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Cracks are formed along the photolithographically pre-determined lines with extremely high yield and repeatability, when Cu clusters are introduced between planarized Au thin films sequentially deposited on a PET substrate. These clusters act as nanometer-sized spacers preventing the formation of contiguous metallic bond between the adjacent Au layers which will render prepatterned-cracking impossible. While the effective gap width is initially zero in the optical sense from microwaves all the way to the visible, outer-bending the PET substrate allows the gap width tuning into the 100 nm range, with the stability and controllability in the ranges of 100 s and Angstrom-scale, respectively. It is anticipated that our wafer-scale prepatterned crack technology with an unprecedented mixture of macroscopic length and Angstrom-scale controllability will open-up many applications in optoelectronics, quantum photonics and photocatalysis.

Metallic nanogaps are being widely used for sensing applications, owing to their ability to confine and enhance electromagnetic field within the hot spots. Since the enhanced field does not confine itself perfectly within the gap, however, fringe fields well away from the gap are of potential use as well in real systems. Here, we extend the concept of near field absorption enhancement by quantitatively analyzing terahertz absorption behavior of water molecules outside the hot spots of sub-20 nm-wide, ∼100 μm-long nanotrenches. Contrary to point-gaps which show negligible field enhancement at distances larger than the gap width, our extended nanogap act as a line source, incorporating significant amount of absorption enhancement at much longer distances. We observe absorption enhancement factors of up to 3600 on top of a 5 nm-wide gap, and still well over 300 at 15 nm away. The finding is well supported by theoretical analyses including modal expansion calculations, Kirchhoff integral formalism and antenna theory. Our results provide means to quantitatively analyze light-matter interactions beyond the hot spot picture and enable application of nanogaps for sensitive surface analyses of various material systems.

A well-designed narrow gap between noble metal nanostructures plays a prominent role in surface-enhanced Raman scattering (SERS) to concentrate electromagnetic fields at the local point, called a "hot spot". However, SERS-active substrate fabrication remains a substantial hurdle due to the high process cost and the difficulty of engineering efficient plasmonic hot spots at the target area. In this study, we demonstrate a simple photolithographic method for generating ultrasensitive SERS hot spots at desired positions. The solid-state dewetting of a Ag thin film (thickness of ∼10 nm) using a continuous-wave laser (∼1 MW/cm

With recent advances in nanofabrication technology, various metallic gap structures with gap widths reaching a few to sub-nanometer, and even ‘zero-nanometer’, have been realized. At such regime, metallic gaps not only exhibit strong electromagnetic field confinement and enhancement, but also incorporate various quantum phenomena in a macroscopic scale, finding applications in ultrasensitive detection using nanosystems, enhancement of light–matter interactions in low-dimensional materials, and ultralow-power manipulation of electromagnetic waves, etc. Therefore, moving beyond nanometer to ‘zero-nanometer’ can greatly diversify applications of metallic gaps and may open the field of dynamic ‘gaptronics.’ In this paper, an overview is given on wafer-scale metallic gap structures down to zero-nanometer gap width limit. Theoretical description of metallic gaps from sub-10 to zero-nanometer limit, various wafer-scale fabrication methods and their applications are presented. With such versatility and broadband applicability spanning visible to terahertz and even microwaves, the field of ‘gaptronics’ can be a central building block for photochemistry, quantum optical devices, and 5/6G communications.

Absorbers for long-wavelength infrared (LWIR) are designed to have a reduced geometry fitted to a gold cross antenna and numerically studied. Compared to the square membrane geometry widely used in conventional microbolometers, the reduced geometry results in smaller thermal capacities of the vanadium dioxide (VO2) and silicon nitride (Si3N4) layers. However, near-field focusing by the cross antenna leads to a high LWIR absorption. Calculations show that the temperature change per incident energy increases with a decrease in the arm width, and the reduced absorber surpasses the square geometry for all incident angles and polarizations. The antenna-based reduced absorber studied here could serve as an alternative geometry for high-performance microbolometers.

