Investigation of device material for functional optimization

The aim of this work package is to investigate relevant material properties in order to select the best material composition for thermometry devices. The results of WP2 will strongly support the activities of WP1. Particular focus will be on maximising the photoelastic effects and minimising the mechanical loss and light absorption in the devices fabricated. In optomechanical devices at least one mechanical mode is coupled to one optical mode. The mechanical vibration induces stress and deformation (strain) in the device which changes its refractive index. The magnitude of this refractive index change is described by the photoelastic coefficients of the related material. The precise control of the coupling of optical and mechanical modes requires the knowledge of these material parameters which are so far unknown.

Furthermore, high precision optomechanical temperature sensors require a high mechanical Q-factor or, in other words, a low mechanical loss. The mechanical loss is influenced by bulk (e.g. impurities) and surface properties (e.g. roughness, surface chemistry), geometry as well as anchor/clamping losses. Small mechanical losses can be determined by measuring the ringdown time of mechanical modes. Large mechanical losses can be measured by performing spectral scans of mechanical modes from whose Q-factor the loss can be deduced. The complexity of factors influencing mechanical loss requires the careful evaluation of this parameter in test structures with properties as similar as possible to the final devices. The photonic thermometry devices in this project operate based on the temperature dependence of their optical properties. The relevant photothermal properties will be investigated as well

This work package will enable the optimisation of material choice and of the geometric design parameters. Materials of interest will be silicon nitride, silicon and diamond. The investigations will be performed within the temperature ranges addressed by the related devices. In the first task, we will perform measurements on the photoelastic properties of Si and Si and photothermal properties of silicon. Therefore measurements on test structures and on devices will be carried out. In the second task, work will focus on measurements of the mechanical loss of Si, SiN and diamond. Here, ringdown measurements of several mechanical modes and - if necessary - spectral measurements of the Q-factor will be carried out. These measurements will be complemented by in-situ loss determination with optical transmission measurements.