P. D. Townsend

Luminescence data offer delicate probes of changes in structure and local environment in insulating materials. Therefore they have long been employed in studies of imperfections and characterisation of lattice distortions. Luminescence techniques are inherently very sensitive so respond to small concentrations of intrinsic defects and impurities, and intentionally added probe ions, such as rare earth or chromium ions, are particularly effective at displaying responsive changes linked to modified structural environments. Parameters of interest are variations in luminescence efficiency, the details of the emission spectra, polarisation, temperature dependence and changes in excited state lifetimes. These are suitable properties to monitor variations in both short and long range lattice structure, composition, pressure and temperature. For modern photonic materials such probes are powerful tools to follow changes introduced by processing to make waveguides, surface layers and new materials. The use of different excitation conditions for the luminescence can resolve differences between near surface and bulk features. A less common approach is to use luminescence as a route to detect phase transitions. This is particularly valuable for rapid survey studies of new materials which are of relevance to modern optics. One of the more surprising results is the observation that phase changes of very small inclusions, such as phase precipitates or impurity nanoparticles, can totally dominate the luminescence response of the bulk material. Specific examples include the frequent observation of trapped nanoparticles of water, N₂, O₂ and CO₂. Further, the excitation techniques, and ion implantation or surface stresses can all induce surface relaxations which, in materials such as SrTiO₃ or ZnO, result in phase changes propagating throughout the entire crystal. This review rapidly recalls the methods used in the older studies of imperfections and then uses a range of examples to show how luminescence studies can be effective in detecting and confirming the presence of phase transitions. Interestingly the examples include not only transparent insulating materials but also fullerenes and superconductors.

Ion beam induced luminescence and cathodoluminescence of albite produce unexpected differences between (010) and (001) aligned samples. The emission spectra are modified by temperature and different components change in their relative intensity as a result of excitation by different ion beams of H⁺, H₂⁺ and He⁺. A consideration of processes which can induce the orientation dependence and the role of excitation density that influences the component features are both discussed.

A simple and general method of using a photon detector to directly distinguish between two or three different polarizations is described. The method is suitable for a pre-selected limited wavelength region, for example as with encoded fibre optic communication. Alternatively, wavelength discrimination via wavelength selective polarization is possible.

A variant route to lock-in amplifier detection of optical emission from radio pulsars is considered. The method uses the radio pulsar frequency signals as a timing clock but splits the optical signals into equal portions of one third the pulsar period. Differences in their intensities offer evidence for an optical signal at the same frequency as the pulsar without the need to know the phase differences between optical and radio signals. The route benefits from sensitivity to pulsed optical signals seen against a steady background and also allows detection of periodic reductions in intensity. The method is insensitive to changes in observational conditions. When used to acquire star images with the viewing field phase split onto three detector areas, the phase block method should provide intensity differences which identify the location of an optical signal with the frequency of the radio pulsar. Supportive data for the method from a simple laboratory based proof of concept experiment are cited. The three phase block route can equally be applied to searches for polarized emission.