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Artist’s conception of a new method that takes advantage of plasmonic metals’ production of hot carriers to boost light to a higher frequency. Bottom: A scanning electron microscope image shows gold-capped quantum wells, each about 100 nm wide. [Image: Rice University]

Upconverting photons from lower to higher energies is more challenging than the reverse process, but scientists have been working on ways to make upconversion easier with plasmonic nanostructures. Now, researchers at two U.S. universities have developed a technique for upconverting photons that uses twice as many “hot carriers” between the tiny nanostructures as previous schemes (Nano Lett., doi:10.1021/acs.nanolett.7b00900).

Illuminating photons

According to Gururaj Naik, an assistant professor at Rice University in Texas, and his former colleagues at Stanford University in California, previous plasmonic upconversion devices made from semiconductors extract energy from either hot electrons or hot holes, but not both. Unlike these previous devices, Naik’s team built an array of nanostructures housing multiple quantum wells that employed both types of carriers.

The nanosized pillars consisted of layers of gallium nitride and indium gallium nitride capped with gold toppers 50- to 100-nm wide and 10-nm thick. A thin layer of silver covering the whole structure blue-shifted the plasmon resonance. The team illuminated the structures with photons of 2.4 eV, which is lower than the bandgap of gallium nitride. The metal-covered pillars upconverted those photons to 2.8 eV. As a control, pillars without the metal covering did not boost the photons’ energy.

Hot carriers lead to upconversion

By performing a series of experiments and modeling the theoretical hot-carrier generation rate of the pillars, Naik and his colleagues established that the hot carriers were indeed responsible for performing the upconversion.

Theoretically, this process could have an internal quantum efficiency of 25 percent, though the group’s proof-of-concept nanoscale array probably had much lower efficiency since the team did not optimize the geometry of the structures or the thinness of the gold cap. In one of their tests, the team made some elliptical nanostructured pillars and measured their upconversion as a function of polarization angle.

The researchers suggest that different combinations of plasmonic materials and quantum wells, built from various metals and semiconductors in optimized geometries, may lead to more efficient solid-state upconversion. Such components, they say, could find applications in many industries, from solar photovoltaic cells to biomedical imaging.