Subwavelength Region Research Articles

Large‐Area Metal Gaps and Their Optical Applications

AbstractRecent technological advances in fabrication methods have allowed researchers to manipulate light–matter interactions in the subwavelength region and develop a wide variety of innovative optical applications from the visible to the microwave region. Metal patterning at a subwavelength scale plays a crucial role in realizing these optical applications. Various standard lithography techniques including laser beam machining, focused ion beam, photolithography, and electron‐beam lithography are used for the subwavelength feature size of the metal patterns. Many recent studies have demonstrated that funneling light into nanometer‐wide gaps in metals gives rise to strong field enhancements and nonlocal electromagnetic effects. However, these standard methods encounter difficulties when one tries to fabricate nanometer feature sizes with macroscopic circumferences, crucial for long‐wavelength applications, over a large area. Here, new lithography techniques that fabricate an array of metal gaps of nanometer‐to‐Ångstrom scale are covered. The corresponding photonic applications in the terahertz and microwave regions are also introduced. These next‐generation metal gaps will have a great impact on the advancement of field enhancement and confinement toward the next level of applications such as metamaterials, quantum tunneling, active switching devices, and ultrasensitive chemical/biological sensors.

Topological interface modes in local resonant acoustic systems

Topological phononic crystals (PCs) are periodic artificial structures which can support nontrivial acoustic topological bands, and their topological properties are linked to the existence of topological edge modes. Most previous studies focused on the topological edge modes in Bragg gaps which are induced by lattice scatterings. While local resonant gaps would be of great use in subwavelength control of acoustic waves, whether it is possible to achieve topological interface states in local resonant gaps is a question. In this article, we study the topological bands near local resonant gaps in a time-reversal symmetric acoustic systems and elaborate the evolution of band structure using a spring-mass model. Our acoustic structure can produce three band gaps in subwavelength region: one originates from local resonance of unit cell and the other two stem from band folding. It is found that the topological interface states can only exist in the band folding induced band gaps but never appear in the local resonant band gap. The numerical simulation perfectly agrees with theoretical results. Our study provides an approach of localizing the subwavelength acoustic wave.

Design and Optimization of a Graphene Modulator Based on Hybrid Plasmonic Waveguide with Double Low-Index Slots

Graphene modulators based on surface plasmonic waveguides enable a strong interaction between light and graphene because great electric field enhancement occurs in the sub-wavelength region. However, a tight field confinement will cause a large metal absorption of light. Thus, graphene modulator base on hybrid plasmonic waveguide has a tradeoff between the propagation loss and the modulation depth. Here, we achieved a good balance between them by designing and optimizing an electro-optic modulator based on hybrid plasmonic waveguide with four graphene layers. The structure of the waveguide is metal/insulator/Si/insulator/metal. The modulation depth and the propagation loss of the modulator are 0.524 and 0.05 dB/μm respectively, which make a relatively high figure of merit about 10.5. Also the obtained modulation bandwidth and power consumption are 150 GHz and 0.607 pJ/bit, respectively.

Plasmonic Hot-Carriers in Channel-Coupled Nanogap Structure for Metal–Semiconductor Barrier Modulation and Spectral-Selective Plasmonic Monitoring

Plasmonic hot-carriers, which are induced by plasmons at metal surfaces, can be used to convert photon energy into excited carriers over a subwavelength region and provide a new means to realize photodetection within the sub-band-gap region of semiconductor materials. However, the barrier height of the metal–semiconductor junction affects the behavior of the plasmon-induced hot-carriers and limits the electrical response of photodetection. High electrical responsivity, achieved by manipulating the barrier height using plasmon-induced hot electrons, is desired to broaden the possible applications. Here we report a plasmonic channel-coupled nanogap structure, where the barrier height of the metal–semiconductor junction is altered upon the excitation of plasmon-induced hot-carriers. The structure consists of semiconductor channels and metal slabs forming nanogaps, which sustain coupled plasmons and confine light to the semiconductor–metal interfaces. In contrast to conventional Schottky barriers and ohmic co...

