All-optical inverter based on carbon nanotubes-polyvinyl alcohol thin film

Experimental setup of all-optical inverter based on CNT-PVA thin film.

All-optical signal processing has attracted wide attention for its potential to overcome the extra loss induced by electrical-optical-electrical conversion. There are a variety of solutions for all-optical signal processing. For example, Kerr nonlinearity enables femtosecond-level signal processing which is far beyond the processing capability of current electronic devices. On the other hand, different from Kerr nonlinearity which typically requires high peak power, thermo-optic effect does not require high peak power and thus plays an important role for the all-optical devices whose states are needed to be hold for a long time, such as a router or a switch. Meanwhile recent development of nanomaterials has shown great values for their abundant photonic properties (e.g., saturable absorption, thermo-optic effect) and high flexibility to incorporate with different photonic platforms (e.g., fiber, integrated devices). Therefore applying nanomaterials to all-optical signal processing may open up a new world for functional photonic devices.

Recently, a research group from Shanghai Jiao Tong University experimentally demonstrated an all-optical inverter based on carbon nanotubes-polyvinyl alcohol (CNT-PVA) thin film. The dispersed CNTs were embedded in 25-μm thick PVA thin film and the thin film was sandwiched between two fiber connectors. When a pump pulse was injected to the thin film, it was absorbed by CNTs and generated a thermal lens in the thin film. Accordingly, the propagation loss of the signal light would be modified because the beam profile no longer matched the fiber numerical aperture. With such a design, the output of signal light was the inversion of pump light.

This design allows very robust output against environmental perturbation. A stable inversion output containing 50 cycles has been recorded in a 1 s time window. The response bandwidth of the inverter was ~7 kHz. Moreover, the thermo-optic effect also allows output integration which may plays a role in an all-optical neural network. The work has been published in Chinese Optics Letters, Vol. 18, Issue 6, 2020 (Zhengpeng Shao, Kan Wu, Jianping Chen. All-optical inverter based on carbon nanotube-polyvinyl alcohol thin film[J]. Chinese Optics Letters, 2020, 18(6): 060603).

"It is a very exciting result that with the help of carbon nanotubes a thermal lens can be formed in a 25-μm thick polymer thin film," said Prof. Kan Wu from Shanghai Jiao Tong University, "This compact all-optical inverter may connect the world of all-optical signal processing and nanomaterials."

Now, the researchers of the group are trying to integrate nanomaterials to an integrated photonic platform so that more functional photonic devices can be put together to realize a more complicated function. They believe that nanomaterials will definitely benefit the development of an all-optical integrated system.

碳纳米管助力全光反相器

基于碳纳米管-聚乙烯醇薄膜的全光反相器实验装置图

全光信号处理因其能够克服电光电转换引入的额外损耗而受到广泛关注。目前，研究人员已经提出了众多全光信号处理的方案。例如，Kerr非线性可以实现飞秒级别的信号处理，远超目前电子器件的处理带宽。另一方面，不同于通常需要较高峰值功率的Kerr非线性，热光效应无需较高峰值功率，在全光路由、全光开关等需要长时间保持状态的全光器件中具有重要作用。

与此同时，因为纳米材料具有丰富的光学特性（如饱和吸收特性、热光效应）和与不同光学平台（如光纤、集成器件）融合的高度灵活性，纳米材料近年来已经展现出巨大价值。因此，将纳米材料应用于全光信号处理有望打开功能性光子器件的新领域。

近日，上海交通大学的研究人员通过实验验证了一种基于碳纳米管-聚乙烯醇薄膜的全光反相器。研究成果发表于Chinese Optics Letters 2020年第6期上（Zhengpeng Shao, Kan Wu, Jianping Chen. All-optical inverter based on carbon nanotube-polyvinyl alcohol thin film[J]. Chinese Optics Letters, 2020, 18(6): 060603）。首先，分散的碳纳米管被嵌入到25 μm厚的聚乙烯薄膜中，随后，薄膜被夹持在两个光纤连接器之间。当一个泵浦脉冲注入到薄膜中时，碳纳米管吸收泵浦，在薄膜中形成一个热透镜。相应的，当信号光在薄膜中传播时，由于其光斑尺寸不再与光纤数值孔径相匹配，传播损耗也发生变化。

利用这种设计，输出的信号光是泵浦光的反相器。这种设计在面对环境扰动时具有很强的鲁棒性。在1 s的测试时间内，连续50个循环周期都保持稳定输出。反相器的响应带宽为~7 kHz。不但如此，热光效应同时支持输出积分，有望促进全光神经网络的发展。

“这是一个令人激动的结果：受益于碳纳米管，我们能在仅有25 μm厚的聚合物薄膜内实现热透镜，”上海交通大学的吴侃老师评价，“这种紧凑的全光反相器将连接全光信号处理与纳米材料领域。”

目前，该课题组正在尝试将纳米材料集成于光学器件中，通过集成更多功能的光学器件来实现复杂功能。该课题组相信，纳米材料将对全光集成系统做出重要贡献。