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.

Direct imaging of the near field and dynamics of surface plasmon resonance on gold nanostructures using photoemission electron microscopy

“…To date, investigations of the optical properties of LSPR have largely relied on far-field spectroscopic techniques or numerical simulations. Several experimental approaches have been utilized to visualize the near field, including scanning photoionization microscopy, 16,17 scanning near-field optical microscopy, [18][19][20][21] nonlinear luminescence or fluorescent microscopy, 22,23 nonlinear photopolymerization 24,25 and near-field ablation of a substrate. [26][27][28][29] However, these approaches have practical limitations; specifically, both scanning photoionization microscopy and scanning near-field optical microscopy require a scanning process to acquire a near-field image, and their spatial resolution barely reaches the sub-50-nm level.…”

Local refractive index sensitivity of gold nanodisks

“…To pursue this approach of controlled analyte access, it is necessary to study in detail the NP field and sensitivity distribution. Plasmonic modes of tailored nanoparticles have been studied with various methods, as with scanning near-field optical microscopy [12], plasmon-field dependent polymerization [13], the position-dependent spontaneous lifetime of fluorophores [14] or electron energy loss spectroscopy combined with electron microscopy [15].…”

Measurement of Nanoplasmonic Field Enhancement with Ultrafast Photoemission

“…A similar concept based on two-photon photoluminescence signal levels confirmed field enhancement values of some dozens for gold nanostrip and bowtie geometries 17 . Alternatively, there are also other approaches that rely on irreversible changes induced by a critical value of the electric field such as (typically two-photon) photopolymerization [18][19][20] and direct ablation 21 of nanopatterned samples with field enhancement estimates ranging from 34 to 600 for different geometries. In addition, a particularly high, three orders of magnitude enhancement was deduced based on the measurement of dc photocurrents in plasmonic sub-nm gaps 22 .…”