Study on transmission enhancement of air-adsorbed graphene by terahertz spectroscopy

Graphene, made of carbon atoms arranged in a honeycomb lattice, has already attracted intense research and commercial interest in recent years. Early research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The unique properties of graphene have also attracted various researches on carrier dynamics using THz spectroscopy. Here we present an experimental demonstration of monolayer CVD grown graphene via THz time-domain spectroscopy, as well as optical pump terahertz probe system. We observe that the maximum transmittance of the graphene is nearly 96% compared to the ambient signal. However, under the excitation of different optical pump fluences, it is found that unlike the semiconductor material, its transmitted amplitude is enhanced accordingly. We observed a wide-band modulation of the terahertz transmission at the range of 0.3-1.6 THz and a large modulation depth of 16.4% with a certain optical excitation. We attributed it to suppression of the air-adsorbed graphene photoconductivity due to an increase in the carrier scattering rate induced by the increase in the free-carrier concentration by photoexcitation. The obtained results not only highlight the influence of air conditions on how THz characterizations would guide the design and fabrication of graphene-based terahertz modulators and optoelectronic devices, but also show that graphene exhibits the potential for terahertz broadband transmission enhancement with photoexcitation.