Three-dimensional sub-wavelength fabrication by integration of additive and subtractive femtosecond-laser direct writing
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Three-dimensional sub-wavelength fabrication by integration of additive and subtractive femtosecond-laser direct writing Wei Xiong1, Yunshen Zhou1, Xiangnan He1, Yang Gao1, Masoud Mahjouri-Samani1, Tommaso Baldacchini1,2, and Yongfeng Lu1 1 Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 685880511, U.S.A. 2 Technology and Applications Center, Newport Corporation, 1791 Deere Avenue, Irvine CA 92606, U.S.A. ABSTRACT Additive nanofabrication by two-photon polymerization (TPP) has recently drawn increased attention due to its sub-100 nm resolution and truly three-dimensional (3D) structuring capability. However, besides additive processes, subtractive process is also demanded for many 3D fabrications. Method possessing both additive and subtractive fabrication capabilities was rarely reported. In this study, we developed a complementary 3D micro/nano-fabrication process by integrating both additive two-photon polymerization (TPP) and subtractive multi-photon ablation (MPA) into a single platform of femtosecond-laser direct writing process. Functional device structures were successfully fabricated including: polymer fiber Bragg gratings containing periodic holes of 500-nm diameter and 3D micro-fluidic systems containing arrays of channels of 1-µm diameter. The integration of TPP and MPA processes enhances the nanofabrication efficiency and enables the fabrication of complex 3D micro/nano-structures that are impractical to produce by either TPP or MPA alone, which is promising for a wide range of applications including integrated optics, metamaterials, MEMS, and micro-fluidics. INTRODUCTION 3D micro/nano-fabrication techniques have recently attracted increased attention due to their promise in a wide range of applications including integrated optics [1,2], plasmonics [3,4], microbiology [5,6], and microelectromechanical systems (MEMS) [7-11]. Similar to 3D fabrication at macro scales, 3D micro/nano-fabrication generally requires two fundamental approaches: additive and subtractive processes. Among additive fabrication methods two-photon polymerization (TPP) is the most prevailing one [12,13]. TPP can be simply thought of using a nanometer-sized pen made of focused femtosecond (fs) laser beam to draw 3D structures inside a photocurable material. It is by far the only process that allows fabricating nearly arbitrary threedimensional micro/nano-structures with a sub-100-nm writing resolution. For the subtractive micro/nano-fabrication, multi-photon ablation (MPA) is a promising method which has been applied to a wide variety of materials [14]. When MPA is applied to transparent materials, structural or phase modification of the materials occurs, leaving behind permanent changes of refractive index or even voids [14,15]. MPA has found applications in a variety of fields such as optical data storage [16], waveguide writing [17], and nanosurgery [18]. Although both additive and subtractive micro/nano-fabrication methods are established separately, they have been largely isolated and difficult t
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