Person:

Yang, Nan

This thesis describes work related to the theme of sculpted electromagnetic fields - engineered fields with particular spatial patterns - and their interactions with molecules. We are motivated by the following questions: what are ways of detecting spatial patterns in electromagnetic fields? What are possible applications of spatially engineered fields? Are there molecular transitions that are dark to plane waves but that can be probed by sculpted fields? The first part of this thesis is in the area of magnetic field effects in chemistry. We focus on magnetic field modulated fluorescence, which provides a convenient method for imaging magnetic field strength. We proposed and demonstrated a fluorescence technique that allows imaging through strongly scattering media. We achieve this by exploiting the fact that most materials do not scatter magnetic field. This allows us to project a magnetic field pattern beyond the scattering surface. The magnetic field dependent fluorescence then allows us to map out the object of interest. We constructed a setup that demonstrates 2D imaging using this technique. We synthesized new molecular systems to enhance the sensitivity to magnetic field. We characterized and compared these molecules with steady state fluorescence spectroscopy, transient fluorescence and transient absorption measurements. The results reveal patterns that point to directions for engineering chemical systems to further enhance their magnetic field sensing properties. The second portion of this thesis is a theoretical study of the molecular multipole transitions and their couplings to local electromagnetic quantities. Using a semiclassical approach, we performed a multipole expansion of molecular transitions driven by monochromatic radiation. We derived the local electromagnetic quantities that couple to different multipole transitions and observables such as circular dichroism and magnetic circular dichroism. It was observed that certain transitions are dark to plane waves, but could be probed by simple spatial arrangements such as superpositions of plane waves. Experiments for their detection are also proposed.

A weak (< 1000 G) magnetic field can influence photochemical processes through its effect on electron spin dynamics in a photogenerated radical pair. In a solution of pyrene and dimethylaniline this effect manifests as magnetic field-dependent exciplex fluorescence. Here we describe magnetofluorescence imaging (MFI). A localized magnetic null defines a fluorescence detection volume, which is scanned through a sample to create an image. MFI forms an image without lenses and in the presence of arbitrarily strong optical scattering. The resolution of MFI is in principle not limited by optical diffraction, although the present implementation is far from the diffraction limit.

Metallic and magnetic nanostructures set electromagnetic boundary conditions which can lead to highly contorted fields in their immediate vicinity. While much attention has been devoted to enhancements in electric field strength, we argue that equally interesting phenomena arise from enhancements in magnetic and electric field gradients. Nonuniform fields near nanostructures can induce molecular transitions that are forbidden by electric dipole selection rules. We illustrate this claim with two examples. ‘‘Superhelical’’ electromagnetic fields are predicted to show enhanced asymmetry in their interaction with chiral molecules, far greater than that due to circularly polarized light. Such fields could be used to induce chiral photochemistry with large enantiomeric excess. Steeply varying DC magnetic fields are predicted to enhance the rate of intersystem crossing in molecular bi-radicals. Such fields could provide a route to new nanomagnetic catalysts and to magnetic control of chemical reactions.