Since the emerging of semiconductor nanowire in 1990s, it has attracted tremendous attention in both electronics and photonics. Typically, semiconductor nanowires have the about 2-200 nmdiameters and are hundreds of nanometers to several micrometers long. The high aspect-ratio of nanostructures allows for the bridging between the nanoscopic and microscopic world, which make a promise to be one of the essential building blocks for optoelectronic device applications, including solar cells, photodetectors and sensors. The nanoscale dimension of nanowires provides opportunities for further miniaturization of these devices, which will lead to many advantages in terms of operating speed, signal-to-noise ratio, and power consumption. However, due to the diffraction limit, the miniaturized devices cannot capture enough incident photons and thus result in unsatisfactory external quantum efficiency. It has been demonstrated that leaky mode resonances in nanowires can enhance light absorption, leading to high efficiency for semiconductor nanowire solar cells and photodetectors.
Recently, the research group of Zhichao Ruan and their collaborators have made new progress in light absorption of semiconductor nanowires. The research result has been published on Nano Letters vol. 15, 5513, 2015 [http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b02044]. The Letter discusses the design principle of nanowire structure under half-space illumination and proposes breaking the centrosymmetry to enhance effective light absorption. Such an absorptionenhancement is essential for solar cells and photodetectors since in a general configuration the incident light illuminates nanowires only from half-space. For centrosymmetric nanowires, the authors demonstrate that there is an upper limit for the integration of absorption cross sections over half-space illumination direction, which is called half-space limit by the authors. The researchers show that breaking of centrosymmetry can make the light absorption exceed the half-space limit. This result provides the fundamental guidance for the design of efficient nanowire solar cells and photodetectors.
As a demonstration, the authors propose a metal nanotrough structure to break the centrosymmetry of the embedded nanowire. It is numerically demonstrated that the wire-trough structure indeed exceeds the half-space limit. The integration of the absorption cross sections for the half-space illumination is enhanced by 39% and 64% as compared with an optimal centrosymmetric single nanowire, in TM and TE polarization, respectively. Moreover, it is shown that the structure enables the enhancement over a broad wavelength range.
The research is supported by Fundamental Research Funds for the Central Universities.
(a) Schematics of wire-trough structures. (b)(c) Effective half-space absorption cross section in the TM and TE polarized case, respectively. The red line and the blue line correspond to the effective absorption cross section of the wire-trough structure and the optimal single nanowire, respectively.
