A new kind of Bragg phenomenon accessed through metadesign

Abstract

I identify a fundamentally new type of Bragg phenomenon, one that occurs in a uniform medium that relies on resonant tuning of the medium parameters, rather than coherent superposition of reflections of monochromatic light from a periodic structure. Since the medium is uniform, reflection occurs over a potentially wide range of wavelengths, much like a metal. Unlike a metal, however, the reflection is polarization dependant: one circular polarization is transmitted, whilst the opposite polarization is reflected. The basis of this remarkable phenomenon is a uniform medium that is simultaneously birefringent and optically active. Solving for the eignmodes for light propagating in the medium, it is found that a resonance occurs when the optical activity is matched to the average transverse refractive index. When this matching occurs, the index associated with one circular polarization is purely imaginary, resulting in it being completely reflected when incident from vacuum. The opposite polarization is, by contrast, transmitted. The ‘bandwidth’ of the resonance is set by the birefringence. Heuristically, one circular polarization progressively, and synchronously, samples the alternating transverse refractive indices as it propagates. It is this alternation that makes the light ‘feel’ a Bragg grating, while the medium remains uniform, an interpretation that has been confirmed by a coupled wave analysis. The resonance condition requires a huge optical activity: a full polarization rotation over a wavelength. This is precisely the condition that triggers the onset of negative refraction in a purely optically active medium, and one that has been observed recently in meta-media at THz frequencies. The medium can provide a low-loss white-light polarization divider. Furthermore, modulation of the medium parameters results in an efficient wide-band polarization switch. We believe that the identified mechanism has wide application in optics and optoelectronics. The proposed medium is hereby christened ‘bire-active’.