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All-optical inverter based on carbon nanotubes-polyvinyl alcohol 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.

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