By tailoring the distance between layers of optically inactive ErAs nano-islands in a semiconductor matrix, researchers at Max Planck Institut für Festkörperforschung in Stuttgart, Germany, and at the University of California, Santa Barbara, have demonstrated the tunability of the electrical response speed in photoconductive switches. They suggested that the approach, which they described in the February issue of Nature Materials, may enable the use of receiver and terahertz technologies at telecommunications wavelengths.In one demonstration, the researchers varied the thickness of GaAs layers that separated 1.2 monolayers of self-assembled ErAs islands from 10 to 400 nm, yielding photoconductive switches with electron lifetimes from 190 fs to 17 ps under 800-nm illumination. A subsequent device that featured InGaAs and ErAs in a 40-nm lattice period displayed an electron lifetime of 1.1 ps and should operate under up to 1.55-µm illumination.