The article reports the development of invisible cryptography writing and latent fingerprint detection techniques using upconversion emission of rare earth doped ceramic for the application in forensic science. The upconversion (UC) emission studies on Yb<sup>3+</sup>/Er+<sup>3+</sup> co-doped Sb<sub>2</sub>O<sub>3</sub>-WO<sub>3</sub>-Li<sub>2</sub>O (SWL) ceramics under 980 nm laser excitation has been done. Due to its intense green and red visible emission, the ceramic finds a great application in developing latent fingermark detection using powder dusting technique. In addition to fingerprint detection, the present ceramic is a promising candidate for coding cyphers in invisible manner. An observed comparison between UV vs. NIR excited fingerprint images shows that fingerprint images exposed by NIR excitation have more clear edges than the UV excitations. Hence the current upconversion emission studies on the present ceramic samples reveal its potential application in forensic science, security writing and cryptography information (cypher).
Yb<sup>3+</sup>/Er<sup>3+</sup> co doped SrTiO<sub>3</sub> ceramic have been prepared through solid state reaction route and then x-ray diffraction, upconversion photoluminescence properties of the sample are investigated. The variation of upconversion intensities with external temperature exhibits a well-fashioned pattern, which suggests that the <sup>2</sup>H <sub>1 ½</sub> and <sup>4</sup>S<sub>3⁄2</sub> levels of Er<sup>3+</sup> ion are thermally coupled. The upconversion mechanism is well explained in terms of phonon-assisted cooperative sensitization. Based on the above upconversion properties the ceramic sample is suggested for near infrared laser based optical imaging and solid state lighting areas.