Soft Report - Dipartimento di Fisica - Sapienza

Facilitieslaserf 2BSf 1FOBSSoftware Toolkit for the Statistical Analysis ofPhotocounts. Photon Correlation Spectroscopy(PCS) has proved to be a major technique to probedynamics in soft materials. Many variationsappeared in the course of the last decades extendingthe range of applications of PCS to: stronglyscattering media (Diffusing Wave Spectroscopy),turbid samples (multi-color and 3D PCS), infra-redand X-ray PCS. However, the core of any of theabove mentioned PCS based experiments remains adigital auto correlator: a device which is able tocompute the autocorrelation function of a timeseries of digital pulses (photocounts). Modernpersonal computers provide far enoughcomputational speed to perform softwareautocorrelation in real time, a task that oncerequired specially designed hardware. We developeda set of software classes (implemented as extensionmodules of the object oriented language Python)designed to perform basic tasks for the statisticalanalysis of digital pulse trains. Photocounts areacquired through a general purpose, counter/timerPCI board. A software approach, having access tothe full photocounts train, allows to efficientlyprototype different analysis protocols, going farbeyond the simple autocorrelation function. Ourphoton statistics toolkit, named PhotonLab, iscurrently used, within the Soft laboratories, toperform: multi-tau PCS, heterodyne Dopplervelocimetry, infra red PCS, time resolved PCS.Author: R. Di Leonardo.Optical Fibers-based Photo-Correlation setup for sheared samples. A photoncorrelationset-up with an optical fiber collecting systemenables measurements on both the dynamicsand the local rheology of a complex fluid. Thesystem is put under shear through a cone-plateor plate-plate cell, whose fixed plate is madeup of an optical window. A polarized laser beam(λ=514 nm) is directed towards the cellcontaining the sample (S) and the scatteredbeam is collected by a mono-mode optical fiber(f 1). Through a beam splitter (BS), part of thelaser beam, representing the local oscillator, iscollected by another fiber (f 2). Both the fibersare polarization-maintaining and theinterference among the scattered field and thelocal oscillator may be achieved through a fiberoptics beam splitter (FOBS), which perfectlymatches the wavefronts of the two beams.Another optical fiber (f 3) propagates theinterfering beam or, optionally, the solescattered beam from the beam splitter to thephotomultiplier. Finally, the digital signal inoutput from the photomultiplier is sent to anhomemade software correlator. In thehomodyne set-up the sole scattered fieldintensity is correlated and information on thedynamics of the system are obtained. In theSOFT Scientific Report 2004-0624