One strategy involves placing a probe close to a sample to either scatter the near field away or “suck” it into a fiber leading to a detector. But even then, the information recovered has historically been a jumble of amplitudes, phases, and polarizations. Recent advances have changed the outlook significantly. Various kinds of interferometry can separate amplitude and phase, and careful control of polarization can help us extract specific information about the field’s vector components.
In this paper, Sfez et al. exercise unprecedented optical control to solve an inverse problem for an ultrathin photonic crystal. The crystal acts as a waveguide that supports optical modes called Bloch surface waves (BSWs). As the name implies, BSWs are confined fields, but they reach somewhat further away from the surface than typical evanescent fields. That makes them viable candidates for sensing applications.
Sfez et al. excite Bloch surface waves by shining two beams of light of different frequencies and polarizations on the waveguide at finely controlled angles. Light from the near field is collected by an optical fiber placed close to the surface. By interfering the signals with two more frequency-shifted beams, they create a beating that contains amplitude and phase information. They can then scan the sample to construct amplitude and phase images of the waveguide for different detected polarizations. They find that the waveguide can support three different spatial modes and show that each one can be selectively excited by tuning the illumination angle.
The authors then solve the inverse problem of finding the near-field vector components given the detector signals at two orthogonal polarizations. To do this, they have to determine how the polarization components are affected by propagation down the fiber. In general, this is a difficult task, but here they can use a priori information about the waveguide modes—namely that one polarization is much weaker than the other—to find a transfer matrix. Then they find each of the field components of the modes and show that they match the theory quite well. They use the amplitude and phase maps of each field component at different wavelengths of light to calculate the dispersion relations of the modes.
This work includes an experimental visualization of BSWs, which have possible applications in sensing. Taking a broader view, Sfez et al. find the amplitude and phase of each component of a sample’s near field. They suggest that by refining the process of finding the transfer matrix or making some more assumptions based on sample symmetries, this method could be applied to near-field imaging in general. It could then give new insight into physical processes, as well as help in characterization of nanoscale devices.
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