Metalenses are ultrathin, flat surfaces that use nanostructures to focus light. They have attracted tremendous attention because they can overcome limitations of conventional bulky, curved and heavy refractive optical lenses and they are poised to revolutionize everything from microscopy to cameras, sensors, and displays.
Despite these advantages, mass production of these flat optics – for instance for waver-level devices – is still not possible due to the low productivity and high cost of electron beam lithography, which has been commonly used to make metalenses.
What is needed are printable and high-refractive index materials for the mass production of high-efficiency metalenses including general metasurfaces. However, so far there has been no material satisfying these requirements.
Researchers in Korea now have developed a high-throughput, scalable nanocomposite printing method to manufacture metalenses at low cost, paving the way to commercializing them.
The team has published their findings in ACS Nano (“Printable Nanocomposite Metalens for High- Contrast Near-Infrared Imaging”).In previous research, titanium dioxide (TiO2) or gallium nitride (GaN) have been used as suitable materials for high-efficiency metasurfaces. However, the fabrication – deposition and etching – of these materials is highly challenging and too expensive for commercialization.
“In contrast” as Prof. Junsuk Rho from Pohang University of Science & Technology (POSTECH), who led this work, explains to Nanowerk, “our nanocomposite material, which is also suitable for high-efficiency metasurfaces, can be molded into metalenses just by one step of printing without any secondary operations such as thin-film deposition or plasma etching.”
The researchers synthesized their nanocomposite by dispersing silicon nanoparticles in the matrix of UV-curable resin to achieve a high-refractive index to increase the efficiency of the metalenses. The printing mold is reusable, so the large-scale metalenses can be printed rapidly and repeatedly.The fabricated metalens shows a focusing efficiency of 47% at λ = 940 nm, and, according to the team, the efficiency can be further increased by optimizing the composition of the silicon nanocomposite.
Although silicon nanoparticles have been used as a key element of this nanocomposite, the team points out that there are still many kinds of nanoparticles – such as quantum dots and upconverting nanoparticles – that potentially could be used instead.
“The effects of these functional nanoparticles with metasurfaces has not been established yet, and we believe that it will be possible to discover new quantum-nanophotonic phenomena or develop novel nanophotonic devices such as nano-lasers,” says Rho.
He notes that the nanocomposite printing process demonstrated by his team can be applied to not only metalenses, but also general metasurfaces such as metaholograms, color filters, perfect absorbers and optical cloaks.Applying their nanocomposite printing technique, the researchers demonstrate a near-infrared (NIR) metalens, which can have applications in LiDAR sensors (receivers), IR cameras, CCTV, night vision devices and biometric devices such as vein recognition.
This printing method is also capable of realizing visible metalenses, and possible applications in this area include ultrathin optical camera modules (smartphones), compact DSLR camera lenses and many other optical devices using conventional optical lenses.
“There have been many attempts to correct various optical aberrations such as spherical aberration, chromatic aberration and coma by using just a single metalens,” says Rho. “Some of the precedent works have shown such promising possibility, but the design strategy has not been perfect. Therefore, we need to develop novel design principles for such aberration-free metalenses.”
“The biggest challenge facing future research in this field is that numerical simulation of metasurfaces is very time consuming,” he concludes. “Therefore, we need to develop novel approaches to designing metasurfaces; deep learning can be a promising candidate for this purpose.”
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In recent times, the sports equipment industry has emerged as a sophisticated yet commercially viable hi-tech industry where advances have revolutionized sports.
The new method of making mixed halide-perovskites results in solar cells with improved stability and performance. The new method results in better control over perovskite crystallization rates. This means the crystal structure is more ordered, in part due to researchers understanding and taking advantage of the faster crystallization of bromide relative to iodide.
The result is a material with fewer defects and less halide migration and thus less segregation of the bromide and iodide. This in turn means uniform mixing of bromide and iodide across the material, which allows the material to absorb light evenly. The end result is that solar cells made using the new method will perform better under real-world conditions.
Typical halide perovskite solution deposition uses an anti-solvent drip procedure to initiate crystallization of the halide film. The standard anti-solvent method for producing bromide-iodide mixed halide perovskite films often leads to excessive defect formation (e.g., bromide vacancies) owing to the rapid crystallization of bromide vs. iodide-perovskite phases. Simulations show that halide migration is enhanced in the presence of a large population of halide vacancies. This limits the stability of bromide-iodide mixed halide perovskites under light and heat.
In comparison to the anti-solvent approach, the gentler gas-quench method better controls crystallization, first producing a bromide-rich surface layer that then induces top-down columnar growth to form a gradient structure with less bromide in the bulk than in the surface region. The anti-solvent method does not produce such a gradient structure.
In this study, researchers from the National Renewable Energy Laboratory, the University of Toledo, and the University of Colorado Boulder demonstrated that the gas-quench method also produces fewer bromide vacancies and results in materials with a higher quality opto-electronic performance. Solar cells made using the gas-quench method retain desirable light absorption properties and provide enhanced performance in the form of a higher charge carrier mobility, higher open circuit voltage, and enhanced stability.
