Seeing Magnetic Light: Quantifying and
Enhancing Magnetic Dipole Emission

Prof. Rashid Zia
Brown University, School of Engineering

Abstract
Although it is often assumed that all light-matter interactions at optical frequencies are mediated by electric dipole transitions, strong optical frequency magnetic dipoles do exist.  In fact, we see magnetic dipole emission everyday from the Lanthanide ions that help to illuminate everything from fluorescent lighting to solid-state lasers. Nevertheless, most applications have overlooked the device implications of these magnetic dipole transitions throughout the visible and near-infrared regime. Moreover, the magnetic dipole contributions of many important transitions (such as the 1550nm line of Erbium) have not been fully characterized.

In this talk, we will illustrate how the naturally occurring magnetic dipole transitions of Lanthanide ions provide a new degree of design freedom for photonic devices. Specifically, we will demonstrate how the different symmetries of electric and magnetic dipoles can be exploited to identify, enhance, and control light emission.

First, we will present a novel spectroscopy technique to directly quantify the electric and magnetic dipole contributions from any mixed transition. Based on self-interference effects near dielectric interfaces, electric and magnetic dipole emission may be readily identified from their angular emission. Using a simple microscope-coupled imaging spectrograph, we have experimentally measured the spectrum of light emission in both momentum and energy space. These measurements have allowed us to directly quantify the line strengths at each wavelength and to directly map the local density of optical states for electric and magnetic dipole emitters. Then, using the spectrally-distinct electric and magnetic transitions of trivalent Europium (Eu3+), we illustrate how self-interference can be used to achieve strong enhancement of magnetic dipole emission and broad spectral tuning.  By scanning a simple metal surface near a Eu3+ doped thin film, we have demonstrated spectral tuning from 580-720nm without the need for a high-quality optical cavity. Finally, if time permits, we will discuss the implications on this work on the design of directly-modulated, high-speed Lanthanide based light sources, which could operate at the sub-lifetime-limited switching rates required for optical communications.

Biography
Rashid Zia is the Manning Assistant Professor of Engineering at Brown University and Director of the Brown Microelectronics Central User Facility. Rashid graduated in 2001 from Brown University with a combined A.B. in English and American Literature and Sc.B. in Engineering. He then went on to receive both his M.S. and Ph.D. in Electrical Engineering from Stanford University, where he was the first graduate student in the laboratory of Professor Mark L. Brongersma. As a student, Rashid was the recipient of a National Defense Science and Engineering Graduate (NDSEG) Fellowship and honorary Stanford Graduate Fellowship. As a faculty member, Rashid has been the recipient of a National Science Foundation CAREER Award and a Department of Defense nominated Presidential Early Career Award for Scientist and Engineers (PECASE).  Rashid is also a Fellow of the National Forum on the Future of Liberal Education.  His current research is supported by grants from the Air Force Office of Scientific Research and the National Science Foundation as well as gifts from the Nanoelectronic Research Initiative of the Semiconductor Research Corporation.