Mapping nanoscale optical fields: A magnetic surprise

Abstract

Near-field scanning optical microscopy (NSOM) using the aperture probe (AP) has helped to uncover exciting phenomena such as single molecule excitation [1], optical polarization singularities [2] and slow light [3]. However, the interpretations of these measurements have all been based on the assumption that what is collected is the in-plane electric near-field [2,3,4]. And, until the recent advent of metamaterials [5] this view has remained unchallenged. Interestingly, the AP has been recently reported yield information on the in-plane magnetic field [6]. In this study we show that contrary to both, the originally held assumption and the assertion of the new study, a typical AP (as shown in Fig. 1a) is sensitive to both electric and magnetic fields. Specifically, we show that the AP collects radiation from the four in-plane components of the near-field.We collect the near-field with a NSOM (shown in in Fig. 1b). We use /2(2) to orient the fields such that Ly detects light from an electric field along y (Ey), and a magnetic field along x (Hx). To identify the APs sensitivity to the in-plane electric (E//) and magnetic field (H//), we measure on a photonic crystal waveguide that has distinctly different electric and magnetic near-fields. In Fig 1c we show how the computed spatial distribution of Ey and Hx evolve with increasing height (the blue arrows in Fig. 1c highlight the differences). At a height of 20 nm above the sample, the measured signal on Ly (shown in Fig 1d, Lx not shown) neatly matches both Ey and Hx. Surprisingly, at increasing heights the signal matches neither purely Ey, nor only Hx. Hence, we investigate the possibility that the AP collects both E// and H//. To find the relative contribution of each of field we fit the measurements with a superposition of the calculated E// and H//. The fits, shown in the right column of Fig. 1d, excellently match the data, when using only the sensitivity to E// and H// on each detector to fit scans at 9 heights simultaneously (only 5 are shown). For 8 probes, with diameters ranging from 130 nm to 370 nm we find that the sensitivity of a coated probe to E// and H// is within a factor three. Based on the excellent agreement between the measurements and the fitted fields, we conclude that we detect all in-plane fields and no vertical field components. This creates a new possibility for a full map of the EM near-field. Moreover the ability to measure the magnetic as well as the electric near-field, opens up new avenues towards a better understanding of light matter interactions on the nanoscale, as well as facilitate the development of magnetically active structures.