In this paper, we use the transfer matrix method to calculate the optical absorptance of vertically-aligned silicon nanowire (SiNW) arrays. For fixed filling ratio, significant optical absorption enhancement occurs when the lattice constant is increased from 100 nm to 600 nm. The enhancement arises from an increase in field concentration within the nanowire as well as excitation of guided resonance modes. We quantify the absorption enhancement in terms of ultimate efficiency. Results show that an optimized SiNW array with lattice constant of 600 nm and wire diameter of 540 nm has a 72.4% higher ultimate efficiency than a Si thin film of equal thickness. The enhancement effect can be maintained over a large range of incidence angles.
Mercury accumulation in montane forested areas plays an important role in global Hg cycling. In this study, we measured stable Hg isotopes in soil and litter samples to understand Hg accumulation on the forest floor along the eastern fringe of the Tibetan Plateau (TP). The low atmospheric Hg inputs lead to the small Hg pool size (23 ± 9 mg m in 0-60 cm soil horizon), up to 1 order of magnitude lower than those found at sites in Southwest China, North America, and Europe. The slightly negative ΔHg (-0.12 to -0.05‰) in the litter at low elevations (3100 to 3600 m) suggests an influence of local anthropogenic emissions, whereas the more significant negative ΔHg (-0.38 to -0.15‰) at high elevations (3700 to 4300 m) indicates impact from long-range transport. Hg input from litter is more important than wet deposition to Hg accumulation on the forest floor, as evidenced by the negative ΔHg found in the surface soil samples. Correlation analyses of ΔHg versus total carbon and leaf area index suggest that litter biomass production is a predominant factor in atmospheric Hg inputs to the forest floor. Precipitation and temperature show indirect effects on Hg accumulation by influencing litter biomass production in the eastern TP.
We design and fabricate a 320 nm slot for an electro-optic (E-O) polymer infiltrated silicon photonic crystal waveguide. Because of the large slot width, the poling efficiency of the infiltrated E-O polymer (AJCKL1/amorphous polycarbonate) is significantly improved. When coupled with the slow light effect from the silicon photonic crystal waveguide, an effective in-device r(33) of 735 pm/V, which to our knowledge is a record high, is demonstrated, which is ten times higher than the E-O coefficient achieved in thin film material. Because of this ultrahigh E-O efficiency, the V(π)L of the device is only 0.44 V mm, which is to our knowledge the best result of all E-O polymer modulators.
We demonstrate a 300 μm long silicon photonic crystal (PC) slot waveguide device for on-chip near-infrared absorption spectroscopy, based on the Beer-Lambert law for the detection of methane gas. The device combines slow light in a PC waveguide with high electric field intensity in a low-index 90 nm wide slot, which effectively increases the optical absorption path length. A methane concentration of 100 ppm (parts per million) in nitrogen was measured.
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