"We have confirmed the potential of using silicon-based optical metasurfaces to achieve third-harmonic generation in infrared imaging, marking an important step towards the development of next-generation infrared imaging technology through nonlinear silicon-based nanophotonics," said Xu Lei, a senior lecturer at Nottingham Trent University in the UK.

Currently, it also happens to be a process of transitioning from theory to application in the study of optical metasurfaces, making this achievement very timely.

At the same time, in the theoretical design of this project, Xu Lei and his team used the concept of bound states in the continuum to achieve control of nanostructures with arbitrary quality factors, providing a good idea for reducing the dependence on light source intensity.

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Firstly, this achievement can be applied to night vision technology, integrating visible light and infrared imaging, and enabling high-performance night-time infrared detection using ordinary silicon-based detectors.

Secondly, this achievement can be applied to medical testing. By shifting infrared light to the visible light spectrum, it is possible to detect protein binding and conformational changes, as well as interactions between drug molecules and target molecules, thereby completely suppressing the infrared light background noise at the detection port, and thus helping to improve the sensitivity and performance of medical testing.Once again, the results of this study can be applied to food testing and national defense security, that is, by combining nonlinear metasurfaces, as well as tunable nonlinear metasurfaces, it is expected to achieve super-resolution imaging technology in the infrared band.

Transform infrared images into visible light

Infrared detection has long been widely used in various fields. For example, by measuring the absorption of materials to infrared radiation, it can provide information about molecular structures and chemical bonds, hence it has great potential in medical diagnostics, video quality control, environmental monitoring, night vision, and security.

The continuous innovation and development of infrared detection technology is expected to promote its application in medical, food, environmental protection, and security fields.

However, the current challenge of infrared detection technology is that most infrared detectors are based on thermal detectors. Although the cost is relatively low, the speed is slow, and the sensitivity is insufficient, which severely limits their performance.Semiconductor detectors, as an alternative choice, have the advantage of high sensitivity, but they often require special cooling and complex processing techniques, or they need extremely low temperatures to maintain appropriate performance levels.

These technical challenges limit the flexibility and reliability of infrared imaging systems, affecting their performance in various application scenarios. Therefore, the field of infrared imaging urgently needs innovative solutions to overcome the limitations of current technology.

This may involve the development of new materials, more efficient detector technology, and the development of new cooling and processing methods.

In the past decade, optical metasurface structures composed of subwavelength dielectric resonators have received widespread attention. These structures can enhance the localized effect of the electromagnetic field of photoelectricity.

By cleverly designing these structures, it is possible to control the phase, amplitude, polarization, and the degree of near-field localization of incident light.At the same time, optical metasurfaces possess a high degree of flexibility and functionality, and have already achieved many new results in the field of optics. For example, they can replace traditional optical components such as lenses, prisms, and polarizers, which not only reduces the volume of traditional optical systems but also brings performance improvements.

Through nonlinear processes, it is possible to achieve frequency conversion of infrared light, providing a means to convert infrared into visible light. In this way, ordinary and inexpensive silicon-based detectors can be used to achieve infrared imaging.

In addition, different nonlinear mixing processes can improve energy utilization efficiency, providing the possibility for the realization of efficient infrared imaging systems.

As for metasurfaces, their unique potential in miniaturization, flexibility, and lightweight is also a good platform for achieving new generation infrared imaging technology.

The above is also the starting point of Xu Lei's team for this topic. In the research, they use the method of combining nonlinear optical metasurfaces. Through the process of nonlinear optics, infrared images can be converted into visible light, making it possible for ordinary silicon-based detectors to directly detect infrared images.When witnessing the green light with one's own eyes...

According to reports, Xu Lei has always been interested in imaging technology and nonlinear optics. When he was working in Australia, he had already used the second-harmonic sum-frequency process with his collaborators to achieve infrared detection. At that time, they were the first research team to attempt this kind of exploration.

Since 2016, Xu Lei has begun to delve into nonlinear nanophotonics. At that time, the development of the Mie resonance mechanism and theory in the field of nanophotonics was becoming faster and faster, which not only provided a framework for the application of various systems but also brought guidance for predicting the propagation characteristics of light.

During this period, Xu Lei accumulated a lot of knowledge about nonlinear nanophotonics. In September 2020, he came to Nottingham Trent University in the UK and, together with Professor Mohsen Rahmani and Lecturer Ying Cui, jointly established the Advanced Optics and Photonics Laboratory.Professor Rahmani focuses on sample processing, and he has a deep foundation in the field of materials and related applications. Ms. Ying Cui has rich experience in nano-optics and biosensing. Our three skills complement each other and each has its strengths, said Xu Lei.

