Magnetic Light-Matter Interactions in a Photonic Crystal Nanocavity

We study the magnetic coupling between a metal-coated near-field probe and a photonic crystal nanocavity. The resonance of the nanocavity shifts to shorter wavelengths when the ringlike apex of the probe is above an antinode of the magnetic field of the cavity. We show that this can be attributed to a magnetic light-matter interaction and is in fact a manifestation of Lenz’s law at optical frequencies. We use these measurements to determine the magnetic polarizability of the apex of the probe and find good agreement with theory. We discuss how this method could be applied to study the electric and magnetic polarizibilities of nano-objects.

Figure 1

(a) Schematic of the experiment. Light is loaded into a photonic crystal nanocavity via a side-coupled photonic crystal waveguide. A metal-coated nanoprobe is used to perturb the cavity. (b) Scanning electron micrograph of the photonic crystal nanocavity and access waveguide. (c) Scanning electron micrograph of the aperture probe, which consists of a silica core coated by aluminum. (d) Schematic of the ring that models the apex of the probe. Here, $r_1=100\mathrm{nm}$ and $r_2=200\mathrm{nm}$ are the inner and outer ring radius, respectively, and $\mathit{j}$ is the magnetically induced current density.

Figure 2

(a) In-plane map of the normalized waveguide transmittance $\overline{T}(x,y,{\lambda }_{r_0})$ at the unperturbed cavity resonance. Black regions indicate unchanged transmission. The dashed circles indicate the position of the holes of the underlying photonic crystal structures. (b) Normalized transmittance spectrum $\overline{T}(x,y,\lambda )$ at probe position indicated by the white arrow in (a). The peak ($\overline{T}>1$) indicates the resonance of the unperturbed cavity whereas the dip ($\overline{T}<1$) arises from the new cavity resonance induced by the presence of the probe. A fit (red solid line) to the spectrum allows us to extract the probe-induced shift of the resonance. (c),(d) Probe-induced resonance shift for tip positions along the horizontal and vertical dashed lines in (a), respectively. A blueshift as indicated by the transmittance dip is evident for all probe positions.

Figure 3

(a) In-plane map of the measured relative shift of the resonance wavelength of the cavity. (b) Calculated modulus $|B_{z0}|$ of the out-of-plane magnetic component of the light field in the unperturbed cavity at $\lambda ={\lambda }_{r_0}$ as calculated by an FDTD approach. (c) Same as (b) for the modulus $|{\mathit{E}}_0|$ of the total electric field. The arrows in (a), (b), and (c) indicate the measured resonance shift and calculated field that we use for estimating the magnetic polarizability of the probe. (d) Calculated relative shift in resonance wavelength based on Eq. (3), the FDTD-calculated and convoluted fields, and the calculated polarizabilities of the ring (see text). In all images, the dashed circles indicate the position of the holes of the underlying photonic crystal structure.