Mimicking plasmonic nanolaser emission by selective extraction of electromagnetic near-field from photonic microcavity.

Plasmonic nanolasers have attracted significant attention owing to their ability to generate a coherent optical field in the deep subwavelength region, and they exhibit promising applications in integrated photonics, bioimaging and sensing. However, the demonstration of lasing in individual metallic nanoparticles with 3D subwavelength confinement represents a significant challenge and is yet to be realized. Herein, we propose to mimic a plasmonic nanolaser via selective scattering off the evanescent tail of a lasing photonic nanobelt using a single silver nanorod (24 nm × 223 nm). The nanorod acts as an optical antenna that selectively extracts the near-field component along the rod axis. The light output from the silver nanorod mimics the emission of a plasmonic nanolaser in its localized near-field and polarization dependence, except for the lasing wavelength and linewidth, which are inherited from the photonic laser. The realization of localized coherent light sources provides promising nanoscale lighting that shows potential in background-suppressed illumination, biosensing and imaging.

Enhancing Upconversion Fluorescence with a Natural Bio-microlens.

Upconversion fluorescence has triggered extensive efforts in the past decade because of its superior physicochemical features and great potential in biomedical and biophotonic studies. However, practical applications of upconversion fluorescence are often hindered by its relatively low luminescence efficiency (<1%). Here, we employ a living yeast or human cell as a natural bio-microlens to enhance the upconversion fluorescence. The natural bio-microlens, which was stably trapped on a fiber probe, could concentrate the excitation light into a subwavelength region so that the upconversion fluorescence of core-shell NaYF4:Yb3+/Tm3+ nanoparticles was enhanced by 2 orders of magnitude. As a benefit of the fluorescence enhancement, single-cell imaging and real-time detection of the labeled pathogenic bacteria, such as Escherichia coli and Staphylococcus aureus, were successfully achieved in the dark fields. This biocompatible, sensitive, and miniature approach could provide a promising powerful tool for biological imaging, biophotonic sensing, and single-cell analysis.

Experimental Demonstration of Tunable Directional Scattering of Visible Light from All-Dielectric Asymmetric Dimers

Due to the presence of strong magnetic resonances, high refractive index dielectric nanoantennnas have shown the ability to expand the methods available for controlling electromagnetic waves in the subwavelength region. In this work, we experimentally demonstrate that an asymmetric dielectric dimer made of silicon can lead to highly directional scattering depending on the excitation wavelength, due to the interference of the excited magnetic resonances. A back focal plane imaging system combined with a prism coupling technique enables us to explore the scattering profile parallel to the substrate. The directivity of scattering along the substrate is high enough to produce selective guiding of light along the substrate. These results showing tunable control of directional scattering will encourage the realization of novel optical applications, such as optical nanocircuitry.

Phononic crystal plate with hollow pillars connected by thin bars

A new type of phononic crystal plate consisting of hollow pillars on a bar-connected plate is proposed. With respect to usual pillar based phononic crystal plates, the Bragg band gap can be tuned to be much wider and extended to a sub-wavelength region, and the low frequency gap can be moved to an extremely low frequency range. Such a structure can generate quadrapolar, hexapolar and octopolar whispering-gallery modes (WGMs) inside the band gaps with very high confinement and quality factors. By filling the hollow pillars with a liquid, these WGMs, together with additional localized compressional and solid–liquid coupling modes, can be tuned either by varying the inner radius of the pillars or controlling the height of the liquid. We discuss some possible functionalities of these phononic crystals for the purpose of sensing the acoustic properties of liquids, multiplexer and wireless communication.

Anomalous reflection from metasurfaces with gradient phase distribution below 2π

Metasurfaces are artificial structures that have been demonstrated to possess the ability to manipulate light within a subwavelength spatial region. Here, we explore another unraised functionality of the energy redistribution of a metasurface by tuning the phase difference over a supercell. We also propose a practical nanorod-based design to achieve an anomalous steering reflection using the finite element method simulation. The proposed phenomena have potential applications in ultracompact nanophotonic systems and high-efficiency flat devices.

