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Digital archive of theses discussed at the University of Pisa


Thesis etd-06212012-115229

Thesis type
Tesi di dottorato di ricerca
Thesis title
A slow and cold particle beam for nanotechnological purposes
Academic discipline
Course of study
tutor Fuso, Francesco
  • focused ion beams
  • nanotechnology
  • optical pumping
Graduation session start date
In this thesis we report the production of a Cesium atomic beam by means of laser cooling techniques and its photoionization, in order to evaluate its applicative potential for specific technological purposes. A comprehensive analysis of the atomic beam is carried out, based on a variety of diagnostics, such as fluorescence imaging at different distances from the pyramidal-MOT, absorption spectroscopy, optical time-of-flight. The results demonstrate that the atomic beam owns peculiar dynamical properties, in particular in terms of longitudinal and transverse velocity distribution. The average value of the longitudinal velocity is on the order of ten m/s, with a spread on the order of m/s, accompanied by a few mrad divergence: such features motivate the names “slow” and “cold” we have attributed to our atomic beam. Thanks to them, the beam can find applications where sources of particles with controlled and rather homogeneous dynamical properties are required.
The main motivation behind photoionization of the Cesium beam was to set the basis for exploring the capabilities of the slow and cold beam in producing an ion beam. This part of the research was carried out within the frame of a European industry-oriented collaboration (FP7-MC-IAPP Project ”COLDBEAMS”) aimed at exploiting laser manipulation tools for the realization of unconventional charged particle beams with superior dynamical properties. The technology presently used for instance in FIB columns is in fact based on beams with “thermal” velocity distribution, that leads to non-monochromatic samples severely suffering chromatic aberration in the focusing stage. To this aim, a two-colour photoionization scheme has been implemented, involving resonant excitation of Cesium 6P atoms and interaction with 405 nm photons. Photoionization was demonstrated and the rate estimated on the order of about 3 · 10^6 s^(−1). The corresponding ion current is on the order of 0.5 pA. A preliminary insight into the dynamical properties of the ion beam has been given by ion time-of-flight measurements upon pulsed laser ionization. Interpretation of the results required a careful description of the electric fields in the collection region, which, while not being optimized by design for this specific purpose, were numerically simulated. The results demonstrate that, owing to the peculiar features of the neutral atom beam, the ions exhibit a rather monochromatic longitudinal energy distribution, with a monochromaticity essentially limited by the size of the ionizing laser beam and by the weak collection field. Such properties are comparable to those of the ion sources used at present.