PHOTON LOCALIZATION IN RESONANT MEDIA

摘要

In analogy with electron localization in insulators, it has been anticipated that photons may be trapped by the constructive interference of waves returning to a point within a strongly scattering medium. Since the closeness to the transition between diffusive and localized waves determines the statistics of multiply scattered waves, charting this transition is of fundamental interest for both statistical optics and electronic mesoscopic physics. However, the very possibility of observing photon localization in random systems has been called into question by the difficulties of achieving strong scattering and of unambiguously detecting electromagnetic localization. Unlike electrons that can be trapped by the Coulomb interaction at atomic sites, photons are not bound by individual particles. They are not strongly scattered by particles either, except at, Mie resonances where the scattering cross section can considerably exceed the geometric cross section. Measurements of the exponential scaling of transmission have not definitively established photon localization since such scaling may also be due to the presence of absorption. However, recent measurements of coherent backscattering in macroporous GaP networks along with theoretical predictions suggest that the approach to localization can be observed in the rounded backscattering peak from weakly absorbing samples. Also recently, the variance of relative fluctuations has been shown to provide a decisive test for localization, even in the presence of strong absorption. This provides a sure guide in the search for photon localization, which can be used to sort out the precise material and structural characteristics that may edge samples towards and potentially across the localization threshold.