Frankfurt 2006 – scientific programme

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Q: Quantenoptik und Photonik

Q 7: Photonik in komplexen und periodischen Strukturen I

Q 7.1: Talk

Monday, March 13, 2006, 11:10–11:25, H14

Anisotropic Light Diffusion in Suspensions of Birefringent Colloids — •T. Gisler, K. Sandomirski, G. Maret, and H. Stark — Universität Konstanz, Fachbereich Physik, Fach M621, 78457 Konstanz

We study multiple scattering of light in suspensions of birefringent, spherical colloids with a large, positive birefringence n_e−n_o≈ 0.2. Due to the anisotropy in the diamagnetic susceptibility, these particles are oriented by a magnetic field B which suppresses the orientation fluctuations.

In the diffusive regime of light transport, the diffuse transmission T_d decreases with increasing magnetic field B when the incident wave vector k_i is parallel to B. On the other hand, T_d measured for k_i⊥ B increases with B. Furthermore, the particle orientation by the magnetic field leads to a pronounced anisotropy in the transmission profiles when k_i⊥B: the photon diffusion coefficient  D_⊥ perpendicular to the magnetic field is larger than  D_||, the photon diffusion coefficient parallel to B, with a maximal value of the anisotropy  D_⊥/ D_||≈ 1.6 at a magnetic field strength B=7 T. Even at high particle densities and with samples thick enough that ballistic light is virtually absent, we find that the diffusely transmitted light is partially polarized, with a degree of polarization that increases with B and reaches a value 0.14 at B=7 T. This indicates that the orientation of the particles by the magnetic field induces a macroscopic uniaxial symmetry into the random medium guiding the diffusive light propagation.