切割空气孔实现高品质因子光子晶体腔

作为最重要的光学共振腔之一,光子晶体腔在控制光与物质相互作用以及光波传播方面具有一系列独特优势:首先,光子晶体腔是公认的具有最高Q/Vmode 因子的光学腔之一(Q和Vmode 分别是共振模式的品质因子和模场体积)。由于光学共振模式的电场密度与Q/Vmode 因子成正比,因此光子晶体腔能够有效控制光与物质的相互作用,可支持高效非线性光学、光力学及经典和量子光发射等行为。其次,光子晶体腔通常在包括硅、氮化硅、III-V族和铌酸锂等材料介电的平板中制备。平板结构使光子晶体腔易与平板光子晶体的其他缺陷结构(如光子晶体波导)进行组合,从而构建超紧凑的光子集成回路,同时也支持其与电子器件的集成以构建光电子器件。此外,与其他光学腔相比,光子晶体腔可以利用半导体微电子加工工艺和设备直接进行制备,可大大促进其未来的实际应用。

在过去的几十年中,已经报道了许多种结构设计来实现具有高Q因子和小Vmode的平板光子晶体腔。一般地,具有三角晶格的空气孔在介电平板上周期性排列,即形成平板光子晶体。如果在光子晶体晶格中引入结构缺陷,则可以在光子晶体的光子带隙中形成缺陷模式,并得到平板光子晶体腔。之前报道的光子晶体腔,其结构设计大都是基于缺失空气孔或者移动空气孔两种方案,如典型的L3、H0、异质结构以及局部宽度调制等类型的光子晶体腔。

近日,西北工业大学赵建林教授领导的研究小组提出了一种设计光子晶体腔的新思路,只需将光子晶体中空气孔简单地切割成半圆形即可形成腔,并且该思路也可以用来优化已报道的其他类型光子晶体腔。相关研究结果发表在Chinese Optics Letters 2020年第6期()。

该工作列举了三个通过切割空气孔实现和优化光子晶体腔的案例。在形成光子晶体腔方面,通过简单地切割平板光子晶体晶格中的两个相邻空气孔或光子晶体波导两侧的两个空气孔,均可获得Q因子超过105的共振模式。在优化光子晶体腔方面,通过将L3型腔边缘的两个空气孔切割为半圆形,其Q因子实现了超过20倍的优化,并且Vmode 几乎是恒定的。这三个案例还表明,这种切割空气孔形成的光子晶体腔,比缺失或移动空气孔形成的腔具有更高的Q因子和更小的Vmode 等优势。

赵建林教授指出:“在传统设计中,无论是缺失还是移动空气孔,光子晶体腔的边缘空气孔都呈圆形,这使得模场处的折射率变化具有凸起的分布。相比之下,按照我们提出的设计,空气孔被切割后,模场处折射率变化趋于平坦分布,从而缓解了空气孔与介电板界面处的光阻抗(或折射率)失配问题,对模式的限制更加柔和。这就是切割空气孔的思路在设计和优化光子晶体腔方面具有优势的原因。实际上,我们小组也正在探索这种切割空气孔的思路是否也可以用于设计具有优异传输损耗和色散特性的光子晶体波导,实现如超低损耗、宽带平坦慢光行为等。”

将空气孔切割为半圆形以实现和优化光子晶体腔

A new strategy for designing high quality photonic crystal cavity with cut air-holes

As one of the most important optical resonators, the photonic crystal cavity has a variety of advantages to resonantly control light-matter interactions and light wave propagations. First, the photonic crystal cavity is well recognized as one of the optical cavities with the highest Q/Vmode factor, where Q and Vmode are quality factor and mode volume of the resonant mode, respectively. The density of electric field of a resonant mode is proportional to the Q/Vmode factor, which therefore enables the photonic crystal cavitie as a promising platform to control light-matter interactions for highly efficient nonlinear optics, optomechanics, classical and quantum light emissions. Second, photonic crystal cavities are normally fabricated in dielectric slabs, including silicon, silicon nitride, III-V groups, and lithium niobate. Their planar structures make them easy to construct ultracompact photonic integrated circuits with the combination of other defect structures in the planar photonic crystal (PPC), such as PPC waveguides. The planar structure also allows their integration with electrical circuits for optoelectronic devices. Third, compared with other optical cavities, these PPC cavities could be straightforwardly fabricated using the standard technologies of semiconductor electronic devices, facilitating their practical applications.

In the past decades, a number of designs have been reported to realize PPC cavities with high Q factor and small Vmode. PPCs are normally periodic air-holes with a triangular lattice in a dielectric slab. If structure-defects were involved into the PPC lattice, defect modes could originate inside the PPC's photonic bandgap, forming a PPC cavity. Summarized from the previously reported PPC cavities, missing air-holes and moving air-holes in PPCs are two of the most widely accepted defect-designs, including the popular types of L3, H0, heterostructure, and local width-modulation.

The research group led by Prof. Jianlin Zhao from Northwestern Polytechnical University proposed that simply cutting air-holes in PPC to semicircles could be considered as a new strategy to form and optimize PPC cavities. The research results are published in Chinese Optics Letters, Vol. 18, Issue 6, 2020 ().

Three examples of realizing and optimizing PPC cavities using cut air-holes were described. In the formations of PPC cavities, by simply cutting two adjacent air-holes in a PPC lattice or two air-holes on the opposite sides of a PPC waveguide, resonant mode with Q factor exceeding 105 are obtained. In the optimization of PPC cavity, by cutting the two air-holes at the edge of a L3-type cavity, the Q factor is optimized by more than 20 times, while Vmode is almost constant. Results of these three examples also indicate that the cavities with cut air-holes have superiorities over cavities with missed or moved air-holes in the higher Q factor and smaller Vmode.

"In the photonic crystal cavities with missed or moved air-holes, the air-holes around the cavity edges have circularly shapes. By contrast, in our proposed cavities, the cut air-holes have flat sides facing to the strong mode distribution, which could relax the impedance mismatch at the interfaces of air-holes and the dielectric slab much more. It therefore enables much gentler mode confinement in the cavities. This is why the strategy of cut air-holes could promise advantages in designing and optimizing cavities." says the corresponding author Prof. Jianlin Zhao from the research group. "Actually, in our group, we are also pursuing to reveal whether this concept of cut air-holes could be employed in designing photonic crystal waveguides with engineered transmission loss and dispersion, which is another important defect structure in PPCs."

Forming and optimizing photonic crystal cavities by cutting air-holes into semicircles