Magnetic resonance imaging (MRI) is a widely used biomedical imaging modality that derives much of its contrast from microscale magnetic field patterns in tissues. However, the connection between these patterns and the appearance of macroscale MR images has not been the subject of direct experimental study due to a lack of methods to map microscopic fields in biological samples. Here, we optically probe magnetic fields in mammalian cells and tissues with submicron resolution and nanotesla sensitivity using nitrogen-vacancy diamond magnetometry, and combine these measurements with simulations of nuclear spin precession to predict the corresponding MRI contrast. We demonstrate the utility of this technology in an in vitro model of macrophage iron uptake and histological samples from a mouse model of hepatic iron overload. In addition, we follow magnetic particle endocytosis in live cells. This approach bridges a fundamental gap between an MRI voxel and its microscopic constituents.
Any chemical process can be, in principle, understood and manipulated through electron dynamics. Such dynamics occur on what is known as the 'ultrashort" time scale, taking place in 10^-15 of a second (a femtosecond). Observing or controlling these processes is extremely challenging, as it requires electromagnetic forces that can be arbitrarily shaped in space and manipulated on the sub-femtosecond time scale, i.e. ultrashort laser pulses. Furthermore, the pulses used in such experiments are typically intense enough to modify the optical properties of the material system under study, thereby changing the way the laser pulses themselves propagate. There is thus a need to better understand this 'nonlinear' regime before having the ability to demonstrate full control. This talk will describe the experiments and simulations we used to study the spatial and temporal physics in the ultrashort nonlinear processes of filamentation and stimulated Raman scattering in solids.