Reliable Fabrication Of Plasmonic Metallic Micro-/Nano-Structures And Its Application

Sub-wavelength metallic micro-/nano-structures exhibit unique and abundant optical properties induced by the excitation,coupling and propagation of surface plasmons.Now,they have attracted a lot of attentions in current device technology,sensing,bioscience and their interdisciplinary research fields.While the burgeoning micro-nano optics is gradually subverting the perception of traditional optics,micro/nano optical devices will play an important role in real-life applications.Hence,fabrication of metallic micro-/nano-structures optical devices with high reliability is highly required.In recent years,with the rapid development of nanotechnology,materials and control technology,different fabrication approaches of functionalized surface plasmon metallic micro-/nano-structures for various application fields are emerging.However,the existing methods are facing the critical challenge in terms of high resolution,efficiency,flexibility,process complexity,and application compatibility.From the perspective of the fabrication requirements of surface plasmon metallic micro-/nano-optical devices for functional applications,we explore and develop new methods to reliably fabricate plasmonic metallic micro-/nano-structures through improved processing,and the optical applications of obtained structures were also demonstrated.The main contents of this paper are as follows:(1)The use of an intervening adhesion layer is essential in reliable fabrication of noble metallic nanostructures for optical and electronic devices,especially in the converntional wet process of electron beam direct writing(EBDW)technique.However,the lossy thin film for adhesion promotion results in worse performance of the plasmonic micro-/nano-optical device.Here,we investigate the fabrication reliability of the adhesion-free plasmon gold nanostructure on a silicon substrate and an optically thick gold substrate using a wet lift-off process.Through statistical results analysis,the patterning behavior of the non-adhesive layer metal nanostructures was comprehensively evaluated,and the intrinsic mechanism of the failure was investigated through mechanical modeling analysis and control experiments.The results provide useful information to the nanofabrication community to obtain adhesion-free gold micro-/nano-structures for plasmonic and transfer printing applications.(2)Three-dimension(3D)donut-like surface plasmon gold nanorings are reliably fabricated as surface-enhanced Raman scattering(surface-enhanced raman scattering,SERS)substrate.These 3D nanoarrays were fabricated directly using EBDW by defining a lithographic template combining with metal film deposition technique.Notably,no additional stripping process is required,thus ensuring an extremely clean metal surface.At the same time,a stronger electric field enhancement can be achieved for a 3D architecture compared to a planar counterpart with a comparable plasmon resonance position.In addition,a 3D architecture has multiple hot spots to improve the performance of the SERS,as well as greater physical cross-section corresponding to the larger surface area to volume.Finally,the enhancement factor(EF)of the 3D donut-like gold nanorings SERS substrate can be as high as~4×10~7.Further finite-difference time-domain(FDTD)simulation results are in good agreement with the experiments.Such a fabrication method of 3D nanorings array has the advantages of simple processing,high reliability and high performance for improved SERS applications.(3)To address the dilemma of EBDW technique in terms of the precision and efficiency during fabrication of multiscale metallic micro-/nano-structures,we proposed a predefined nanotreches-based process,in which target multiscale structures can be realized by first creating nanotrech contours on positive resist via single-pixel exposure of EBDW,and then selectively peeling the isolated film outside the contours after metallization.By using this approach,we can reliably fabricate multiscale plasmon metallic structures by mitigating the proximity effect and impoving processing efficiency.The anti-peeling mechanism of the target structures was well revealed by finite-element-method-based modelling using interfacial fracture mechanics,which provides a theoretical basis for reliable fabrication.(4)By using the proposed novel EBDW processing strategy based on predefined nanotrenches,various adhesion-free multiscale plasmon metallic structures with tiny gaps were reliably fabricated,such as micro-scale patterned metal optical devices and nano-scale gold plasmon dimers with sub-10 nm gaps.We explored the feasibility in near-field optics and SERS applications.In addition,the special selective peeling behavior of this process provides a rapid fabrication strategy to obtain for obtaining exfoliable labels incorporating both covert and overt features for steganography.The potential application in anti-counterfeiting field of this tag was also demonstrated.(5)Aiming at the high spectral tunability applications of germanium(Ge)-metal gap-plasmon nanoresonators based on the anomalous dispersion property of Ge in the visible light region,two different architectures were realized by EBDW technique combining with direct metal deposition and atomic layer deposition to intuitively verify the spectral tunability of the nanoresonators.First,we analyze the spectral tunability of the Ge-Au gap plasmon by measuring the single particle scattering spectra of Ge nanodisks on a Au substrate.After optimizing the fabrication processing,an inversed Ge-Ag gap-plasmon resonators array was fabricated to demonstrate the plasmonic color generation and high spectral tunability.This work reveals utility of anomalous material dispersion for improving the spectral tunability of gap plasmons,while the new explored configurations provides a useful design and fabrication method of future tunable plasmon devices.