title: Quantitative measurement of the near-field enhancement of nanostructures by two-photon polymerization.
abstract: The quantitative determination of the strength of the near-field enhancement in and around nanostructures is essential for optimizing and using these structures for applications. We combine the gaussian intensity distribution of a laser profile and two-photon-polymerization of SU-8 to a suitable tool for the quantitative experimental measurement of the near-field enhancement of a nanostructure. Our results give a feedback to the results obtained by finite-difference time-domain (FDTD) simulations. The structures under investigation are gold nanotriangles on a glass substrate with 85 nm side length and a thickness of 40 nm. We compare the threshold fluence for polymerization for areas of the gaussian intensity profile with and without the near-field enhancement of the nanostructures. The experimentally obtained value of the near-field intensity enhancement is 600 ± 140, independent of the laser power, irradiation time, and spot size. The FDTD simulation shows a pointlike maximum of 2600 at the tip. In a more extended area with an approximate size close to the smallest polymerized structure of 25 nm in diameter, we find a value between 800 and 600. Using our novel approach, we determine the threshold fluence for polymerization of the commercially available photopolymerizable resin SU-8 by a femtosecond laser working at a wavelength of 795 nm and a repetition rate of 82 MHz to be 0.25 J/cm(2) almost independent of the irradiation time and the laser power used. This finding is important for future applications of the method because it enables one to use varying laser systems.
authors.affiliation: Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan.
authors.forename: Tobias
authors.initials: T
authors.lastname: Geldhauser
authors.forename: Andreas
authors.initials: A
authors.lastname: Kolloch
authors.forename: Naoki
authors.initials: N
authors.lastname: Murazawa
authors.forename: Kosei
authors.initials: K
authors.lastname: Ueno
authors.forename: Johannes
authors.initials: J
authors.lastname: Boneberg
authors.forename: Paul
authors.initials: P
authors.lastname: Leiderer
authors.forename: Elke
authors.initials: E
authors.lastname: Scheer
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Misawa
mesh_terms: D004852:Epoxy Compounds / Q000737:chemistry*; D049329:Nanostructures / Q000737:chemistry*; D010316:Particle Size; D017785:Photons; D058105:Polymerization; D011108:Polymers / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D004852:Epoxy Compounds; D011108:Polymers; C520895:SU-8 compound