Dipole Response of Spaser on an External Optical Wave
  We find the conditions upon the amplitude and frequency of an external electromagnetic field at which the dipole moment of a Bergman-Stockman spaser oscillates in antiphase with the field. For these values of the amplitude and frequency the losses in metal nanoparticles is exactly compensated of by gain. This shows that spasers may be used as inclusions in designing lossless metamaterials. 
E. S. Andrianov, A.A. Pukhov, A.V. Dorofeenko,2 A.P. Vinogradov, and   A.A. Lisyansky
cond-mat.mes-hall