title: Spectral evidence for multi-pathway contribution to the upconversion pathway in NaYF4:Yb3+,Er3+phosphors.
abstract: Although upconversion phosphors have been widely used in nanomedicine, laser engineering, bioimaging, and solar cell technology, the upconversion luminescence mechanism of the phosphors has been fiercely debated. A comprehensive understanding of upconversion photophysics has been significantly impeded because the number of photons incorporated in the process in different competitive pathways could not be resolved. Few convincing results to estimate the contribution of each of the two-, three-, and four-photon channels of near-infrared (NIR) energy have been reported in yielding upconverted visible luminescence. In this study, we present the energy upconversion process occurring in NaYF4:Yb3+,Er3+phosphors as a function of excitation frequency and power density. We investigated the upconversion mechanism of lanthanide phosphors by comparing UV/VIS one-photon excitation spectra and NIR multi-photon spectra. A detailed analysis of minor transitions in one-photon spectra and luminescence decay enables us to assign electronic origins of individual bands in multi-photon upconversion luminescence and provides characteristic transitions representing the corresponding upconversion channel. Furthermore, we estimated the quantitative contribution of multiple channels with respect to irradiation power and excitation energy.
authors.affiliation: Department of Chemistry, Kookmin University, 77, Jeongneung-ro, Seongbuk-gu, Seoul, 02707, Republic of Korea. crpark@kookmin.ac.kr hyungkim@kookmin.ac.kr and Sensor System Research Center, Korea Institute of Science and Technology (KIST), 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul, 02792, Republic of Korea.
authors.forename: Youngho
authors.initials: Y
authors.lastname: Cho
authors.affiliation: Department of Chemistry, Kookmin University, 77, Jeongneung-ro, Seongbuk-gu, Seoul, 02707, Republic of Korea. crpark@kookmin.ac.kr hyungkim@kookmin.ac.kr.
authors.forename: Si Won
authors.initials: SW
authors.lastname: Song
authors.affiliation: Department of Chemistry, Kookmin University, 77, Jeongneung-ro, Seongbuk-gu, Seoul, 02707, Republic of Korea. crpark@kookmin.ac.kr hyungkim@kookmin.ac.kr.
authors.forename: Soo Yeong
authors.initials: SY
authors.lastname: Lim
authors.affiliation: Sensor System Research Center, Korea Institute of Science and Technology (KIST), 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul, 02792, Republic of Korea.
authors.forename: Jae Hun
authors.initials: JH
authors.lastname: Kim
authors.affiliation: Department of Chemistry, Kookmin University, 77, Jeongneung-ro, Seongbuk-gu, Seoul, 02707, Republic of Korea. crpark@kookmin.ac.kr hyungkim@kookmin.ac.kr.
authors.forename: Chan Ryang
authors.initials: CR
authors.lastname: Park
authors.affiliation: Department of Chemistry, Kookmin University, 77, Jeongneung-ro, Seongbuk-gu, Seoul, 02707, Republic of Korea. crpark@kookmin.ac.kr hyungkim@kookmin.ac.kr.
authors.forename: Hyung Min
authors.identifier: http://orcid.org/0000-0002-7400-5336
authors.initials: HM
authors.lastname: Kim