Materials Today
ResearchUltrathin Fresnel lens based on plasmene nanosheets
Graphical abstract
Introduction
Flat and ultrathin fresnel zone plate (FZP) lens have been demonstrated to be a core diffractive multi-focus optical component implemented in a myriad of optoelectronic applications due to its compactness, lightweight construction, and excellent distortion-free light-manipulating and focusing capabilities [2], [3], [4]. To build up a specified lens surface profile, current fabrication techniques predominantly rely on multi-step top-down processes involving photolithography, electron-beam deposition, and anisotropic etching of thick metal films or graphene layers [4], [7], [10], [11], [12], [13]. However, these tedious fabrication processes introduce severe limitations toward scalability, structure, and performance quality that restrict potential real-world applications besides proof-of-concept demonstrations [13], [14], [15]. For evaporated metal-film lens, inhomogeneous deposition of metal layers may lead to poor control over surface features and batch-to-batch variances; for graphene-based lens, their simplicity and compactness are greatly offset by the low focusing efficiency and limited mid-infrared/terahertz working wavelength ranges apart from graphene defects [4], [5].
Herein we report a new strategy to fabricate Fresnel lens using our recently reported plasmene nanosheets which are tailor-made two-dimensional (2D) assemblies of plasmonic nanoparticles [8], [9], [16], [17]. Unlike evaporated metallic film or graphene, plasmene is obtained from a bottom-up self-assembly process and offers the freedom of tuning its plasmonic properties simply by adjusting sizes, shapes, and orientations of constituent building blocks. In this model system, we demonstrated ∼23% focusing efficiency of ∼37-nm-thin FZP plasmene lens at the operating wavelength of 637 nm. In contrast, continuous gold, silver films, and disordered nanoparticle film with similar thickness only achieved ∼7.5%, ∼10%, and ∼9.6%, respectively. Our combined experimental and theoretical modeling studies reveal that the higher lens efficiency is due to the additional Huygens’ effects from plasmene-constituent building blocks. These add-on effects can be adjusted by the choices of building blocks and number of plasmene layers, hence, maximizing the lens efficiency.
Section snippets
Results and discussion
We began with the model system based on Au@Ag nanocubes (NC) which were used as the building blocks to fabricate plasmene on ITO glass. This was followed by focused ion beam (FIB) lithography to mill plasmene into a series of concentric zones at optimized conditions (see Methods for fabrication process and Supplementary Section 2). According to the classical zone plate law, the zone radius at relatively large focal lengths can be determined using [4], [18]:where n = 1, 2, 3,.…, f is the
Plasmene lens fabrication
The fabrication process consists of two stages. First, plasmene nanosheets are deposited on an ITO glass substrate by means of our previously developed bottom-up polystyrene-based drying-mediated self-assembly approach [8], [17]. The assembled nanosheets are then patterned into custom-designed Fresnel zones via top-down focused ion beam (FIB) lithography. See also Supplementary Section S1 & S2 for detailed procedure.
Characterization of plasmene lens
The morphology and assembled structure of plasmene nanosheet were observed
Acknowledgments
M.P., and W.L.C. acknowledge Discovery Grants DP110100713, DP140100883, DP170102208, and DP180101715. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). D.D. would like to acknowledge scholarship support from The Monash Centre for Atomically Thin Materials. W.Z would like to acknowledge supports from National Natural Science Foundation of China (51777168 and 61701303), Natural Science
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Kae Jye Si and Dashen Dong contributed equally to this work.