Optical fibers have significance due to their exclusive abilities, such as efficient long-distance light transportation, strong light captivity, and flexible handling. These features provide a variety of advantages in advanced optics, including endoscopic imaging, non-linear light generation, optical trapping and fiber communication. The applications are advantageous in medical diagnosis, telecommunication, and other scientific fields. For example, flexible fiber endoscopes enable the scanning of internal organs for medical purposes.
Chromatic Aberration of Optical Lens
During fiber imaging, light from the sample is gathered by several lenses. Restraints in the index profile of these lenses cause a group delay, resulting in chromatic aberration that disrupts optical images of a wide range of wavelengths.
With current technologies, the control over the chromatic dispersion of an end face focusing or imaging optical lens has not been achieved until now.
What is a Metalens?
Metalenses are flat lenses made from optical parts that focus light using meta-surfaces. They can be used in optical devices that need a flat surface and less thickness than the traditional, curved refractive lenses that are most commonly used today.
A metalens allows for diffraction-limited light focusing and aberrations correction without additional optical elements. Previous achromatic diffractive metalenses exhibit considerably reduced form factor compared to a conventional achromatic doublet. However, these lenses use a complex design of meta-atoms (unit cell of meta-surface) for acquiring various group delay responses, and the modulation capacity of the group delay in their complicated meta-atom designs which limits achromatic performance. Hence, the maximum upper bound of the time-bandwidth product (TBP) is 11.5.
Different Lithography and Nanoimprinting Techniques
Advanced wavefront manipulation of the fiber output requires interfacing functional meta-surfaces with optical fibers.
Hydrofluoric chemical-etching and Ion-beam lithography methods implement end-face meta-surfaces enabling light focusing and bending. However, these fabrication methods cannot create the required structures for efficient light modulation, which limits their photonic functionality and applications.
Similarly, despite offering manufacturing resolution in the subwavelength regime, nanoimprinting and electron-beam lithography methods confront complex problems when preparing the fiber end face for planar surface patterning and predesigned-pattern transfer.
Two-Photon Polymerization-Based 3D Laser Nanoprinting
Two-photon polymerization-based 3D laser nanoprinting provides an ideal platform for optically implementing 3D diffractive microstructures on a fiber facet, opening the door to functionalizing optical fibers for various photonic applications.
3D Nanoprinting of Polymer-Based Achromatic Metalense
The technique used in this study for the 3D laser nanoprinting of polymer-based achromatic metalens on-fiber was two-photon polymerization through a tightly focused femtosecond laser beam. This was carried out via a commercial photolithography system.
The polymer meta-surface samples were initially created in photosensitive resin on a silica substrate using a high numerical aperture objective in the dip-in mode.
After laser exposure, the samples were produced by immersion for 20 minutes in propylene glycol monomethyl ether acetate, five minutes in isopropanol, and two minutes in Methoxy-nonafluorobutane. The mechanical strength of polymer nanopillars was increased through high aspect ratios and employing small hatching and slicing in 3D nanoprinting.
Advantages of 3D Nanoprinted Achromatic Metalens
Researchers designed 3D nanoprinted polarization-insensitive achromatic metafiber by interfacing 3D achromatic diffractive metalense with telecommunication single-mode fiber (SMF-28).
The height degree of a 3D achromatic metalense significantly increased the upper bound of the time-bandwidth product to 21.34, increasing the modulation range of group delay from -8 to 14 femtoseconds.
Subwavelength meta-atoms of the achromatic metalens show significant birefringence that can imprint a hyperbolic lens profile through the geometric phase. This 3D achromatic metalens designed on a single-mode fiber allows achromatic and diffraction-limited focus over the complete near-infrared telecommunication range from 1.25 to 1.65 μm.
A unique significance of this study is the employment of achromatic metafiber for confocal scanning without the help of a regular microscope, offering an unparalleled solution for highly compact confocal endoscopic system establishment.
Conclusion
Optical fibers allow us to communicate data, audio, pictures, and laser radiations by transmitting light via tiny transparent fibers.
In telecommunications, optical fiber technology has practically replaced copper wire in long-distance telephone lines and is used to link computers in local area networks.
Optical fiber is also the foundation of fiberscopes used in the inspection of the body's internal parts or analysis of the interior of produced structural goods.
The 3D laser nanoprinting method is expected to open doors for various photonic applications. It is a massive leap towards more advanced optical fiber-based technologies. It solves the problems of distorted images due to chromatic aberration.
The results achieved by this study may unlock the full potential of fiber meta-optics for a variety of applications, including fiber lasers, fiber sensing, wavelength-multiplexing fiber-optic communications, deep tissue imaging, femtosecond laser-assisted treatment, hyperspectral endoscopic imaging.
Reference
Ren, H., Jang, J., Li, C., Aigner, A., Plidschun, M., Kim, J., ... & Maier, S. A. (2022). An achromatic metafiber for focusing and imaging across the entire telecommunication range. Nature Communications. 13, 4183. https://www.nature.com/articles/s41467-022-31902-3
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