Reports on underlying technology application and innovation

  • Scholars from Zhejiang University have realized pixel-level three-dimensional programmable structural colors inside lithium niobate crystals

    Recently, Professor Qiu Jianrong and Dr. Zhang Bo of Zhejiang University and researcher Tan Dezhi of Zhijiang Laboratory proposed a multi-dimensional optical storage strategy based on ultrafast laser-induced microphase transformation of lithium niobate crystal vitrification. Using the intrinsic birefringence effect of lithium niobate crystal, the ultrafast laser pulse is excited to couple the material modification in the crystal, and the pixel-level programmable three-dimensional structural color is constructed inside the crystal, which realizes the efficient and low-cost permanent preservation of large-scale data.

    The results were published in the international authoritative journal Advanced Materials, and were selected as “Editor’s Choice” and cover paper.

    Data is a factor of production in the new era and an important strategic resource of the country. With the development of big data, artificial intelligence and other technologies, the total amount of data generated by human society has exploded. In the face of large-scale data preservation, the existing storage technology has problems such as poor reliability, short lifespan, and low storage density, and the resulting energy consumption is quite huge. Faced with these problems, ultrafast laser engraving inorganic transparent media optical storage offers an attractive solution for preserving massive amounts of information in a simple process, extremely stable and long lifetime. However, due to the limitation of the inherent high damage threshold of inorganic transparent media, this kind of storage technology generally has problems such as high write energy consumption, slow read and write speed, and data extraction relies on complex optical systems, and the problems of insufficient uniformity and low standardization of storage media also seriously restrict the popularization and large-scale application of this technology.

    Facing the national “Overall Layout Plan for the Construction of Digital China” and the “14th Five-Year Plan” for the Development of Big Data Industry, in response to the development needs of low-energy, high-efficiency and long-life optical storage, Qiu Jianrong’s team developed an optical storage based on ultrafast laser selectively inducing lithium niobate single-crystal microphase transition, and proposed a crystal birefringence effect to excite the intra-pulse coupling material modification mechanism, so that a single nJ-level ultrafast pulse can induce micro-amorphous phase transformation in the crystal matrix, combined with the color polarization effect. Pixel-level programmable structural colors are generated in the laser-modified region (Figure 1).

    Fig.1 Ultrafast laser-induced microphase change optical storage mechanism

    Due to the high flexibility of ultrafast laser direct writing, micro-phase transition regions can be manipulated anywhere in 3D space to achieve customized pattern and tonal style control (Figure 2). Ultrafast laser selectively induced single crystal microphase change optical storage has excellent comprehensive performance. The local phase modulation of the glass phase transition and the transparency of the lithium niobate crystal medium support the multi-channel extraction of information, and the written information can be compatible with a variety of mature data reading systems. The data point diameter can be reduced to 500 nm, and the data can be efficiently written with a single ultrafast pulse of 30 nJ, which is comparable to the laser modification energy of organic media. The microphase change structure has extremely high stability, and the stored information can withstand a variety of extreme environments such as strong magnetic field (42 T), high temperature (700 °C), strong acid (60% HNO3), and X-ray (50 kGy). Accelerated aging experiments show that the data storage life is as long as trillions of years at room temperature, which can truly achieve low energy consumption permanent storage of massive data.

    It is worth noting that the data signal based on pixel-level structural color can be read at high speed through image recognition, which solves the problem of low information extraction efficiency and dependence on complex and expensive optical systems in multi-dimensional optical storage technology. By writing parameter codes to the laser, the wavelength and intensity signal of the structural color can be flexibly manipulated, and it can be used as a new information reuse channel to achieve multi-dimensional data storage.

    Fig.2 Programmable color pixels are printed across dimensions

    This work discovers a new mechanism of the interaction between ultrafast laser and matter, improves people’s understanding of the interaction between light and matter in the dielectric environment, proposes an intrapulse-coupled material modification strategy for ultrafast laser in inorganic crystal media, and establishes a new optical storage method-ultrafast laser selectively induced microphase change optical storage of lithium niobate crystals, following the three cutting-edge data storage technologies in the world: holographic storage, DNA storage, and glass storage. It provides a new technical route for the permanent storage of ultra-large-scale data in the future.

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  • DeepMind AI beats humans in every game

    An AI that can defeat human players in chess, Go, poker, and other games that require multiple strategies to win. This artificial intelligence, called “Game Student” (SoG), was created by Google DeepMind. The company says this is a step towards general artificial intelligence capable of performing any task with superhuman performance. The paper was recently published in Science Advances.

    Shall we play the game?

    Martin Schmid, who worked on artificial intelligence at DeepMind, now works for a start-up called Equalization Technology. He said the SoG model can be traced back to two projects. One of them is DeepStack, an AI developed by teams such as Schmid at the University of Alberta in Canada, and the first AI to beat a human pro in a poker match. The other is DeepMind’s AlphaZero, which beats the best human players in games like chess and Go.

    The difference between the two models is that one focuses on imperfect knowledge games – where the player does not know the state of other players, such as the hand in a poker game, and the other focuses on perfect knowledge games, such as chess, where both players can see the position of all the pieces at any given time. These two require fundamentally different approaches. DeepMind hired the entire DeepStack team with the goal of building a model that could promote both types of games, thus giving birth to SoG.

    According to Schmid, the SoG starts out as a “blueprint” for how to learn the game and then improve it through practice. This beginner model can then play freely in different games and teach yourself how to play against another version of yourself, learn new strategies and gradually become more capable. While DeepMind’s previous AlphaZero could adapt to the perfect knowledge game, the SoG can adapt to both the perfect and imperfect knowledge games, making it more versatile.

    Researchers tested SoG on chess, Go, poker and a board game called Scotland Yard, as well as Leduc Poker and a customized version of Scotland Yard, and found that it could beat several existing AI models and human players. Schmid said it should be able to learn to play other games as well. “There are a lot of games that you can just throw at it, and it’s really, really good at it. ”

    This wide range of capabilities is a slight drop in performance compared to DeepMind’s more specialized algorithms, but the SoG can easily beat the best human players in most of the games it learns. Schmid says that the SoG learns to go against itself in order to level up in the game, but also to explore what might happen from the current state of the game, even if it’s playing an imperfect game of knowledge.