Momentum space topology can be exploited to manipulate radiation in real space. Here we demonstrate topological control of 2D perovskite emission in the strong coupling regime via polaritonic bound states in the continuum (BICs). Topological polarization singularities (polarization vortices and circularly polarized eigenstates) are observed at room temperature by measuring the Stokes parameters of photoluminescence in momentum space. Particularly, in symmetry-broken structures, a very large degree of circular polarization (DCP) of ∼0.835 is achieved in the perovskite emission, which is the largest in perovskite materials to our knowledge. In the strong coupling regime, lower polariton modes shift to the low-loss spectral region, resulting in strong emission enhancement and large DCP. Our reciprocity analysis reveals that DCP is limited by material absorption at the emission wavelength. Polaritonic BICs based on 2D perovskite materials combine unique topological features with exceptional material properties and may become a promising platform for active nanophotonic devices.

Electromagnetic absorbers based on ultra-thin metallic film are desirable for many applications such as plasmonics, metamaterials, and long-wavelength detectors. A metallic film will achieve a maximum 50% of electromagnetic wave absorption, frequency independent, at a thickness defined by its conductivity, typically in the sub-Angstrom range for good metals if bulk conductivity is maintained throughout. This makes it extremely difficult to obtain substantial absorption from thin metal films, in contrast to 2D materials such as graphene. Luckily, however, from a practical point of view, metal conductivity is drastically reduced as the film becomes sub-100 nm, to make it a race between the thinnest possible metal thickness experimentally achievable vs the conductivity reduction. Here, we demonstrate a near-50% absorption at a gold film thickness of 6.5 nm, with conductivity much reduced from the bulk value, down to the range of 106 Siemens per meter. Studying the effect of the substrate thickness, we found that the common cover glass, with its thickness much smaller than the wavelength, achieves symmetric absorption of 44%, implying that a pseudo-free-standing limit is achieved. Our work may find applications in infrared sensing as in bolometers and biomedical sensing using microwaves.

Metal nanostructures supporting plasmonic resonances offer pronounced modifications of an electromagnetic environment for efficient light harvesting in solar energy conversion applications. Since t...

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.

We demonstrated that an incident terahertz peak field amplitude below 0.01 MV/cm can trigger Zener tunneling in a semi-insulating GaAs. Moreover, a transmission decrease with an extinction ratio of 60% was observed in the semi-insulating GaAs with an electric field strength of up to 46 MV/cm (maximum incident peak field of ∼0.29 MV/cm). These experimental results were realized by taking advantage of the nonlinear effects, such as Zener tunneling, impact ionization, and metal–insulator–metal tunneling in 5 nm metallic nanogaps on the GaAs; a strong field was locally confined in the vicinity of these gaps. The 5 nm gap enabled us to lower the voltage across the gap to suppress impact ionization while allowing Zener tunneling. Simulation results indicated that the effective thickness of the semiconductor increased as a function of the gap size. The approach used in this study decreases the threshold incident electric field for nonlinear responses as well as paves the way toward ultrathin high-speed electronic devices and ultrafast light pumps.

In this work, we demonstrated a phonon-polariton in the terahertz (THz) frequency range, generated in a crystallized lead halide perovskite film coated on metamaterials. When the metamaterial resonance was in tune with the phonon resonance of the perovskite film, Rabi splitting occurred due to the strong coupling between the resonances. The Rabi splitting energy was about 1.1 meV, which is larger than the metamaterial and phonon resonance line widths; the interaction potential estimation confirmed that the strong coupling regime was reached successfully. We were able to tune the polaritonic branches by varying the metamaterial resonance, thereby obtaining the dispersion curve with a clear anticrossing behavior. Additionally, we performed

Terahertz (THz) nanoantennas have significant potential for versatile applications in THz spectroscopy because of their capability for strong electromagnetic field localization. Electron-beam lithography or focused ion beam machining is typically employed to fabricate nanoantenna structures. These nanolithography methods present limitations in the widespread utilization of THz nanoantennas because of their high cost and low productivity. In this work, we proposed graphene-based crack lithography as a high throughput fabrication method for nanoantenna structures. A double-layer graphene interface was introduced to enable independent control of the nanoantenna dimensions and provide graphene-based nanoantenna structures. We analyzed the underlying mechanism of graphene-based cracking and developed an analytical model governing the geometric parameters of the fabricated nanostructures. As a vital application of the fabricated nanoantenna structures, we demonstrated the highly sensitive detection of d -Glucose molecules. Graphene-based crack lithography can provide a cost-effective method for generating nanoantenna structures with the desired characteristics and can accelerate the development of practical applications of electromagnetic metamaterials.