After studying material properties, structural design, and methods such as Mie resonance, and achieving the enhancement of nonlinear optical fields and the manipulation of light fields, the three of them and Ph.D. student Zheng Ze began to consider how to combine nonlinear optics with solving practical problems, and soon after, they launched this project.

Structural material and parameter design are the first issues at hand. To achieve application, it is necessary to consider the integration of later devices from the perspective of nonlinear materials.

Considering that silicon material itself has good nonlinear effects, coupled with the relatively mature processing technology of silicon, they chose silicon as the research material.

The reason is: this can not only consider the nonlinear effect but also make full use of the processing technology of silicon to process complex structures, thereby enhancing the conversion efficiency from infrared light to visible light.After proving the feasibility of the above plan, they began to enter the experimental phase. Due to Xu Lei's own research direction, which lies between theory and experiment.

Therefore, he usually conducts theoretical simulations before the experiment. However, the experiments were not smooth sailing, especially when the signals obtained at the beginning did not match the expectations.

Xu Lei said: "Most people might feel frustrated at this time. But these seemingly unsuccessful experimental data are actually the most interesting part to me, because they may point out the areas for improvement in theory and experiment."

In his view, if all experimental results were consistent with theoretical expectations, it would not be the best. Many key advances in the history of science have been inspired by some failed experimental data.

For example, when they initially designed the device structure, they tried to achieve light localization enhancement through high-quality factor structures. However, the experimental results showed that a high-quality factor was not the best choice.The unexpected experimental results have also prompted them to further refine the theoretical models and improve the experimental plans. This has also sparked their thinking about the use of continuous light and ultrafast light in imaging and sensing, and has provided some inspiration for the development of infrared imaging technology.

At the same time, the process of completing this project group is also the process of Xu Lei cultivating his first Ph.D. student. This Ph.D. student is Zheng Ze, who was mentioned earlier.

During the research, a good interaction of mutual learning was formed between teachers and students. Xu Lei also consciously let Zheng Ze participate more in the construction of the optical path to cultivate the ability to design experiments independently.

"Especially important is that I have always focused on cultivating his scientific research confidence, encouraging him to put forward independent ideas and believe in his own abilities," said Xu Lei.

When building the nonlinear testing system, Zheng Ze was the first time to be involved in the construction of such a system. When he first saw the generation of nonlinear signals and was able to see the green light with his own eyes, Zheng Ze's excitement infected the entire laboratory.Xu Lei said: "As a mentor, seeing his so devoted and satisfied expression makes Professor Mohsen and Ms. Ying Cui Feng and me feel extremely gratified."

With the efforts of the three teachers and Zheng Ze, the results of this study have shown the application prospects of silicon-based optical metasurfaces in the field of nonlinear nanophotonics. This not only provides new ideas for the practical application of nonlinear optics but also lays a foundation for more in-depth research to follow.

Finally, the related paper was published in Opto-Electronic Advances (IF 14.1) with the title "Third-harmonic generation and imaging with resonant Si membrane metasurface."

Zheng Ze is the first author, and Professor Xu Lei from Nottingham Trent University in the UK and Professor Mohsen Rahmani are the co-corresponding authors.

Next, they will continue to delve deeper into the theoretical aspects, with the aim of enhancing the conversion efficiency of infrared light and continuously reducing the requirements for light source energy in infrared imaging.At the same time, there will be a focus on the integration and multifunctionality of devices, exploring how to combine image signal processing and spectral information extraction, and how to utilize metasurfaces to achieve multifunctional imaging chip devices, thereby better moving towards applications.

Xu Lei continued to say: "Additionally, I would like to mention that everyone has different skills and perspectives on things. Sometimes a casual remark from a pure experimentalist may inspire an important idea for a theorist." A pure theorist may also play a role in adding the finishing touch to the experimental plan.

Take mathematical research and physical research as an example: There are many different phenomena and mechanisms in physics. However, a mathematician may not pay attention to different phenomena, but directly see the connections between various phenomena and mechanisms from the formulas. At the same time, these connections are often the key points for achieving breakthroughs in physics.

"Sometimes the so-called non-professionals give more profound insights. Therefore, collaborating with people from different knowledge backgrounds is very important for scientific research," Xu Lei concluded.