Electromagnetic Saturation of Angstrom-Sized Quantum Barriers at Terahertz Frequencies

Abstract

Metal-graphene-metal hybrid structures allow angstrom-scale van der Waals gaps, across which electron tunneling occurs. We squeeze terahertz electromagnetic waves through these λ/10 000 000 gaps, accompanied by giant field enhancements. Unprecedented transmission reduction of 97% is achieved with the transient voltage across the gap saturating at 5 V. Electron tunneling facilitated by the transient electric field strongly modifies the gap index, starting a self-limiting process related to the barrier height. Our work enables greater interplay between classical optics and quantum tunneling, and provides optical indices to the van der Waals gaps.

Anomalous extinction in index-matched terahertz nanogaps

“…With the aid of field enhancement, the local field at the gap reaches ~10 V/nm to create tunneling current density -the amount of which is determined by the gap width, barrier height, and applied voltage. The generated free carrier density absorbs THz waves, expressing itself as an effective imaginary permittivity of the gap material, aluminum oxide [20,21,33]. The consequent decrease in transmission can, therefore, function as a direct probe for the local field enhancement at the metallic nanogaps [22].…”

“…Among them negative slot or slit antenna structures possess the advantage of backgroundfree exclusive electromagnetic funneling when operated in transmission geometry, and is, therefore, capable of experimentally determining the near-field enhancement at the gap [19]. Such properties are advantageous in applications where quantitative analyses are highly required, such as in nonlinear experiments [10,[20][21][22] or when realizing molecular absorption or scattering enhancement [23][24][25]. Physical origins of the observed phenomena are mostly interpreted in terms of changes in transmission spectra, while the reflection counterpart, an excellent additional source of information, is usually not considered in the transmission type nanogaps.…”

“…Extraordinary transmission and field enhancement of THz waves in nanogaps have been theoretically studied [26] and experimentally realized, recently at gap sizes down to 1 nm [27] and below [20]. We especially focus on the effect of illumination side, from the substrate side or the air side, for wider applications in future research.…”

Selection rule engineering of forbidden transitions of a hydrogen atom near a nanogap

“…While the spatial variation of electromagnetic waves in free space occurs within the wavelength scale, close to the induced sources such as surface current and surface charges which naturally occur in metallic nano objects, electric field vectors can vary in length scale much smaller than their vacuum wavelength, in the length scale of the nano objects themselves or the gap size between the metallic objects [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Of particular interest in the present paper is the one-dimensional metallic nano-and subnanogaps whose widths can be in the 1-0.1 nm regime [17][18][19], comparable to the spatial extents of hydrogen atom wavefunctions while maintaining a macroscopic length of 1 mm to 1 cm.…”

“…Of particular interest in the present paper is the one-dimensional metallic nano-and subnanogaps whose widths can be in the 1-0.1 nm regime [17][18][19], comparable to the spatial extents of hydrogen atom wavefunctions while maintaining a macroscopic length of 1 mm to 1 cm. Electric fields emanating from these gaps possess rapidly varying electric fields, both in magnitude and in direction, in the length scale of the gap itself, creating a potentially very useful field configuration for the purpose of breaking down well-known selection rules, thereby facilitating forbidden transitions in large enough volumes to be experimentally detectable.…”