Doppler shift generated by diffraction gratings under time-dependent incidence angle near a Wood anomaly

Abstract: Diffraction gratings are famous for their ability to exhibit, near a Wood anomaly, an arbitrarily large angular dispersion, e.g., with respect to the incidence angle or wavelength. For a diffraction grating under incidence by a plane wave at a fixed frequency, by rotating the incidence angle at a given angular velocity, the field propagated by a nonzero diffraction order will rotate at increasingly fast angular velocity as the incidence angle approaches the angle where Wood anomaly occurs. Such a fast rotating diffracted field has the potential to generate a substantial Doppler shift. Indeed, under the assumption of a grating with infinite extent, the expression for the instantaneous frequency shift perceived by an observer, who is looking into the light radiated by the diffraction order, is derived and it is in full agreement with the prediction from an interpretation based on the Doppler shift generated by a rotation of light sources. In particular the classical (non-relativistic) Doppler shift can take arbitrarily high values as the incidence angle approaches a Wood anomaly. It is also found that gratings of a finite size can have a similar property. In order to have a physically detectable frequency shift, it is important to use a grating which can maintain a significant reflectance into higher diffraction orders near their Wood anomaly cut-off. Interestingly, we have found that the geometry of the nanostructures of a \emph{Morpho} butterfly wing scale is aptly suited for such a function because it can strongly reflect into higher diffraction orders while minimising the reflection into the specular order.