Laser threshold of Mie resonances

We show that time-independent scattering coefficients calculated from the standard extrapolation of Mie theory to the gain regime have physical meaning up to the laser threshold. The theoretical width of a resonance decreases linearly with increasing gain and becomes zero at the laser threshold. We performed experiments on dielectric microspheres with gain, trapped with optical tweezers. The width of the mode was measured to narrow as a function of the gain up to the lasing threshold, confirming both the validity of the extrapolation of Mie theory to the gain regime below threshold and our interpretation of its point of divergence as the laser threshold.     

Monte Carlo study of the two-dimensional spin-1/2 XY model

The generalized Trotter formula is used to map the two-dimensional spin-1/2 XY model onto a three-dimensional Ising model with complicated many-spin interactions. This relationship is used to construct an efficient Monte Carlo algorithm. Simulation data for the specific heat and vortex correlation functions give strong evidence for the existence of a phase transition.     

Anisotropic diffusion of light in a strongly scattering material

Light transport in a strongly scattering, strongly anisotropic material is studied experimentally using both static and time-resolved techniques. Both the static and the dynamic results are well characterized by a diffusion equation with an anisotropic diffusion tensor and a scalar absorption term. Light diffuses 4.0 times faster along the uniaxial axis of the material compared with diffusion in the orthogonal directions.     

Accumulation of dwell times in light diffusion

We discuss various speeds of propagation that are relevant for the dynamics of classical waves in random media: phase velocity, group velocity and transport velocity. The transport velocity can be much smaller than the phase velocity due to the long dwelling of classical waves inside resonant scatterers. We show that the transport velocity of light can be obtained experimentally by study both transient and steady-state diffusion of light.     

Comment on the NMR lineshape in low-dimensional paramagnets

It will be demonstrated that the model used by some workers to describe the NMR lineshape in low-dimensional magnetic systems is of academic interest only. Furthermore, we will show that this model was not treated properly. The NMR lineshape in low-dimensional systems has a normal single-peaked structure.     

What is measured in a time-resolved stimulated Raman experiment?

We have performed four-beam time-resolved stimulated Raman measurements on liquid CS₂. To interpret our results we have applied the response formalism for the third-order polarization to such an experiment. It turns out that the measured quantity is proportional to (a convolution of) Abs[χ⁽³⁾(-ω_s, ω_s, ω_l, -ω_l)]. As a consequence in this four-beam e xperiment one picks up electronic contributions in contrast to a two-beam experiment where Im[χ⁽³⁾] is observed.     

Control of light transmission through opaque scattering media in space and time

We report the first experimental demonstration of combined spatial and temporal control of light transmission through opaque media. This control is achieved by solely manipulating spatial degrees of freedom of the incident wave front. As an application, we demonstrate that the present approach is capable of forming bandwidth-limited ultrashort pulses from the otherwise randomly transmitted light with a controllable interaction time of the pulses with the medium. Our approach provides a new tool for fundamental studies of light propagation in complex media and has the potential for applications for coherent control, sensing and imaging in nano- and biophotonics.     

Computer simulation study of bipolaron formation

Monte Carlo computer simulation techniques are used to study the formation of bipolarons on a lattice. The transition between the three possible states, extended, twopolaron, and bipolaron is studied. The phase diagram as a function of the strengths of the electron-phonon coupling and repulsive interaction is determined.