A pair of parallel metallic plates with nanometer-scale separations, or a ‘metallic nanotrench’, creates strongly enhanced electric field with uniform spatial distribution when a long wavelength radiation is incident. This property is not only useful for quantitative analysis of light-matter interactions, but also for potential electrochemical studies on nanoconfined molecules. Here, we show our progress on realizing sub-10 nm-wide metallic nanotrenches filled with various liquids to study interaction of nano-confined molecules with terahertz radiation. Large height-to-width aspect ratio and strong field enhancement of the nanotrenches enable sensitive detection of the nano-confined molecules, from which optical properties of the molecules can be determined. We demonstrate fabrication of the nanotrenches with widths as small as 2 nm, and study changes in their terahertz optical properties upon integration with various liquids. Also, we discuss anomalous optical properties of water molecules confined within sub-10 nm-wide metallic nanotrenches.

Metallic nanostructures play an essential role in electromagnetic manipulations due to the localization and enhancement of electromagnetic waves in nanogaps. Scaling down the dimensions of the gap, such as the gap width and the thickness, is an effective way to enhance light-matter interaction with colossal field enhancement. However, reducing the thickness below 10 nanometers still suffers from fabrication difficulty and unintended direct transmission through metals. Here, we fabricate effective-zero-thickness slot antennas by stepping metals in the vicinity of the gaps to confine electromagnetic waves in tiny volumes. We analyze and simulate terahertz transmission, and demonstrate the absorption enhancement of molecules in the slot antennas. Our fabrication technique provides a simple but versatile tool for maximum field enhancement and molecular sensing.

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.

Critical factors for terahertz polarizers include broadband operation, high transmittance, and a good extinction ratio. In this paper, using a 5 nm-wide metallic slit array with a 200 nm periodicity as a wire grid polarizer, we achieved over 95% transmittance with an average extinction ratio of 40 dB, over the entire spectrum as defined by the terahertz time-domain spectroscopy (0.4 ∼ 2 THz). Theoretical calculations revealed that the slit array can show 100% transmission up to 5 THz, and wider bandwidths with a higher cutoff frequency can be achieved by reducing the slit periodicity. These results provide a novel approach for achieving a broadband THz polarizer and open a new path for seamless integration of the polarizers with nanophotonic applications.

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.

In the terahertz (THz) electromagnetic wave regime, which has recently received great attention in the fields of communication and security, shielding of THz waves is a significant issue. Therefore, carbon-based nanostructures or polymer-carbon nanocomposites have been widely explored. Herein, significantly enhanced THz shielding efficiency is reported for silver nanowires coated with reduced graphene oxide (rGO) and nanoscale THz metamaterials, as compared to the cases without nanoscale metamaterials. Using a nanoslot-patterned metamaterial with strong resonances at certain frequencies, THz transmission in intensity is enhanced up to three orders of magnitude. Enhanced transmission by nanopatterns substantially increases the shielding performance to the external THz waves, even for ultrathin films (several tens of nanometers) produced by a simple spray of rGO (a few nm of flakes) on a complex random nanowire network. Excellent shielding performance is presented and the shielding mechanism is investigated by the nanoprobing configuration at the same time.

Metallic nanostructures have important roles in electromagnetic field manipulations due to the localization of electromagnetic waves, for which fabrication techniques in nanotechnology have been developed. To achieve higher field enhancement, it is required to decrease the dimensions of the nanostructures such as thickness or width of the gap. Here, we fabricated the slant nano-antennas where a portion of metals on each side is overlapped. The slant nano-antennas realize higher electromagnetic field enhancement while keeping background noise almost zero.

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.