    “When you’re playing a game like poker, it’s hard to figure out how to find the best next move if you don’t know what cards your opponent has in hand. “So there’s some ideas from AlphaZero, and some ideas from DeepStack that form this huge mix of ideas, and that’s the game students.” ”

    Michael Rovatsos of the University of Edinburgh in the United Kingdom, who was not involved in the study, said that while the research is impressive, there is still a long way to go before AI can be considered universal intelligence, because games are an environment where all rules and behaviors are clearly defined, not the real world.

    “The important point to emphasize here is that this is a controlled, self-contained artificial environment in which the meaning of everything and the result of every action is very clear. “The problem is a toy problem because as complex as it can be, it’s not real.” (Source: China Science Daily Li Huiyu)

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  • A wireless epidermal biosensing wristband based on monolithic fabric integration was developed

    Recently, the research group of Lin Yuanjing, assistant professor of the Shenzhen-Hong Kong School of Microelectronics of Southern University of Science and Technology, and the research group of Zheng Zijian, professor of the Hong Kong Polytechnic University, have made research progress in the field of flexible wearable sensor devices, and the related research has been published in Science Advances.

    Sweat contains many biomarkers, including electrolytes, metabolites, amino acids, and hormones, and continuous monitoring of these biomarkers can help enable early disease detection and management. Electronic fabrics have great potential in sweat biosensing due to their unique comfort and breathability. However, due to the large inherent resistance of the fabric and the low degree of electronic integration, the existing methods usually only integrate the sensor on the fabric, which cannot achieve the high-compatibility integrated interconnection of the entire electronic system.

    Polymer-assisted metal deposition (PAMD) is a low-cost, high-yield process that can be used to fabricate highly conductive textiles, providing a new idea for fabricing electronic fabric systems for flexible sensing.

    In this study, the research team proposed a hybrid bracelet integrated on a single-layer fabric for real-time wireless detection of biomarkers in sweat, and achieved the same circuit pattern on the single-layer fabric as a traditional printed circuit board through the PAMD process and improved double-sided lithography technology. Based on a specially designed circuit pattern, the research team fabricated the bracelet consisting of three parts: a highly selective sensor for ion detection, a circuit for signal extraction and processing, and a Bluetooth module and application software for wireless data transmission. The integrated bracelet continuously monitors sweat potassium concentrations from 0.3 to 40 mm, enabling reliable wireless real-time epidermal biosensing.

    In addition, the bracelet has excellent air and moisture permeability, which is more than an order of magnitude higher than the elastomers commonly used in commercial medical tapes and flexible electronics, ensuring a comfortable fit. This work has great potential as a unique strategy for manufacturing wearable e-fabrics in areas such as mobile health and telemedicine. (Source: Diao Wenhui, China Science News)

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    Use the bracelet for on-site sweat monitoring during exercise Courtesy of the scientific research team

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  • Scientists have made progress in the field of multi-focus image fusion

    Depth of field is the distance between the closest and farthest clear imaging planes in an optical system. The greater the depth of field, the wider the range of clear images that can be imaged by the optical system. Due to the depth of field limitation of optical lenses, it is difficult to obtain a fully focused image, i.e., there is often a part of the blurred area. One of the effective ways to solve this problem is Multifocal Image Fusion (MFIF). MFIF aims to fuse multiple locally focused images obtained by focusing different objects in the same scene separately to obtain a fully focused image where all objects are clear. MFIF can effectively extend the depth of field of the optical lens, so that the imaging system can break through the depth of field limitation to obtain higher quality images. 

    At present, in the field of MFIF, the effect of deep learning methods is significantly better than that of traditional algorithms. In recent years, deep learning-based MFIF algorithms have developed rapidly, but scientists often focus on designing increasingly complex network structures, modules, and loss functions to improve the fusion performance of algorithms. This means that a lot of time has to be spent designing clever network structures and doing enough comparative experiments. However, this is not conducive to the improvement of algorithm performance, resulting in a bottleneck in the performance of the current MFIF algorithm. 

    To this end, Fu Weiwei’s team at the Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, reconsidered the image fusion task and modeled it as a conditional generation model. Combined with the diffusion models with the best effect in the current image generation field, the team proposed an MFIF algorithm based on the diffusion model, FusionDiff (the image fusion principle of FusionDiff is shown in Figure 1). This is the first application of the diffusion model in the field of multi-focus image fusion, which provides a new idea for the research in this field.Schematic diagram of the image fusion principle of the FusionDiff algorithm

    Experiments show that FusionDiff is better than other MFIF algorithms in terms of fusion effect and small-shot learning performance. FusionDiff was compared with 13 representative MFIF algorithms on 8 evaluation indexes, and the best fusion results were achieved (Tables 1 and 2). At the same time, FusionDiff is a small-shot learning MFIF algorithm, which only needs 100 pairs of training sets to achieve good fusion results. Table 3 shows the training set size of different MFIF algorithms, and the training set size of FusionDiff is reduced to less than 2% of that of other algorithms. This means that the algorithm may be suitable for use cases where samples are scarce, such as microscopic image fusion.

    Table 1. The average score of all algorithms on the Lytro public test set

    Table 2. The average score of all algorithms on the MFFW public test set

    Table 3. The size of the training set for different MFIF algorithms

    The research results were published in Expert Systems with Applications, titled FusionDiff: Multi-focus image fusion using denoising diffusion probabilistic models. The research work was supported by the Natural Science Foundation of Shandong Province and the Youth Innovation Promotion Association of the Chinese Academy of Sciences. (Source: Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences)

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  • Scientists are making progress in the field of microrobots

    Recently, the research group of Shang Wanfeng, Intelligent Bionic Center, Shenzhen Institute of Advanced Integrated Technology, Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, has cooperated with the Intelligent Manufacturing Center of the Hong Kong University of Science and Technology to make new progress in the field of microrobots. Faced with the challenges of difficult counter-current swimming and insufficient control of micro-medical robots in fluid environments such as blood vessels, the team proposed a unique soft film capsule structure of untethered micro-robots and a control strategy for their wall-mounted precession, which provided new research ideas and solutions for the application of micro-magnetic robots in actual blood vessels. The research results were published in IEEE Transactions on Robotics under the title An On-Wall-Rotating Strategy for Effective Upstream Motion of Untethered Millirobot: Principle, Design, and Demonstration.

