Concentration of Er3+ ion effect on optical bistability and multistability in Er3+: YAG crystal Seyyed Hossein Asadpour and H. Rahimpour Soleimani Eur. Phys. J. Plus, 129 5 (2014) 77 Published online: 02 May 2014 DOI: 10.1140/epjp/i2014-14077-3