Nanocomposite materials have become very promising in the field of optical sensing. A variety of inorganic and
hybrid (polymer/inorganic) material structures are prepared and evaluated as potential sensors for physicochemical
monitoring applications (e.g. reducing/oxidising gases, NH<sub>3</sub>, alcohols hydrocarbons and other). Materials response is
based on their optical properties alteration such as scattering effects, refractive index changes or absorbance shifts
incurred upon ambient physicochemical parameters modification. First results of response, together with surface
analysis and stoichiometric and morphological characteristics of the developed materials are presented.
NiCl<sub>2</sub>/SiO<sub>2</sub> sol gel materials are proposed for the first time for ammonia sensing. Thin films are prepared by spin coating or casting the solution on appropriate substrates and tested for sensitivity in ammonia environment. We report a uniform optical response over the visible spectrum in optical transmission. The main mechanism of operation is based on reversible physico-chemical complexation effects, which produce morphology alteration and result in enhanced optical scattering of the thin film material. Such ammonia sensor can find applications in industrial and environmental engineering fields including "on-line" process and quality control, environment monitoring and safety.
Diffractive optical elements are designed and demonstrated as elemental units in photonic gas sensors. Diffraction gratings are written on specially designed photosensitive polymers using photolithographic techniques, as well as on multilayer metal/metal oxide thin film structures. Photonic sensors are implemented using grating structures as the elemental units for the detection of the external agent. These gratings are designed from such materials that show response to the external agent and the sensitivity is increased through the design of the grating. The principle of operation is based on the grating's diffraction efficiency variations due to index of refraction alterations and/or geometrical changes of the grating structure (e.g., groove depth, groove spacing) to external factors. The advantageous characteristics of the presented integrated sensor are the fully reversible behavior at ambient operating conditions, without the need for additional heating or light exposure. Applications of these sensitive photonic sensors so far include water vapor, hydrocarbons, and alcohol detection. The optical designs are based on diffraction efficiency measurements, and incorporate a monochromatic optical source and simple optoelectronic detection components. The photonic sensor integration is based on bulk optics approach.
The ability to engineer polymer materials that have special response to external factors as well as to incorporate nano- and/or micro- materials in these polymer matrices make these materials perfect candidates for physicochemical sensors. The incorporation of the nano-/micro-materials help the polymer materials become more sensitive to a variety of external factors. Different polymer designs for the detection of humidity, alcohols and hydrocarbons are described. Special diffractive photonics structures are implemented to offer increased sensitivity to physicochemical changes. The final photonic sensor design is based on an optimization of chemical as well as optical design.
The chemical polymer designs are based on selecting and synthesizing the appropriate polymeric structure that will facilitate interaction with the analyte, through a number of physical and chemical processes (adsorption, solubilization, entrapment, coulombic interaction and hydrogen bonding). These functions are determined by the chemical and structural features of the polymer used i.e. functional groups, glass transition, porosity, etc. In the case of polymer/nano- and/or micro-inorganic hybrid materials, interactions of the polymer matrix with the inorganic component(s) and dispersion of the nanomaterials within the matrix have to be taken into account.
Suitable photonic interfaces based on transmissive and/or diffractive techniques are designed to provide the medium with interface tailoring and interrogation methodologies. Novel photonic information processor prototype devices based on free space configurations are demonstrated to extract/recover the captured information from the sensing material. The advantageous characteristics of the presented integrated sensor are the fully reversible behavior, at ambient operating conditions, without the need for additional heating or light exposure.