    In recent years, in order to achieve the goal of minimally invasive treatment of cardiovascular diseases (CVD), scientists have proposed more magnetic untethered robots for blood vessels. However, due to the fluidity of blood, the untethered and untethered microrobots in the blood vessels bear great resistance, which makes it difficult to remain stationary in the free state and more difficult to achieve fixed-point drug administration control against the current. In order to reduce the fluid resistance of the wireless robot in blood vessels, the team proposed a streamlined structure design and an adherent motion strategy that is easier to apply in clinical practice. As shown in Figure 1, the study combined an elliptical arc and a parabolic design to reduce the fluid resistance of the robot by about 58.5% compared to the traditional structure. The adherent motion mode allows the robot to advance at the tube wall with low fluid resistance, which is further reduced by about 30.7% compared to the classic method of advancing in the center of the tube.

    Figure 1. Design and optimization of self-vector streamlined capsule robot 

    The rotation uniform magnetic field drive mode is limited by the cut-off frequency, which cannot provide sufficient power to realize the high-speed countercurrent motion of the robot, thus limiting the further application of this kind of magnetic drive robot in clinical practice. The adherent rotating magnetic drive strategy was established, and the fluid resistance was overcome by the high-efficiency magnetic rotation “dragging” force generated by the rotating gradient magnetic field at a uniform rotational velocity on the surface of the streamlined robot, so that the robot was subjected to uniform dynamic friction during the movement, so that the wireless robot could be controlled to move forward at a uniform speed in the tube, which solved the problems of the robot motion jamming and instability due to the constant change of static friction when the traditional gradient magnetic field drove the robot, and reached a relative countercurrent velocity of about 143mm/s. To explore the clinical potential of the new approach, the researchers tested the robot’s locomotor ability in pig blood vessels. In this study, a 130 mm section of porcine abdominal aorta was connected to a peristaltic pump to simulate a 2700 mm3/s blood flow environment (Figure 2). The robot successfully passed through the above blood vessels within 26s, which verified the robot’s countercurrent movement ability in real blood vessels, and made the clinical application of intravascular wireless robots possible.

    Figure 2. Robot counterflow control system and its in vitro experimental verification. (a) Counterflow control system of the capsule robot;(b) Comparison of control strategies: insufficient gradient force, unstable movement of stuttering, continuous movement at uniform speed;(c) Counterflow of the robot through the abdominal aorta of pigs at a uniform speed. 

    The team has made a series of achievements in the fields of micromanipulation and microrobotics, and proposed a paradigm for rapid preparation of robots at the general microscale-the theory of magnetic spray microskeleton empowerment.

    The research work was supported by the National Natural Science Foundation of China and the Research Grants Council of Hong Kong, and the Shenzhen Basic Research Special Project. (Source: Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences)

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  • Perovskite cells with 26.1% photoelectric conversion efficiency were born

    Recently, the team of Pan Xu and Tian Xingyou, researchers of the Institute of Solid State Physics, Hefei Institute of Physical Sciences, Chinese Academy of Sciences (hereinafter referred to as the Institute of Solid State) and the Key Laboratory of Photovoltaic and Energy-saving Materials, Chinese Academy of Sciences, cooperated with Professor Nam-Gyu Park of Sungkyunkwan University in South Korea and Professor Dai Songyuan of North China Electric Power University to find for the first time that the uneven cation distribution is the main reason affecting the performance of perovskite solar cells, and successfully prepared a “homogeneous” perovskite solar cell, obtaining 26.1% The certified efficiency is 25.8%. The research results were recently published online in the journal Nature.

    It is worth mentioning that it took just over a week for the paper to be published in the journal from the official acceptance.

    “This study provides a clear direction for further improving the efficiency and stability of perovskite solar cells, and is of great significance for promoting their commercial development. Pan Xu, the first corresponding author of the paper, introduced to China Science News.

    A reviewer in the journal Nature commented on the results, saying that “this work provides valuable insights into the effective inhibition of ion phase segregation in the field of perovskites, which will help promote the commercialization of perovskite solar cells”.

    Xu Pan (right), the first corresponding author of the paper and a researcher at the Institute of Solid State Physics, discusses the problem with the students. Photo by Yao Jie

    “Rookie” in the field of solar cells

    Solar energy is the main source of energy for life on Earth. It is inexhaustible and cleaner and more environmentally friendly than traditional energy sources such as coal and oil. Researchers have invented solar power generation technology, which converts light energy into electrical energy for production and living needs.

    Perovskite solar cells are a “rookie” in the field of solar cells.

    “Here we want to popularize the concept that perovskite is not a mineral, but a crystal structure. Perovskite materials used in solar cells have very high absorption and conversion efficiency for visible light, and inherently have the characteristics of producing high-efficiency solar cells. Pan Xu said.

    “The increase in the efficiency of perovskite solar cells is unprecedented. Pan Xu made a comparison, it took nearly 80 years for crystalline silicon solar cells to increase from the initial 3% to the current 26%, while it took only more than 10 years for perovskite solar cells to increase from 3.8% to the current 26%.

    In addition to excellent efficiency, perovskite solar cells are simple to fabricate and low in cost. At room temperature, several chemicals are mixed in the solution, and then the solution is “brushed” on the substrate like “brushing the wall” to obtain a perovskite film. Finally, with the addition of functional layers such as electron transport layers and metal electrodes, a perovskite solar cell is fabricated.

