TY  - RPRT
TI  - Lasing without or with inversion in a closed four-level system.
AU  - Y Zhu
PY  - 1993
DO  - 10.1103/physreva.47.495
UR  - https://pubmed.ncbi.nlm.nih.gov/9908941/
ID  - 9908941
ER  -