## 3 Aperture-Type SNOM
### Imaging Electrical and Magnetic Near-Fields  
In addition to probing chemical and morphological properties, SNOM has shown the capability to map electromagnetic near-fields in both c-SNOM and s-SNOM modes using careful tip design. Both the amplitude and phase of the near-field can be mapped, with phase-sensitive c-SNOM measurements allowing detection of the mode properties of light as it propagates through nanoscale waveguides composed of nanoparticles or nanowires [28; 29]. With these capabilities, construction of electric and magnetic vector field maps in the near-field of exotic nanostructures, nanoparticles and nano-antennas has been accomplished [8; 30; 31]. Several variations of electric and magnetic field mapping are available in the literature using c-SNOM, including the ability to examine either the electric or magnetic near-field components individually, or together to form the complete electromagnetic spectrum [29]. Direct mapping of electric fields near sub wavelength holes acting as nanoantennas has been demonstrated using tips with 30 nm circular apertures [30; 31]. Optical mapping of magnetic fields is more challenging, and was first demonstrated using c-SNOM with a slit etched into the tip of an aperture probe [32]. Etching a slit in the side of the probe in this way converts the aperture probe into split-ring resonator. This architecture effectively converts the normally inaccessible out-of-plane component of the magnetic field (\(H_{2}\)) into detectable in-plane far-field radiation (\(E_{x}\) or \(E_{y}\)). Careful probe design has also provided access to the in-plane magnetic fields (\(H_{x}\) or \(H_{y}\)), such as a pyramidal shaped probe that has been used to map the magnetic response of plasmonic nanoantennas shown in Figure 2a [11; 33]. Scanning electron microscope (SEM) images of these plasmonic disk (320 nm) and ring (470 nm outer diameter, 300 nm inner diameter) antennas are shown in the top row of Figure 2a. The antennas were excited by an \(H_{y}\) magnetic dipole source, positioned 75 nm below the antennas, and SNOM imaging was used to reveal the \(\,|Hy|^{2}\) near-field distribution near the antennas. The bottom row of Figure 2a shows experimental SNOM images for the disk and ring architectures. The disk displays a dipole mode which produces a strong plasmon excitation due to the proximity of the probe, represented by a darkened elliptical spot. For the ring, two separated dark lobes are observed on opposite edges of the ring. Both cases correspond well to the simulated field, enabling directed design of nanoantennas [33]. The realization that an unspecialized aperture probe could be sensitive to in-plane magnetic fields indicated that the magnetic near-field could be studied in greater detail. It has been since shown that normal circular aperture c-SNOM probes can in fact be sensitive to magnetic near-fields [29; 34]. Through judicious choice of aperture size and metal coating thickness, SNOM mapping of \(H_{x}\), \(H_{y}\) and \(E_{z}\) of a plane wave has been through the mapping of standing wave patterns on gold films [29]. Using photonic crystal waveguides as a test environment, c-SNOM images have shown that typical circular aperture probes are roughly equally sensitive to electric and magnetic fields, not just to the electric component [34]. The left of Figure 2b shows the use of this technique to map the in-plane map of the resonance shift in a photonic crystal nanocavity, which is made possible by the shift of the nanocavity resonance when the apex a normal circular aperture, metal coated c-SNOM probe is above an antinode of the magnetic field of the cavity [29]. The right part of Figure 2b shows the associated out-of-plane magnetic component of the light field (\(H_{z}\)) that can be calculated for the unperturbed cavity [29]. These types of electromagnetic field maps are crucially important in the design and characterization of nano scale optoelectronic devices.