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One of the most straightforward methods to actively control optical functionalities of metamaterials is to apply mechanical strain deforming the geometries. These deformations, however, leave symmetries and topologies largely intact, limiting the multifunctional horizon. Here, we present topology manipulation of metamaterials fabricated on flexible substrates by mechanically closing/opening embedded nanotrenches of various geometries. When an inner bending is applied on the substrate, the nanotrench closes and the accompanying topological change results in abrupt switching of metamaterial functionalities such as resonance, chirality, and polarization selectivity. Closable nanotrenches can be embedded in metamaterials of broadband spectrum, ranging from visible to microwave. The 99.9% extinction performance is robust, enduring more than a thousand bending cycles. Our work provides a wafer-scale platform for active quantum plasmonics and photonic application of subnanometer phenomena.

Plasmonic gap governs much of the electromagnetic response of metamaterials. Meanwhile, nano and subnanometric gap control achieved by exceptional advancement of nanotechnology has paved the way for quantum plasmonics. However, practical applications have been hindered by difficulties of active nano-control over a broad spectral range. We report on mechanically nano-controllable plasmonic metamaterials fabricated on flexible substrate with a broad spectral response from the visible to the terahertz waves. By closing and opening the metallic nanogap via macroscopic control, we observed both classical and quantum plasmonic responses. Using our devices functioning between the two extreme regimes of classical gaps and full-contact mode, we achieve unprecedented performances of light modulation in a broad spectral range.

We demonstrated a switchable terahertz (THz) resonator by bending it outward with different radius curvatures (RCs), accordingly changing the shape of a diabolo array to a bowtie array. The resonance frequency switched to 0.5 THz and 1.08 THz as the flexible substrate of the resonator was flattened and bent. We analyzed the electrical properties in the I- V curve and optical properties in THz transmission spectra for the cases on flattening/bending/re-flattening after bending the resonator. A switchable THz resonator remains functional even though the operation originates from distorting the metallic array.

Terahertz (THz) nanogap structures have emerged as versatile platforms for THz science and applications by virtue of their strong in-gap field enhancements and accompanying high levels of sensitivity to gap environments. However, despite their potential, reliable fabrication methods by which to create THz structures with sub-10 nm gaps remain limited. In this work, we fabricated THz split-ring resonator (SRR) arrays featuring a sub-10 nm split gap. Our fabrication method, involving photolithography, argon ion milling, and atomic layer deposition, is a high-throughput technique that is also applicable to the fabrication of other THz structures with sub-10 nm gaps. Through THz-time domain spectroscopy and a numerical simulation, we identified the fundamental magnetic resonances of the nanogap SRRs, at which the electric field enhancement factor is experimentally estimated to be around 7000. This substantial field enhancement makes SRRs with a sub-10 nm gap suitable for the study of high-field phenomena and ...

We investigated optical nonlinearity induced by electron tunneling through an insulating vertical gap between metals, both at terahertz frequency and at near-infrared frequency. We adopted graphene and alumina layers as gap materials to form gap widths of 3 A and 1.5 nm, respectively. Transmission measurements show that tunneling-induced transmittance changes from strong fields at the gaps can be observed with relatively weak incident fields at terahertz frequency due to high field enhancement, whereas nonlinearity at the near-infrared frequency is restricted by laser-induced metal damages. Even when the same level of tunneling currents occurs at both frequencies, transmittance in the terahertz regime decreases much faster than that in the near-infrared regime. An equivalent circuit model regarding the tunneling as a resistance component reveals that strong terahertz nonlinearity is due to much smaller displacement currents relative to tunneling currents, also explaining small nonlinearity of the near-inf...

We demonstrate microwave funneling through metallic gaps of nanometer-scale width, corresponding to λ/10 000 000. For achieving both resonant transmission and strong confinement of microwaves, we fabricate two types of samples with an extreme aspect ratio: 300 nm wide, 3.5 mm long slots and sub-10 nm wide rectangular rings with a perimeter of 6.5 mm. Considering the peak transmittance value of 45% and the small coverage ratio of transparent area in the nanogap surface, we can infer a giant intensity enhancement factor of up to 25 million inside the nanogaps. The polarization extinction ratio up to 20 dB indicates that the microwave transmission originates from capacitive coupling of the induced charges at the sidewalls of a metallic gap. We also measure terahertz transmittance and observe a convergence to the microwave range. Our work represents the highest field enhancement recorded for the microwave regime, made possible by wafer-scale-length nanogaps matching the wavelengths, with future applications i...

