随着5G通信时代的发展，射频滤波器作为其中必不可少的一环而日趋重要。在集成的微机电系统（MEMS）中，基于压电材料能够在电场的作用下产生形变的特性而制成的表面声波（SAW）或体声波（BAW）滤波器，已经作为重要的滤波器组件运用在人们的手机上。目前，互补金属氧化物半导体（CMOS）工艺兼容的氮化铝（AlN）压电材料沉积技术日渐成熟，掀起了基于氮化铝材料的射频滤波器的研究热潮。此外，科研人员发现在氮化铝压电材料（d33 = 5 pC N-1）中掺入钪元素而制成的铝钪氮（AlScN）压电材料（d33 = 25 pC N-1）能够大大地提升材料的压电性能，进而提升了表面声波或体声波滤波器的性能。随着信息的载体逐渐从电到光转换，基于压电材料的表面声波或者体声波与光波导中光波的相互作用可实现声光调制和微波光子滤波等功能，得到了科研人员的广泛关注。然而，由于氮化铝（n = 2.0）的折射率低于硅衬底（n = 3.5），主流的硅基氮化铝需要设计悬空波导结构才能在实现声光相互作用，给器件的稳定性和实用性带来了一定的挑战。

With the development of 5G communication, radiofrequency (RF) filters are becoming more and more important as an indispensable part. In integrated micro-eletromechanical systems (MEMS), based on piezoelectric materials that can deform in the electric field, surface acoustic wave (SAW) or bulk acoustic wave (BAW) filters have been applied into people's mobile phones as one of the important filtering components. Nowadays, as the deposition of piezoelectric aluminum nitride (AlN) material is becoming more and more mature, which is compatible with complementary metal-oxide-semiconductor (CMOS) technology, it set off a wave of research on RF filters based on AlN. Researchers also found that aluminum scandium nitride (AlScN) material (d33 = 25 pC N-1) made by doping scandium element into AlN material (d33 = 5 pC N-1) can greatly improve the piezoelectric properties and the performances of SAW or BAW filters as well. As the information carrier gradually transforms from electricity to light, the interaction between SAW or BAW based on piezoelectric materials and light wave in optical waveguides can realize acousto-optic modulation, microwave photonic filtering, etc, which has attracted extensive attention of researchers. However, due to the lower refractive index of AlN (n = 2.0) than silicon substrate (n = 3.5), the mainstream silicon-based AlN requires the design of suspended waveguide structures to implement acousto-optic interaction, posing a challenge to the stability and practicability of the devices.

图 1 声光器件的模型与设计

Figure 1 Model and Design of the acousto-optic device

  近日，华中科技大学武汉光电国家研究中心孙军强课题组在非压电的绝缘体上硅（SOI）平台上沉积一层电性能优异的铝钪氮压电薄膜，通过在压电薄膜上设计叉指换能器实现表面声波的激励，波导中的材料折射率在时谐的表面声波作用下受到扰动，进而使波导中光场模式的有效折射率受到调制。并且，该课题组对器件的声光调制性能进行详细的阐述。测试结果表明，声光调制响应曲线在3.044GHz和3.375GHz处达到峰值，其3dB带宽分别为4 MHz和11MHz，以及对应的半波电压与作用区长度的乘积VπL分别为1.06Vcm和0.95Vcm。此外，通过设计螺旋波导结构来延长表面声波与光波的相互作用长度，可以进一步提升器件的声光调制性能。最终，窄的3dB射频响应带宽可应用于微波光子滤波，与探测光波长基本无关的调制响应特性可应用于宽光谱的声光调制，而且该声光器件的制作工艺能够与互补金属氧化物半导体工艺兼容，具有广阔的应用前景。相关研究工作在线发表在Advanced Optical Materials上。

Recently, Junqiang Sun Group, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, proposed acousto-optic devices that a layer of AlScN thin film with excellent piezoelectric properties is deposited on the non-piezoelectric silicon-on-insulator (SOI) paltform, where SAW is actuated through the design of interdigital transducers upon the piezoelectric film. The refractive index of waveguide material is perturbed by the time-harmonic actuated SAW, then the effective index of optical mode in waveguide is modulated. In addition, the Group elaborated on the performances of the acousto-optic device. As the results show, the peaks of acousto-optic modulation response curve appear at 3.044 GHz with 3-dB bandwidth of 4 MHz and 3.375 GHz with 3-dB bandwidth of 11 MHz, and the corresponding products of half-wave voltage and interaction length VπL are 1.06 V cm and 0.95 V cm, respectively. Meanwhile, spiral waveguide structures are designed to extend the interaction length of SAW and optical wave, which can further improve the acousto-optic modulation performances of the devices. Consequently, the narrow 3-dB bandwidth of RF response is able to applied into microwave photonic filtering, while the modulation characteristic that is largely independent of probe wavelength is capable of implementing broad-spectrum acousto-optic modulation. Moreover, the fabrication process of the acousto-optic devices is compatible with CMOS technology, which has a broad prospect in practical applications. Relevant research is published online in Advanced Optical Materials.

图 2 硅直波导中的声光调制测试结果

Figure 2 Results of acousto-optic modulation in the straight silicon waveguide

论文信息：

Acousto-Optic Modulation in Silicon Waveguides Based on Piezoelectric Aluminum Scandium Nitride Film

Chukun Huang, Haotian Shi, Linfeng Yu, Kang Wang, Ming Cheng, Qiang Huang, Wenting Jiao, Junqiang Sun

Advanced Optical Materials

DOI: 10.1002/adom.202102334