Copper is a low-cost material compared to silver and gold, having high reflectivity in the near infrared spectral range as well as good electrical and thermal conductivity. Its properties make it a good candidate for metal-based low-cost multilayer thin-film devices and optical components. However, its high reflectance in the devices is reduced because copper is easily oxidized. Here, we suggest a copper-based Fabry-Perot optical filter consisting of a thin dielectric layer stacked between two copper films, which can realize low-cost production compared to a conventional silver-based etalon filter. The reduced performance due to the inherent oxidation of the copper surface can be overcome by passivating the copper films with monolayer graphene. The anti-oxidation of copper film is investigated by optical microscopy, x-ray photoelectron spectroscopy, and transmission measurement in UV-vi spectral ranges. Our results show that the graphene coating can be expanded for various metal-based optical devices in terms of anti-corrosion.

We demonstrate photoexcited carrier response of metallic nanogap-patterned semiconductor using optical pump-terahertz probe spectroscopy. Metallic nanogap facilitates observing surface carrier dyna...

Understanding light interaction with metallic structures provides opportunities of manipulation of light, and is at the core of various research areas including terahertz (THz) optics from which diverse applications are now emerging. For instance, THz waves take full advantage of the interaction to have strong field enhancement that compensates their relatively low photon energy. As the THz field enhancement have boosted THz nonlinear studies and relevant applications, further understanding of light interaction with metallic structures is essential for advanced manipulation of light that will bring about subsequent development of THz optics. In this review, we discuss THz wave interaction with deep sub-wavelength nano structures. With focusing on the THz field enhancement by nano structures, we review fundamentals of giant field enhancement that emerges from non-resonant and resonant interactions of THz waves with nano structures in both sub- and super- skin-depth thicknesses. From that, we introduce surprisingly simple description of the field enhancement valid over many orders of magnitudes of conductivity of metal as well as many orders of magnitudes of the metal thickness. We also discuss THz interaction with structures in angstrom scale, by reviewing plasmonic quantum effect and electron tunneling with consequent nonlinear behaviors. Finally, as applications of THz interaction with nano structures, we introduce new types of THz molecule sensors, exhibiting ultrasensitive and highly selective functionalities.

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 charact...

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 ...

Split-ring resonator (SRR) based metamaterials have been studied for the development of highly sensitive, small-sized, low-power chemical and biomolecular sensors. However, the anisotropic behavior arising from their two-dimensional (2D) structure presents substantial challenges leading to ambiguity in their transmission spectra. In this paper, we present the design of a three-dimensional (3D) isotropic octagram split-ring resonator (OSRR) demonstrating a three-dimensionally coupled resonance behavior that overcomes the anisotropic response of conventional 2D SRRs, leading to a strong, distortion-free, and polarization-invariant transmission response. The OSRR undergoes 3D coupling through the splits at the corners of the 3D structure (cube), which remains invariant under any polarization along the coordinate axes. The strong coupling between resonant segments provides the OSRR with 25 times higher sensitivity than the corresponding 2D structure, allowing the resonant frequency to be reliably monitored fo...

We perform an analytical study on the allowance of forbidden transitions for a hydrogen atom placed near line dipole sources, mimicking light emanating from a one-dimensional metallic nanogap. It is shown that the rapid variation of the electric field vector, inevitable in the near zone, completely breaks the selection rule of Δl=±1. While the forbidden transitions between spherically symmetric S states, such as 2S to 1S or 3S to 1S (Δl=0), are rather robust against selection rule breakage, Δl=±2 transitions such as between 3D and 1S or 3D and 2S states are very vulnerable to the spatial variation of the perturbing electric field. Transitions between 2S and 3D states are enhanced by many orders of magnitude, aided by the quadratic nature of both the perturbing Hamiltonian and D wavefunctions. The forbidden dipole moment, which approaches one Bohr radius times the electric charge in the vicinity of the gap, can be written in a simple closed form owing to the one-dimensional nature of our gap. With large enough effective volume together with the symmetric nature of the excited state wavefunctions, our work paves way towards atomic physics application of infinitely long nanogaps.

