Mechanochromic Photonic Crystals as Strain Sensors for structural Applications

Structural Health Monitoring is an important aspect in civil engineering, dedicated to monitor and maintain the structural conditions of civil architecture objects. It results in extension of their life time and appropriateness for human use. Present, commercially available sensors for SHM are complex, sophisticated and multicomponent systems. Although, they provide high precision of measurements, their total cost (the price and costs of exploitation) has been still too high to be commonly applicable. There are also other disadvantages such as distributed architecture, heavy cables or their sensitivity to electromagnetic interference like it is in case of conventional electrical sensors. Unlike them, more advance fiber optic sensors are robust to external fields. However, they involve the infrared light for data transmission, therefore they desire additional support of other devices for data processing. Now a day, there is a lack of portable sensing instruments supporting more sophisticated technologies, whose applications can be reduced by failure assessing with those instruments. Current investigations have been focused on development of structures that can be used as an independent sensing tool without a power supplier, such as mechanochromic photonic crystals with three-dimensional structure. Their mechanochromic properties are visible with naked eye as a color variation on their top surface stimulated by mechanical deformation of the structure. However, their fabrication desires high precision to obtain sample with the high sensitivity to stretching and omit some limitation corresponding to its composition (deeply described in chapter 4). Hence, there is need to find alternative solutions. One of them refers to two-dimensional photonic crystals, which were intensively investigated as a components of sensing systems such as MOEMSs (micro-opto-electro-mechanical microsystems). However, their main disadvantage is the fabrication that involves the lithography techniques, which are quiet expensive and time-consuming. Furthermore, the lithographic techniques desire clean room conditions. Hence, the number of produced specimens is limited. In this thesis, there is proposed completely new approach to develop a strain sensor, including fabrication of strain-sensitive sample and methodology of measurements. The sample was fabricated as two-dimensional finite structure of hexagonally arranged voids on the PDMS substrate. The applied fabrication protocol was cost-effective and not time-consuming. The final product was a PDMS replica of monolayer colloidal crystal obtained by self-assembly of polystyrene colloidal spheres. Further investigations involved diffractive properties of its periodic structure. Its strain sensitivity was investigated by monitoring the parameters of diffracted (transmitted or reflected) light such as the diffracted wavelength (chapter 6) and the polarization (chapters 7 and 8), which vary by stretching the sample. Moreover, there was tested another approach, which involved shape changes in diffraction pattern. The diffraction pattern is a result of interaction between a periodic structure and an illuminating light. The obtained data confirmed strong relationship between optical response and the geometry of diffractive structure. However, the experiments require further optimization of fabrication protocol, methodology (conditions of measurements, sample parameters, an appropriate arrangement of components in the experimental setups)