Ultra-Subwavelength and Low Loss in V-Shaped Hybrid Plasmonic Waveguide

A new kind of hybrid plasmonic waveguide is proposed, and its propagation properties are investigated using the finite-element method. This waveguide consists of a V-shaped silver nanowire embedded in a low-index dielectric cladding above a semiconductor substrate, which can confine light in the subwavelength region with a long propagation length. The field distribution, the mode effective index, the propagation length, and the normalized mode area of the hybrid mode supported by the waveguide are investigated at the wavelength of 1550 nm, which are dependent on the geometric parameters.

Experiments on localized wireless power transmission using a magneto-inductive wave two-dimensional metamaterial cavity

In this letter, we propose a magneto-inductive wave (MIW) metamaterial cavity for enhanced mid-range wireless power transfer (WPT) applications. Cavity operation is achieved by controlling the propagation of MIWs at lower megahertz frequencies. The cavity is realized by omitting a cell and thereby breaking the periodicity of the closely coupled metamaterial slabs. The cavity in the proposed metamaterial effectively confines the MIWs into a subwavelength region. Consequently, it localizes the magnetic field in the WPT region and provides enhanced power transmission. When the proposed MIW metamaterial cavity is used, the measured efficiency improves significantly from 8.7 to 54.9%.

The physical understanding on dynamic readout/detection of super-resolution pits with nonlinear reverse saturation absorption thin films

The physical mechanism and understanding behind dynamic readout/detection of super-resolution pits with a nonlinear reverse-saturation absorption active layer, such as an InSb active layer, is presented on the basis of experimental results of open-aperture z-scan measurements and pump–probe transient time response analysis. The super-resolution of an InSb active layer is a result of the formation of a sub-wavelength scatterer region at the center of the focused spot. The frequency response function also verifies that the cutoff frequency with an InSb active layer is clearly extended compared to when an InSb active layer is not used. The findings are useful for understanding the physical process of the far-field super-resolution effect with nonlinear reverse-saturation absorption characteristics.

Quasi-3D plasmonic coupling scheme for near-field optical lithography and imaging.

Near-field optical imaging and lithography rely on achieving both high resolution and efficient coupling. Particularly conventional near-field scanning optical microscopy (NSOM) suffers from the trade-off between resolution and efficiency. Planar plasmonic lens schemes can partially solve this issue utilizing plasmonic resonances, but the performance is not robust over a large range of sample materials. In this work we show a novel quasi-3D plasmonic scheme to focus light into the extreme subwavelength region in the near field with an efficiency orders higher than NSOM. The superb performance comes from the strong coupling between the localized mode with an off-plane E-field component and the sample being processed. Our scheme can efficiently focus light to a spot with a diameter down to 1/20 of its wavelength, and the coupling efficiency can be as high as 10%. Theoretically, we demonstrate that the FWHM of the focus spot can be 7nm with an enhancement of about 800 at the UV region. The focusing performance is constantly good over a large variety of materials and the illumination and collection imaging scheme has been demonstrated by simulation. An example design of this quasi-3D coupling scheme is fabricated and its imaging performance is characterized by the apertureless optical near-field measurement. The high coupling efficiency at extreme subwavelength resolution of this quasi-3D coupling scheme opens the door to many applications, such as optical lithography, nanoscale imaging, heat-assisted magnetic recording, plasmon-enhanced Raman spectroscopy, etc.

Barrier-free absorbance modulation for super-resolution optical lithography.

Absorbance-Modulation-Optical Lithography (AMOL) enables super-resolution optical lithography by simultaneous illumination of a photochromic film by a bright spot at one wavelength, λ1 and a node at another wavelength, λ2. A deep subwavelength region of the transparent photochromic isomer is created in the vicinity of the node. Light at λ1 penetrates this region and exposes an underlying photoresist layer. In conventional AMOL, a barrier layer is required to protect the photoresist from the photochromic layer. Here, we demonstrate barrier-free AMOL, which considerably simplifies the process. Specifically, we pattern lines as small as 70nm using λ1 = 325nm and λ2 = 647nm. We further elucidate the minimum requirements for AMOL to enable multiple exposures so as to break the diffraction limit.

