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For Scientific Research & Industry Modernisation.
Lenses are one of the most common optical components used in everyday life. Whether it is the glasses that we normally wear, the lens that we use for taking pictures, the microscope that we use for research, or the magnifying glass that we use for the elderly, they are all lenses. The lenses used in daily life are spherical lenses, generally made of optical glass, optical crystals, optical resin and other optical materials. It uses the principle of optical refraction, using different materials, different spherical surfaces and different spatial positions to achieve control of light.
The thickness of most planar lenses is still comparable to the wavelength, especially the equivalent wavelength within the material. From conventional lenses to planar ultra-thin lenses, there are still many engineering and optimization-oriented questions, such as the design of the introduction of adjustable mechanisms to improve prism focusing efficiency and broadband, color aberration aberration correction, and so on. But the most exciting scientific question is: how thin can the lens be? If the lens is a single atomic layer of two-dimensional material, is it still possible to make a lens? Currently, due to the limitations of the modulation principle, the need to achieve sufficiently strong field and phase modulation requires materials with sufficient thickness, in the order of optical wavelength, so that high-quality imaging can be achieved. Although some research works have been introduced to achieve focusing using lenses or mirrors with sub-nanometer thickness, their focusing efficiency is limited to less than 1%, which cannot meet the demand of high quality imaging. Often single atomic layer materials are considered too thin to provide sufficient phase or amplitude modulation to achieve the lens function, let alone a high efficiency focusing lens, and the classical design principles of diffractive lenses and superlenses are no longer applicable, so there are no available theoretical principles to guide, and even some sound contrary to common sense.
Fig. 1. Femtosecond laser fabrication of single atomic layer 2D materials for ultra-thin flat lenses (Source from: Lin, H., et al., Diffraction-limited imaging with monolayer 2D material-based ultrathin flat lenses. Light: Science & Applications, 2020. 9(1). )
This solution is based on the special interaction between the femtosecond laser and the monolayer 2D transition metal dihalide. When the femtosecond laser irradiates the surface of the monolayer 2D transition metal dihalide, its material decomposes and produces nanoparticles of transition metal oxides. Since the laser pulse of the femtosecond laser processing is very short, the material is not heated during the interaction and, therefore, is a cold process, and the resulting nanoparticles can be effectively adsorbed on the substrate surface to realize a pattern composed of nanoparticles.
The process is shown in Figure 1, with the focused femtosecond laser irradiating a single layer of transition metal dihalide material. The thickness of this monolayer is only 7Å, as shown in the inset in Figure 1. As seen in the enlarged view therein, nanoparticles of metal oxides were produced in the region irradiated by the laser. The produced nanoparticles have a strong scattering effect on light, which enables the modulation of the field intensity of light. Therefore, by arranging such nanoparticles into a structure of concentric rings by the femtosecond laser, a lens effect is achieved, enabling the focusing of light and the imaging of objects. The line width of each ring depends on the spatial resolution of the femtosecond laser focus. In this study, a high numerical aperture microscope head was used to focus the femtosecond laser to achieve high spatial resolution up to 400 nm.
Fig. 2. Microscope picture of the fabricated single atomic layer lens and its focused light intensity distribution (Source from: Graphene Multilayer Photonic Metamaterials: Fundamentals and Applications)
Fig. 3 Imaging of large-area atomic-level ultra-thin lenses (Source from: Lin, H., et al., Diffraction-limited imaging with monolayer 2D material-based ultrathin flat lenses. Light: Science & Applications, 2020. 9(1). )
An optical micrograph of a lens fabricated using laser fabrication is shown in Figure 2(a). The lens consists of concentric rings with unequal spacing. The area irradiated by the femtosecond laser is darkened due to the generation of nanoparticles that form a strong scattering of light. The light intensity distribution of its focused focus in the xy plane and xz plane is shown in Fig. 2(b), which achieves subwavelength resolution in the xy plane and wavelength-level resolution in the xz plane. The fabrication method can achieve good focusing effect in arbitrary single-layer transition metal dihalide materials. Its focusing efficiency can reach 31%, which is much larger than that of the current sub-nanometer-thick planar lenses.
Meanwhile, by fabricating a large-area monolayer transition metal dihalide lens, the first high-resolution imaging of a single atomic layer material lens was achieved in this study, and the results are shown in Figure 3. The letter F and a USAF standard plate were used to characterize the imaging effect. Two of the parallel lines at 1.1 μm can be clearly distinguished in the figure. More interestingly, since the lens can achieve both positive primary and positive secondary imaging, this allows for effective implementation of variable-focus lenses with different magnifications.
INNOFOCUS
By integrating this lens into cameras, VR/AR devices and microfluidic devices, we can effectively reduce the size and thickness of the devices and achieve the special functions required by customers. It can be widely used in micro-spectrometer, ultra-thin digital camera, microfluidic device, optical communication device, phase modulation device, transparent phase plate, holographic optical element, VR/AR device.