    “A perovskite solar cell is about 1 micron thick, which is equivalent to one hundredth of the thickness of a sheet of A4 paper. Pan Xu explained that thin means that the overall weight of the cell itself is very light and can be superimposed on existing crystalline silicon solar cells, and thin means that it has good light transmittance and can obtain more light energy. Thin, but flexible, it is expected to be used in aerospace and wearable devices in the future.

    However, there are still some problems with such a “perfect” perovskite solar cell. For example, the poor stability, the current outdoor service life is only 2 to 3 years, and the improvement rate of photoelectric conversion efficiency has slowed down significantly, which are the core problems restricting the industrialization of perovskite solar cells, and they are also the difficulties for researchers to overcome.

    In this work, Pan Xu et al. found for the first time that the cations in the perovskite film are unevenly distributed in the vertical direction, proposed a “homogeneous” cation phase distribution strategy, and successfully prepared a high-efficiency perovskite solar cell, obtained a photoelectric conversion efficiency of 26.1%, and the continuous light stability test reached 2500 hours.

    Dr. Jiajiu Ye (right), co-corresponding author of the paper, and Dr. Zheng Liang (left), the first author of the paper, are testing the performance of battery devices. Courtesy of Solids

    “Cool high resolution”

    After the paper was launched, Pan Xu’s mobile phone information “exploded”, and many industry peers sent congratulatory messages. One of the messages reads: “This high resolution is cool”.

    was “pointed out” by his peers as soon as he was “pointed out” the research highlights, and Pan Xu had some small “satisfaction”.

    In the past ten years of development, a lot of research work has mainly focused on the properties and optimization of the plane of perovskite films, and the inside of perovskite films is like a “black box”, and people lack in-depth understanding of their crystal growth and component distribution, and basically rely on inference.

    “You can’t rely on inference, scientific research needs to be evidence. Pan Xu said bluntly.

    Based on years of research on the properties of high-performance perovskite solar cells and perovskite thin films, Pan Xu et al. have tackled key problems.

    They first deeply analyzed the X-ray photoelectron spectroscopy, and really clearly observed the internal element distribution of the perovskite film from a microscopic perspective. Then, through high-resolution electron microscopy, the difference in crystal plane spacing was directly “seen”, which showed that cations of different sizes exist in different positions, that is, cation inhomogeneity. Large-sized cations are enriched at the upper interface of the film, and small-sized cations are enriched at the bottom of the film.

    “The electron transport channel inside the perovskite film is like a road, and these cations of different sizes are obstacles, which hinder the advance of electrons and naturally cannot improve the efficiency of the cell. The co-corresponding author of the paper, Dr. Ye Jiajiu of the Institute of Solid Solids, introduced.

    Furthermore, the team and the Shanghai synchrotron radiation source developed a new test method, i.e., in-situ grazing-incidence wide-angle X-ray diffraction, which monitors the crystal growth inside perovskite films.

    It is found that the crystallization rate of cations of different sizes is too different in the process of crystal formation, and the crystallization rate of large-size cations is slow, and the crystallization rate of small-size cations is fast, which leads to the uneven distribution of perovskite films.

    Eventually, they devised an additive that synchronized the crystallization rates of different cations so that they were evenly aligned and promoted charge transfer.

    “This result shows that excellent cell performance can be obtained by homogenizing the vertical distribution of perovskite cations, opening up a new way to improve the stability of cell devices, and is expected to break the efficiency bottleneck of perovskite solar cells.” ”

    It took just over a week from receipt to publication

    In this paper, the reporter noticed a message that on October 25, 2023, the paper was officially accepted and published online on November 2. In other words, it took just over a week for the paper to be published.

    “In general, there are two forms of publication in academic journals, one is a modest queue, and the other is a fast-track online publication. The reviewers believe that the research results are suitable for the latter form. Pan Xu admitted that they were also pleasantly surprised by such a quick process, and at the same time they were under pressure to prepare various materials within a week, including providing raw data, pictures, etc., to ensure the reliability and rigor of the paper.

    On January 1, 2023, the paper was officially submitted. “This date is easy to calculate. Pan Xu said that they have been waiting for the email since the submission.

    Soon, on January 20 (Chinese New Year’s Eve), they received a reply from the editor, and made changes to the editor’s comments, returned to the editor in March, and successfully submitted it for review.

    Just when they felt that the start of the submission seemed to be going well, they received a new email in April, with nearly 40 comments from reviewers and requests for “major revisions”. So, they worked overtime for two whole months, carefully replied to every comment, and wrote more than 100 pages of content.

    On June 19, they submitted for the third time. The reviewers highly praised the content of the responses, saying that “the responses were very patient and professional, and addressed all their comments”.

    On October 25, the paper was finally accepted and published online “at an accelerated pace” a week later. Pan Xu said, “The reason for the rapid publication is that the work itself is meaningful and has direct reference value for the industry. ”

    “Practical research is to be done”

    Pan Xu was one of the earliest engaged in perovskite research in China. “The reason why we chose this direction is that because of the excellent optoelectronic properties of perovskite materials themselves, the potential for initial development and the theoretical limit efficiency of up to 33%, there is an irresistible attraction for researchers who have been engaged in the photovoltaic industry. Pan Xu said with a smile.

    However, he was also very unconfident at first. “At that time, we held the perovskite thin-film cell in our hands, and it was visible to the naked eye that it slowly turned from black to yellow, and the light absorbance deteriorated. Pan Xu recalled the early days of perovskite research.

    After more than ten years of deep cultivation, Pan Xu and others have made a major breakthrough. On November 2, the research results were published in the journal Nature. He said, “Perseverance is the most important quality in doing scientific research.”

    At 1 o’clock in the morning of the same day, Pan Xu, who was sitting in the office, posted such a circle of friends – “The road ahead is still very long, there are still heavy rains, and there are bumps, keep going.” It’s nice to enjoy the scenery along the way, and it’s a treat even if you can’t see the end in sight. ”

    “I want to tell my students that doing scientific research is not only for publishing articles, but also for practical research, doing research that has an impact on the national economy and people’s lives, even if it plays a small role. Pan Xu said that this is what he thinks is the greatest significance of doing scientific research.

