Abstract

Random perturbations highly alter the propagation of light propagation in periodic potentials which causes localized modes in the vicinity of photonic band-gaps [1]. Localization in photonic crystal waveguides (PhCWs) has garnered special attention due to the important limitations it imposes on the realization of slow-light photonic devices [2], but also for the opportunities it offers for strong light-matter interaction [3]. It is widely accepted that localization is enhanced when the group velocity decreases, i.e. when approaching the band-edge [1], and that tiny perturbations are sufficient to create defect modes [4]. Very little is known on the properties of individual localized modes. Finding out how small a localized mode can be for a given small disorder level and on what structural characteristics this minimal size depends brings deeper knowledge into the physics of light localization at band-edges of periodic media. Here, we investigate the formation of wavelength-scale localized states (WLS) at the band-edge of photonic structures at very small disorder levels both numerically and experimentally (see Fig. 1). Instead of the group index being the key quantity impacting localization near band-edges, we find that the relation between the disorder level and the WLS extent is driven by the effective photon mass. WLS are observed using near-field measurements. The observation is supported by extensive numerical simulations, which unambiguously show that only periodic media with flat dispersion bands ("heavy photons'') are capable of forming very small localized modes at very small disorder levels (σ<λ/1000). The present work provides new understanding into the physical mechanism involved in the formation of localized modes at band-edges of periodic media, underlining that the relation between the disorder level and the spatial extent of individual localized states is driven to a large part by the effective photon mass.

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