We take advantage of the recent advances in terahertz-nano technology to study quantum scale light-matter interaction. Terahertz waves can be squeezed down to extreme aspect ratio nanogaps which are composed of metal-insulator-metal barriers. Noble metals such as gold or silver can serve as good conductors at this terahertz frequencies, and the electric field intensity inside the metallic nanogaps can be orders of magnitudes larger than the incident one. Cross sections of molecules can be hugely enhanced and the probing depth decrease dramatically. As the gap size decreases down to the nanometer regime, quantum mechanical effects such as electron tunneling across the nanogaps are almost inevitable, rendering different dielectric constants to the gap material than that without tunneling. These efforts originated from the nearly perfect transmission through terahertz slot antennas with tens of microns of feature sizes, together with its nanometer-sized counterparts. In this work, we will discuss our recent results of extreme terahertz phenomena on plasmonic nanoantenna structures. On the one hand, we demonstrate ultrafast control of tunneling currents using macroscopic loops of terahertz antenna. Light-field induced surface currents projects upon the barrier loops fabricated on a metallic film, spatiotemporally changing the local electric potentials. The total tunneling currents flowing through the loops are critically affected by the symmetry of the loop, enabling ultrafast full-wave rectification of electromagnetic waves in sub-picosecond scale. On the other hand, we demonstrate our terahertz nanoresonator can support nearly up to 70 % absorption of incident terahertz radiation by direct Ohmic loss in metal at this long wavelength limit, breaking the good conductor approximation which is generally considered in terahertz frequency.

Arrays of van der Waals gaps were manufactured by synthesizing the vertically aligned graphene layer stacked between two copper (Cu) catalytic films. The Cu-graphene-Cu laminated structure was obtained by directly synthesizing graphene on a patterned Cu film followed by depositing a second copper layer for optical measurements. The synthesis of graphene on the Cu surface was optimized by adjusting the synthesis temperatures and pre-annealing time using plasma enhanced chemical vapor deposition (PECVD). Resonant Raman spectroscopy measurements reveal that graphene can be synthesized on both bulk Cu foil and relatively thin Cu film under the same growth mechanism using PECVD. Structural and optical characterizations of the array of graphene van der Waals gaps were implemented by the transmission electron microscope (TEM) and terahertz-time domain spectroscopy (THz-TDS). In THz-TDS, the measured THz amplitude transmitted through the graphene van der Waals gap slit array was constant regardless of the gap width determined by the number of graphene layers between the Cu thin films in a single slit. These results imply that the optical dielectric constant of graphene at THz frequencies in the out-of-plane direction is linearly proportional to the gap width. Our results of the manufacturing method can be adopted to investigate mechanical, electrical, and optical properties of other 2D materials such as h-BN, MoS2, and others. Furthermore, metal-graphene-metal structures with vertical orientations can be used in many electronic, optic, and optoelectronic applications.

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...

Surface plasmon photonics (“plasmonics”) is an expanding field at the frontiers of optical science and engineering, concerned with the interaction of light with metallic structures. Surface plasmons are coupled electromagnetic/charge-density waves propagating along metal-dielectric interfaces or localized at metal nanostructures. Light suitable for exciting surface plasmons is typically within or near the visible but may extend into the infrared and ultraviolet regions. Metallic structures that support surface plasmons are highly varied, including planar arrangements of metal films, stripes or grooves, metal gratings, and metal nanoparticles such as islands, spheres, rods, or antenna-inspired structures. Surface plasmons can be localized at subwavelength scales, and, for example, are involved in optical transmission through one or several subwavelength holes in a metal film, in what is now referred to as “extraordinary optical transmission.” Surface plasmons are intimately involved in the response of “metamaterials” and “metasurfaces” constructed from deep subwavelength metallic features, producing esoteric macroscopic properties such as a negative refractive index, or a permittivity/permeability near zero.

Terahertz waves are focused onto extreme aspect ratio slot antennas. In the middle of the nano slot antenna, the electric field intensity can be a million times stronger than the incident one. Thereby, at a large enough incident field, tunneling of electrons are inevitable [1]–[3]. These efforts originated from nearly perfect transmission through terahertz slot antennas with tens of microns of feature sizes, which evolved into its nanometer sized and Angstrom sized cousins [4]–[5]. We will discuss our recent results on tunneling through closed, macroscopic rings of quantum barriers. Surface current driven tunneling provides a new paradigm compared with the conventional voltage driven tunneling, being ultrasensitive to the global 2D geometry and its symmetry. Macroscopic geometries can be turned into another useful tool to govern total tunneling currents.

