We present experimentally a low-complexity ultralow switching energy and highlinearity all-optical sampling gate based on a terahertz optical asymmetric demultiplexer, constructed by placing a polarization-insensitive multiple-quantum-well semiconductor optical amplifier (PI-MQW-SOA) asymmetrically within a fiber loop mirror. Using a picosecond pulse train as the trigger clock, we analyze, in detail, the influence of the sampling pulse power and the offset of the PI-MQW-SOA from the loop midpoint on the shape, width, and amplitude of the sampling window, respectively. Furthermore, the switching energy and linearity of this sampling gate under different widths of sampling windows are specifically investigated. Results demonstrate that our sampling gate possesses a switching energy as low as 25 fJ and a high linearity above 0.99.

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The site discrimination and the size effect on the formation energy Ef of the B atoms in B-doped Si nanocrystals (SiNCs) were studied by introducing a new parameter, the adjusted coordination number (ACN), which includes second-nearest neighbor atoms. A series of Ef values were obtained by using the first-principle calculation. It was found that the B-doping sites in the SiNCs can be well discriminated by ACN, and the change trends of Ef with either size or site become clear if one uses ACN as a describer. When the site is given, Ef has a decreasing trend when the size increases. However, when considering the sites with the same coordination number in a given Si nanocrystal, their Ef values increase with increasing ACN. Moreover, it was observed that sites with ACN = 3.375 are the usually preferred B-doping sites in these SiNCs when they exist.