title: Theory of Half-Space Light Absorption Enhancement for Leaky Mode Resonant Nanowires.
abstract: 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. Here we show that the absorption capability of these symmetrical nanowires has an upper limit under the half-space illumination. Based on the temporal coupled-mode equation, we develop a reciprocity theory for leaky mode resonances in order to connect the angle-dependent absorption cross section and the radiation pattern. We show 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, we 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.
authors.affiliation: †State Key Laboratory of Modern Optical Instrumentation and Department of Physics, Zhejiang University, Hangzhou 310027, China.
authors.forename: Yiming
authors.initials: Y
authors.lastname: Jia
authors.affiliation: ‡State Key Laboratory of Modern Optical Instrumentation and Department of Optical Engineering, Zhejiang University, Hangzhou 310027, China.
authors.forename: Min
authors.initials: M
authors.lastname: Qiu
authors.affiliation: §State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Wu
authors.affiliation: ∥Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States.
authors.forename: Yi
authors.initials: Y
authors.lastname: Cui
authors.affiliation: ⊥Department of Electrical Engineering and Ginzton Laboratory, Stanford University, Stanford, California 94305, United States.
authors.forename: Shanhui
authors.initials: S
authors.lastname: Fan
authors.affiliation: †State Key Laboratory of Modern Optical Instrumentation and Department of Physics, Zhejiang University, Hangzhou 310027, China.
authors.forename: Zhichao
authors.initials: Z
authors.lastname: Ruan
keywords: Nanowire; absorption; coupled-mode theory; leaky mode resonance; photodetector; solar cell