Electric-induced enhancement and modulation of upconversion photoluminescence in epitaxial BaTiO3:Yb/Er thin films.

Spectroscopic tuning and enhancement of upconversion photoluminescence (PL) are highly desirable for understanding the physical processes of energy transition and for widespread applications, such as laser mediums, optical waveguides, display lighting, and biomedicine. To date, modification of the upconversion PL in phosphors excited by a given excitation source can normally be achieved through a conventional chemical approach, that is, changing the composition of host materials and/or doping ions, thus giving only a limited understanding of the detailed process of luminescence and applications. For instance, the variation of the PL in different hosts can be ascribed to the different crystal field around dopant ions of various symmetries. Low symmetry hosts typically exert a crystal field containing more uneven components around the dopant ions compared to high symmetry counterparts. However, the tuning of PL by chemical way is essentially an irreversible and ex-situ process. Therefore, it is unlikely to know from the kinetic process how the luminescence changes with structural symmetry through the conventional approach. Moreover, it is almost impossible to isolate the pure crystal-field effect from other extrinsic effects present in different samples such as chemical inhomogeneities and defects. Hence, it is much needed to find a pathway in varying the host s symmetry in the same material, leading to the modification of PL. In particular, an enhancement of upconversion still presents a significant challenge, although a breakthrough of plasmon-enhanced upconversion through very precise manipulation was recently presented. 11] To date, there are no reports of an established approach to modulate upconversion emission in an in-situ and real-time way. The unique crystal structure of ferroelectric materials provides us an opportunity to couple variables including electric field and temperature to crystal symmetry in a single compound. Approaches through the use of ferroelectric characteristics have recently proven successful in controlling the ferromagnetism, spin polarization, and photovoltaic effects. Comparatively, ferroelectric control of luminescence has not been exploited. Therefore, it is of great interest to take advantage of the properties of ferroelectric host in combination with the luminescent doping ions acting as sensitive probes for the structure and symmetry. PL in the specific materials system should be modulated in reversible, real-time and dynamical way under ferroelectric control that can be realized by electrically changing the structural symmetry. Herein, BaTiO3 (BTO) is selected as ferroelectric host because it has been regarded as a model system for investigating crystal structure transformation under mechanical stress, electric field, and temperature. Lanthanide Er ion is composed of an incompletely filled 4f inner shell and two closed outer shells. Actually, Er ion in doped hosts has been a spectroscopic probe for many applications, including ferroelectric domain imaging. Very recently, we have observed the effects of Er doping and measuring temperature on the upconversion emission in BTO:Er powders. Therefore, it is interesting to observe the influence of electric field on the upconversion spectra of Er doping ions. Herein, we present a new approach to enhance and modulate upconversion emission through applying a relatively low bias voltage to the lanthanide-doped BTO thin films.