Optical Broadband Angular Selectivity

Abstract

Abstract:Light selection based purely on the angle of propagation is a long-standing scientific challenge. In angularly selective systems, however, the transmission of light usually also depends on the light frequency. We tailored the overlap of the bandgaps of multiple one-dimensional photonic crystals, each with a different periodicity, in such a way as to preserve the characteristic Brewster modes across a broadband spectrum. We provide theory as well as experimental realization with an all-visible-spectrum, p-polarized angularly selective material system. Our method enables transparency throughout the visible spectrum at one angle, the generalized Brewster angle, and reflection at every other viewing angle. 1The ability to control light has long been a major scientific and technological goal. In electromagnetic theory, a monochromatic electromagnetic plane wave is characterized (apart from its phase and amplitude) by three fundamental properties: its frequency, its polarization, and its propagation direction. The ability to select light according to each of these separate properties would be an essential step in achieving control over light (Fig. 1).Tremendous progress has been made toward both frequency selectivity and polarization selectivity. Frequency selectivity (Fig. 1A) can be obtained, for example, by taking advantage of photonic bandgaps in photonic crystals (1-5). Polarization selectivity (Fig. 1B) is accomplished for example by means of a "wire grid" polarizer (6) or by exploiting birefringent materials (7,8).Methods based on interference and resonance effects have been explored for angular selectivity, but they have limited applications because they are sensitive to frequency.An angularly selective material-system should ideally work over a broadband spectrum.Such a system could potentially play a crucial role in many applications, such as high efficiency solar energy conversion (9, 10), privacy protection (11), and detectors with high signal-to-noise ratios. Some progress has been made towards achieving broadband angular selectivity by means of metallic extraordinary transmission (12, 13), anisotropic metamaterials (14), combined use of polarizers and birefringent films (11), or geometrical optics at micrometer scale (15). The first two of these mechanism are difficult to realize in the optical regime; the other two work only as angularly selective absorbers.Here, we introduce a basic principle to achieve optical broadband angular selectivity. Our result rests on i) the fact that polarized light transmits without any reflection at the Brewster angle, ii) the existence in photonic crystals of band gaps that prevent light propagation for given frequency ranges, and iii) the bandgap broadening effect of heterostructures. First, we prove our fundamental idea theoretically for a single polarization and oblique incident angles, and also for both polarizations and normal angle of incidence. Second, we experimentally demonstrate the concept in the case of all-visible spectrum, p-polarized light. The dem...

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“…This is achieved by controlled variation of the thickness of each polymer pair from substrate to surface [76]. A similar effect was used in an all-inorganic pair stack used recently to confine transmittance to one incident angle [75]. Alldielectric sputtered stacks of two oxides on silver on silicon have also enabled good cooling [38].…”

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