Evaluating Incoherent Control of Population Dynamics as a Tool for Next Generation Photochemistry

Abstract

Light has a long history of successful use in the initiation of photochemical reactions.\(^{1-5}\) However, recent optical technologies that allow the arbitrary shaping of an excitation source’s spectral and temporal characteristics have yet to be utilized in this field. Since sophisticated light sources have enabled major advances in other areas such as coherent quantum control\(^{6-11}\) and optogenetics,\(^{12}\) it seems possible that their application to photochemistry could yield enormous dividends. The shaping of an incoherent light source in a process called incoherent control has successfully manipulated a quantum system’s population dynamics in simulations.\(^{13}\) Our goal was to develop this proposed technique with applications to photochemistry in mind. We used a lanthanide doped material—Gd\(_{2}\)O\(_{2}\)S with 6% Er\(^{3+}\)—as a developmental testbed because its long excited state lifetimes made it sensitive to lower excitation powers and its ions in excited states have been shown to interact\(^{14}\) in mechanisms that can be formally compared to chemical reactions. Additionally, studying this material had intrinsic value as better understanding the interactions between its various excited states could improve the many devices that utilize lanthanide doped crystals. In our work, we constructed a theoretical model of our system, analyzed the effects of our shapeable light source’s spectral components on its visible excited states, and fit our model to the sample’s response to a simple excitation source. After analyzing our sample’s properties, we were then able to control its population dynamics by shaping incoherent radiation with an adaptive feedback control loop. Specifically, we were able to modulate the relative populations of two of its excited states by a factor of 1.56.