Toroidal moment is an electromagnetic excitation that lies outside the familiar picture of electric and magnetic multipoles. It has recently been a topic of intense research in the fields of nanophotonics and metamaterials due to its weakly radiating nature and its ability to confine electromagnetic energy. Among extensive studies on toroidal moments and their applications, high quality factor (Q) toroidal resonances have been experimentally realized only in a very limited set of geometries and wavelengths. In this study, we demonstrate that a metasurface consisting of arrays of hollow dielectric cuboids supports a high Q-factor resonances at near-infrared and visible wavelengths due to the destructive interference between toroidal dipoles and magnetic quadrupoles. Using silicon as the high index dielectric, an experimental Q-factor of 728 is realized at a wavelength of 1505 nm, which is one of the highest values reported in the near-infrared using a dielectric metasurface. Importantly, our resonator geometry enables very efficient coupling of the toroidal resonance to the environment. This makes our metasurface design useful for refractometric sensing, where we measure a sensitivity of 161 nm per refractive index unit with a line width of 2.01 nm, efficiently distinguishing an index change of less than 0.02. We also find that a metasurface made of a relatively low-index dielectric, titanium dioxide (n < 2.4), is also capable of supporting the same toroidal mode with an observed Q-factor of 160 at visible wavelengths. With the versatility and robustness that dielectric metasurfaces provide, toroidal resonances are expected to be a powerful tool for investigating strong light–matter interaction and nonlinear phenomena at the nanoscale.

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 related applications.

In pursuit of higher field enhancement and applications in terahertz frequency regime, many techniques have been developed and reported for fabrication of high-aspectratio 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.Keywords Nano-slit . Fabrication . Terahertz spectroscopy . Terahertz nanotechnology . Field enhancement Metallic structures with nanometer-size gaps are effective in strong electric field enhancement, due to capacitive coupling between surface charges of the two metallic sides [1][2][3]. In terahertz frequencies, where metals can safely be treated as perfect electric conductors [4], the gaps can incorporate even higher field enhancement factor and find applications in molecule sensing [5], nonlinear phenomena [6], and etc.[3] For a gap to operate efficiently in terahertz frequencies, however, the lateral size of the structure must exceed several millimeters with the width remaining in nanometer scale. Such large-area, high-resolution patterning is

The ideals of reconfigurable metasurfaces would be operation in a broad frequency range with a high extinction ratio and fatigue resistivity. In this paper, all the above is achieved in the microwave regime by transforming a bare metallic film into well‐controlled nanometer sized gaps in a fully reversible manner. It is shown that adjacent metallic patterns deposited at different times can form “zero‐nanometer gaps,” or “zerogaps,” while maintaining the optical and electrical connectivity. The zerogaps readily open and recover with gentle bending and relaxing of the flexible substrate, precisely along the rims of the pre‐patterns of centimeter lengths. In a prototypical pattern of densely packed slit arrays, these gaps when opened serve as antennas achieving transparency for polarizations perpendicular to the length of the gap and shut off all the incident lights when closed. In such transformation between a polarizer and a mirror, 75% of transmission is observed with polarization extinction ratio of 7500 coming back down to 5 orders of magnitude extinction repeatable over 10 000 times. This work has long‐standing implications to metamaterials and metasurfaces as well as the fundamental aspect of extending a picometer scale distance controllability toward the wafer scale.

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.

Strong demand for plasmonic devices with an enormously enhanced electric field and desired resonance frequencies has led to extensive investigations of metallic slot structures. While strong field enhancement can be achieved by reducing the width of the slot, the effect of the gap surface plasmon limits the maximum achievable field enhancement at higher frequencies. Specifically, the effect of the gap surface plasmon becomes stronger as the gap width decreases and strongly suppresses the transmission while causing a red-shift of the resonance. Here, we overcome these issues and realize strong field enhancements at higher frequencies, by managing the metal thickness of the nanoslots. We show that, as the nanoslots become as thin as 10 nm, they show a giant electric field enhancement of up to 7600. Moreover, the resonances are strongly blue-shifted to above 1 THz from 0.33 THz. Our work provides a novel route to achieving high field enhancements at desired frequencies, as well as a means by which to characterize the slot as the gap-sensitive or substrate-sensitive type.

Slot-type nanogaps have been widely utilized in transmission geometry because of their advantages of exclusive light funneling and exact quantification of near-field enhancement at the gap. For further application of the nanogaps in electromagnetic interactions with various target materials, complementary studies on both transmission and reflection properties of the nanogaps are necessary. Here, we observe an anomalous extinction of terahertz waves interacting with rectangular ring-shaped sub-30 nm wide gaps. Substrate works as an index matching layer for the nanogaps, leading to a stronger field enhancement and increased nonlinearity at the gap under substrate-side illumination. This effect is expressed in reflection as a larger dip at the resonance, caused by destructive interference of the diffracted field from the gap with the reflected beam from the metal. The resulting extinction at the resonance is larger than 60% of the incident power, even without any absorbing material in the whole nanogap structure. The extinction even decreases in the presence of an absorbing medium on top of the nanogaps, suggesting that transmission and reflection from nanogaps might not necessarily represent the absorption of the whole structure.

Ohmic absorption of light is an indication of a light−matter interaction within metals, where many interesting phenomena and application potentials can be found. To realize the ohmic absorption of light at long wavelengths, where metals are highly reflective, one can use a metamaterial absorber design to concentrate the electromagnetic field within a thin metal film. This concept has enabled thinning of perfect absorbers from a quarter-wave thickness to several tens of nanometers, greatly improving the utility and efficiency of light−metal interactions. Further improvements on the performance are expected if the absorption can be additionally focused laterally, which is a possibility not yet explored. In this study, we report that nanoslot antennas can be a unique ohmic absorber of the low-frequency radiations, where it can incorporate 70% of incident light to ohmic absorption, focused laterally onto 1% of the unit cell area. The inductive field that drives both field enhancement and ohmic absorption is localized within a skin depth distance from the slots with amplitude being as large as 30% of the incident field. Mode-matching calculations and terahertz spectroscopy measurements confirm the inductive and localized nature of the absorption. The strong confinement of the inductive field and of the resulting ohmic absorption is expected to open a new venue in nanocalorimetry, optical nonlinearities of metals, and bolometer applications.