Optical properties of germania and titania at 1064 nm and at 1550 nm

One of the main noise sources in current gravitational wave detectors is the thermal noise of the high-reflectivity coatings on the main interferometer optics.
Coating thermal noise is dominated by the mechanical loss of the high-refractive index material within the coating stacks, Ta2O5 mixed with TiO2. For upgrades to room-temperature detectors, a mixture of GeO2 and TiO2 is an interesting alternative candidate coating material. While the rather low refractive index of GeO2 increases with increasing TiO2 content, a higher TiO2 content results in a lower threshold temperature before heat treatment leads to crystallisation, and potentially to a degradation of optical properties. For future cryogenic detectors, on the other hand, a higher TiO2 content is beneficial as the TiO2 suppresses the low-temperature mechanical loss peak of GeO2. In this paper, we present the optical properties of coatings -- produced by plasma-assisted ion-beam evaporation -- with high TiO2 content at 1550nm, a laser wavelength considered for cryogenic gravitational-wave detectors, as a function of heat-treatment temperature. For comparison, the absorption was also measured of pure GeO2. Furthermore, results at the currently-used wavelength of 1064nm are presented

MBE growth, characterisation and epitaxial lift-off processing of II – VI semiconductors

This thesis explores the MBE growth of two different II – VI magnetic semiconductor compounds, MnS and MnSe, and seeks to introduce a new technique for quantitative measurements of the adhesion between very thin films. A technique based on fracture mechanics and thin-film interference has been applied and the adhesion between ZnSe lifted epilayers and six different substrates (glass, ZnSe, GaAs, GaP, InAs and InP) calculated. The obtained results indicate that the bonding at the new surface is predominantly chemical. The impact of the oxide layer on the surface of the new host substrate in the bonding process has also been investigated and preliminary results are presented. MnS has been grown by MBE in the metastable zinc blende (ZB) crystal structure on GaAs (100) substrates over a wide range of growth conditions by varying the growth temperature from 220 °C to 300 °C and ZnS:Mn flux ratio from 5 to 20. Double-crystal X-ray rocking curves have been used to determine the crystallinity and residual zinc content in the layers. This allowed a phase diagram to be produced for ZB MnS giving zinc incorporation as a function of the growth temperature and the ZnS:Mn flux ratio. An optimum growth region has been identified where ZB MnS with less than 1% residual zinc incorporation can be obtained. Under these optimum growth conditions the maximum ZB MnS layer thickness obtained was double that obtained in previous studies, with layers up to 250 nm thick produced. The MBE growth and the structural properties of the metastable ZB MnSe has been studied. A set of growth parameters, corresponding to substrate temperature of 240 °C and Mn:Se flux ratio of 1:10, has been established where MnSe can be grown up to 230 nm in the ΖΒ crystal structure. By using both (004) and (115) reflections, the lattice constant of ZB MnSe was determined to be 5.876 ± 0.002 Å, the value of 0.435 Poisson’s ratio was experimentally obtained and the growth rate obtained from XRI analysis was 0.43 Ås-1 . A common window for the MBE growth of both MnS and MnSe in the metastable ΖΒ crystal structure has been demonstrated and verified.Engineering and Physical Sciences Research Council (EPSRC) fundin