Light Absorption and Reflection in Nanostructured GaAs Metal–Semiconductor–Metal Photodetectors

Nanostructured metal-semiconductor-metal photodetectors (MSM-PDs) can assist in future high-speed communication devices for achieving high responsivity-bandwidth characteristics. We numerically evaluate the periodic nanotextured structures that provide for the confinement of light and concentration of energy near the designed wavelength, a feature attributed to surface plasmon polaritons. This plasmonic-based miniaturization provides a new approach to investigate remarkable optical properties of nanoscale device structuring. A finite-difference time-domain method is used to calculate the maximum light absorption and reflection of nanograting-assisted MSM-PDs by varying detector's physical parameters in comparison with a conventional device. The horizontal and vertical surface resonances are identified as the key mechanisms to excite the surface waves with the aid of normally incident TM-polarized light. The extraordinary optical transmission is notably enhanced by optimizing the grating parameters and light incident angles. The simulation results are presented for light absorption and reflection in the subwavelength region of nanograting-assisted MSM-PDs compared with the conventional MSM-PDs.

Symmetry-protected mode coupling near normal incidence for narrow-band transmission filtering in a dielectric grating

A narrow-band transmission filter is demonstrated near normal incidence that operates through relaxation of supported-mode selection rules and is explained in the context of group theory. We calculated the transverse magnetic and transverse electric dispersion relations of a dielectric grating in the subwavelength and near-wavelength region using finite element modal analysis and determine the modes' corresponding irreducible representations. Coupling to select transverse magnetic modes at normal incidence is optimized to yield broadband high reflectance that acts as the background for the transmission filter. While some modes couple at normal incidence, others are shown to remain inaccessible due to symmetry mismatch. Away from normal incidence, the reduced symmetry relaxes the selection rules, enabling weak coupling between the incident field and these symmetry-protected modes. This weak coupling produces narrow transmission bands within the opaque background. Furthermore, by choosing the plane of incidence to include or exclude the grating periodicity, we show that orthogonal mode sets can independently be selected to couple to the incident light, yielding separate transmission bands. This spectral filtering is experimentally demonstrated with a suspended silicon grating in the infrared spectrum ($7--14\ensuremath{\mu}\mathrm{m}$), which agrees well with simulated transmittance spectra and modal analysis.

SIL 기반 플라즈모닉 리소그래피에서 주파수 적응형 필터를 이용한 나노간극 제어의 성능향상

Plasmonic lithography is the latest technique to overcome diffraction limit of previous optical lithography. In the plasmonic lithography, the nano gap between nano metal wave guide and photoresist should be in sub-wavelength region. SIL-based plasmonic lithography is the one of the solutions to maintain small air gap. However, the nano gap control is so sensitive that a little disturbance is able to have a large effect on the nano gap control. So, we analyzed the characteristics of disturbance, and then modified the previous controller to suppress the disturbance. We applied two peak filters which were fixed one and adaptively changeable one. We experimentally confirmed the improvement of the nano gap control, which reduced nano gap error by 30 %. The proposed control will improve the quality of lithography pattern.

Topological properties of microwave magnetoelectric fields.

