We have developed a new technique for spectrally-resolved enhanced backscattering over a large
optical bandwidth from 530 nm to 1000 nm, employing a supercontinuum light source and a fiber spectrometer. This technique allows broadband investigation of random media, giving access to spectral resonances, transitions from weak to large photonic strength, and eventually Anderson localization of light, in a single realization of the disorder.
The principle of enhanced backscattering is explained in Figure 1. A random material contains many multiple scattering paths. For every such path (red arrows) there is one other path which has exactly the opposite direction (purple arrows). Light scattered along these two reciprocal paths interfere constructively in the exact backscattering direction (q=0).
Figure 1 Illustration of the principle of enhanced backscattering.
For angles q slightly different from the backscattering condition the contributions from the two paths dephase. (top right of Figure 1) Paths with a large distance d between first and last scattering event dephase the quickest. The sum of all possible paths result in a cone of backscattered intensity with an enhancement factor of 2 with respect to the diffuse background (bottom right of Figure 1). This cone has a cusp at exact backscattering, corresponding to the contribution of very long light paths involving millions of scattering events. The width of the cone is determined by the photonic strength 1/kl, where k denotes 2p over the wavelength and l is the transport mean free path in the medium.
Figure 2 SEM image of a porous GaP material, showing that it consists of a complex network of pyramid-shaped nanostructures.
To demonstrate the new technique, enhanced backscattering spectra were measured for some of the most strongly scattering random media to date, such as porous gallium phosphide (Figure 2). Figure 3 shows the enhanced backscattering spectrum of this porous GaP. From this spectrum we can obtain the photonic strength 1/kl of the medium over a large range in the visible and infrared.
Figure 3 Enhanced backscattering spectrum of a photonanodically-etched porous GaP sample.
References:
O. L. Muskens, S. L. Diedenhofen, R. Algra, E. P. A. M. Bakkers, B. Kaas, A. Lagendijk, Large Photonic Strength of Highly Tunable Resonant Nanowire Materials,
Nano Lett. 9, 930:934 (2009)
O. L. Muskens, A. Lagendijk, Method for broadband spectroscopy of light transport through opaque scattering media,
Opt. Lett. 34, 395:397 (2009)
O. L. Muskens, A. Lagendijk, Broadband enhanced backscattering spectroscopy of strongly scattering media, Opt. Express 16(2), 1222:31 (2008)
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