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Active control properties and coupling effect in the terahertz metamaterials

We modulate the electromagnetic response of terahertz metamaterials by exciting photogenerated carriers with ultra-short pulsed laser, and realize the active control of terahertz transmission and resonances in a wide band. The experimental results show that when the terahertz electric vector is perpendicular and parallel to the bar direction with the open gap in the circular split ring resonators, respectively, the transmission of the non-resonant region and resonance absorption are significantly tuned by the irradiated pump pulse. With the increase of the pump power, the resonance absorption peak decreases and the frequency of resonance dip has a remarkable blue-shift. Meanwhile, the transmission of the non-resonant region decreases correspondingly. It is also found that the blue-shift of the resonance is mainly determined by the change of conductivity and dielectric constant in the photoexcited layer of the substrate. Through the numerical simulation, we have further proved the introduction of pump light has a significant modulation effect on the electromagnetic properties of terahertz metamaterials. Additionally, it can be observed that the LC resonance annihilates earlier than the resonance of the dipole resonance with the increase of pump power, implying the former is particularly sensitive to the variation of the active control. In the theoretical analysis, based on the Lorenz oscillator model, we have derived and calculated the resonance response intensity of the coupled oscillators under an external field. The calculated results indicate the coupling coefficient and damping rate have an impact on the spectra evolution, showing that the resonant response peak has a remarkable blue-shift with the increase of coupling coefficient and the resonance response intensity decreases with the increase of damping rate.