MERIT · Modulated electrooptic response imaging technique

Final Activity Report Summary - MERIT (Modulated electrooptic response imaging technique)

Within this project, we have designed, constructed and tested a new kind of scanning probe microscope (SPM), optimised for the investigation at the nanometre scale of materials possessing two or more stable states. Ferroelectrics and ferromagnetics are the most known examples of such materials. The strong motivation to design a novel SPM is given by the fact that there are not contrast mechanisms in existing microscopes to discriminate such physically different two states. Both the fundamental properties and technological applications of ferroelectrics and ferromagnetic materials and devices depend strongly on their domain structure. This novel SPM is apt to investigate nanometre scale ferroelectric or ferromagnetic domains and to observe switching between them induced by proper external fields. The essence of our method is the exploitation of the linear electro-optic or magneto-optic effect in near-field scanning microscopy (SNOM) by applying an external electric or magnetic field, respectively. As outlined in the proposal, MERIT has validated this approach for the electric case and therefore it is suitable for the investigation of ferroelectric materials. Ferroelectrics are essential components in nonvolatile memories. Components based on ferroelectric films are also being developed for various sensor and actuator applications and for tuneable microwave circuits. In thin film form, ferroelectrics add functionality to micro electro mechanical systems (MEMS) and polar thin films have contributed to the miniaturisation and improved performance of mass produced portable telephones during the late 1990s. The electro-optical near-field microscope (EO-SNOM) developed in this project, provides imaging of the local linear electro-optic effect. Thus, it combines the advantages of the piezo atomic force microscope (PM-AFM) that provides the sign of the ferroelectric polarisation, with those of the optical microscopy that provides the local optical index of the material. The EO-SNOM has been realized in three configurations: 1) diffraction-limited confocal microscope; 2) aperture SNOM; and 3) apertureless SNOM. Electro-optical images of the domain structure of model ferroelectric materials have been obtained for all three configurations and features of each of them were found out. SNOM super-resolution is however, restricted to objects with nanometre scale thickness. For samples extended in the direction of the light propagation, the far field scattering takes over the near-field and spoils the resolution. As an unforeseen outcome of this project, we have found a way to enhance the near-field contribution in extended objects in presence of an external planar electric field by proper modulation of the optical properties localized at the surface. In full accordance with this idea, comparison of images taken with different scanning techniques has shown that the spatial resolution of our modulated electro-optic microscope is significantly better with respect to conventional SNOM. This remarkable result opens the way to use SNOM beyond the current limit of very few classes of specific objects like thin films. The developed technique is expected to be useful in the pattern imaging of ferroelectric polarisation, including inhomogeneous phase transitions and complicated systems like ceramics, films, or relaxor materials. We have addressed this field by acquiring the electro-optic images in two of such systems, namely PMN-PT crystals and PLZNT ceramics. The EO-SNOM and its magnetic analogue - to be developed in future projects - will likely play a crucial role also in the investigation of a new class of materials called multiferroic. They display more than one ferroic property, i.e. ferroelectric, ferromagnetic, and ferroelastic, in a single phase or individually within separated phases. Although in its infancy, this field is expected to yield even more striking applications.

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