Piezophotonics: From fundamentals and materials to applications
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Introduction Many emerging photonics and optoelectronics applications require them to be directly triggered or tuned by mechanical inputs from their environment. Some inorganic or organic materials exhibit light emission when an external mechanical stimulus is applied, which is termed mechanoluminescence (ML). Xu first reported intense and reproducible luminescence during exposure of certain materials to mechanical stimuli, including ZnS:Mn2+ and SrAl2O4:Eu2+, and introduced the principle and applications of hybrid inorganic–organic composites especially for sensors and sensitive artificial skins.1–3 Wang introduced the field of piezophotonics in semiconductors, regarded as a two-way coupling of piezoelectric– photoexcitation.4,5 Therefore, metal ions as activators in ML may serve for demonstrating the piezophotonic effect since they are capable of responding to photoexcitation and subsequently emitting light. Luminescence is a phenomenon in which the excitation energy of a substance excited by external energy is given off as a photon, resulting in light emission. One may classify luminescence according to the excitation source used. Typically, photoluminescence (PL) is stimulated by photon energy while electroluminescence (EL) is triggered by an electric field. PL- and EL-based materials have been extensively
applied in display systems, light-emitting diodes (LEDs), solidstate lighting, and biomedicine.6,7 A useful form of ML, elasticoluminescence (ESL), recoverable light emission generated during elastic deformation that is also called piezoluminescence for piezoelectric materials, has attracted considerable attention because it can be repeatedly used for mechano-optical conversion. ESL materials and devices offer advantages of wireless detection and nondestructive analysis, making it a promising candidate for various applications such as stress sensing and damage diagnosis.8–13 Here, we first introduce the physical mechanisms of piezophotonics, followed by the relevant materials considerations and processing. We review the development of recent prototypical demonstrations and applications. Finally, a brief perspective on piezophotonics is outlined.
Fundamental principles and materials considerations The piezophotonic effect is a two-way coupling between piezoelectricity and photoexcitation properties as shown in Figure 1a.14 The piezophotonic effect uses the piezopotential to tune the energy band, carrier detrapping, and recombination and hence induces photon processes such as light emission. Apart from the two-way coupling effect mentioned, coupling
Jianhua Hao, Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong; [email protected] Chao-Nan Xu, National Institute of Advanced Industrial Science and Technology, Japan; [email protected] doi:10.1557/mrs.2018.296
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