Since its inception in the 1990s, the semiconductor nanowire has been greeted with tremendous attention. Its cross section is 2-200nm in diameter while its length adds up to several micrometers. Thus, it is marked by a high aspect ratio, thereby serving as a link between nano and micro worlds.
Recently, the research team led by Prof. RUAN Zhichao from the Department of Physics achieved a breakthrough in research into light absorption of semiconductor nanowires. Relevant findings were officially published on the August issue of Nano Letters in 2005. Semiconductor nanowires supporting leaky mode resonances have been used to increase light absorption in optoelectronic applications from solar cell to photodetector and sensor. The light conventionally illuminates these devices with a wide range of different incident angles from half space. Currently, most of the investigated nanowires have centrosymmetric geometry cross section, such as circle, hexagon, and rectangle. It is revealed that the absorption capability of these symmetrical nanowires has an upper limit under the half-space illumination. Based on the temporal coupled-mode equation, the research team develops a reciprocity theory for leaky mode resonances in order to connect the angle-dependent absorption cross section and the radiation pattern. They demonstrate that in order to exceed such a half-space limit the radiation pattern should be noncentrosymmetric and dominate in the direction reciprocal to the illumination.
As an example, they design a metal trough structure to achieve the desired radiation pattern for an embedded nanowire. In comparison to a single nanowire case the trough structure indeed overcomes the half-space limit and leads to 39% and 64% absorption enhancement in TM and TE polarizations, respectively. Also the trough structure enables the enhancement over a broad wavelength range.
