2023.08.02

Structural origin of anisotropy in the luminescence of ultraviolet laser crystals

Ultraviolet (UV) lasers are lasers that generate ultraviolet light. These lasers are widely used due to their high output power and high precision in micromachining,UV Raman spectroscopy, disinfection for the inactivation of pathogens, etc. Traditional UV lasers such as argon laser (gas-based) have been exceptionally expensive and bulky. But advances in UV laser technology nowadays have resulted in smaller and less expensive lasers with the use of a solid crystal doped with particular rare earth elements. Many existing oxide and fluoride host crystals can be tailored for compact UV laser systems. However, their laser performance is often degraded by the existence of the so-called excited state absorption (ESA). In ESA, electrons that are promoted to a state of higher energy (also termed excited state) are prevented from returning back to a state of lower energy. Instead, they are further promoted to a much higher energy state in the conduction band of the host crystal. As a consequence, they do not contribute to the generation of UV light. Fortunately, lithium calcium aluminum fluoride (LiCaAlF6, LiCAF) crystal doped with cerium ion has been found to be less susceptible to ESA. Thus, by far, it is the most prominent and excellent material for compact UV laser. I joined the group of Professor Toshihiko Shimizu of the Osaka University Institute of Laser Engineering to carry out an in-depth investigation on the optical properties of the cerium-doped LiCAF crystal. Prof. Shimizu is an expert in optical spectroscopies and laser materials as well as advance equipment used in characterizing crystals. Optical properties of cerium-doped crystals relate to how the crystals absorb and emit UV light and it depend on the local environment of the cerium ion in the crystal. The absorption and emission spectra of cerium-doped LiCAF crystal in the UV region have been observed to exhibit anisotropy. In this regard, we complemented our optical experiments with X-ray absorption spectroscopy to obtain information on the changes in the valence state of the cerium ion and its coordination environment. Our results confirm that cerium substitutes at the calcium site of LiCAF crystal. Cerium maintains its original 3+ oxidation state and the trigonal octahedron environment of calcium is retained as well. However, the bond distances between cerium and fluorine are stretched. We also found that coupling interaction exists between cerium and its neighbors, fluorine and aluminum ions which are stacked along the c-axis of the crystal. This leads to anisotropy in the optical properties of the crystal and likewise to ESA. These results demonstrate the influence of cerium ionʼs local environment on the optical properties of LiCAF crystal. Charge compensation still ensues but it is assumed to be via a remote lithium vacancy.

派遣先滞在期間

Date of Departure: 2022/09/26
Date of Return: 2023/02/28

国、都市等

Osaka, Japan

機関名、受入先、会議名等

Osaka University Institute of Laser Engineering

‍派遣中に学んだことや得られたもの

Through the Dispatch Program, I have received rigorous trainings on the use of various advanced spectroscopy and laser systems. It also provided me with the opportunity to further hone my collaboration skills in research with experts in the field of optics and lasers in and outside Osaka University.

高エネルギー加速器科学研究科 物質構造科学専攻　UY Mayrene Allam

I am Mayrene Allam Uy, a third year PhD student at the Department of Materials Structure Science. I am studying cerium-doped fluoride and oxide crystals for UV scintillator and laser applications using combined optical and x-ray spectroscopies.