Submicron dust clouds for optical charge measurements

Dusty plasmas consist of particles immersed in gaseous plasmas. The charge of the dust particles that they attain due to the inflow of plasma electrons and ions is a pivotal parameter: the particle charge determines the interaction with the plasma species and among the particles themselves. So far, charge measurements exploit the dust-plasma interaction or the analysis of wave-motion of the particles. Recently, a completely different, optical approach has been suggested, where the charge is extracted from the wavelength shift of the optical phonon resonance due to the charge-modified polarizability of the material. This ``detuning'' of the phonon resonance increases with particle charge and is prominent for nanometer-sized particles. In first experiments, we demonstrate the trapping of nanometric dust particles made of Al$_2$O$_3$. The dust is injected by a gas stream into the plasma. Clouds of particles with diameters around 100~nm can be trapped in the bulk plasma of the discharge. There, the phonon resonance of Al$_2$O$_3$ is measured in-situ in an FTIR spectrometer.