Regine Frank

Theory of strong localization effects of light in disordered loss or gain media

We present a systematical theory for the interplay of strong localization effects and absorption or gain of classical waves in three-dimensional, disordered dielectrics. The theory is based on the self-consistent Cooperon resummation, implementing the effects of energy conservation and its absorptive or emissive corrections by an exact, generalized Ward identity. Substantial renormalizations are found, depending on whether the absorption or gain occurs in the scatterers or in the background medium. We find a finite, gain-induced correlation volume which may be significantly smaller than the scale set by the scattering mean-free path, even if there are no truly localized modes. Possible consequences for coherent feedback in random lasers as well as the possibility of oscillatory in time behavior induced by sufficiently strong gain are discussed.

Inelastic scattering puts in question recent claims of Anderson localization of light

The interplay between nonlinear effects and Anderson localization in disordered optical fibres1 has recently attracted great interest, and it is important in the action of random lasers in which closed multiple scattering loops have enhanced intensity. As optical nonlinearities in TiO2 can give valuable information on the nature of light transport in strongly scattering powders, we studied these effects in an extended experimental and theoretical investigation (to be published). Now, Scheffold and Wiersma have ...

Coherent control of Floquet-mode dressed plasmon polaritons

We study the coherent properties of plasmon polaritons optically excited on periodic nanostructures. The gold grains are coupled to a single mode photonic waveguide which exhibits a dramatically reduced transmission originating from the derived quantum interference. With a nonequilibrium description of Floquet-dressed polaritons we demonstrate the switching of light transmission through the waveguide due to sheer existence of intraband transitions in gold from right above the Fermi level driven by the external laser light.

Quantum criticality and population trapping of fermions by non-equilibrium lattice modulations

An ultracold gas of interacting fermionic atoms in a three-dimensional optical lattice is considered, where the lattice potential strength is periodically modulated. This non-equilibrium system is non-perturbatively described by means of a Keldysh?Floquet?Greens function approach for Mott?Hubbard systems employing a generalized dynamical mean field theory (DMFT). Strong repulsive interactions between different atoms lead to a Mott insulator state for the equilibrium system, but the additional external driving at zero temperature yields a non-equilibrium quantum critical behavior, where an infinite number of Floquet states arise and a transition to the liquid and conducting phase is given.

Population trapping and inversion in ultracold Fermi gases by excitation of the optical lattice: non-equilibrium Floquet–Keldysh description

A gas of ultracold interacting quantum degenerate Fermions is considered in a three-dimensional optical lattice which is externally modulated in the frequency and the amplitude. This theoretical study utilizes the Keldysh formalism to account for the system being out of thermodynamical equilibrium. A dynamical mean field theory, extended to non-equilibrium, is presented to calculate characteristic quantities such as the local density of states and the non-equilibrium distribution function. A dynamic Franz–Keldysh splitting is found which accounts for the non-equilibrium modification of the underlying bandstructure. The found characteristic Floquet-fan like bandstructure accounts for the quantized nature of the effect over all frequency space.

Non-equilibrium polaritonics - non-linear effects and optical switching: Non-equilibrium polaritonics - non-linear effects and optical switching

We report a theoretical non-equilibrium description of polaritonics and we propose ultrafast all-optical switching due to highly nonlinear polaritonics. The electronic band structure within gold (Au) nano grains is modified by external laser light. The Au grains are coupled to a single mode photonic waveguide and we derive a strong transmission reduction of switching originating from the establishd quantum interference with a finite lifetime.

Light Propagation in Anisotropic Disordered Media

We present a semi‐analytical theory for light propagation in three dimensional, strongly scattering, disordered, anisotropic dielectrics. The anisotropy of the system is incorporated by a tensor dielectric function. By starting at Maxwell’s equations, we derive a general transport theory for light including transport quantities such as energy transport velocity, transport mean free path and diffusion coefficient. This approach is based on a fully vectorial treatment of the generalized kinetic equation and also incorporated a generalized Ward identity for these systems. Furthermore, self‐interference contributions to the transport are included by means of a generalized localization theory based on a cooperon resummation first derived for electrons by Vollhardt and Wolfie [1]. Numerical evaluations will be presented.

Theoretical Approach to Random Lasing in thin Systems on reflecting Substrates

We develop a theory for random laser systems especially for different experimentally relevant setups [1]. A systematical semi‐analytical transport theory for amplifying random media is presented. The optical gain is described self‐consistently by coupling the transport theory to the semiclassical rate equations and solving this system. In order to stabilize a stationary lasing mode, we include necessarily a loss of photons through the surface of the sample. In experiments [1, 2] the disordered laser active material is placed on a substrate and then optically pumped. The free surface on the one side of the sample and the substrate on the other side of the sample yield different conditions for the lasing mode, which we include by considering asymmetric boundary conditions by means of loss through the free surface and reflection on the substrate side, respectively. We calculate all relevant quantities such as the mean photon number and the correlation length for these systems, and compare these results with ...

Tuning the Quantum Efficiency of Random Lasers - Intrinsic Stokes-Shift and Gain

We report the theoretical analysis for tuning the quantum efficiency of solid state random lasers. Vollhardt-Wölfle theory of photonic transport in disordered non-conserving and open random media, is coupled to lasing dynamics and solved positionally dependent. The interplay of non-linearity and homogeneous non-radiative frequency conversion by means of a Stokes-shift leads to a reduction of the quantum efficiency of the random laser. At the threshold a strong decrease of the spot-size in the stationary state is found due to the increase of non-radiative losses. The coherently emitted photon number per unit of modal surface is also strongly reduced. This result allows for the conclusion that Stokes-shifts are not sufficient to explain confined and extended mode regimes.

Bi-functional nonlinearities in monodisperse ZnO nano-grains – Self-consistent transport and random lasing

We report a quantum field theoretical description of light transport and random lasing. The Bethe-Salpeter equation is solved including maximally crossed diagrams and non-elastic scattering. This is the first theoretical framework that combines so called off-shell scattering and lasing in random media. We present results for the self-consistent scattering mean free path that varies over the width of the sample. Further we discuss the density dependent correlation length of self-consistent transport in disordered media composed of semi-conductor Mie scatterers.