: Mechanoluminescence (ML) is the phenomenon describing the emission of light during mechanical action on a solid, leading to applications such as pressure sensing, damage detection and visualization of stress distributions. In most cases, this mechanical action releases energy that was previously stored in the crystal lattice of the phosphor by means of trapped charge carriers. A drawback is the need to record the ML emission during a pressure event. In this work, we provide a method for adding a memory function to these pressure-sensitive phosphors, allowing an optical readout of the location and intensity of a pressure event in excess of 72 h after the event. This is achieved in the BaSi₂O₂N₂:Eu²⁺ phosphor, where a broad trap depth distribution essential for the process is present. By merging optically stimulated luminescence (OSL), thermoluminescence (TL) and ML measurements, the influence of light, heat and pressure on the trap depth distribution is carefully analysed. This analysis demonstrates that mechanical action can not only lead to direct light emission but also to a reshuffling of trap occupations. This memory effect not only is expected to lead to new pressure sensing applications but also offers an approach to study charge carrier transitions in energy storage phosphors.

The monitoring of stress changes in structural components under various kinds of dynamical loading is crucial for the assessment of their integrity and lifetime. In addition to many methodologies available, such as strain gauges, optical fiber sensors, X-Ray diffraction and digital image correlation, we introduce a novel non-contact method to visualize stress distributions based on mechanoluminescence (ML). ML is a phenomenon occurring in some materials that emit light upon an applied stress level. In this paper, we develop the ML material (Ca_0.4Sr_0.6)Al₂Si₂O₈:1%Eu²⁺,1%Ho³⁺, a glow-in-the-dark material, to visualize stress distribution in a disc, as well as the stress field of an ultrasonic transducer. The properties of defects in the ML phosphors, which are responsible for ML in this material, are vital for stress visualization.

Ultrasonic transducers are used in many fields of application, including medical imaging/treatment, non-destructive testing and material characterization. To assure the quality of the ultrasonic investigation transducers require regular checks for possible deterioration and accurate calibration. Current methods rely on point-by-point scanning of the ultrasound field with a needle hydrophone, which is expensive and time consuming. Recently, we have developed a new concept, in which a fast full-field visualization of the radiation field is achieved through Acoustically induced PiezoLuminescence (APL). Here, we report on an improved ultrasonic beam visualization and provide further insights into the mechanism underlying APL and mechanoluminescence.