We demonstrate an ultrafast reversible modulation of resonant terahertz (THz)response in strongly photoexcited metamaterials. The transientspectral-temporal response of the dipole transition ~1.6 THz exhibits adistinct non-monotonic variation as a function of pump fluence. The transitionenergy shift, strength, spectral width and density-dependent ultrafastrelaxation manifest a remarkable re-emergence of the resonances after initialquenching. Our simulation, incorporating the first-order diffraction from thephotoinduced transient grating, reproduces the salient features, providing anew avenue for designing nonlinear and frequency-agile THz modulators.
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