  We use an analogue of nanoindentation on a colloidal glass to elucidate the
incipient plastic deformation of glasses. By tracking the motion of the
individual particles in three dimensions, we visualize the strain field and
glass structure during the emerging deformation. At the onset of flow, we
observe a power-law distribution of strain indicating strongly correlated
deformation, and reflecting a critical state of the glass. At later stages, the
strain acquires a Gaussian distribution, indicating that plastic events become
uncorrelated. Investigation of the glass structure using both static and
dynamic measures shows a weak correlation between the structure and the
emerging strain distribution. These results indicate that the onset of
plasticity is governed by strong power-law correlations of strain, weakly
biased by the heterogeneous glass structure.
