A spider uses up to seven different types of silk, all having specific functions, to build its web. For scientists, native silk - directly extracted from spiders - is a tough, biodegradable and biocompatible thread used mainly for tissue engineering and textile applications. Blessed with outstanding optical properties, this protein strand can also be used as an optical fibre and is, moreover, intrinsically sensitive to chemical compounds. In this communication, a pioneering proof-of-concept experiment using spider silk, in its pristine condition, as a new type of fibre-optic relative humidity sensor will be demonstrated and its potential for future applications discussed.
Whilst being thoroughly used in the textile industry and biomedical sector, silk has not yet been exploited for fibre optics-based sensing although silk fibres directly obtained from spiders can guide light and have shown early promises to being sensitive to some solvents. In this communication, a pioneering optical fibre sensor based on spider silk is reported, demonstrating for the first time the use of spider silk as an optical fibre sensor to detect polar solvents such as water, ammonia and acetic acid.
High-resolution infrared absorption spectroscopy of acetylene gas is demonstrated in dispersion-engineered photonic
crystal waveguides under slow light propagation. Individual absorption profiles are obtained for both TE and TM
polarizations for group indices ranging from 1.5 to 6.7. Experimental enhancement factors of 0.31 and 1.00 are obtained
for TE and TM polarization, respectively, and are confirmed by time-domain simulations. We experimentally
demonstrate that molecular absorption is a function of the electric field distribution outside the photonic crystal slab and
the group index under structural slow-light illumination.
A couple of experiments are here presented to clarify the impact of slow light on light-matter interaction. The
experiments are designed, so that the process generating slow light and the probed light-matter interaction only present a
marginal cross-effect. The impact of slow light on simple molecular absorption could be separately evaluated under
either material or structural slow light propagation in the same medium and led to an entirely different response.
This paper presents helpful expressions predicting the filling time of gaseous species inside photonic crystal fibres.
Based on the theory of diffusion, our gas-filling model can be applied to any given fibre geometry or length by
calculating diffusion coefficients. This was experimentally validated by monitoring the filling process of acetylene gas in
several fibre samples of various geometries and lengths. The measured filling times agree well within ±15% with the
predicted values for all fibre samples. In addition the pressure dependence of the diffusion coefficient was
experimentally verified by filling a given fibre sample with acetylene gas at various pressures. Finally ideal conditions
for gas light interaction are determined to ensure optimal efficiency of the sensor by considering the gas flow dynamics
in the design of microstructured fibres for gas detection and all-fibre gas cell applications.
The absorption of light by a gas molecule has been measured comparatively using light propagating in normal conditions
and in a slow light regime. The experiment is designed to make the 2 measurements possible without modifying the
interaction conditions, so that the sole effect of slow light is unambiguously observed. A 26% group velocity reduction
induced by stimulated Brillouin scattering in a gas-filled microstructured fiber caused no observable change in the
measured absorption, so that it is proved that material slow light does not enhance Beer-Lambert absorption and has a
null impact on gas sensing or spectroscopic applications.
We have thoroughly studied and modelled many important aspects for the realization of gas-light interactions
in suspended-core fibres. The fraction of the optical field propagating in holes could be calculated from the fibre
geometry to predict the total absorption for a given molecular absorption line and fibre length. In addition, the
gas diffusion into the fibre holes could be modelled to precisely anticipate the filling time for a given fibre geometry
and length. This was experimentally validated by preparing several samples of suspended-core fibres showing
various lengths. These samples were filled with acetylene at low pressure (< 50 mbar) and were hermetically and
permanently sealed by fusion splicing each fibre end to a plain single-mode silica fibre. The adequacy between
the modelling and the experimental results turned out to be excellent. Several physical parameters essential
for the fibre characterization could be extracted from a set of measurements, sketching a specific metrological
approach dedicated to this type of fibre. Finally, applications and advanced experiments that can be specifically
carried out using these fibres are discussed.