Squeezing light through nanometre-wide gaps in metals can lead to extreme field enhancements, nonlocal electromagnetic effects and light-induced electron tunnelling. This intriguing regime, however, has not been readily accessible to experimentalists because of the lack of reliable technology to fabricate uniform nanogaps with atomic-scale resolution and high throughput. Here we introduce a new patterning technology based on atomic layer deposition and simple adhesive-tape-based planarization. Using this method, we create vertically oriented gaps in opaque metal films along the entire contour of a millimetre-sized pattern, with gap widths as narrow as 9.9 Å, and pack 150,000 such devices on a 4-inch wafer. Electromagnetic waves pass exclusively through the nanogaps, enabling backgroundfree transmission measurements. We observe resonant transmission of near-infrared waves through 1.1-nm-wide gaps (l/1,295) and measure an effective refractive index of 17.8. We also observe resonant transmission of millimetre waves through 1.1-nm-wide gaps (l/4,000,000) and infer an unprecedented field enhancement factor of 25,000.

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 >10(3) 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 mm(2), 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.

We experimentally show that terahertz (THz) waves confined in sub-10-nm metallic gaps can detect refractive index changes caused by only a 1-nm-thick (~λ/10 6 ) dielectric overlayer. We use atomic layer lithography to fabricate a wafer-scale array of annular nanogaps. Using THz time-domain spectroscopy in conjunction with atomic layer deposition, we measure spectral shifts of a THz resonance peak with increasing Al 2 O 3 film thickness in 1 nm intervals. Because of the enormous mismatch in length scales between THz waves and sub-10-nm gaps, conventional modeling techniques cannot be used to analyze our results. We employ an advanced finite-element-modeling (FEM) technique -Hybridizable Discontinuous Galerkin (HDG) scheme -for full three-dimensional modeling of the resonant transmission of THz waves through an annular gap that is 2 nm in width and 32 µm in diameter. Our multi-scale 3D FEM technique and atomic layer lithography will enable a series of new investigations in THz nanophotonics that has not been possible before.

Utilizing highly oriented multiwalled carbon nanotube aerogel sheets, we fabricated micrometer-thick freestanding carbon nanotube (CNT) polarizers. Simple winding of nanotube sheets on a U-shaped polyethylene reel enabled rapid and reliable polarizer fabrication, bypassing lithography or chemical etching processes. With the remarkable extinction ratio reaching ∼37 dB in the broad spectral range from 0.1 to 2.0 THz, combined with the extraordinary gravimetric mechanical strength of CNTs, and the dispersionless character of freestanding sheets, the commercialization prospects for our CNT terahertz polarizers appear attractive.

We demonstrate an active metamaterial device that allows to electrically control terahertz transmission over more than one order of magnitude. Our device consists of a lithographically defined gold nano antenna array fabricated on a thin film of vanadium dioxide (VO(2)), a material that possesses an insulator to metal transition. The nano antennas let terahertz (THz) radiation funnel through when the VO(2) film is in the insulating state. By applying a dc-bias voltage through our device, the VO(2) becomes metallic. This electrically shorts the antennas and therefore switches off the transmission in two distinct regimes: reversible and irreversible switching.