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One critical factor for bolometer sensitivity is efficient electromagnetic heating of thermistor materials, which plasmonic nanogap structures can provide through the electric field enhancement. In this report, using finite element method simulation, electromagnetic heating of nanorod dimer antennas with a nanogap filled with vanadium dioxide (VO2) was studied for long-wavelength infrared detection. Because VO2 is a thermistor material, the electrical resistance between the two dimer ends depends on the dimer’s temperature. The simulation results show that, due to the high heating ability of the nanogap, the temperature rise is several times higher than expected from the areal coverage. This excellent performance is observed over various nanorod lengths and gap widths, ensuring wavelength tunability and ultrafast operating speed, thereby making the dimer structures a promising candidate for high sensitivity bolometers.

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.

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.

Nanogap slits can operate as a plasmonic Fabry–Perot cavity in the visible and infrared ranges due to the gap plasmon with an increased wavenumber. Although the properties of gap plasmon are highly dependent on the gap width, active width tuning of the plasmonic cavity over the wafer length scale was barely realized. Recently, the fabrication of nanogap slits on a flexible substrate was demonstrated to show that the width can be adjusted by bending the flexible substrate. In this work, by conducting finite element method (FEM) simulation, we investigated the structural deformation of nanogap slit arrays on an outer bent polydimethylsiloxane (PDMS) substrate and the change of the optical properties. We found that the tensile deformation is concentrated in the vicinity of the gap bottom to widen the gap width proportionally to the substrate curvature. The width widening leads to resonance blueshift and field enhancement decrease. Displacement ratio ((width change)/(supporting stage translation)), which was identified to be proportional to the substrate thickness and slit period, is on the order of 10−5 enabling angstrom-scale width control. This low displacement ratio comparable to a mechanically controllable break junction highlights the great potential of nanogap slit structures on a flexible substrate, particularly in quantum plasmonics.

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.

Various material properties change considerably when material is thinned down to nanometer thicknesses. Accordingly, researchers have been trying to obtain homogeneous thin films with nanometer thickness but depositing homogeneous few nanometers thick gold film is challenging as it tends to form islands rather than homogenous film. Recently, studies have revealed that treating the substrate with an organic buffer, (3-mercaptopropyl) trimethoxysilane (MPTMS) enables deposition of ultra-thin gold film having thickness as low as 5 nm. Different aspects of MPTMS treatment for ultra-thin gold films like its effect on the structure and optical properties at visible wavelengths have been investigated. However, the effect of the MPTMS treatment on electrical conductivity of ultra-thin gold film at terahertz frequency remains unexplored. Here, we measure the complex conductivity of nanometer-thick gold films deposited onto an MPTMS-coated silicon substrate using terahertz time-domain spectroscopy. Following the MPTMS treatment of the substrate, the conductivity of the films was found to increase compared to those deposited onto uncoated substrate for gold films having the thickness less than 11 nm. We observed 5-fold enhancement in the conductivity for a 7 nm-thick gold film. We also demonstrate the fabrication of nanoslot-antenna arrays in 8.2-nm-thick gold films. The nanoslot-antenna with MPTMS coating has resonance at around 0.5 THz with an electric field enhancement of 44, whereas the nanoslot-antenna without MPTMS coating does not show resonant properties. Our results demonstrate that gold films deposited onto MPTMS-coated silicon substrates are promising advanced materials for fabricating ultra-thin terahertz plasmonic devices.

A novel plasmonic interferometric sensor intended for application to biochemical sensing has been investigated experimentally and theoretically. The sensor was included a slit surrounded by rectangular grooves using a thick gold film. A three-dimensional finite difference time-domain commercial software package was applied to simulate the structure. The Focused ion beam milling has been used as a mean to fabricate series of rectangular plasmonic interferometer with varying slit-groove distance L. Oscillation behavior is shown by transmission spectra in a broadband wavelength range between 400 nm and 800 nm in the distance between slit and grooves. Red-shifted interference spectrum is the result of increasing refractive indices. The proposed structure is functional from visible to near-infrared wavelength range and yields a sensitivity of 4923 nm/RIU and a figure of merit as high as 214 at 729 nm wavelength. In conclusion, this study indicates the possibility of fabricating a low cost, compact, and real-time high-throughput plasmonic interferometer.

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.

A metallic nano-trench is a unique optical structure capable of ultrasensitive detection of molecules, active modulation as well as potential electrochemical applications. Recently, wet-etching the dielectrics of metal–insulator–metal structures has emerged as a reliable method of creating optically active metallic nano-trenches with a gap width of 10 nm or less, opening a new venue for studying the dynamics of nanoconfined molecules. Yet, the high surface tension of water in the process of drying leaves the nano-trenches vulnerable to collapsing, limiting the achievable width to no less than 5 nm. In this work, we overcome the technical limit and realize metallic nano-trenches with widths as small as 1.5 nm. The critical point drying technique significantly alleviates the stress applied to the gap in the drying process, keeping the ultra-narrow gap from collapsing. Terahertz spectroscopy of the trenches clearly reveals the signature of successful wet etching of the dielectrics without apparent damage to the gap. We expect that our work will enable various optical and electrochemical studies at a few-molecules-thick level.

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...

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.