Collective excitations of electron spins in a ferromagnetic sample dominated by the magnetic dipole-dipole interaction strongly influence the field structure of microwave radiation. A small quasi-two-dimensional ferrite disk with magnetic-dipolar-mode (MDM) oscillation spectra can behave as a source of specific fields in vacuum, termed magnetoelectric (ME) fields. A coupling between the time-varying electric and magnetic fields in the ME-field structures is different from such a coupling in regular electromagnetic fields. The ME fields are characterized by strong energy confinement at a subwavelength region of microwave radiation, topologically distinctive power-flow vortices, and helicity parameters [E. O. Kamenetskii, R. Joffe, and R. Shavit, Phys. Rev. E 87, 023201 (2013)]. We study topological properties of microwave ME fields by loading a MDM ferrite particle with different dielectric samples. We establish a close connection between the permittivity parameters of dielectric environment and the topology of ME fields. We show that the topology of ME fields is strongly correlated with the Fano-resonance spectra observed at terminals of a microwave structure. We reveal specific thresholds in the Fano-resonance spectra appearing at certain permittivity parameters of dielectric samples. We show that ME fields originated from MDM ferrite disks can be distinguished by topological portraits of the helicity parameters and can have a torsion degree of freedom. Importantly, the ME-field phenomena can be viewed as implementations of space-time coordinate transformations on waves.

Optical orbital angular momentum conservation during the transfer process from plasmonic vortex lens to light

We demonstrate the optical orbital angular momentum conservation during the transfer process from subwavelength plasmonic vortex lens (PVLs) to light and the generating process of surface plasmon polaritons (SPPs). Illuminating plasmonic vortex lenses with beams carrying optical orbital angular momentum, the SP vortices with orbital angular momentum were generated and inherit the optical angular momentum of light beams and PVLs. The angular momentum of twisting SP electromagnetic field is tunable by the twisted metal/dielectric interfaces of PVLs and angular momentum of illuminating singular light. This work may open the door for several possible applications of SP vortices in subwavelength region.

Dyakonons in hyperbolic metamaterials

We have analyzed surface-wave propagation that takes place at the boundary between an isotropic medium and a semi-infinite metal-dielectric periodic medium cut normally to the layers. In the range of frequencies where the periodic medium shows hyperbolic space dispersion, hybridization of surface waves (dyakonons) occurs. At low to moderate frequencies, dyakonons enable tighter confinement near the interface in comparison with pure SPPs. On the other hand, a distinct regime governs dispersion of dyakonons at higher frequencies. Full Text: PDF References Z. Ruan, M. Qiu, "Slow electromagnetic wave guided in subwavelength region along one-dimensional periodically structured metal surface", Appl. Phys. Lett. 90, 201906 (2007). CrossRef A. Yariv, P. Yeh,"Electromagnetic propagation in periodic stratified media. II. Birefringence, phase matching, and x-ray lasers", J. Opt. Soc. Am. 67, 438 (1977). CrossRef D.R. Smith, D. Schurig, "Electromagnetic Wave Propagation in Media with Indefinite Permittivity and Permeability Tensors", Phys. Rev. Lett. 90, 077405 (2003). CrossRef I.I. Smolyaninov, E. Hwang, E. Narimanov, "Hyperbolic metamaterial interfaces: Hawking radiation from Rindler horizons and spacetime signature transitions", Phys. Rev. B 85, 235122 (2012). CrossRef Z. Jacob, E.E. Narimanov, "Optical hyperspace for plasmons: Dyakonov states in metamaterials", Appl. Phys. Lett. 93, 221109 (2008). CrossRef M.I. D'yakonov, "New type of electromagnetic wave propagating at an interface", Sov. Phys. JETP 67, 714 (1988). DirectLink I.I. Smolyaninov, Y.J. Hung, C.C. Davis, "Magnifying Superlens in the Visible Frequency Range", Science 315, 1699 (2007). CrossRef J. Elser, V.A. Podolskiy, I. Salakhutdinov, I. Avrutsky, "Nonlocal effects in effective-medium response of nanolayered metamaterials", Appl. Phys. Lett. 90, 191109 (2007). CrossRef E. Popov, S. Enoch, "Mystery of the double limit in homogenization of finitely or perfectly conducting periodic structures", Opt. Lett. 32, 3441 (2007). CrossRef B. Wood, J.B. Pendry, D.P. Tsai, "Directed subwavelength imaging using a layered metal-dielectric system", Phys. Rev. B 74, 115116 (2006). CrossRef