New Lens for High Resolution Imaging

Monolayer TMDC lens: Schematic of femtosecond laser-induced generation of MOx nanoparticles. (Source: H. Lin et al., Swinburne U.)

Conventional lenses are based on the principle of light refraction, using dif­ferent materials, spherical surfaces and spatial positions to achieve the control of light. The fabrication of conven­tional lenses including the processes of material selection, cutting, rough grinding, fine grinding, polishing, and testing. In order to minimize the aber­rations including the chromatic aberration, spherical aberration and astig­matisms, it is necessary to stack multiple layers of lenses to form compound lenses, leading to the complexity and cumber­someness of current camera equipment. There­fore, tremendous effort has been devoted into the development of ultrathin flat lenses. Unlike conven­tional lenses, flat lenses use nanostructures to modulate light. By controlling the optical proper­ties and the spatial position of each nano-element, advanced functions, such as achromatic and aber­ration-free focusing, high spatial resolution and special focal intensity distri­butions can be achieved. However, when the material thickness is reduced to the subwave­length scale, the insuf­ficient phase or amplitude modu­lation based on the intrinsic refractive index and absorption of the materials results in poor lens performance.

Now, a team of scientists, led by Baohua Jia at Centre for Trans­lational Atomaterials, Swinburne Uni­versity of Technology, Australia, Qiaoliang Bao formerly at Monash University, Chengwei Qiu at National Univer­sity of Singapore and co-workers have developed an inno­vative method to fabricate high per­formance lenses in monolayer two dimensional transi­tional metal dichal­cogenide (TMDC) material by using a femtosecond laser to pattern nano­particles. The lens has a sub-wavelength resolution and a focusing effi­ciency of 31%, laying the foundation for ultimately thin optical devices for use in nano-optics and on-chip photonic appli­cations.

Although lenses made from multi­layer TMDCs have been demons­trated before, when their thickness is reduced to the sub-nanometer scale, their insuf­ficient phase or amplitude modu­lation results in focusing effi­ciencies of less than 1%. The inter­national team discovered that it is possible to generate nanoparticles by using a femto­second laser beam to interact with the monolayer TMDC material, which is signi­ficantly different from the process produced by a continuous wave laser. When the laser pulse is so short that the entire material remains cold after laser process, the nano­particles can firmly attach to the substrate. The nano­particles show very strong scat­tering to modulate the amplitude of light. Therefore, the lens made from the nano­particles can provide subwave­length resolution and high efficiency, which allows the team to demonstrate diffrac­tion-limited imaging by using the lenses.

Monolayer is the thinnest form of a material, which is the ultimate physical thickness limit. By using the monolayer for the lens fabri­cation, the process demonstrated in this study consumed the least material meeting the theoretical limitation. More impor­tantly, the femtosecond laser fabrication technique is a one-step simple process, without the require­ments of high vacuum or special environment, thus it provides the simplest way to fabricate an ultrathin flat lens. As a result, the lens can be easily inte­grated into any photonic or micro­fluidic devices for broad appli­cations. “We have used the thinnest material in the world to fabricate a flat lens, and prove that the good per­formance of the ultrathin lens can lead to high resolution imaging. It shows enormous potential in different appli­cations, such as eyeglasses, microscopy lenses, telescopes and camera lenses. It is fore­seeable that by using this technique, the weight and size of camera lenses can be signi­ficantly reduced in the near future,” said Han Lin from the Centre for Trans­lational Atomaterials, Swinburne Univer­sity of Technology.

“We are excited to see unique outcome from femto­second laser processing 2D materials. It opens up new possi­bility to fabrication photonic devices using scalable method,” added Baohua Jia, Director of Centre for Trans­lational Atomaterials. “We can integrate the monolayer 2D material lens onto desired devices by simply attaching the material then using a femtosecond laser to perform fabri­cation. The entire process is simple, and the method is flexible and low cost. Thus, we also see the great application potential of the method,” commented Qiaoliang Bao formerly at Monash University. “We design our lens in such a way that image can be found at different focal planes, with different magni­fications. This mechanism can be readily used to develop an optical zoom lens that is required in all cellphone cameras. Currently, lenses with different focal lengths are used to achieve different zoom function. However, our lenses can achieve different zoom rates simply with one design,” forecasts Chengwei Qiu from National Univer­sity of Singapore. (Source: LPC-CAS)

Reference: H. Lin et al.: Diffraction-limited imaging with monolayer 2D material-based ultrathin flat lenses, Light Sci. Appl. 9, 137 (2020); DOI: 10.1038/s41377-020-00374-9

Link: Centre for Translational Atomaterials, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Australia

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