Strongest Coupling of Light and Matter by Synthetic Gold Crystals

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Research Team Led by Freie Universität Designs New Materials

No 133/2020 from Jul 29, 2020

Scientists at Freie Universität Berlin, Universität Hamburg, and Universidade Federaldo Ceará in Brazil showed that crystals of tiny gold spheres bind light. The new material shows the strongest coupling to light reported so far. In future, it may be used for computers that operate with light.

"The material we synthesized consists of gold, but it appears pink and turquoise," says physicist Dr. Niclas Müller, a researcher at Freie Universität Berlin. The synthetic crystals consist of tiny gold spheres, so-called nanoparticles, and they bind light. "Inside the crystal, a beam of light becomes a mix of light and matter, in this case, excited gold electrons," explains research group leader Prof. Stephanie Reich. "To understand the color and other properties of the nanoparticle crystals, we had to include light in our calculations." Theoretical physicists have studied systems that extremely strongly couple with light for years, but synthesizing them proved difficult and expensive. "We designed a very simple system. The exotic conditions for light emerge naturally while the crystal forms," says Stephanie Reich.

For the publication, Reich and her group collaborated with the physical chemists Prof. Holger Lange and Dr. Florian Schulz at Universität Hamburg. Originally, the joint work aimed at novel materials for catalysis. Together with the theoretical physicists Prof. Eduardo Barros at the State University of Ceará in Brazil the groups realized that the real beauty of the crystals was their coupling with light. Before demonstrating the mixed light-matter state, the groups had to overcome multiple challenges. The researchers in Hamburg developed a method to grow high-quality artificial crystals of nanoparticles with 50 nanometers diameter - about the size of small viruses. The scientists in Berlin built special optical setups, and numerical simulations were run in Brazil.

"This particular crystal is our first step into this novel research field," says Stephanie Reich. "We want to synthesize more materials by changing the microscopic crystal structure. In this way, we will fabricate materials with various properties from one building block." The scientists envision materials in which two light quanta interact. That will be a key material for an optical information technology that works with light. This technology will require less energy than our current solutions and will allow revolutionary concepts like quantum computing. The three research groups have already submitted a proposal to the German Research Foundation (DFG) for their future work on the mixed light-matter states.

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