Colossal Absorption of Molecules Inside Single Terahertz Nanoantennas

Molecules have extremely small absorption cross sections in the terahertz range even under resonant conditions, which severely limit their detectability, often requiring tens of milligrams. We demonstrate that nanoantennas tailored for the terahertz range resolves the small molecular cross section problem. The extremely asymmetric electromagnetic environment inside the slot antenna, which finds the electric field being enhanced by thousand times with the magnetic field changed little, forces the molecular cross section to be enhanced by >10³ accompanied by a colossal absorption coefficient of ∼170 000 cm^–1. Tens of nanograms of small molecules such as 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and lactose drop-cast over an area of 10 mm², with only tens of femtograms of molecules inside the single nanoslot, can readily be detected. Our work enables terahertz sensing of chemical and biological molecules in ultrasmall quantities

Plasmon Enhanced Terahertz Emission from Single Layer Graphene

We show that surface plasmons, excited with femtosecond laser pulses on continuous or discontinuous gold substrates, strongly enhance the generation and emission of ultrashort, broadband terahertz pulses from single layer graphene. Without surface plasmon excitation, for graphene on glass, ‘<i>nonresonant laser-pulse-induced photon drag currents</i>’ appear to be responsible for the relatively weak emission of both <i>s</i>- and <i>p</i>-polarized terahertz pulses. For graphene on a discontinuous layer of gold, only the emission of the <i>p</i>-polarized terahertz electric field is enhanced, whereas the <i>s</i>-polarized component remains largely unaffected, suggesting the presence of an additional terahertz generation mechanism. We argue that in the latter case, ‘<i>surface-plasmon-enhanced optical rectification</i>’, made possible by the lack of inversion symmetry at the graphene on gold surface, is responsible for the strongly enhanced emission. The enhancement occurs because the electric field of surface plasmons is localized and enhanced where the graphene is located: at the surface of the metal. We believe that our results point the way to small, thin, and more efficient terahertz photonic devices