07 Sep 2022
New device makes practical use of polaritons, pointing to a "goldmine of polariton applications."University of Michigan.
It may pave the way for large-scale artificial photosynthetic systems integrating organic photonic and opto-electronic devices, according to the project team.
Published in Optica, the findings also point towards new practical applications for polaritons, quasiparticles that combine a molecular excited state with a photon.
On the molecular level, photosynthesis in many plants involves the transport of molecules in electronically excited states from a large light absorbing region to a reaction center where the energy is converted to a charge. Recreating this arrangement in man-made structures requires efficient energy transport in an organic material, which has proven difficult to accomplish.
"Numerous efforts have been made to mimic natural photosynthesis using organic materials," noted the team in its paper. "However, short-range energy transport due to material disorder has hindered their success."
Previous research into polaritons has included their potential value in all-optical data processing and in devices capable of producing single photons on demand. The Michigan team had itself previously considered polaritons while looking for ways to make better solar cells.
"After observing polariton propagation over long distances in simple structures such as a mirror with an organic film on its surface, we thought it might be possible to make a photosynthetic analog using polaritons," said Stephen Forrest from the University of Michigan. "However, it was quite difficult to figure out how to build such a device. This photodetector is one of the first demonstrations of a practical optoelectronic device based on polaritons."
Practical polariton applications
The eventual architecture involved selected thinfilm materials, including a 40-nanometer-thick layer of tetraphenyldibenzoperiflanthene (DBP) and molybdenum trioxide, mounted on a distributed Bragg reflector (DBR) formed from multiple layers of silicon dioxide and silicon nitrides.
"We borrowed from structures that we previously designed to create efficient organic photovoltaic cells," said Forrest.
"It was a bit fortuitous that these structures allowed efficient harvesting of the energy carried by polaritons. Polaritons still hold some mysteries and this is a new way of using them, so we weren't sure if it would work."
Tests showed that the new photodetector was more efficient at converting light to electrical current than a comparable silicon photodiode, gathering light from areas of 0.01 square millimeters and achieving conversion of light to electrical current over exceptionally long distances of 0.1 nanometers - three orders larger than the energy transfer distance of photosynthetic complexes.
"Our work shows that polaritons, in addition to being interesting science, are also a goldmine of applications yet to be discovered," said Forrest. "Devices such as ours provide an unusual, and possibly unique, method to understand the fundamental properties of polaritons and to enable yet-to-be imagined ways to manipulate light and charge."
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