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We report the modeling results of an all-fiber gas detector that uses photonic crystal fiber (PCF). The relative sensitivity of the PCF as a function of the fiber parameters is calculated. Gas-diffusion dynamics that affect the sensor response time is investigated theoretically and experimentally. A practical PCF sensor aiming for high sensitivity gas detection is proposed.

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We report the fabrication process of isolated one-dimensional Se chains incorporated in the matrix of AlPO4-5 single crystals and the experimental investigation of the geometry, phase stability, electronic properties, and electron-phonon coupling effect of these Se chains. The structure of the helical Se chains inside the channels is discussed on the basis of X-ray scattering measurements. Thermal analysis and temperature-dependent micro-Raman measurements show that Se single chains are flexible and can convert from a weak distorted phase into another phase with strongly disordered structure ("melting" state) around 340 K. Since the electrons are confined in the one-dimensional channels, the absorption band of the Se chain is obviously blue shifted compared with that of trigonal Se. With increasing temperature, this band shifts linearly to the lower energy side, in sharp contrast to the nonlinear temperature coefficient of trigonal Se, which is attributed to the greatly diminished interchain interaction and the weakening of the electron-optical phonon coupling in a low-dimensional system. In the vicinity of the absorption band, both first-order and second-order Raman signals for the Se chain are enhanced, due to the strong electron-phonon coupling when the excitation laser energy matches the electronic transition in isolated Se chains.

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