August 02, 2023

Structural origin of anisotropy in the luminescence of ultraviolet laser crystals

SOKENDAI Student Dispatch Program program year: 2022

UY Mayrene Allam

Materials Structure Science

Local restructuring in a LiCaAlF6 crystal doped with 1.0mol% cerium ion

When a cerium ion substitutes the calcium ion in LiCaAlF6 crystal, the trigonal octahedron environment of calcium is maintained but the bond distance between cerium and fluorine is stretched. This substitution should be accompanied by a charge compensator due to the imbalance of charge brought about by substituting calcium (which has a 2+ charge) with cerium (which has a 3+ charge). In this case,charge compensation is assumed to be via a remote lithium vacancy.

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.

Period of Stay

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

Country and/or City

Osaka, Japan

Visiting Institute, Host, or Meeting

Osaka University Institute of Laser Engineering

Title of your Presentation

What you learned and achieved during the visit

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.

Department of Materials Structure Science,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.