Localization of light
Photons are rather elusive particles. When interacting with matter, they are absorbed, reflected or refracted. When talking about light-matter interaction in solar cells, reflection in particular is a rather unwanted process, since it decreases the efficiency of converting photon energy into electrical current.
Ideas of increasing the conversion efficiency report of roughening the surface of the solar cell in order to enforce the incoming light to be strongly scattered within the solar cell .
Within this project, we are investigating the interaction of ultrashort laser pulses with arbitrary arranged, vertically aligned zinc oxide nanoneedles. The light incident onto the needle array is forced to be stored, i.e. “localized” within the needles, until it is finally emitted into far field again. Within our experiment, we both investigate the spatial and temporal localization of localization effects. The direct investigation of the temporal signatures of localization is rather difficult, since it takes place on a femtosecond time scale .
The image above shows the experimental set-up (a). Dispersion-compensated, 6-fs laser pulses from a Ti:Sapphire oscillator are focused by an all-reflective Cassegrain objective onto an array of 30 nm thick, 500 nm long ZnO needles. The pulse duration of 6 fs is preserved within the diffraction-limited focus diameter of the laser pulse. The Second Harmonic (SH) within the needle array is measured in reflection geometry using a spectrometers as a function of sample position is taken as a measure for the local electric field strength (b). Strong fluctuation of the SH indicate that some light is localized within the needle array.
By implementing a Mach-Zehnder interferometer into the set-up, the time structure of the local electric can be deduced. As shown in (c, on top), this field is elongated with respect to incident, transform-limited pulse duration of 6 fs. Three-dimensional FTDT simulations and a theoretical model indicate, that the light shows indeed evidence for weak localization .
This novel time-resolving SH microscope gives us the opportunity to investigate the evolution of electromagnetic fields on a time scale of a few femtoseconds and a spatial scale of less than a micrometer.
In the future we will focus our research both on the localization of light in nanostructures of different shape and size and on the propagation of light-induced Surface Plasmon Polaritons (SPP).
 C. Rockstuhl et al., Appl. Phys. Lett. 91, 171104 (2007)
 D. Wiersma et al., Nature 390, 671-673 (1997)