Researcher at Indian Institute of Technology (IIT) Guwahati have found a way to get more light out of LED die using nano-scale metal particles.

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This has nothing to do with making the particles into phosphors. Instead, is is a way of preventing light bouncing back into into a conventional LED die from the die-encapsulant boundary – a reflection that occurs due to the difference in refractive index between the die and the transparent epoxy.

By casting a regular array of particles just above the die surface, light reflected from the particles can be adjusted to destructively interfere with light reflected from the chip-epoxy interface – reflections are negated.

The team, which included researchers from Imperial College London, studied and optimised the arrangement by modelling: the height of the array above the die surface, particle size, particle material and inter-particle spacing within the array – finding that up to 20% more light could leave the packaged LED for the same input power.

Silver nano-spheres look to be the best material – combining strong plasmonic resonance with minimal energy absorption, and a hexagonal array looks favourite to both work and self-assemble.

“While improvements to the casing have been suggested previously, most make the LED bulkier or more difficult to manufacture,” according to Dr Debabrata Sikdar, now on the IIT Guwahati team, and formerly of Imperial. “We think that our innovation, based on fundamental theory and optimisation analysis, could be introduced into existing manufacturing processes with little disruption or added bulk.”

So far, results are only theoretical, although: “The predicted effect is a result of theory related to nano-particle arrays at interfaces experimentally tested in switchable mirror-windows, tuneable-colour mirrors and optical filters,” added Imperial chemist Professor Alexei Kornyshev.

The next step is to manufacturing prototype LEDs with a nanoparticle layer.