Scientists develop a new type of flexible integrated photonic device, which is e

In recent years, laser direct writing technology, based on its high precision and flexibility, has gradually become one of the effective means for processing micro-nano optoelectronic devices.

The liquid crystal material platform has always attracted attention due to its electro-optical tunability and photosensitive curability. At present, liquid crystal photonic devices have served as key technical supports in many fields.

Recently, the Soft Matter Optoelectronics Laboratory at the University of Oxford, in collaboration with the Integrated Optoelectronic Chip and Photonic Network Research Group at the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, has developed flexible integrated photonic devices with both stable and efficient performance on the liquid crystal photonic platform, relying on two-photon laser direct writing technology.

These photonic devices are expected to meet the performance requirements of photoelectric reconfigurability, flexibility, and foldability in various application systems.

The related paper, titled "Laser-written liquid crystal waveguides," was recently published in Ultrafast Science [1].Chen Bohan, a doctoral student in the Department of Engineering Science at the University of Oxford, and Xie Peng, a young leading researcher at the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, are co-first and corresponding authors. Professor Stephen Morris from the University of Oxford serves as a co-corresponding author.

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The research is a forward-looking exploration aimed at developing new types of photonic devices needed for optoelectronic reconfigurable computing/optical reconfigurable information processing, brain-computer interfaces, and other fields, with the goal of promoting the development of related technology applications through these innovative photonic devices.

Xie Peng stated that the team took the two-photon laser direct writing technology as a starting point and developed electrically tunable optical waveguides on a liquid crystal soft matter photonic integration platform.

By using photosensitive curing film, they have developed wearable, foldable, and stretchable flexible integrated photonic devices. The relevant experimental results have demonstrated the reliability and practicality of the technical route.

The reviewers commented on the research, saying: "The new type of flexible integrated photonic devices realized by the authors are highly original and have significant importance in the fields of optoelectronic information reconstruction, biophotonics, and augmented reality/virtual reality."Original Innovation from 0 to 1

In the early stages of research, the team plans to develop on-chip electrically tunable optical waveguides based on the liquid crystal polymer material platform and the electrically controllable properties of liquid crystal molecules, to realize a new type of on-chip optical logic gate.

The technical approach can be understood as: relying on two-photon laser direct writing technology, in conjunction with an external high electric field, to fabricate micro-nano optical waveguides in liquid crystal device samples.

During the fabrication process, the liquid crystal polymer undergoes cross-linking polymerization through two-photon absorption, locking the spatial orientation of some specific liquid crystal molecules.

Therefore, when the external electric field is removed, the unfixed liquid crystal molecules will return to their original positions, and the processed structure forms an optical waveguide, which realizes the localization and transmission of energy for specific polarized incident light.When a high electric field is reapplied, the refractive index difference disappears, the device structure loses its optical waveguide function, and the light localization capability is lost, thereby achieving the electrically tunable nature of the liquid crystal optical waveguide.

This is a study that is entirely original in both concept and technology. During the research exploration, the researchers encountered two major challenges.

First, how to prepare a liquid crystal optical waveguide with stable performance and response. Unlike traditional rigid optical waveguides, micro-nano processing in soft materials is prone to structural drift or distortion, which reduces the stability and reference value of the test results.

To solve this problem, the research team started from the aspects of material ratio, sample preparation, and preparation parameters to optimize, and then explored a technical path for preparing high-stability liquid crystal optical waveguides.

Second, the effective coupling test of light. The researchers finally used an optical coupling platform and obtained stable coupling and reliable data through edge coupling.

Please note that the translation is provided with the assumption that the text is about a scientific study related to liquid crystal optical waveguides and their electrical tunability.Chen Bohan reminisced, "After successfully overcoming the two major issues of the preparation and coupling testing of the liquid crystal optical waveguide, on a day in March 2023, we once again verified the repeatability and reliability of the experiment. Finally, in the early morning of that day, we successfully achieved effective optical coupling and observed stable phenomena."

At that moment, the entire research team was extremely excited, working through the night to process the data and images, and reported the results to the team's senior professor, who replied, "Perfect!!!"

1 technology, 2 application directions

After the demonstration of the electrically controlled liquid crystal optical waveguide, Xie Peng proposed: based on the existing photonic devices, to further attempt to prepare stretchable, foldable, and wearable flexible photonic devices through ultraviolet light curing film formation, which may be applied in the field of non-invasive brain-computer interfaces.However, the challenge in fabricating such flexible thin-film photonic devices lies in: how to safely separate them from the rigid substrate?

This is a highly engineering and skill-testing process. Chen Bohan made hundreds of attempts before finally mastering the safe separation of liquid crystal thin-film photonic devices from the glass substrate.

Subsequently, the stretchability and foldability of the device were successfully demonstrated, and effective coupling and localization of light were achieved under various conditions.

In 2021, Xie Peng, who was conducting research in the Morris research group, proposed the research direction of "laser direct writing of liquid crystal optical waveguides", which is a very cutting-edge topic, even a "no man's land".

At that time, although preliminary judgments had been made through theoretical calculations or simulation, many researchers in the laboratory were skeptical about the feasibility of the topic, and even the most experienced Professor Morris pointed out several very difficult key technical points.Xie Peng overcame objections and demonstrated comprehensive simulation design and relevant research results, which ultimately earned the support of the laboratory director. The project was initiated under the premise of "let's give it a try" and a research team was established. Starting from scratch, they conducted scientific exploration in the absence of effective reference literature.

The subject spanned the entire doctoral stage of Chen Bohan, who joined the Soft Condensed Matter Optoelectronics Laboratory at the University of Oxford in 2020 to pursue his Ph.D., coinciding with the time when Xie Peng proposed the subject. Driven by his own research interests, he quickly joined the research team.

"Fortunately, we have made substantial progress in over two years, completing the process from 0 to 1. Moreover, this research has become an important part of my doctoral thesis," said Chen Bohan.

It is also reported that Xie Peng has returned to China full-time and joined the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, as a young leading researcher, doctoral supervisor, and independent PI, establishing a research group on integrated optical chips and photonic networks to conduct research on optical chips and their applications.

In addition, he was also selected as one of the "2023 China Intelligent Computing Innovators" by the Chinese edition of the MIT Technology Review."Previously, we have made a lot of accumulation on the path of optical computing technology. This research has enriched the team's technical system in the field of reconfigurable optoelectronic computing. In the future, we will continue to explore more interesting scientific work," said Xie Peng.