In pursuit of higher field enhancement and applications in terahertz frequency regime, many techniques have been developed and reported for fabrication of high-aspect-ratio metallic nanostructures. While techniques utilizing spacer deposition has successfully overcome the size limit of conventional fabrication tools, they suffer from low throughput or vulnerability to mechanical and chemical treatment, limiting their further application to various fields. In this Letter we report a high-throughput scheme for fabricating metallic gap structures, free from all the aforementioned shortcomings. Vertically aligned gaps are first defined with photolithography and atomic layer deposition, and then made suitable for transmission measurements by etching out predefined sacrificial layers. Existence of the sacrificial layers alleviates many requirements associated with fabrication steps, thereby increasing the overall reliability of the whole process. Using this method we fabricate arrays of 10 nm wide metallic slits whose length is only limited by the substrate size, here 1 cm, and then characterize the sample with terahertz time domain spectroscopy. The sample show steady performance of up to 2500-fold field enhancement even after sonication under various solvents.

Transmission of Terahertz (THz) electromagnetic wave through a substrate is encumbered because of scattering, multiple reflections, absorption, and Fabry–Perot effects when the wave interacts with the substrate. We present the experimental realization of nonresonant electromagnetic field enhancement by a factor of almost 104 in substrate-free 5-nm gold nanoslits. Our nanoslits yielded greater than 90% normalized electric field transmission in the low-frequency THz region; the slit width was 5 nm, and the gap coverage ratio was 10−4 of the entire membrane, 0.42 mm2. This large field enhancement was attributed to gap plasmons generated by the THz wave, which squeezes the charge cross-section, thus enabling very highly dense oscillating charges and strong THz field transmission from the nanoslits.

A single metallic slit is the simplest plasmonic structure for basic physical understanding of electromagnetic field confinement. By reducing the gap size, the field enhancement is expected to first go up and then go down when the gap width becomes subnanometer because of the quantum tunneling effects. A fundamental question is whether we reach the classical limit of field enhancement before entering the quantum regime, i.e., whether the quantum effects undercut the highest field enhancement classically possible. Here, by performing terahertz time domain spectroscopy on single slits of widths varying from 1.5 nm to 50 \textmu{}m, we show that ultimate field enhancement determined by the wavelength of light and film thickness can be reached before we hit the quantum regime. Our paper paves way toward designing a quantum plasmonic system with maximum control yet without sacrificing the classical field enhancements.

Transmission of Terahertz (THz) electromagnetic wave through a substrate is encumbered because of scattering, multiple reflections, absorption, and Fabry–Perot effects when the wave interacts with the substrate. We present the experimental realization of nonresonant electromagnetic field enhancement by a factor of almost 104 in substrate-free 5-nm gold nanoslits. Our nanoslits yielded greater than 90% normalized electric field transmission in the low-frequency THz region; the slit width was 5 nm, and the gap coverage ratio was 10−4 of the entire membrane, 0.42 mm2. This large field enhancement was attributed to gap plasmons generated by the THz wave, which squeezes the charge cross-section, thus enabling very highly dense oscillating charges and strong THz field transmission from the nanoslits.

We demonstrate electron tunneling-induced colossal nonlinear response of vertically oriented angstrom-metallic gaps by irradiation with intense terahertz pulses. We also investigate the sub-picosecond time evolution of the induced tunneling current extracted from the transmission measurement of light passing through the metallic gap. Our technology of angstrom gap lithography enlarges the boundary of quantum plasmonics down to terahertz frequencies, enabling both linear and nonlinear angstrom optics in a broad spectrum domain. Also, our method has a huge potential of merging the knowledge of conventional dc electronic transport measurements and the study of ultrafast dynamics and nonlinear terahertz spectroscopy.

Quantum tunnelling becomes inevitable as gap dimensions in metal structures approach the atomic length scale, and light passing through these gaps can be used to examine the quantum processes at optical frequencies. Here, we report on the measurement of the tunnelling current through a 3-Å-wide metal-graphene-metal gap using terahertz time-domain spectroscopy. By analysing the waveforms of the incident and transmitted terahertz pulses, we obtain the tunnelling resistivity and the time evolution of the induced current and electric fields in the gap and show that the ratio of the applied voltage to the tunnelling current is constant, i.e., the gap shows ohmic behaviour for the strength of the incident electric field up to 30 kV/cm. We further show that our method can be extended and applied to different types of nanogap tunnel junctions using suitable equivalent RLC circuits for the corresponding structures by taking an array of ring-shaped nanoslots as an example.

