Deep Subwavelength Scale Research Articles

Optical skyrmions and tunable fine spin structures in deep-subwavelength scale at metal/graded index material interface

Optical skyrmions and tunable fine spin structures in deep-subwavelength scale at metal/graded index material interface

On-chip phonon-enhanced IR near-field detection of molecular vibrations

Phonon polaritons – quasiparticles formed by strong coupling of infrared (IR) light with lattice vibrations in polar materials – can be utilized for surface-enhanced infrared absorption (SEIRA) spectroscopy and even for vibrational strong coupling with nanoscale amounts of molecules. Here, we introduce and demonstrate a compact on-chip phononic SEIRA spectroscopy platform, which is based on an h-BN/graphene/h-BN heterostructure on top of a metal split-gate creating a p-n junction in graphene. The metal split-gate concentrates the incident light and launches hyperbolic phonon polaritons (HPhPs) in the heterostructure, which serves simultaneously as SEIRA substrate and room-temperature infrared detector. When thin organic layers are deposited directly on top of the heterostructure, we observe a photocurrent encoding the layer’s molecular vibrational fingerprint, which is strongly enhanced compared to that observed in standard far-field absorption spectroscopy. A detailed theoretical analysis supports our results, further predicting an additional sensitivity enhancement as the molecular layers approach deep subwavelength scales. Future on-chip integration of infrared light sources such as quantum cascade lasers or even electrical generation of the HPhPs could lead to fully on-chip phononic SEIRA sensors for molecular and gas sensing.

Deep subwavelength sound absorption by a buckling plate resonator

Conventional sound-absorbing materials face great difficulties in achieving deep subwavelength broadband sound absorption due to the causal constraint on the minimum thickness. A buckling plate resonator is proposed to alleviate the causal constraint on the thickness of sound-absorbing materials. The resonator consists of a back cavity sealed by an elastic circular thin plate. When the plate is subjected to a uniform in-plane compressive force up to its critical load, it buckles and behaves as a negative stiffness spring. The positive stiffness of the back cavity is counteracted by the negative stiffness of the buckling plate, resulting in resonance absorption at the deep subwavelength scale. The sound absorption performance of the buckling plate resonator under different in-plane loads was measured in an impedance tube. Quasi-perfect sound absorption was observed in a buckling plate resonator with a thickness less than 1% of the wavelength corresponding to the resonance frequency. This work provides a solution for the design of ultra-thin broadband sound absorbers.

Ultrafast Laser Manipulation of In-Lattice Plasmonic Nanoparticles.

Plasmonic nanoparticles enable manipulation and enhancement of light fields at deep subwavelength scales, leading to structures and devices for diverse applications in optics. Despite hybrid plasmonic materials display remarkable optical properties due to interactions between components in nanoproximity, scalable production of plasmonic nanostructures within a single-crystalline matrix to achieve an ideal plasmon-crystal interface remains challenging. Here, a novel approach is presented to realize efficient manipulation of in-lattice plasmonic nanoparticles. Employing ultrafast-laser-driven plasmonic nanolithography, metallic nanoparticles with controllable morphology are precisely defined in the crystalline lattice of yttrium aluminum garnet (YAG) crystal. Through direct ion implantation, hybrid plasmonic material composed of nanoparticles embedded in a sub-surface amorphous YAG layer is created. Subsequently, femtosecond laser pulses guide formation and reshaping of plasmonic nanoparticles from the amorphous layer into the single-crystalline matrix along direction of light propagation, facilitated by a plasmon-mediated evolution of laser energy deposition. By tailoring resonance modes and optimizing the coupling between structured particle assemblies, a range of applications including polarization-dependent absorption and nonlinearity, controllable photoluminescence, and structural color generation is demonstrated. This research introduces a new approach for fabricating advanced optical materials featuring in-lattice plasmonic nanostructures, paving the way for the development of diverse functional photonic devices.

Uncovering low-frequency vibrations in surface-enhanced Raman of organic molecules

Accessing the terahertz (THz) spectral domain through surface-enhanced Raman spectroscopy (SERS) is challenging and opens up the study of low-frequency molecular and electronic excitations. Compared to direct THz probing of heterogenous ensembles, the extreme plasmonic confinement of visible light to deep sub-wavelength scales allows the study of hundreds or even single molecules. We show that self-assembled molecular monolayers of a set of simple aromatic thiols confined inside single-particle plasmonic nanocavities can be distinguished by their low-wavenumber spectral peaks below 200 cm−1, after removal of a bosonic inelastic contribution and an exponential background from the spectrum. Developing environment-dependent density-functional-theory simulations of the metal-molecule configuration enables the assignment and classification of their THz vibrations as well as the identification of intermolecular coupling effects and of the influence of the gold surface configuration. Furthermore, we show dramatically narrower THz SERS spectra from individual molecules at picocavities, which indicates the possibility to study intrinsic vibrational properties beyond inhomogeneous broadening, further supporting the key role of local environment.

