包括布儒斯特角连接器、镀薄膜涂层、以及采用纳米蛾眼减反结构等。2-10 μm的常规As40Se60硫系玻璃的布儒斯特角在70°左右,实施起来相对简单。然而,其AR只对p线性偏振光有效,对偏振的波动异常敏感,但对圆偏振和非偏振光的减反效果并不理想。

在光纤端面沉积薄膜可以改善硫系光纤的传输性能,但由于沉积的涂层材料和光纤材料的热机械性能不同,光纤损伤阈值相应降低,造成在光纤端面在加热时涂层开裂和脱层。此外,单层涂层只能在几百纳米的窄带波长范围内抗反射,且中心波长随图层厚度的变化剧烈改变;多层涂层可以扩展抗反射带宽覆盖范围,却增加了工艺的复杂性,降低了热机械的稳健性。

(a)热纳米压印原理示意图,(b,c)光纤照片:(b)压印期间(c)压印之后;不同的颜色来源于纳米结构的角度和不同波长的反射

相比之下,蛾眼减反结构可以直接压印在光纤的端面上实现宽带宽,不受偏振影响的高吞吐量传输,且不会因开裂和脱层造成光纤损伤。此前的研究已证实,纳米压印技术可在宽带宽范围内增加单材料硫系光子晶体光纤(photonic crystal fiber, PCF)的传输性能。然而,对于传统阶跃折射率光纤(step-index fibers, SIF)来说,还存在另一个问题:即纤芯和包层玻璃的热机械性能不同。迄今为止,有关在SIF端面上纳米压印AR结构的文献报道仅限于硫系玻璃型的大芯径多模光纤,这种光纤的纤芯和包层玻璃之间组分差异非常小。而对于玻璃材质差别非常大的小芯径阶跃折射率光纤不同热机械性能的影响,在已知范围内尚未有相关研究成果发表。

丹麦技术大学光子工程系、国家纳米制造与表征中心的Christian R. Petersen及同事与英国诺丁汉大学乔治·格林电磁研究所的中红外光子学小组合作,通过实验测试了不同的纤芯和包层玻璃成分的纳米压印SIFs的动态响应和热机械性能,相关研究成果发表在Chinese Optics Letters第19卷第3期(C. R. Petersen, et. al., Thermo-mechanical dynamics of nanoimprinting anti-reflective structures onto small-core mid-IR chalcogenide fibers [Invited])。该研究挑战了小芯径SIFs上进行纳米压印的难题,特别是压印过程中纤芯玻璃的收缩问题,提出了一种在不过度变形光纤尖端的情况下实现较高质量压印的方法,从而大大提高传输的性能。此外,研究人员还验证提出了在聚合物/硫族化合物多材料光纤上可压印AR结构。

该研究指出,纳米压印是一种通用且实用的方法,可以提高小芯径和大芯径硫系SIFs的传输性能,无需无尘室沉积设备即可进行工业规模制造。此项技术在由不同的聚合物、玻璃和金属组成的多材料光纤端面制备中具有未来应用的潜力,尤其可以在各种硬度不同的光纤中大显身手。

 

Thermo-mechanical Dynamics of Nanoimprinting Anti-Reflective
Structures onto Small-core Mid-IR Chalcogenide Fibers

 

Thermal nanoimprint lithography, or nanoimprinting in short, is the process by which nano- and microscopic structures are transferred onto a material by heating it up to a specific temperature, such that it becomes malleable and can be molded into a desired shape. The technique has proven particularly useful for chalcogenide glasses due their low glass transition temperatures (e.g. 185 ºC), that allow for structuring of the glasses at temperatures that are easily obtained by conventional heaters. For this reason, thermal nanoimprinting has been applied in the fabrication of various photonic devices, including planar waveguides, diffraction gratings, and ring resonators.

Unfortunately, due to the high refractive indices of chalcogenide glasses, transmission of light in chalcogenide waveguides and optical fibers are limited by strong Fresnel reflections at the interfaces with air or other low index materials. Several methods have been proposed to obtain anti-reflective (AR) properties, including Brewster angle connectors, thin-film coatings, and nanoimprinting of moth-eye structures. The Brewster angle for typical As40Se60 at. % chalcogenide glass is around 70 degrees from 2-10 µm, which although inconvenient is relatively simple to implement. However, the AR effect works only for linearly p-polarized light, making it sensitive to polarization fluctuations and reducing its effectiveness for circular or unpolarized light sources.

Thin film deposition on the fiber end faces has been demonstrated to improve the transmission of chalcogenide fibers, but tends to reduce the damage threshold of the fiber due to differences in thermo-mechanical properties between the deposited coating material and the optical fiber. This causes the coating to crack and delaminate as the fiber end face heats up during use. Furthermore, single layer coatings can only provide AR properties over a relatively narrow wavelength region of a few hundred nanometers, and the peak wavelength is very sensitive to the layer thickness. Multilayer coatings are therefore needed to extend the bandwidth, which increases the complexity and reduce the thermo-mechanical robustness.

(a) Illustration of the thermal nanoimprinting principle. (b), (c) Photograph of the fiber (b) during and (c) after imprinting. The different colors are due to the angle and wavelength dependent reflection of the nanostructures.

In contrast, so-called moth-eye AR structures can be directly imprinted on the end face of optical fibers to achieve broadband-, polarization-independent-, and high throughput transmission without the risk of damage due to cracks and delamination. In a previous report, an increase in transmission over a broad bandwidth was achieved in single-material chalcogenide photonic crystal fiber (PCF) via nanoimprinting. However, traditional step-index fibers (SIF) have the additional challenge of having different core and cladding glasses, each with its own thermo-mechanical properties. Reports from the literature on nanoimprinting of AR structures onto SIF end faces have so far been limited to large-core multi-mode fibers of the sulphide-glass type, with small variation between core and cladding glass compositions. Consequently, the effect of different thermo-mechanical properties in highly disparate glasses and small-core SIFs has, to our knowledge, not been explored until now.

Christian R. Petersen and co-workers from Technical University of Denmark, Department of Photonics Engineering and the National Centre for Nanofabrication and Characterization, experimentally tested the dynamics of nanoimprinting SIFs with different core/cladding glass compositions, and therefore different thermo-mechanical properties. The work was performed in collaboration with the Mid-Infrared Photonics group at the George Green Institute for Electromagnetics Research, Nottingham University, United Kingdom, and published in Chinese Optics Letter, Volume 19, Issue 3, 2021 (C. R. Petersen, et. al., Thermo-mechanical dynamics of nanoimprinting anti-reflective structures onto small-core mid-IR chalcogenide fibers [Invited]). The study highlights some of the challenges of nanoimprinting on small-core SIFs, in particular the contraction of the core glass during imprinting, and propose a method for achieving good imprints and greatly improved transmission without excessive deformation of the fiber tip. The conclusion of this investigation is, that nanoimprinting is a versatile and practical method for achieving high transmission in both small-core and large-core chalcogenide SIFs, that can be scaled to industrial-scale fabrication without the need for cleanroom deposition facilities.

The research team hints at a future application of the technology within end face preparation of multi-material fibers, composed of different polymers, glasses, and metals. Such fibers are very difficult to polish due to the different hardness of the materials, and would therefore benefit greatly from thermal nanoimprinting. Here, the researchers present a proof-of-concept result with imprinting the AR-structure on a polymer/chalcogenide multi-material fiber.