@misc{indiciae75737dd06b60,
  title = {Lasing without or with inversion in a closed four-level system.},
  author = {Y Zhu},
  year = {1993},
  doi = {10.1103/physreva.47.495},
  url = {https://pubmed.ncbi.nlm.nih.gov/9908941/},
  note = {Source identifier: 9908941}
}