Quantitative measurement of the near-field enhancement of nanostructures by two-photon polymerization

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.