Loss compensation by spasers in metamaterials
  We show that exact loss compensation can be achieved in active metamaterials containing spasers pumped over a wide range of pumping values both below and above the spasing threshold. We demonstrate that the difference between spaser operation below and above spasing threshold vanishes, when the spaser is synchronized by an external field. As the spasing threshold loses its significance, a new pumping threshold, the threshold of loss compensation, arises. Below this threshold, which is smaller than the spasing threshold, compensation is impossible at any frequency of the external field. 
E. S. Andrianov, D. G. Baranov, A. A. Pukhov, A. V. Dorofeenko, A. P.   Vinogradov, and A. A. Lisyansky
cond-mat.mes-hall physics.optics