Advances in photolithographic processes have allowed semiconductor industries to manufacture smaller and denser chips. As the feature size of integrated circuits becomes smaller, there has been a growing need for a photomask embedded with ever narrower patterns. However, it is challenging for electron beam lithography to obtain

We observed a ninety-seven percent decrease of terahertz transmittance in five-millimeter long, angstrom-sized metallic gaps of copper-graphene-copper hybrid structure. The giant nonlinearity originates from terahertz funneling-induced electron tunneling across the angstrom gap.

We investigate terahertz transmission through nanogap-patterned vanadium dioxide thin film. It is found that the nanogap structure allows the temperature shift of the apparent phase transition described by the normalized terahertz transmission.

Atomic layer deposition is an efficient method for coating a few nanometer-thick alumina over a wafer scale. This method combined with the standard photolithography process was presented to fabricate metallic nanometer gaps that optically act in terahertz regimes. However, the cross-sectional view of the gap shape of the metal–insulator–metal nanogap structure varies depending on the conditions from the stepwise procedure. In specific, selecting photoresist materials, adding ion milling and chemical etching processes, and varying metal thicknesses and substrates result in various optical gap widths and shapes. Since the cross-sectional gap shape affects the field enhancement of the funneled electromagnetic waves via the nanogap, the control of tailoring the gap shape is necessary. Thus, we present five different versions of fabricating quadrangle-ring-shaped nanometer gap arrays with varying different kinds of outcomes. We foresee the usage of the suggested category for specific applications.

Quantum tunneling in plasmonic nanostructures has presented an interesting aspect of incorporating quantum mechanics into classical optics. However, the study has been limited to the subnanometer gap regime. Here, we newly extend quantum plasmonics to gap widths well over 1 nm by taking advantage of the low-frequency terahertz regime. Enhanced electric fields of up to 5 V/nm induce tunneling of electrons in different arrays of ring-shaped nanoslot antennas of gap widths from 1.5 to 10 nm, which lead to a significant nonlinear transmission decrease. These observations are consistent with theoretical calculations considering terahertz-funneling-induced electron tunneling across the gap.

We report that vanadium dioxide films patterned with λ/100000 nanogaps exhibit an anomalous transition behavior at millimeter wavelengths. Most of the hybrid structure's switching actions occur well below the insulator to metal transition temperature, starting from 25 °C, so that the hysteresis curves completely separate themselves from their bare film counterparts. It is found that thermally excited intrinsic carriers are responsible for this behavior by introducing enough loss in the context of the radically modified electromagnetic environment in the vicinity of the nanogaps. This phenomenon newly extends the versatility of insulator to metal transition devices to encompass their semiconductor properties.

We observe microwave funneling through few nanometer-wide gap arrays. Nanogaps having rectangular ring shaped geometries are fabricated by atomic layer lithography technique. Microwave transmittance in 10∼40 GHz range is measured by vector network analyzer (VNA) connected with pairs of rectangular waveguides supporting TE 10 mode. The peak transmittance of 2 nm width gap is about 45% and it corresponds to the electric field enhancement factor of over 10,000. Terahertz transmittance is also measured by time domain spectroscopy to further verify the funneling phenomena in microwave regime.

We present CdSe quantum dots nanogap metamaterial fabrication over large scale, resonant funneling of terahertz waves across 10 nm gap with giant terahertz intensity enhancements and quenching of photoluminescence of QDs inside the gap.

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.

We present a new and versatile technique of self-assembly lithography to fabricate a large scale Cadmium selenide quantum dots-silver nanogap metamaterials. After optical and electron microscopic characterizations of the metamaterials, we performed spatially resolved photoluminescence transmission measurements. We obtained highly quenched photoluminescence spectra compared to those from bare quantum dots film. We then quantified the quenching in terms of an average photoluminescence enhancement factor. A finite difference time domain simulation was performed to understand the role of an electric field enhancement in the nanogap over this quenching. Finally, we interpreted the mechanism of the photoluminescence quenching and proposed fabrication method of new metamaterials using our technique. (C) 2015 Optical Society of America

We study the electric near-field enhancement of a metallic nanogap by far-field transmission measurement in the 0.6–2.3 µm wavelength range. The electric field is resonantly enhanced at the gap and the enhancement factor is quantified experimentally. The resonance condition of field enhancement can be controlled to various wavelengths by changing the gap size, which is confirmed by theoretical calculation using a mode expansion method.