More than 30% of the incident radiation from the sun is invisible infrared light, or “heat”, which cannot be absorbed by solar cells designed to convert light into electricity. Converting this infrared radiation efficiently into visible light can significantly increase the efficiency of photovoltaic conversion of sunlight into electricity, and open up avenues for using other waste-heat sources to generate electricity. Moreover, since the infrared light can also penetrate biological tissues, it can also provide a pathway for simple biodetection and bioimaging applications simply by using invisible infrared light sources instead of ionizing X-rays or other invasive procedures.
The biggest hurdle towards converting “heat” radiation into visible light, known as upconversion, is low efficiency.
Intense infrared light from a laser can be easily used to energetically combine two light particles, or photons, into single visible photon or light radiation. However, diffuse sunlight or low intensity infrared sources (intense infrared radiation can damage tissue) have very low upconversion efficiencies, making this process infeasible.
Researchers from Assistant Professor Prashant Nagpal’s group have utilized low intensity infrared light to generate quasiparticle Surface Plasmon waves on inexpensive nanofabricated metal chips.
While we can all use lenses to focus light, the dual nature of light as particle and wave prohibits using simple lenses to focus light beyond their wave-like length scales. These surface plasmon waves can squeeze light into a spot million times smaller than the incident light wavelength volume, thereby focusing diffuse sunlight into a “laser-like” spot on top of metal pyramids.
Nagpal's group placed doped-lanthanide nanoparticles on the pyramid tips, which absorb these infrared waves, and enhanced the transfer of energy to higher energy Erbium levels, which emits the visible radiation. The researchers showed that on these chip scale devices, not only the light intensity gets enhanced into the focused light spot, but these plasmon waves also enhance the transfer of energy to higher energy levels by several-fold, leading to 80 or 100 fold enhancement in emitted visible light.
This research can open pathways for improving solar energy conversion, imaging infrared radiation, using waste-heat energy sources, and developing new bioimaging techniques using simple infrared light sources.
This research was recently published in the journal Nano Letters. Along with Prashant Nagpal, the study was conducted by Chemical Engineering postdoctoral associates Qi Sun (first author) and Vivek Singh, graduate student Josep Ribot, along with colleagues Haridas Mundoor and Ivan Smalyukh in the Physics Department.
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