Ultrathin and pressure-resistant meta-coating with embedded tree-shaped acoustic black hole for broadband low-frequency underwater sound absorption

Acoustic coating is the main technical means to absorb incoming sound from detection sonars. Considering the advance of sonar technology to cope with low frequencies and the underwater working environment, acoustic coatings are required to provide effective broadband low-frequency sound absorption (SA) under hydrostatic pressure. However, current underwater acoustic coating materials have poor low-frequency broadband SA performance, especially under hydrostatic pressure. Herein, we propose an ultrathin meta-coating with tree-shaped acoustic black hole (ABH) units embedded in an elastomer matrix. Each tree-shaped ABH unit consists of multi-scale ABH plates as scatterers, a center-support column as “trunk” and a frame as “skeleton”. This design combines the rich dynamics of multi-scale ABH plates, the pressure-resistance characteristics of the frame and the column, and the damping of the matrix, to collectively lead to superior performance. Capitalizing on the rich local and coupled large-amplitude local resonances of the unit, and strong resistance to static-pressure-induced deformation, high and quasi-perfect SA is achieved in multiple and ultra-wide low-frequency bands at a deep subwavelength scale under different hydrostatic pressures. Notably, quasi-perfect SA (>0.92) is achieved at 500 Hz with a meta-coating whose thickness is 2 % of the sound wavelength. The quasi-perfect SA is also demonstrated experimentally within 1200–7500 Hz, which persists till 1.0 MPa. The SA is still above 0.6 from 3–10 kHz at 4.5 MPa. For SA model under hydrostatic pressures, hyperelastic constitutive model and pressurized dynamic mechanical parameters of the matrix were established. Numerical results are then verified by experimental tests under different hydrostatic pressures. With both high local-modal densities and hydrostatic-pressure resistance, this design represents a new type of acoustic metamaterials and promises broad underwater applications.

Unidirectional guided-wave-driven metasurfaces for arbitrary wavefront control

Metasurfaces are capable of fully reshaping the wavefronts of incident beams in desired manners. However, the requirement for external light excitation and the resonant nature of their meta-atoms, make challenging their on-chip integration. Here, we introduce the concept and design of a fresh class of metasurfaces, driven by unidirectional guided waves, capable of arbitrary wavefront control based on the unique dispersion properties of unidirectional guided waves rather than resonant meta-atoms. Upon experimentally demonstrating the feasibility of our designs in the microwave regime, we numerically validate the introduced principle through the design of several microwave meta-devices using metal-air-gyromagnetic unidirectional surface magneto-plasmons, agilely converting unidirectional guided modes into the wavefronts of 3D Bessel beams, focused waves, and controllable vortex beams. We, further, numerically demonstrate sub-diffraction focusing, which is beyond the capability of conventional metasurfaces. Our unfamiliar yet practical designs may enable full, broadband manipulation of electromagnetic waves on deep subwavelength scales.

Steering and cloaking of hyperbolic polaritons at deep-subwavelength scales

Polaritons are well-established carriers of light, electrical signals, and even heat at the nanoscale in the setting of on-chip devices. However, the goal of achieving practical polaritonic manipulation over small distances deeply below the light diffraction limit remains elusive. Here, we implement nanoscale polaritonic in-plane steering and cloaking in a low-loss atomically layered van der Waals (vdW) insulator, α-MoO3, comprising building blocks of customizable stacked and assembled structures. Each block contributes specific characteristics that allow us to steer polaritons along the desired trajectories. Our results introduce a natural materials-based approach for the comprehensive manipulation of nanoscale optical fields, advancing research in the vdW polaritonics domain and on-chip nanophotonic circuits.

Coupled mode theory for plasmonic couplers

Photonic integrated circuits play an increasingly important role in several emerging technologies. Their functionality arises from a combination of integrated components, e.g., couplers, splitters, polarization rotators, and wavelength selective filters. Efficient and accurate simulation of these components is crucial for circuit design and optimization. In dielectric systems, design procedures typically rely on coupled-mode theory (CMT) methods, which then guide subsequent refined full-wave calculations. Miniaturization to deep sub-wavelength scales requires the inclusion of lossy plasmonic (metal) components, making optimization more complicated by the interplay between coupling and absorption. Even though CMT is well developed, there is no consensus as to how to rigorously and quantitatively implement it for lossy systems. Here we present an intuitive coupled-mode theory framework for quantitative analysis of dielectric–plasmonic directional and adiabatic couplers, whose large-scale implementation in 3D is prohibitively slow with full-wave methods. This framework relies on adapting existing coupled mode theory approaches by including loss as a perturbation. This approach will be useful in designing dielectric–plasmonic circuits, providing a first reference point for anyone using techniques such as inverse design and deep learning optimization methods.

