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"Unlike visible light that can be easily seen by the eye, THz light is challenging to detect," said Gao.

THz waves are electromagnetic waves within the THz range, which falls between the microwave and infrared portions of the electromagnetic spectrum, typically spanning frequencies from 300 gigahertz to 3 THz.

For example, at 4 K, there are effectively no photons that have frequencies larger than 0.2 THz.

Optical properties of metal–dielectric–metal microcavities in the THz frequency range.

The researchers used further THz pulses at various intervals to witness this kind of vibration for the first time.