    “This work is just the beginning, and we will continue to explore further in this direction, hoping to develop improved additives to improve the efficiency and stability of perovskite solar cells. When it comes to the development of perovskite solar cells in the future, Pan Xu is full of confidence. (Source: Wang Min, China Science News)

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  • Large-area liquid crystal microlens arrays are prepared by single-step exposure

    As a tunable core optical device, liquid crystal microlens array is widely used in 2D/3D switchable glasses-free 3D display, adaptive optics and other fields. However, the design and fabrication of liquid crystal microlens arrays is difficult and costly, and usually involves precision machining technologies such as photolithography, making it difficult for large-scale mass production.

    In view of this, Yanjun Liu’s team from Southern University of Science and Technology and Yanqing Lu’s team from Nanjing University proposed a technology for preparing liquid crystal microlens arrays based on photopolymerization-induced phase separation. This technique requires only a single exposure step to fabricate large-area, high-quality liquid crystal microlens arrays. Experiments show that the liquid crystal microlens array has good focusing and imaging effect, and can be used to expand the imaging depth of integrated imaging 3D display. This technology is expected to greatly reduce the difficulty and production cost of liquid crystal microlens arrays.

    At present, this work is presented as “Optically Anisotropic, Electrically Tunable Microlens Arrays Formed via Single-Step Photopolymerization-Induced Phase Separation in Polymer/Liquid-Crystal Composite”. Materials” was published in Light: Advanced Manufacturing. Cai Wenfeng, a Ph.D. student at Southern University of Science and Technology, is the first author of the paper, and Professor Lu Yanqing of Nanjing University and Associate Professor Liu Yanjun of Southern University of Science and Technology are the corresponding authors.

    Microlens arrays are widely used in optical systems, such as light diversion, wavefront detection, fiber coupling, three-dimensional display, etc. Liquid crystals, on the other hand, are anisotropic materials that endow them with features that traditional microlens arrays do not have, such as adjustable focal length and beam deflection. Compared with mechanical zoom lenses, liquid crystal microlens arrays are more light and thin, conducive to integration, short response time and good stability, and are widely used in 2D/3D switchable naked-eye 3D display, automatic focus imaging system, automatic projection system, etc., which has great application value. However, the fabrication of liquid crystal microlens arrays usually involves precision machining techniques such as photolithography, and the preparation process is relatively complex and costly.

    In view of the above problems, the researchers proposed a technique to fabricate large-area liquid crystal microlens arrays based on Polymerization induced phase separation (PIPS), as shown in Figure 1. In this method, Phase-separated composite films (PSCOF) were formed in liquid crystal cells by single-step ultraviolet exposure. The microstructure topography of the composite film is controlled by a grayscale mask above. In the picture, the top is a grayscale mask with a microlens array pattern. Below is a liquid crystal box with a composite structure of a polymer layer and a liquid crystal layer with the morphology of the microlens array.

    Figure 1: Schematic diagram of the preparation of a liquid crystal microlens array

    The process of photopolymerization-induced phase separation to form a liquid crystal microlens array is shown in Figure 2. First, the prepolymer is poured into the liquid crystal cartridge. Prepolymers are made from a homogeneous mixture of liquid crystals and polymerizable monomers. Since the prepolymer has a strong absorption of UV light, the UV light intensity is distributed in a gradient in the direction of the vertical liquid crystal cell. Under the action of ultraviolet light, the monomer in the prepolymer first undergoes a polymerization reaction on the side close to the light source, and the molecular weight of the prepolymer increases. When a critical value is reached, the mutual solubility of the liquid crystal and the prepolymer decreases until phase separation occurs and gradually precipitates. In order to maintain the concentration equilibrium between the monomer and the liquid crystal molecule, the liquid crystal molecule will move to the lower layer, and the monomer will move to the upper layer, which is the process of diffusion. This process continues until the liquid crystal and polymer are completely separated, resulting in the formation of a polymeric phase separation composite film (PSCOF). Finally, when the temperature drops below the clear point, the liquid crystal is oriented by the orientation layer of the lower layer.

    Due to the grayscale mask, the UV light intensity is distributed in a periodic gradient parallel to the LCD cell, as shown in the purple area in Figure 2. The monomer first undergoes a polymerization reaction in the region with strong light intensity, so the morphology of the final polymer layer is related to the gray distribution of the reticle. By designing the grayscale distribution of the reticle, the researchers successfully prepared a liquid crystal microlens array.

    Figure 2: The process of ultraviolet photopolymerization phase separation to form an array of liquid crystal microlenses

    The research team prepared a 5 cm × 5 cm liquid crystal microlens array sample, as shown in Figure 3. Under a polarizing microscope, the sample exhibits a rich variety of interference colors. The color of each microlens unit gradually transitions from red in the center to blue at the edges, indicating that the thickness of the liquid crystal layer gradually changes from the center to the edges.

    Figure 3: Sample plot of a liquid crystal microlens array

    In addition, the research team characterized the optical properties of liquid crystal microlens arrays. The results show that the prepared liquid crystal microlens array has a uniform focusing effect. In addition, the liquid crystal microlens array is close to the diffraction limit and has a good focusing effect. Due to the polarization-dependent and electrically adjustable nature of liquid crystals, researchers also use these two methods to control the focusing and imaging effect of the prepared samples. Videos 1 and 2 show the focusing effect of a liquid crystal microlens array under polarization modulation and voltage modulation, respectively.

    Finally, the research team prepared a liquid crystal microlens array for integrated imaging 3D display, as shown in Figure 4. Among them, the light source is combined with the mask as the three-dimensional display source. The researchers took advantage of the adjustable focal length of the liquid crystal microlens array to adjust the position of the central depth imaging plane and expand the imaging depth of the three-dimensional display. When a sufficiently large voltage is applied, the focusing effect of the liquid crystal microlens array disappears. This enables 2D/3D switchable glasses-free 3D display.


