Three-dimensional mapping of optical near field with scattering SNOM

Abstract

Scattering-type scanning near-field optical microscopy (s-SNOM) is applied to investigate three-dimensional optical near field distribution, including both amplitude and phase information. A method analogous to the force volume mode of the atomic force microscopy (AFM) technique is adapted for the measurement. The results show high lateral resolution of tens of nanometers, and even higher vertical resolution of only a few nanometers. The experiment results provide a straightforward illustration of the optical near fields in the space above the sample surface. Numerical computations support the experimental data. The ability to map the 3D optical near filed helps to reveal the factors that influence the performance of the designed near-field structures.

A Review of Three-Dimensional Scanning Near-Field Optical Microscopy (3D-SNOM) and Its Applications in Nanoscale Light Management

“…From the 3D SNOM scans the near-field can be seen to extend up to 20 nm from the film surface, at which point it drops off sharply in intensity. The optical phase image (Figure 11b) clearly identifies a 180° phase shift that is characteristic of bowtie apertures, and extends past the near-field boundary seen in the amplitude image [50,51].…”

“…In the following section, applications of 3D SNOM compared to traditional 2D scanning are discussed, with examples of the use of 3D SNOM to study and improve the design of organic solar cells, nano-aperture antennas, nanoparticle arrays, and photonic crystal waveguides [28,29,34,[48][49][50][51][52]. Figure 5 illustrates the equipment setup that can be used for 3D SNOM measurements.…”

“…pattern observed in the SNOM image is a consequence of light originating from the surface that is back scattered from the tip itself and reflected from the Au surface to be collected by the tip. Nano antennas or apertures with more exotic shapes such as the bowtie antenna [50,51] have been sought after for their light scattering properties. Bowtie shaped antennas are designed to produce enhanced electric fields in the aperture between the two bowtie halves.…”

“…Figure 11 shows the optical amplitude distribution in the space above a bowtie antenna of this type, using s-SNOM imaging. This bowtie antenna was manufactured using focused ion beam milling to produce an aperture with an outline of 150 nm and a gap size of 20 nm in a 60 nm gold film to [50]. From Figure 11a, the electric (Ez) field is concentrated along the edges of the bowtie-shaped aperture gap.…”

Mapping three-dimensional near-field responses with reconstruction scattering-type scanning near-field optical microscopy

“…To overcome this obstacle, method of approach curve (or s-SNOM Z-plot) has been used to infer the vertical signal behavior of s-SNOM or three-dimensional mapping. 12,[21][22][23] In collection of the approach curve, a non-fundamental lock-in demodulation is recorded as the oscillating tip is approaching the sample surface. However, it is proved that the shape of the approach curve is not the direct near-field response (see Figs.…”