Australian researchers at the University of Technology in Sydney have found a way to use nanoparticles to allow light through glass but block most of the heat.
Current technology uses thin silver films to block infrared light but allow the transmission of visible light. The researchers doped a polyvinyl butyral laminate sheet with rare-earth lanthanum hexaboride (LaB6) nanoparticles and sandwiched it between two sheets of clear glass. They found that at nanoparticle concentrations of 0.03 wt%, visible sunlight was reduced by 41% and transmitted heat was reduced by 71%. They have been experimenting with concentrations as low as 0.01 wt%.
The LaB6 works well as it has a resonance peak around 1000nm and therefore absorbs much of the near infrared light in the wavelengths that contribute to heat around 750 to 1300nm.
Higher concentrations of nanoparticles would decrease the transmitted hit but also lower the amount of visible light passing through the glass. The low concentrations currently being used still allow the windows to look transparent but they have a slight green tinge and appear blueish at certain angles.