a, The tunable resonant response of an array of metallic split-ring resonators on a thin semiconductor substrate can be exploited as a modulator, letting more or less of an incident radiation beam through. b, When no external voltage is applied, the doped semiconducting substrate conducts. Current circulates around the two lobes of the resonator in response to the applied terahertz field. c, When an external voltage is applied, the semiconductor ceases to conduct. Current can no longer flow through the ring gap; this gap instead behaves as a capacitor, storing up charge. The capacitance gives rise to a resonant behaviour at a frequency in the terahertz range, in which the electrons slosh back and forth between the upper and lower gap electrodes. The incident terahertz radiation drives this resonance efficiently, leading to the transfer of more energy from the beam to the electrons of the substrate. Thus, less radiation is transmitted through the device.