Optical near-fields of triangular nanostructures

Abstract

We compare simulations of optical near-fields of single triangular nanostructures with experimental results from a near-field ablation technique on a periodic arrangement of triangles. We find good agreement of the lateral near-field distributions; nevertheless their dependency on the polarization of the incident light differs by 90 • . Upon increasing the lateral distances of the nanotriangle arrangement in the experiment, the polarization dependence agrees with the simulation. We conclude that this at first sight unexpected behaviour stems from the coupling of near-fields by scattered surface waves and their interaction with the incoming beam. The ongoing miniaturisation is one of the driving forces for the field of nanotechnology. In that field it would be useful to have all kinds of optics available that are frequently used on a micrometer scale, e.g., spectroscopy but also exposition of photoresists. For that purpose light must be focussed to a tiny spot well below the diffraction limit. To achieve this goal different types of nanoantennas have been proposed and examined during the last years [1][2][3][4][5][6]. For a further improvement of the efficiency of these nanoantennas progress has to be made on both sides, the theoretical description of these nanoantennas and the imaging of the near-fields with highest resolution [6][7][8][9][10]. The SNOM (scanning optical near-field microscopy) is an instrument used frequently, but alternative imaging methods have been developed as well which do not involve the problem of interaction of the SNOMtip with the optical near-field [11][12][13][14][15][16]. Here, we study experimental simple realizable nanoantennas, Au-triangles produced by colloidal lithography, and compare near-field simulations with the experimental results of a near-field ablation technique [14], where the optical near-field is used to pattern a silicon surface. SimulationThe optical properties have been investigated by using the discrete dipole approximation (DDA) method for u Fax: +49-7531-883127, E-mail: johannes.boneberg@uni-konstanz.de solving for the scattering of light from nanostructures. Details of this method have been described previously [17,18]. In the present application, we focus on the field intensity distribution, as defined in terms of the the square of the local electric field, |E| 2 , in studies of a gold triangle on a silicon substrate. Dielectric constants for gold and for silicon are taken from Johnson and Christy [19] and Palik [20], respectively. In this calculation, the particle is discretized using a grid spacing of 3 nm for 240 nm edge length and 3.75 nm for 380 nm edge size, and the silicon substrate is treated using an effective medium approximation that uses a weighted average dielectric constant, where the weight factor is determined by the area of the particle that is exposed to the silicon. Past studies [21] have suggested that this level of discretization is capable of determining the qualitative shape of the intensity distribution in the near-field regio...

Femtosecond laser near field ablation

“…Such methods, which do not need the symmetry of the system, are the finite difference time domain (FDTD) method [56] or the discrete dipole approximation (DDA) [57,58]. In the latter the object is divided by a fine grid (a few nanometers wide) on which interacting dipoles are located [59][60][61].…”

Structural study of near-field ablation close to plasmon-resonant nanotriangles

“…Using discrete dipole approximation (DDA) simulations (c), the complex field distribution of some larger-scale nanotriangles could not be predicted correctly. 7 Recently, we could show that the FDTD simulations (d) can model the field distribution that is suggested by the ablation pattern almost perfectly. 9 Examples of both field distributions in a plane on the substrate surface are displayed in Fig.…”

“…The ablation of the silicon substrate by tailored nanostructures, in our case the nanotriangles, has been addressed earlier, 6,7,18 however, not fully quantitatively. In practice, this would require to elucidate structural properties such as thermal behavior on the ultrashort time scale.…”

“…This can either be a photoactive material, such as a photoresist that is cast onto the nanostructures [28][29][30][31] or it can be imprinted in an imaging layer below the particles 32,33 or directly into the surface, where the nanoparticles are lying on. 6,7,10,34 These techniques have the advantage of a higher resolution than SNOMs, as they are only limited by the resolution of the (classical) detection method for the surface morphology (e.g., SEM and AFM). Of practical interest, especially if one is interested in material modification, is the possibility to imprint the field distribution below the nanoparticles as a permanent surface modification.…”

“…5 Shape and size tailoring can lead to an enhancement factor of several orders in magnitude. It has also been shown that this enhancement can lead to drastic interaction such as ablation close [6][7][8][9] to or on the nanostructures. 10,11 The mechanism of this process deserves some interest.…”