|This STREP project aims to study the underlying physical mechanisms of optical switching based on single-particle phase transitions triggered by light or electron-beam excitation and to demonstrate new types of nanophotonic switches based on this principle. The implementation of these ambitious research aims will require the development of new techniques to study active nanophotonic structures. We will for the first time combine nanophotonics with electron microscopy - employing sub-nanometre resolution diagnostic capabilities to achieve the deepest possible insight into the photonic switching process and to exercise electron-beam controlled switching.This technique will be particularly suited to the study of optical switches and will employ the evanescent field of the electron beam to truly probe the near-field regime of these devices in a non-intrusive fashion. More specifically, a new all-optical switch will be developed using light-induced phase transitions in nanoparticles where two or more phases can co-exist in metastable form.The reversibility of the phase transisions will be addressed, and the effect of the nano-environment will be investigated. Nanoparticles will be trapped by tips and analysed both by measuring(1) scattering and absorption of light driven to the nanoparticles down optical fibres,(2) electron energy losses within transmission electron microscopes, and(3) light induced by those electrons.Our ultimate goals are:(1) to demonstrate the ability of some specific nanoparticles to act as single-photon switches based upon optically-driven phase transitions and(2) to provide a general photonic nanostructures characterisation technique based upon electron microscopy that can have a significant impact in both the nanophotonics and the electron microscopy communities.