Potassium upconversion violet light generation under two-color two-photon excitation to 4D, 6S level Chenchen Liu, Jinsheng Liu, Baodong Gai, Ying Chen, Jinbo Liu, Jingwei Guo and Shu Hu Eur. Phys. J. D, 76 5 (2022) 88 Published online: 17 May 2022 DOI: 10.1140/epjd/s10053-022-00411-9