    Figure 4: Integrated imaging 3D display based on liquid crystal microlens array

    In summary, this technique requires only a single exposure step to fabricate large-area, high-quality liquid crystal microlens arrays. Therefore, this technology is expected to greatly reduce the difficulty and production cost of liquid crystal microlens arrays. In addition, this technology can be used as a general platform for the fabrication of liquid crystal micro-optical devices with other functions, such as liquid crystal column lens arrays, liquid crystal gratings, etc. (Source: Advanced Manufacturing WeChat public account)

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  • Holographic laser processing: rapid fabrication and image reconstruction of artificial bionic compound eyes

    In recent years, inspired by the insect compound eye, the artificial bionic compound eye has attracted more and more attention in overcoming the limitations of existing imaging devices such as large, bulky and heavy, and improving the visual performance of medical endoscopy, panoramic imaging, micronavigation and robot vision with its unique optical imaging solutions such as small size, distortion-free imaging, wide field of view and high-sensitivity motion tracking ability.

    The existing artificial bionic compound eye manufacturing mainly faces two challenges: one is that the existing process is relatively complex, the manufacturing efficiency is limited, and it is difficult to meet the commercial standard, and the other is that the curved surface characteristics of the artificial bionic compound eye do not match the commercial plane imaging sensor, which makes it difficult to integrate the two.

    In view of this, the research group of Professor Chen Qidai of Jilin University proposed a wet-assisted holographic laser processing method, which greatly improved the processing efficiency of the artificial bionic compound eye by customizing the preparation of the bionic compound eye and large-area transcription, and combined with the artificial intelligence method, it solved the difficulty of mismatch between the artificial bionic compound eye and the plane imaging sensor from the algorithm.

    The research results were published online in Light: Advanced Manufacturing under the title of “Holographic laser fabrication of 3D artificial compound μ-eyes”.

    Figure 1: Flow diagram of wet-assisted femtosecond laser parallel fabrication of artificial bionic compound eye

    In the experiment, a femtosecond laser without space light field modulator (SLM) modulation was used to expose the surface of the quartz substrate, and the compound eye main lens was formed by wet etching, and then the femtosecond laser beam was split by SLM and combined with wet etching to realize the parallel processing of multiple small eyes in the compound eye, and the polydimethylsiloxane (PDMS) micro-nano structure transcription technology was used to realize the large-scale production of compound eye microlens arrays. The compound eye microlens array prepared by this method has the characteristics of high resolution and wide field of view. In order to overcome the problem that artificial bionic compound eyes are difficult to integrate with planar cameras, high-quality image reconstruction was achieved by using Generative Adversarial Network (GAN), which laid a foundation for future device integration.

    Figure 2: Large-scale fabrication sample of an artificial bionic compound eye

    Complex optics manufactured by holographic laser processing technology are scalable. To address the complexity and time-consuming nature of the process, Figure 2 illustrates the mass production of polydimethylsiloxane (PDMS) soft miniature optical components using quartz glass-based fly-eye microlenses as hard templates. In this process, the microoptics maintain a high surface quality (Figure 2a for scanning electron microscopy and Figure 2b for 3D depth of field images).

    Figure 3: Image reconstruction based on Generative Adversarial Network (GAN) deep learning algorithm

    The curved profile gives the compound eye a large field of view, but at the same time limits its focus position, which can only be positioned in a curved focal plane. For a true biological compound eye, there is an optical fiber that picks up light and directs it directly into the retina. However, this is difficult to be compatible with current sensor programs and to integrate optics and detectors on the chip. Theoretically, the parameters of each lens, including height, curvature, and focal length, should be redesigned, but it is difficult to determine the plane based on its position on the curved profile. To this end, we propose a deep learning algorithm based on Generative Adversarial Network (GAN) for image processing. In this study, we utilize two neural networks to maximize the generative power of the discriminator and minimize its loss function, while the discriminator is trained to maximize its loss function. As shown in Figure 3a, the neural network can be trained to perform image restoration of all eyes using the image shown in Figure 3c. Image restoration is independent of incident wavelength, material refractive index, or singlet thickness. With this technology, fly-eye imaging can preserve a large field of view and significantly improve image quality, making it suitable for a wider range of application scenarios (Figure 3B).

    In this study, a method for the preparation of artificial bionic compound eyes by efficient femtosecond holographic laser was proposed, and an artificial intelligence method was introduced to reverse the image reconstruction, which solved the pain point of low manufacturing efficiency of artificial bionic compound eyes, and laid a foundation for the matching and integration of artificial bionic compound eyes and planar imaging sensors in the future. (Source: Advanced Manufacturing WeChat public account)

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  • On-chip nonlinear photonics: hybrid integration of two-dimensional materials

    At present, the performance of silicon-based electronic systems has approached its physical limit, and it is difficult to meet the requirements of information processing in the era of big data. Optical information processing has unmatched speed and energy consumption advantages. For example, all-optical information processing devices based on nonlinear optics can achieve ultra-high response speed in the femtosecond range, far exceeding that of electronic devices of the same class. Recently, the on-chip integrated nonlinear photonic devices have adopted the design idea of hybrid integration of two-dimensional new semiconductor materials and optical waveguides to achieve enhanced nonlinear effects, which is expected to further promote the development of photonic devices such as ultrafast optical switching, optical parametric amplification, all-optical logic computing, and quantum light sources.

    Vincent Pelgrin and his collaborators from Aalto University in Finland recently published a review article in Light: Advanced Manufacturing titled Hybrid integration of 2D materials for on-chip nonlinear photonics.

    This paper first summarizes the linear and nonlinear properties of different integrated optical systems and the nonlinear effects of different 2D materials, systematically summarizes the modeling and characterization of 2D material-on-chip photonic structure hybrid integrated systems, analyzes the hot applications of 2D material-integrated nonlinear optics in detail, and discusses the main challenges and future development prospects of the current hybrid integration of 2D materials and optical waveguides. The authors believe that with the exploration of various new 2D materials and the continuous improvement of optical waveguide integration technology, 2D material-optical waveguide integrated photonics will bring new breakthroughs in cutting-edge scientific research, industrial production and daily life.

