Plasmonics breakthrough offers advances in quantum computing

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Researchers at the Lawrence Berkeley National Laboratory in California have shown that plasmonic properties can be achieved in the semiconductor nanocrystals known as quantum dots. They claim the breakthrough could have significant implications for future light harvesting, nonlinear optics and quantum information processing.

"The term 'plasmonics' describes a phenomenon in which the confinement of light in dimensions smaller than the wavelength of photons in free space makes it possible to match the different length scales associated with photonics and electronics in a single nanoscale device," explained Berkeley Lab director Paul Alivisatos, who led the research. "We believe that through plasmonics, it should be possible to design computer chip interconnects that are able to move much larger amounts of data, much faster than today's chips." Alivisatos believes it could also be possible to create microscope lenses that can resolve nanoscale objects with visible light, as well as a new generation of highly efficient leds and supersensitive chemical and biological detectors. He says there is even evidence that plasmonic materials can be used to bend light around objects, thereby rendering them invisible. "We have demonstrated well defined localised surface plasmon resonances arising from p-type carriers in vacancy doped semiconductor quantum dots that should allow for plasmonic sensing and manipulation of solid state processes in single nanocrystals," said Alivisatos. "Our doped semiconductor quantum dots also open up the possibility of strongly coupling photonic and electronic properties." Alivisatos and his team made their quantum dots from the semiconductor copper sulfide, a material that is known to support numerous copper deficient stoichiometries. Initially, the copper sulfide nanocrystals were synthesised using a common hot injection method. But, while this yielded nanocrystals that were intrinsically self doped with p-type charge carriers, there was no control over the amount of charge vacancies or carriers. Transmission electron micrographs showing the electron diffraction patterns of three quantum dot samples with average size of (a) 2.4nm (b) 3.6nm, and (c) 5.8nm. (Image courtesy of Alivisatos group) "We were able to overcome this limitation by using a room temperature ion exchange method to synthesise the copper sulfide nanocrystals," explained Alivisatos "This froze the nanocrystals into a relatively vacancy free state, which we could then dope in a controlled manner using common chemical oxidants." By introducing enough free electrical charge carriers via dopants and vacancies, the team was able to achieve localised surface plasmon resonances (LSPRs) in the near infrared range of the electromagnetic spectrum. "Unlike a metal, the concentration of free charge carriers in a semiconductor can be actively controlled by doping, temperature, and/or phase transitions," Alivisatos continued. "Therefore, the frequency and intensity of LSPRs in dopable quantum dots can be dynamically tuned. The LSPRs of a metal, on the other hand, once engineered through a choice of nanostructure parameters, such as shape and size, is permanently locked in." The scientist envisions quantum dots being integrated into a variety of future film and chip based photonic devices that can be actively switched or controlled. He also believes the strong coupling that is possible between photonic and electronic modes in such doped quantum dots holds exciting potential for applications in solar photovoltaics and artificial photosynthesis. "Quantum dot plasmonics also hold intriguing possibilities for future quantum communication and computation devices," he concluded. "Our long term goal now is to generalise plasmonic phenomena to all doped quantum dots, whether heavily self doped or extrinsically doped, with relatively few impurities or vacancies." The full research can be found in the online journal Nature Materials.