Kwang Jun Ahn

Atomic layer lithography of wafer-scale nanogap arrays for extreme confinement of electromagnetic waves

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 background-free transmission measurements. We observe resonant transmission of near-infrared waves through 1.1-nm-wide gaps (λ/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 (λ/4,000,000) and infer an unprecedented field enhancement factor of 25,000.

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 >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.

Electrical control of terahertz nano antennas on VO2 thin film

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.

Controlling Terahertz Radiation with Nanoscale Metal Barriers Embedded in Nano Slot Antennas

Nanoscale metallic barriers embedded in terahertz (THz) slot antennas are shown to provide unprecedented control of the transition state arising at the crossover between the full- and half-wavelength resonant modes of such antennas. We demonstrate strong near-field coupling between two paired THz slot antennas separated by a 5 nm wide nanobarrier, almost fully inducing the shift to the resonance of the double-length slot antenna. This increases by a factor of 50 the length-scale needed to observe similar coupling strengths in conventional air-gap antennas (around 0.1 nm), making the transition state readily accessible to experiment. Our measurements are in good agreement with a quantitative theoretical modeling, which also provides a simple physical picture of our observations.

Plasmon Enhanced Terahertz Emission from Single Layer Graphene

Emission of terahertz radiation is observed when a single layer graphene on a glass slide is excited with femtosecond near-infrared laser pulses. Furthermore, we can enhance the amplitude of terahertz emission by surface plasmon resonance excitation on a thin gold film in the Kretschmann geometry.

Selective enhanced resonances of two asymmetric terahertz nano resonators.

We studied the electromagnetic interaction between two asymmetric terahertz nano resonators, rectangular holes which have a few hundred micron lengths but nanoscale widths. We report that the dominant resonant transmission of the structures can be modulated by the horizontal distance between two rectangles due to the different oscillation strength of the asymmetric coupling at two different resonance frequencies. Our results are significant for an optimum design of rectangular holes in terahertz frequency regime for applications such as sensitive nanoparticle detection and terahertz filters.

Terahertz pinch harmonics enabled by single nano rods

A pinch harmonic (or guitar harmonic) is a musical note produced by lightly pressing the thumb of the picking hand upon the string immediately after it is picked [J. Chem. Educ. 84, 1287 (2007)]. This technique turns off the fundamental and all overtones except those with a node at that location. Here we present a terahertz analogue of pinch harmonics, whereby a metallic nano rod placed at a harmonic node on a terahertz nanoresonator suppresses the fundamental mode, making the higher harmonics dominant. Strikingly, a skin depth-wide nano rod placed at the mid-point turns off all resonances. Our work demonstrates that terahertz electromagnetic waves can be tailored by nanoparticles strategically positioned, paving important path towards terahertz switching and detection applications.

Enhanced in and out-coupling of InGaAs slab waveguides by periodic metal slit arrays

We studied the in- and the out-coupling efficiencies of photons with a thin InGaAs slab covered by periodic gold nano-slit arrays, by measuring transmission and photoluminescence (PL) spectra. While the maximum in-coupled photons into the InGaAs slab waveguide were found at dip positions in transmission spectra, the mostly out-coupled photons were observed as peaks in PL spectra. For different periods of slit arrays and incident angles we discussed spectral positions of transmission dips and efficiency of the in-coupling influenced by the absorption coefficient of InGaAs. In PL spectra we measured overall enhanced PL intensities from the InGaAs slab covered by slit arrays compared to that of a bare InGaAs, where the peak positions are determined by the period of slit arrays as well. Our findings are important for designing semiconductors both as an optically passive waveguide and active light emitter.

Sub-wavelength measurements of the near-field of a metal aperture

We present results on THz near-field measurements of sub-wavelength apertures in metal films. Our results allow us to test, with unprecedented detail, the near-field predictions made by the well-known Bethe-Bouwkamp model.