Aharon Gero

We study multiple scattering of photons in disordered atomic media while taking into account cooperative effects, which originate from the interaction between atoms through the radiation field. We show that in atomic gases cooperative effects, like superradiance and subradiance, lead to a potential between two atoms that decay as the inverse inter-atomic distance, where in the case of superradiance, this potential is attractive for close enough atoms. The contribution of superradiant pairs to multiple scattering properties of a dilute gas, such as photon elastic mean free path and group velocity, is significantly different from that of independent atoms. Near resonance, it leads to a finite and positive group velocity, unlike the one obtained for light interaction with independent atoms. We also study the photon propagation in a gas of N atoms, using an effective Hamiltonian that accounts for photon mediated atomic dipolar interactions. The density of photon escape rates is obtained from the spectrum of the N x N random matrix U_ij = sin x_ij / x_ij, where x_ij is the dimensionless random distance between any two atoms.  A scaling function is defined to study photon escape rates as a function of disorder and system size. Photon localization is described using statistical properties of random networks whose mean field solution displays a "small world" behavior.