TiO2 Boosts Efficiency, Photoluminescene of White Laser Diodes

Light-scattering nano-elements, also known as nanoantennas due to their ability to harness light, are used as an optical control technology in nanophotonics. In combination with a phosphor plate, and upon irradiation, these 2D structures — in which nanopoarticles are arranged periodically on a substrate — achieve a mix of blue and yellow light and enable spatial and spectral control over luminescence.

As a light source, white LEDs have already been improved upon in the form of white laser diodes (LDs). These LDs offer reduced coherence noise and enable very precise measurements, which benefit certain sensing, measurement, and imaging applications.

White LDs consist of yellow phosphors and blue LDs. The blue LDs are highly directional, and the yellow phosphors radiate in all directions. Unfortunately, this results in an undesired mixing of colors.

To address this issue, researchers have developed phosphor plates combined with nanoantennas — once combined, called nanoantenna phosphors — using metallic aluminum, which enables increased photoluminescence. Aluminum nanoparticles effectively scatter light and improve light intensity and directionality.

However, this is not a perfect solution. Aluminum also absorbs light, which reduces output. This characteristic represents a bottleneck, especially in high-intensity lighting applications.

By replacing the aluminum material with titanium dioxide, researchers at Kyoto University have now reported a tenfold enhancement in forward directed luminescence from nanoantenna phosphors. In addition to improved photoluminescence, the materials substitution demonstrated increased efficiency and charted a path to a potential replacement for white LEDs.

“It turns out that titanium dioxide is a better choice for its high refractive index and low-light absorption,” said study lead author Shunsuke Murai.

Nanoantenna phosphors comprising a periodic array of nanoparticles on a phosphor plate tailored spatial radiation patterns of the photoluminescence, supporting smaller, lighter, and brighter solid-state lighting devices. Courtesy of Kyoto University/Shunsuke Murai.

The researchers fabricated a hexagonal array of titanium dioxide nanoparticles on a phosphor plate of yttrium aluminum garnet doped with cerium. Experimental results showed that the fabricated nanoantenna increased the forward emission and decreased the side emission, while the total emission intensity remained unchanged. The researchers then deposited a Bragg reflector on the bottom of the plate, which enhanced the forward emission.

Although the light-scattering intensity of titanium oxide initially appeared inferior to metallic aluminum, the researchers used computer simulations to devise the optimal nanoantenna design.

“The new nanoantenna phosphors are advantageous for intensely bright yet energy-saving solid-state lighting because they can suppress temperature rise when irradiated,” Murai said.

In addition to yielding an alternative light source to white LEDs, the research allows the flow of photons inside nanoantenna phosphors to be understood, according to the researchers, which supports the efficient use of photoluminescence.

“While the emission enhancement in a specific direction has been reported in nanoantenna studies, the evaluation of the total distribution of radiation as well as the conversion efficiency is largely missing,” the researchers said in their paper.

The research was published in Journal of Materials Chemistry C (www.doi.org/10.1039/D2TC03076D).