Photon upconversion is the process of converting two or more low-energy photons into a single higher-energy photon. An example is the conversion of infrared light into the visible, which has potential application in photovoltaics, infrared sensing, biological imaging, and 3D display. Being a nonlinear process, upconversion typically requires coherent, high-intensity laser excitation. In this talk, I will present an approach that is capable of achieving upconversion under incoherent, low-intensity excitation such as the sunlight. We rely on the interactions among the long-lived excited states, known as excitons, in organic and low-dimensional materials, to manipulate the flow of the photon energy. We demonstrate that a device made of stacked thin films of colloidal nanocrystals (aka quantum dots) and organic molecules is able to convert infrared light with wavelength up to 1100 nm into visible light with wavelength around 610 nm. When incorporated with optical structures such as a Fabry-Pérot microcavity, the upconversion device reaches its optimal efficiency under an input photon flux even lower than that of the sunlight. Our work paves the way towards the integration of infrared-to-visible upconversion in sensing and photovoltaic applications.
Dr. Mengfei Wu is a research scientist at the Institute of Materials Research and Engineering, A*STAR. Mengfei received her B.A. in Engineering and M.Eng. in Electrical Engineering from the University of Cambridge in 2012, and her Ph.D. in Electrical Engineering from MIT in 2018. Mengfei’s research seeks to understand and exploit the unique properties of excitons in organic and low-dimensional materials, as well as to complement materials with photonic designs, in order to realise novel and efficient optoelectronic devices. Mengfei has won a number of awards, including the National Science Scholarship from A*STAR in 2008, and the Best Paper of the Symposium at the MRS Spring Meeting in 2017.