Luminescent materials: techniques of optical and spectroscopic characterization

Today, luminescent materials are part of our everyday lives. But whether we're talking about lighting, displays or public signage, high-speed optical telecommunications or quantum computers, solar cells or lasers for civil or military applications, there are still many improvements to be made, and many original and innovative materials and systems to be discovered, both to increase yields and lower manufacturing costs, while limiting, if not reducing, the risks to health and the environment. To this end, many public and private laboratories and institutes, both in highly industrialized and emerging countries, are devoting ever greater financial and human resources. Numerous physical chemists, opticians, spectroscopists and laser physicists have to be trained to use a wide range of methods and techniques to characterize the materials they have designed or manufactured themselves, or that have been supplied to them for integration into the systems they are to test. These usually involve chemical and structural characterization, thermal and thermomechanical characterization and/or optical and spectroscopic characterization.

Several articles have already been devoted to the optical and spectroscopic properties of organic luminescent materials for OLEDs, and to inorganic materials doped with rare earths or transition ions for lighting, 3rd generation solar cells and lasers. In this article, we focus on the latter category of luminescent materials, especially from the point of view of optical and spectroscopic characterization methods and techniques. The aim is not only to present what is available commercially, but also to describe the operation and characteristics of the tools used, be they light excitation sources, detectors or signal analysis and processing systems. All the classic spectroscopic characterization methods and techniques (absorption, emission, excitation, fluorescence lifetime, quantum yield) will be reviewed, each time taking care to detail the aspects of calibration and exploitation of the results according to the systems studied.

For example, we'll look at how to correct spectra for the spectral response of devices, how to take polarization into account in the case of birefringent crystals, how to avoid the problems posed by radiative trapping, how to determine branching ratios, how to describe the thermal evolution of observed spectra, and finally how to calibrate spectra in units of effective cross-section. Another section will be devoted to more specific measurement mechanisms and techniques. These include pump-probe techniques for recording and calibrating absorption spectra in the excited states of ions, methods for characterizing multi-photon excitation and "cooperative" luminescence spectra, and pump-probe techniques for recording refractive...