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 report surface plasmon (SP) lasing in metal/semiconductor nanostructures, where one-dimensional periodic silver slit gratings are placed on top of an InGaAsP layer. The SP nature of the lasing is confirmed from the emission wavelength governed by the grating period, polarization analysis, spatial coherence, and comparison with the linear transmission. The excellent performance of the device as an SP source is demonstrated by its tunable emission in the 400-nm-wide telecom wavelength band at room temperature. We show that the stimulated emission enhanced by the Purcell effect enables successful SP lasing at high energies above the gap energy of the gain. We also discuss the dependence of the lasing efficiency on temperature, grating dimension, and type of metal.

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.

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.

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.

Most semiconductors have surface dynamics radically different from its bulk counterpart due to surface defect, doping level, and symmetry breaking. Because of the technical challenge of direct observation of the surface carrier dynamics, however, experimental studies have been allowed in severely shrunk structures including nanowires, thin films, or quantum wells where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nanoprobing system to investigate the surface dynamics of bulk semiconductors, using metallic nanogap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically decrease close to the limit of THz time resolution (∼1 ps) as the gap size decreases down to nanoscale and that they return to their original values once the nanogap patterns are removed. Our THz nanoprobing system will open up pathways toward direct and nondestructive measurements of surface dynamics of bulk semiconductors.

Metamaterials with desired resonance frequency and high field enhancement are highly demanded in many applications including high harmonic generation, molecular sensing, and non-linear optics. For such purposes, especially sub-wavelength gaps, various parameters of the nano-resonators including gap width, length, period have been optimized, while the effect of height on the performance of the resonators still remains ambiguous. Here, we experimentally investigate resonance frequencies and field enhancements with different metal thicknesses. Scope of this study encompasses thicknesses as small as 10 nm, accessing by far the thinnest terahertz nano-resonators reported in the literature. Thinner sub-wavelength gaps restore theirs resonance frequencies accompanying higher field enhancements.

Most semiconductors have surface dynamics radically different from its bulk counterpart due to the surface state. However, direct observation of the surface carrier dynamics has been limited to nano structures where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nanoscopy to investigate surface dynamics of bulk semiconductors, using metallic nano gap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically as the gap size decreases, and that they return to their bulk values once the nano patterns are removed.

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 show that surface plasmons, excited with femtosecond laser pulses on continuous or discontinuous gold substrates, strongly enhance the generation and emission of ultrashort, broadband terahertz pulses from single layer graphene. Without surface plasmon excitation, for graphene on glass, 'nonresonant laser-pulse-induced photon drag currents' appear to be responsible for the relatively weak emission of both s- and p-polarized terahertz pulses. For graphene on a discontinuous layer of gold, only the emission of the p-polarized terahertz electric field is enhanced, whereas the s-polarized component remains largely unaffected, suggesting the presence of an additional terahertz generation mechanism. We argue that in the latter case, 'surface-plasmon-enhanced optical rectification', made possible by the lack of inversion symmetry at the graphene on gold surface, is responsible for the strongly enhanced emission. The enhancement occurs because the electric field of surface plasmons is localized and enhanced where the graphene is located: at the surface of the metal. We believe that our results point the way to small, thin, and more efficient terahertz photonic devices.

We present a new way for the thermal modulation of terahertz wave through 5nm wide nanogaps by means of the thermal expansion. Resonance amplitude is decreased by 20% and the resonance frequency is red-shifted from 0.45THz to 0.35THz, which is similar to the resonance features of terahertz wave through nanogaps with smaller gap width. The modulation under temperature changes results from the thermal expansion and the extreme width-to-length ratio of the nanogaps resonant at terahertz frequency can compensate the small thermal expansion coefficient, enabling the width of the nanogap to be changed. COMSOL simulation and coupled-mode method calculation confirm the effect of the thermal expansion on the nanostructures.

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 by10(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.

We manufactured an array of three angstrom-wide, five millimeter-long van der Waals gaps of copper-graphene-copper composite, in which unprecedented nonlinearity was observed. To probe and manipulate van der Waals gaps with long wavelength electromagnetic waves such as terahertz waves, one is required to fabricate vertically oriented van der Waals gaps sandwiched between two metal planes with an infinite length in the sense of being much larger than any of the wavelengths used. By comparison with the simple vertical stacking of metal-graphene-metal structure, in our structure, background signals are completely blocked enabling all the light to squeeze through the gap without any strays. When the angstrom-sized van der Waals gaps are irradiated with intense terahertz pulses, the transient voltage across the gap reaches up to 5 V with saturation, sufficiently strong to deform the quantum barrier of angstrom gaps. The large transient potential difference across the gap facilitates electron tunneling through the quantum barrier, blocking terahertz waves completely. This negative feedback of electron tunneling leads to colossal nonlinear optical response, a 97% decrease in the normalized transmittance. Our technology for infinitely long van der Waals gaps can be utilized for other atomically thin materials than single layer graphene, enabling linear and nonlinear angstrom optics in a broad spectral range.

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 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.

We demonstrate adiabatic nanofocusing of few-cycle light pulses using ultrasharp and ultrasmooth single-crystalline gold tapers. We show that the grating-induced launching of spectrally broad-band surface plasmon polariton wavepackets onto the shaft of such a taper generates isolated, point-like light spots with 10 fs duration and 10 nm diameter spatial extent at its very apex. This nanofocusing is so efficient that nanolocalized electric fields inducing strong optical nonlinearities at the tip end are reached with conventional high repetition rate laser oscillators. We use here the resulting second harmonic to fully characterize the time structure of the localized electric field in frequency-resolved interferometric autocorrelation measurements. Our results strongly suggest that these nanometer-sized ultrafast light spots will enable new experiments probing the dynamics of optical excitations of individual metallic, semiconducting, and magnetic nanostructures.

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.