Introduction

Many optoelectronic devices and systems exhibit a large mismatch between critical optical and electronic length scales that limit their performance. Particularly severe issues in this regard have emerged in scaling electronic circuitry for information technology and in the development of ultra-thin devices for solar energy harvesting. For example, the stringent electronic power and speed requirements on photodetectors used in an optical link set demanding limits on the size of these components. Ideally, one would scale these detectors to the size of an electronic transistor (~10 nm) or in fact build optically controlled transistors. The fundamental laws of diffraction – which state that light waves cannot be focused beyond about half a free-space wavelength (typically a few hundreds of nanometers) – seems to indicate that an efficient coupling to such tiny devices is physically impossible. Similar challenges occur in ultra-thin film solar cells that are realized with the aim of reducing processing and materials costs compared with thicker crystalline cells. Unfortunately their low energy conversion efficiencies still prevent rapid large-scale implementation. The key reason for their relatively poor performance is that the absorption depth of light in the most popular, deposited semiconductors films used in these cells is significantly longer than the electronic (minority carrier) diffusion length (particularly for photon energies close to the bandgap). As a result, charge extraction from optically thick cells is challenging due to carrier recombination in the bulk of the semiconductor.