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Over the last few decades, gas membrane diffusion has been applied to elaborate chemical analyses, leading to the development of a series of gas sensing techniques for environmental monitoring. This work reviews the gas sensors that incorporate the gas membrane diffusion mechanism with either electrochemical or optical transducers, and concludes the theoretical relationship between the detection signal and the mass transfer parameters across the membrane, such as membrane thickness, gas diffusion coefficient and driving force. It also envisages that, with the availability of modern electronic and computing technology, the in-situ membrane diffusion rate of a target species is proportional to its real-time concentration in the sample and can be readily measured. Such a measuring principle is promising in developing the next generation of gas sensors based on membrane diffusion to achieve real-time and continuous monitoring of important trace gases (e.g. CO 2 , SO 2 , NH 3) in the natural environment (water, soil and air).

151 Eu 3+ -doped yttrium silicate ( 151 Eu 3+ : Y 2 SiO 5 ) crystal is a unique material that possesses hyperfine states with coherence time up to 6 h. Many efforts have been devoted to the development of this material as optical quantum memories based on the bulk crystals, but integrable structures (such as optical waveguides) that can promote 151 Eu 3+ : Y 2 SiO 5 -based quantum memories to practical applications, have not been demonstrated so far. Here we report the fabrication of type II waveguides in a 151 Eu 3+ : Y 2 SiO 5 crystal using femtosecond-laser micromachining. The resulting waveguides are compatible with single-mode fibers and have the smallest insertion loss of 4.95 dB. On-demand light storage is demonstrated in a waveguide by employing the spinwave atomic frequency comb (AFC) scheme and the revival of silenced echo (ROSE) scheme. We implement a series of interference experiments based on these two schemes to characterize the storage fidelity. Interference visibility of the readout pulse is 0.99 ± 0.03 for the spin-wave AFC scheme and 0.97 ± 0.02 for the ROSE scheme, demonstrating the reliability of the integrated optical memory.

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[1] Generation of electric fields at the scale of the local electron Debye length in collisionless magnetic reconnection is studied through two-dimensional Darwin particle-in-cell simulation. For asymmetric initial temperature and density profile across the Harris current sheet, intense perpendicular electrostatic structure with the local electron Debye-length scaling ($2.9 l De ) is observed near the edge of the magnetic island in the high-temperature/low-density region. It is also found that a weak electron jet with return electron flow on the high-density side results in the formation of an electron current loop. However, in the low density region only a strong electron outflow is detected. Because of a singlelooped electric current on each side of the outflow region, the usual quadrupole structure appearing in the earlier symmetric-profile simulations is replaced by a dipole-like structure of the guiding magnetic field. Possible applications of the present results in the dayside magnetopause are discussed.

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