基于各向异性超材料的超宽带硅基集成起偏器

起偏器是一类基本光学器件,它可以将输入光中的特定偏振态滤除,从而保持单一偏振态输出。起偏器被广泛应用于各类光学系统中,用于降低偏振串扰。传统起偏器大多基于双折射光纤、薄膜滤波器以及线栅偏振片等结构,然而这些起偏器通常尺寸较大且造价昂贵。硅基集成光子器件具有损耗低、结构紧凑的特点,并且硅基纳米波导可以采用CMOS工艺加工,具有实现高性能、小尺寸、低成本起偏器的潜力。硅基集成起偏器可以通过浅刻蚀波导、光子晶体微腔、不对称定向耦合器等构型实现。然而,由于硅基纳米波导的模式色散较强,导致绝大多数硅基集成起偏器的光学工作带宽较窄(< 100 nm),这限制了硅基集成起偏器在光学层析成像以及大容量光通信等领域的应用。

针对这一缺陷,浙江大学时尧成教授等研究人员提出一种基于各向异性超材料的新型硅基集成起偏器,其光学带宽可达415 nm以上,可同时覆盖O、E、S、C、L、U波段,打破了传统硅基集成起偏器的光学带宽限制。相关研究结果发表于Photonics Research 2019年第7卷第12期上(Hongnan Xu, Daoxin Dai, Yaocheng Shi. Anisotropic metamaterial-assisted all-silicon polarizer with 415-nm bandwidth[J]. Photonics Research, 2019, 7(12): 12001432),并被美国光学学会选为OSA Spotlight on Optics。

这种新型起偏器由弯曲半径较小的硅基纳米波导与亚波长光栅包层组成。亚波长光栅是一种具有各向异性的超材料,在亚波长光栅中,不同的电场取向对应不同的有效折射率。对于TE偏振态,其电场取向与光轴方向平行,此时亚波长光栅的等效折射率显著低于硅材料的折射率。因此,可以将TE模式有效限制在弯曲波导中,并由输入端口几乎无损地传输至输出端口。对于TM偏振态,其电场取向与光轴方向垂直,此时亚波长光栅的等效折射率接近硅材料的折射率。因此,TM模式会泄漏至亚波长光栅包层中,并产生显著的弯曲传播损耗。通过调节亚波长光栅超材料的结构尺寸,可以有效增强弯曲波导中模场传输的偏振相关性,使得TM偏振态在较短的传输距离内被完全滤除,同时保证TE偏振态几乎不被损耗。值得注意的是,亚波长光栅超材料的双折射一般不随波长变化,另外起偏过程中不涉及相位匹配,因此这种新型起偏器具备宽带工作特性。研究人员利用电子束曝光与干法刻蚀工艺,在绝缘硅平台上加工实现了这一结构,在415 nm的光学带宽范围内(波长1.26 μm至1.675 μm),实验测得小于 1 dB的低损耗与大于 20 dB的高偏振消光比,同时整体器件尺寸仅为13 μm × 6.5 μm。

本文通讯作者时尧成教授相信,该器件在大容量光通信领域具有广泛应用前景,也为超大带宽硅基偏振调控器件的设计提供了崭新的思路。该器件可以应用于片上光互联系统,以降低多维度复用过程中产生的偏振串扰,在近红外光学层析成像等领域也有应用。

基于各向异性超材料的超宽带硅基集成起偏器。器件由弯曲波导与亚波长光栅超材料组成。对于TE偏振态,亚波长光栅的等效折射率显著低于硅材料的折射率,使得TE模式可以被有效限制在弯曲波导中无损传播。对于TM偏振态,亚波长光栅的等效折射率接近硅材料的折射率,使得TM模式泄漏至亚波长光栅包层中,产生显著的弯曲传播损耗。

 

Anisotropic metamaterial-assisted all-silicon polarizer with 415-nm bandwidth

When talking about photonic integrated circuits, one of the first questions we are asked about is their polarization sensitivity. The main reason is that it is extremely challenging to realize photonic devices that are polarization insensitive, especially when we use high-index-contrast platforms like silicon photonics. On the other hand, it is also true that making a device with an ultra-high polarization dependence can be equally difficult. Polarizers should be as transparent as possible to one polarization state while being as lossy as possible to the orthogonal state. However, it’s still a challenge to produce integrated polarizers with a small size, low loss, high polarization selectivity, and operating on a broad band.

In this work, Xu and coworkers demonstrate that it is actually possible to face all these issues in a rather simple way. They conceived a compact silicon device consisting of a sharply bent waveguide, which is assisted by an anisotropic sub-wavelength grating (SWG), that is a series of dielectric nano-stripes with deep sub-wavelength scale. This SWG structure provides enhanced polarization sensitivity, because only the TE polarization can propagate through the bend, while the TM polarization is radiated away regardless of the wavelength. As a result, high polarization extinction ratio (>20 dB) is achieved over a record bandwidth >415 nm (from 1.26 μm to 1.675 μm) in a less than 100 μm2 footprint device.

This polarization sensitive "guardrail" is a new tool for on-chip management of polarization and could find useful applications in integrated devices for optical communication systems, sensing, and imaging.