    Theoretical basis and preparation method of two-dimensional material-optical waveguide hybrid integrated system

    Designing a hybrid two-dimensional material-waveguide integrated system is one of the key steps to effectively realize its optical function. In general, the eigenmode calculation is used to analyze its effective refractive index, transverse mode field distribution, and group velocity dispersion of the working wavelength to design the applicable waveguide and integration. Figure 1 simulates the TE mode field distribution of a planar buried strip silicon nitride (SiN) waveguide covering molybdenum disulfide (MoS2) at an operating wavelength of 1550 nm.

    Figure 1: Mode field distribution of a hybrid integrated MoS?-SiN waveguide

    According to the effective nonlinear properties of the waveguide (such as third-order nonlinear effects, the number of free carriers, and linear passive losses, etc.), the pulse propagation in the waveguide can be simulated in combination with the generalized nonlinear Schrödinger equation. Figure 2 simulates the propagation of nonlinear pulses in a SiN waveguide system, where the blue rectangular box region is the MoS2-SiN hybrid region, which greatly broadens the spectral width of the input pulse due to the enhanced nonlinear effect.

    Figure 2: Transmission of nonlinear pulses in a hybrid integrated MoS?-SiN waveguide system

    In addition, the authors summarize the main preparation methods (e.g., mechanical exfoliation, liquid phase exfoliation, and chemical vapor deposition) and transfer methods (e.g., wet transfer, dry transfer, or semi-dry transfer) of 2D materials to prepare integrated 2D material waveguide devices. In recent years, scientific researchers have also realized the direct growth of two-dimensional materials in waveguides, such as photonic crystal fibers.

    Nonlinear optics of two-dimensional materials

    The authors briefly review the nonlinear optical phenomena such as SHG, THG, Raman (CARS & SRS) and saturable absorption of typical 2D materials (such as graphene, black phosphorus, hexagonal boron nitride, transition metal chalcogenide, etc.) (Fig. 3b), explain the reasons for the significant differences in the conclusions of experimental studies on the nonlinear coefficients of 2D materials, and point out that the physical mechanism behind the nonlinear phenomena of 2D materials still needs to be improved.

    Figure 3: Typical 2D materials and their nonlinear effects

    Research progress of two-dimensional material-optical waveguide hybrid integrated system

    The authors introduce in detail the latest research progress of two-dimensional material-optical waveguide hybrid integrated systems. Fig. 4a illustrates the enhancement of the four-wave mixing effect by using a graphene-silicon ring resonator system, Fig. 4b illustrates the tuning of a graphene-planar waveguide system with an applied electric field, and Fig. 4c and Fig. 4d illustrate the enhancement of second harmonics in transition metal dichalcogenides using planar silicon waveguides and optical fibers, respectively.

    Fig. 4: Experimental progress on nonlinear effect enhancement in two-dimensional material-waveguide hybrid integrated systems

    Figures 5a and 5b illustrate the enhanced Raman response of graphene using photonic crystal microcavities, while Figures 5c and 5d illustrate the saturable absorption of graphene by designing different waveguide structures.

    Fig. 5: Research progress on Raman enhancement and saturable absorption experiments in two-dimensional material-waveguide hybrid integrated systems


    The two-dimensional material-waveguide hybrid integration technology shows that it can bypass the physical limits of different integrated photonic systems, enrich and improve the performance of micro-nano photonic systems, and introduce new functions in physical, chemical and biological regulation. Finally, the authors analyze the use of novel characterization tools, novel two-dimensional materials with high nonlinear coefficients and their heterojunctions, complementary metal-oxide-semiconductor (CMOS)-compatible fabrication techniques, and new integrated platforms to achieve higher performance integrated optoelectronic devices, and look forward to their prospects in emerging directions such as broadband on-chip light sources, optical frequency comb generation, all-optical computing, optical parametric amplification, and optical quantum applications.

    In summary, the authors believe that the research on hybrid integrated photonic technology for two-dimensional materials has just begun, and with the development of new structures and new two-dimensional layered materials, more high-efficiency on-chip optical functions are possible. At the same time, the authors point out some key scientific problems that need to be solved urgently in this field (such as the influence of propagation loss on the selection, reliability, and integration methods of 2D materials). (Source: Advanced Manufacturing WeChat public account)

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  • Two-photon lithography: breaking through the packaging bottleneck of optoelectronic chips

    Dr. Shaoliang Yu’s team from Zhijiang Lab and collaborators from the Massachusetts Institute of Technology (MIT) published a review article titled Two-photon lithography for integrated photonic packaging in Light: Advanced Manufacturing.

    As a rising star in the semiconductor industry, optoelectronic chips have important application prospects in the fields of optical interconnection, optical computing, optical sensing, and lidar, and have become the focus of attention from all walks of life, opening up a new track for the development of the post-Moore’s Law era. Benefiting from the mature semiconductor CMOS process, the fabrication capacity of optoelectronic chips has been rapidly developed.

    However, there are still key technical bottlenecks in the packaging process of optoelectronic chips. In addition to the well-known interconnection of electrical signals, the coupling of optical signals between different modules needs to be taken into account in the packaging process of optoelectronic chips, which requires solving the optical interconnection problems between optical fiber-chip and chip-chip. There are two main challenges:

    1. Optical chips involve a variety of material systems and structures, and there are great differences in the size and distribution of different beam mode fields, so the efficient coupling between multiple material systems and structures can be achieved by cleverly solving the problem of mode field mismatch in the packaging process.

    2. The beam size of the waveguide on the chip is in the micron range, which requires high-precision alignment to achieve efficient coupling, which puts forward higher requirements for the alignment accuracy of the packaging process.

    Figure 1: Schematic diagram of optoelectronic chip packaging. Source: Research group

    Two-photon lithography based on two-photon absorption process, as a three-dimensional printing process at micro-nano size, can prepare arbitrary three-dimensional structures with high precision, which is expected to solve the bottleneck of the optoelectronic chip packaging process.

    On the one hand, a three-dimensional curved surface or gradient waveguide structure can be integrated on the chip, and the beam can be shaped by reflection or adiabatic compression to achieve the mode field transformation of the ultra-wide waveband.

