Reversible modulation and ultrafast dynamics of THz resonances in   strongly photoexcited metamaterials
  We demonstrate an ultrafast reversible modulation of resonant terahertz (THz) response in strongly photoexcited metamaterials. The transient spectral-temporal response of the dipole transition ~1.6 THz exhibits a distinct non-monotonic variation as a function of pump fluence. The transition energy shift, strength, spectral width and density-dependent ultrafast relaxation manifest a remarkable re-emergence of the resonances after initial quenching. Our simulation, incorporating the first-order diffraction from the photoinduced transient grating, reproduces the salient features, providing a new avenue for designing nonlinear and frequency-agile THz modulators. 
I. Chatzakis, L. Luo, J. Wang, N-H. Shen, T. Koschny, J. Zhou, and C.   M. Soukoulis
cond-mat.mtrl-sci cond-mat.mes-hall physics.optics