In this work, we show a ring-shaped metallic nanogap exposed to electromagnetic fields to manipulate ultrafast tunneling electrons. Eddy currents induced by incoming terahertz pulses project upon the ring, changing the local potentials in vectorial way. Contour-integrated along the perimeter of the ring, the total tunneling currents are critically dependent upon the ring shape. Furthermore, the spatiotemporal dynamics of the fully-rectified terahertz pulses can be visualized by the femtosecond optical pulses.

Plasmon-mediated polymerization has been intensively studied for various applications including nanolithography, near-field mapping, and selective functionalization. However, these studies have been limited from the near-infrared to the ultraviolet regime. Here, we report a resist polymerization using intense terahertz pulses and various nanoantennas. The resist is polymerized near the nanoantennas, where giant field enhancement occurs. We experimentally show that the physical origin of the cross-linking is a terahertz electron emission from the nanoantenna, rather than multiphoton absorption. Our work extends nano-photochemistry into the terahertz frequencies.

Tunneling is the most fundamental quantum mechanical phenomenon with wide-ranging applications. Matter waves such as electrons in solids can tunnel through a one-dimensional potential barrier, e.g. an insulating layer sandwiched between conductors. A general approach to control tunneling currents is to apply voltage across the barrier. Here, we form closed loops of tunneling barriers exposed to external optical control to manipulate ultrafast tunneling electrons. Eddy currents induced by incoming electromagnetic pulses project upon the ring, spatiotemporally changing the local potential. The total tunneling current which is determined by the sum of contributions from all the parts along the perimeter is critically dependent upon the symmetry of the loop and the polarization of the incident fields, enabling full-wave rectification of terahertz pulses. By introducing global geometry and local operation to current-driven circuitry, our work provides a novel platform for ultrafast optoelectronics, macroscopic quantum phenomena, energy harvesting, and multi-functional quantum devices., Many experiments have been performed that study rectification by exciting electrons to tunnel across a single quantum barrier. Here, the authors take a 2-dimensional approach by combining 2D closed-loop tunneling barriers with broken symmetry to enable geometrically controllable rectification of THz fields.

An electromagnon in the magnetoelectric (ME) hexaferrite ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{2.5}{\mathrm{Co}}_{2}{\mathrm{Fe}}_{24}{\mathrm{O}}_{41}$ (${\mathrm{Co}}_{2}\mathrm{Z}$-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on the electric field (${E}^{\ensuremath{\omega}}$) of the THz light parallel to the $c$ axis ($\ensuremath{\parallel}\text{ }[001]$), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes $\ensuremath{-}8.7%$ and $+5.8%$ under external magnetic field ($H$) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from antiphase motion of spins which are coupled with modulating electric dipoles through the exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of the Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different $H$.

We take advantage of recent advances in nano technology to study quantum scale light matter interaction. Terahertz waves are squeezed into extreme aspect ratio slot antennas. In the middle of a nano slot antennas, the electric field intensity can be a million times stronger than the incident one. Thereby, cross sections of molecules can be hugely enhanced [1] while the probing depth decreases [2]. Quantum tunneling of electrons through nanometer and Angstrom sized gaps are inevitable, rendering different dielectric constants compared with bulk counterparts [3-5]. These efforts originated from nearly perfect transmission through terahertz slot antennas with tens of microns of feature sizes, together with its nanometer sized counterparts [6, 7]. We will discuss our recent results on tunneling through closed, macroscopic rings of quantum barriers. Surface current driven tunneling provides a new paradigm compared with the conventional voltage driven tunneling, being ultrasensitive to the global geometry and symmetry of the planar geometry. Macroscopic geometries can be used to govern total tunneling currents, to be used to sensing as well.

We report UV emission from a ZnO nanorod (NR) highly enhanced by coupling of excitons to cavity-enhanced second harmonic generation (SHG). A Ag cavity concentrates the exciting field into the center of the NR, producing an enhanced second harmonic (SH) response through the nonlinearity of ZnO, that spatially overlaps with an ensemble of excitons. Strong coupling to the excitons at room temperature is achieved when the SH response is spectrally tuned to the exciton transition through the NR diameter, yielding total SHG enhancements of 2 orders of magnitude. Our approach highlights a unique and simple platform for enhancing coherent, nonlinear light–matter interactions in a size regime that is challenging for plasmonic architectures, yet important for optoelectronics.

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.