P. E. Wolf

Multiple light scattering from disordered media : the effect of brownian motion of scatterers

We have measured the time autocorrelation function of the light intensity multiply scattered from turbid aqueous suspensions of submicron size polystyrene spheres in directions near backscattering. It is found strongly non-exponential at short times revealing the very fast decay of coherence in extended scattering loops due to the thermal motion of the many spheres involved; the longest living decay time is found remarkably close to the single particle backscattering relaxation time even under conditions of interparticle interactions. These features are only weakly affected by the particular interference effect between time-reversed pairs of loops giving rise to the coherent backscattering enhancement. A simple argument is presented which accounts for these observations.

Photoactivation turns green fluorescent protein red

In the few years since its gene was first cloned, the Aequorea victoria green fluorescent protein (GFP) has become a powerful tool in cell biology, functioning as a marker for gene expression, protein localization and protein dynamics in living cells. GFP variants with improved fluorescence intensity and altered spectral characteristics have been identified, but additional GFP variants are still desirable for multiple labeling experiments, protein interaction studies and improved visibility in some organisms. In particular, long-wavelength (red) fluorescence has remained elusive. Here we describe a red-emitting, green-absorbing fluorescent state of GFP that is generated by photoactivation with blue light. GFP can be switched to its red-emitting state easily with a laser or fluorescence microscope lamp under conditions of low oxygen concentration. This previously unnoticed ability enables regional, non-invasive marking of proteins in vivo. In particular, we report here the use of GFP photoactivation to make the first direct measurements of protein diffusion in the cytoplasm of living bacteria.

Measurements of the Kapitza resistance and Onsager cross-coefficient for the4He crystal-superfluid interface

AbstractThe effect of a heat current across the atomically rough crystal-superfluid interface in4He has been studied below 1 K. The chemical potential difference Δμ and temperature difference ΔT were measured simultaneously, the ΔT by means of a “superfluid melting-curve thermometer.” The results give the mean phonon transmission coefficient across the interface

A sensitive and versatile torque magnetometer for use in high magnetic fields

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Static and dynamic multiple scattering of light

and a dimensionless quantity β that determines the Onsager coefficient linking Δμ with the heat current. The transmission probability

Low-temperature polarized helium-3 for MRI applications

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