[Show abstract][Hide abstract] ABSTRACT: We have investigated the extraordinary optical transmission of terahertz waves through an array of nanogaps with varying dimensions and periodicities, and used this platform to demonstrate terahertz sensing of a thin film of single-walled carbon nanotubes. We have used atomic layer lithography to fabricate periodic arrays of nanogap loops that have a gap size of 2 nm and a loop length of 100 μm (aspect ratio of 50 000). These sub-mm-scale loops of nanogaps can sustain terahertz electromagnetic resonances along the contour. We have characterized the transmission of terahertz waves through the nanogap arrays and investigated the influence of inter-gap electromagnetic coupling as the array periodicity shrinks from 100 μm to 4 μm. While the gaps occupy only 0.1% of the surface area, we have measured an amplitude (|E|) transmittance of over 50% due to the strong and broadband field enhancement inside the nanogaps. The absolute transmission through the 2 nm gaps along the rectangular loops can be boosted up to 25%, while it is only 1% for annular gaps with the same perimeter. Furthermore, the extremely tight field confinement and strong field enhancement near the 2 nm gap lead to 43% extinction of THz waves in a 10 nm-thick film of single-walled carbon nanotubes over the gaps. On the other hand, THz extinction by the same nanotube film on a bare glass substrate is only 2%. These nanogaps pave the way toward developing sensitive terahertz detectors for biological and chemical targets.
Full-text · Article · Mar 2015 · Faraday Discussions
[Show abstract][Hide abstract] ABSTRACT: 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/106) dielectric overlayer. We use atomic layer lithography to fabricate a wafer-scale array of annular nanogap arrays. THz time-domain spectroscopy is used to monitor transmission spectra through these samples in conjunction with sequential depositions of 1-nm-thick Al2O3 overlayers. Using sub-10-nm annular aperture arrays, we can unambiguously measure THz resonance shifts due to a 1-nm-thick Al2O3 film. Because of the enormous mismatch in length scales between THz waves (millimeter-scale wavelength) and sub-10-nm gaps, conventional modeling techniques cannot be used to analyze our experiments. We employ an advanced finite-element-modeling (FEM) technique - Hybridizable Discontinuous Galerkin (HDG) scheme - for full three-dimensional modeling of resonant transmission of THz waves (millimeter-scale wavelength) 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.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate broadband non-resonant squeezing of terahertz (THz) waves through an isolated 2-nm-wide, 2-cm-long slit (aspect ratio of 10(7)), representing a maximum intensity enhancement factor of one million. Unlike resonant nanogap structures, a single, effectively infinitely-long slit passes incident electromagnetic waves with no cutoff, enhances the electric field within the gap with a broad 1/f spectral response, and eliminates interference effects due to finite sample boundaries and adjacent elements. To construct such a uniform, isolated slit that is much longer than the millimeter-scale spot of a THz beam, we use atomic layer lithography to pattern vertical nanogaps in a metal film over an entire 4-inch wafer. We observe an increasing field enhancement as the slit width decreases from 20 nm to 2 nm, in agreement with numerical calculations.
Full-text · Article · Oct 2014 · Scientific Reports
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We study terahertz transmission through nano-patterned vanadium dioxide thin
film. It is found that the patterning allows the lowering of the apparent
transition temperature. For the case of the smallest width nano antennas, the
transition temperature is lower by as many as ten degrees relative to the bare
film, so that the nano patterned hysteresis curves completely separate
themselves from their bare film counterparts. This early transition comes from
the one order of magnitude enhanced effective dielectric constants by nano
antennas. This phenomenon opens up the possibility of transition temperature
[Show abstract][Hide abstract] ABSTRACT: We show that sub-skin depth nano metallic objects embedded in terahertz slot antennas strongly modulate their resonance features. Our findings have fundamental implications for sensitive nanoparticle detections using millimeter wavelength light.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.