    On the other hand, the topography of the 3D structure has a high degree of geometric freedom, which increases the flexibility of the manipulation of the on-chip tool field, resulting in more efficient coupling interconnection.

    In addition, two-photon lithography can also prepare the connection structure after the submodule is assembled, which effectively reduces the alignment accuracy requirements in the packaging process.

    Therefore, in the packaging of optoelectronic chips, two-photon lithography technology has important application value and has been widely explored, and there are currently three main technical routes.

    Optical packaging method based on two-photon lithography

    1. Photonic wire bonding

    Drawing on the widely used wire bonding and photonic wire bonding technology in microelectronics, the two-photon lithography process is used to directly print polymer waveguides between the waveguides to be connected. Through the gradual change of the waveguide cross-section, the adiabatic transformation of the mode field is completed, so as to realize the efficient interconnection between different waveguides. This method has been verified in optical interconnection and coherent communication, and is suitable for a variety of application scenarios such as fiber-chip, chip-chip, etc.

    2. Miniature freeform surfaces

    A miniature optical freeform surface is printed on the end surface of the waveguide, and the light field emitted by the waveguide is shaped in the form of reflection or refraction, and the mode field distribution and propagation direction are adjusted to complete the mode field transformation. The structure used has low dispersion and is not wavelength sensitive, and ultra-wideband coupling from visible to near-infrared has been validated, and it is compatible with wafer-level test and packaging, enabling high-density interconnect packaging.

    3. Mechanical alignment and guidance structure

    Two-photon lithography can also be used to print mechanical alignment guide structures to assist in high-precision alignment of the coupling process. Printing an inverted cone structure on the grating coupling region to guide the alignment process of the fiber can achieve sub-micron alignment accuracy without introducing significant additional losses, which is expected to be applied in pluggable devices.

    Figure 2: (a), Schematic diagram of two-photon lithography. (b), Photon leads. (c), Freeform surface. (d), alignment of the guidance structure. Source: Light: Advanced Manufacturing 4, 32(2023)

    Commercialization exploration

    With the gradual development of optoelectronic chips to the market, packaging technology based on two-photon lithography has also begun to be commercialized. For large-scale commercial applications, more factors need to be considered in addition to coupling characteristics such as bandwidth and insertion loss. For example, whether two-photon lithography can stably and reliably produce high-quality 3D structures, whether it can meet the industry’s processing speed and accuracy requirements, and whether it has user-friendly ease of use and maintenance.

    At present, several companies have opened up the commercial market of two-photon lithography. NanoScribe, Vanguard, Heidelberg and other companies have launched commercial two-photon lithography equipment, and have made great progress in scanning speed, processing accuracy, alignment accuracy, etc., while Dream Photonics, PHIX, etc., mainly provide process services, and can directly provide packaging services based on two-photon lithography. The application of two-photon lithography technology in optoelectronic chip packaging has taken a solid step towards large-scale commercialization.

    Figure 3: Three types of slicing: uniform slicing, adaptive slicing, and smart slicing. Source: Light: Advanced Manufacturing 4, 32(2023)

    Future outlook

    After more than ten years of exploration, the packaging method based on the two-photon lithography process has made a lot of progress and has been widely recognized by all walks of life. In the era of explosive growth in communication capacity, it is necessary to judge whether the two-photon lithography process can occupy an important position in the packaging of optoelectronic chips, and whether it can meet the needs of large-scale applications in the future. Based on this, the author also sorts out the future development trend of this field.

    1. Greatly improve the preparation efficiency

    The current point-by-point scanning method has a slow preparation speed and is difficult to meet the efficiency requirements of large-scale production. On the one hand, new two-photon exposure methods such as multi-beam and layer-by-layer can be used to improve the preparation speed. On the other hand, other fabrication processes can also be explored, such as nanoimprint, which can upgrade the serial processing method to parallelism to meet the fabrication needs at the wafer level.

    Figure 4: Three exposure methods: point-by-point, layer-by-layer, and multi-beam exposure. Source: Light: Advanced Manufacturing 4, 32(2023)

    2. Develop multiple types of lithography materials

    Two-photon lithography mostly acts on photopolymer materials. Compared with conventional semiconductor or dielectric materials, polymer materials have a large coefficient of thermal expansion, a limited range of refractive index options, and poor long-term stability. At the same time, the shrinkage of polymers in the cross-linking process also brings certain challenges to the morphology control of three-dimensional structures. Exploring organic-inorganic hybrid composite photosensitive materials can solve the above problems to a certain extent.

    3. Optimize the design and modeling methods

    The geometric degree of freedom of the three-dimensional structure is high, which brings great convenience to the wavefront control. However, the design regulates many parameters, which brings a lot of pressure to the simulation design process. It is necessary to combine geometric optics and wave optics methods to explore computational exploration and construct a new modeling method. Data-driven and physics-driven machine learning methods can also play an important role in the design and characterization of 3D miniature optical structures.

    4. Develop new methods for structural characterization

    The miniature three-dimensional optical structure, the scale is between the macro and the micro, the structure is small, the curvature is large, and the conventional measurement methods, such as white light interferometry, electron microscopy, atomic force microscopy, etc., are difficult to carry out effective measurements, and new characterization methods are urgently needed. The three-dimensional reconstruction of electron microscopy based on multi-quadrant is expected to achieve accurate measurement of the topography of microfreeform surfaces. X-ray microtomography is also a promising characterization method.


    Two-photon lithography technology can accurately prepare three-dimensional structures and integrate them on optoelectronic chips, which can build large-bandwidth and low-loss optical signal links between fiber-chip and chip-chip, realize the efficient interconnection of optical signals, reduce the alignment accuracy of the packaging process, and bring new opportunities to the packaging process of optical chips. With the iterative evolution of technology and the further development of the industry, we expect that the packaging architecture of optoelectronic chips based on two-photon lithography will be applied on a large scale to solve the packaging problems of optoelectronic chips. (Source: Advanced Manufacturing WeChat public account)

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