Nano Lett. http://doi.org/9pt (2015)

Changing the charge density in small silicon or germanium spherical resonators can tune their resonant Mie wavelength across the mid-infrared (2 to 16 μm), say researchers in the US. The approach, demonstrated by Tomer Lewi and co-workers from the University of California at Santa Barbara, could offer a route to wide, dynamic tuning of mid-infrared antennas. The team fabricated Si and Ge spheres of various sizes (0.5 to 4 μm diameter) and charge carrier densities using femtosecond laser ablation of Si and Ge wafers with different doping levels. Experiments indicate that the resonant wavelength of the resonators gets longer as the size of the semiconductor particles increases, but gets shorter as the doping concentration increases. The researchers say that the use of optical or electrical schemes to dynamically modulate the charge density of the particles could thus offer a means for fast tuning of their wavelength response.