Appl. Phys. Lett. 100, 101108 (2012)

Light travelling through a nanowire solar cell is repeatedly scattered by different nanowires, causing its path to resemble that of a diffusing particle and giving it many chances to be absorbed. Understanding this diffusive transport can aid device design. However, measuring the time dynamics of light diffusion has generally required advanced interferometry experiments. Now, Jaap Dijkhuis and colleagues at Utrecht University have shown how to calculate the time it takes for light to diffuse through a nanowire forest using only electron microscope images.

The researchers varied the delay between two ultrashort laser pulses that were passed through a forest of ZnO nanowires. When the delay was short enough for both pulses to be present in the forest simultaneously, two-photon absorption occurred and the transmission of the second pulse was reduced. Measurements on a 22-μm-thick nanowire forest revealed that the transit, or dwell, time of violet light was about 1 ps. This was much longer than would be expected if the light passed through in a straight line, confirming that the transport through the sample was diffusive.

Furthermore, by calculating the average distance between adjacent nanowires from an electron microscope image of the nanowire forest and using a ray-optics model, Dijkhuis and colleagues successfully reproduced the dwell times they had measured experimentally.