A compact circuit-based metasurface for enhancing magnetic resonance imaging

Herein, we propose a compact 0.36 T MRI-enhancing circuit-based metasurface working at its third order mode. Different from most MRI-enhancing metasurface designs which exploit the fundamental mode with the highest enhancement, our choice is a trade-off between the enhancement and homogeneity. The metasurface is organized with capacitively-loaded metal wires where the capacitors decrease the electric length of the wires thus enabling a deep subwavelength scale. The working frequency of metasurface is tuned to the Larmor frequency, contributing to the redistribution of transmitted field. Full-wave simulations based on CST Microwave Studio compare the magnetic field in a mimicked MRI environment with and without the metasurface. The utilization of metasurface leads to a field enhancement ratio of 9.36-fold over a 28 × 28 cm2 area at 2 cm height while exceeding unity till almost 12 cm. Meanwhile, the variation of the quasi-homogenous magnetic field is less than 1/3 over a relatively large area. The impact of metasurface is further demonstrated by simulations with a head bio-model to evaluate the transmitted field strength and electromagnetic energy absorption. A preliminary measuring experiment is also conducted to validate the special mode pattern. The proposed metasurface effectively enhances the transmitted efficiency thus can be employed in clinical MRI to enhance imaging quality or reduce the input power. Moreover, this design paradigm is compatible with other enhancing approaches due to the nonmagnetic inclusions and frequency-dependent response and can be adapted for higher-field MRI systems by adjusting the length of metal wires and the value of loaded capacitors.

Resonantly Enhanced Optical Birefringence in Ultrathin High‐Index WS2 Metasurfaces

AbstractOptical birefringence plays an important role in the manipulation of polarization states of light. However, the weak birefringence in common birefringent materials restricts the device thickness to tens of microns for desired phase retardation. Although recent advances in dielectric metasurfaces have enabled remarkable birefringence using structurally anisotropic nanostructures, the refractive index contrast between the dielectric and the air cladding fundamentally limits the thickness of metasurfaces to a fraction of a micron to obtain the required phase retardation. Here, birefringent resonances in high‐index tungsten disulfide (WS2) metasurfaces are utilized to push the thickness limit down to the deep subwavelength scale. The inherent high‐index property of WS2 enables birefringent resonances in WS2 metasurfaces which consist of anisotropic nanostructures with a thickness of only 50 nm. Such birefringent resonances enhance the light‐matter interaction and produce an unprecedented birefringence (Δneff) about 4. As a result, it is experimentally realized that circular‐to‐linear polarization conversion with a normalized efficiency of ≈90% within an incident angle range up to ±10°. The results break the fundamental limit of birefringent devices and provide strategies for creating ultimate thin polarization optical devices.

Manipulating chiral photon generation from plasmonic nanocavity-emitter hybrid systems: from weak to strong coupling

Abstract By confining light into a deep subwavelength scale to match the characteristic dimension of quantum emitters, plasmonic nanocavities can effectively imprint the light emission with unique properties in terms of intensity, directionality, as well as polarization. In this vein, achiral quantum emitters can generate chiral photons through coupling with plasmonic nanocavities with either intrinsic or extrinsic chirality. As an important metric for the chiral-photon purity, the degree of circular polarization (DCP) is usually tuned by various scattered factors such as the nanocavity design, the emitter type, and the coupling strategy. The physical mechanisms of the chiral photon generation, especially when plasmons and emitters step into the strong coupling regime, are less explored. In this paper, we extended the coupled-oscillator and Jaynes–Cummings models to their chiral fashion to account for the above factors within a single theoretical framework and investigated the chiroptical properties of a plasmonic nanocavity-emitter hybrid system from weak to strong coupling. It was demonstrated that both the circular differential scattering and prominent scattering DCP rely on the intrinsic chirality generated by breaking the mirror symmetry with the emitter, and is thereby tunable by the coupling strength. However, the luminescence DCP (as high as 87 %) is closely related to the extrinsic chirality of the bare nanocavity and independent of the coupling strength. The results thus reveal two different physical mechanisms of generating chiral photons in scattering and luminescence. Our findings provide a theoretical guideline for designing chiral photon devices and contribute to the understanding of chiral plasmon-emitter interaction.

Optimized design of nano-photonic devices for temperature self-regulating on vanadium dioxide thin films

Phase change materials can enable temperature self-regulation due to their drastic changes in optical properties accompanying the phase transition. Significant reduction of the optical absorption after the transition is the key ingredient for an enhanced regulating performance. However, the absorptivity of unpatterned vanadium dioxide (VO2) thin films can hardly be reduced after phase transition at visual-to-infrared band. In this work, we combine the direct binary search (DBS) and particle swarm optimization (PSO) algorithms for an optimized design of temperature self-regulating nano-photonic devices on VO2 thin films. For a given incident wavelength, a pixelated structure is firstly inverse-designed by the DBS algorithm which maximizes the absorption contrast before and after the transition. To overcome fabrication challenges as pixel size is at deep sub-wavelength scale, the pixelated structure can then be replaced by geometric shapes which are more tractable in manufacturing processes. The geometrical parameters are optimized by the PSO algorithm where our optimized device brings the absorptivity down to 33% after the transition. These results provide an effective way for the inverse design of optimized nano-photonic structures based on phase change materials.

Negative capacitors and inductors enabling wideband waveguide metatronics

Waveguide metatronics, known as an advanced platform of metamaterial-inspired circuits, provides a promising paradigm for millimeter-wave and terahertz integrated circuits in future fifth/sixth generation (5/6G) communication systems. By exploiting the structural dispersion properties of waveguides, a lumped type of waveguide integrated elements and circuits could be developed in deep subwavelength scales with intrinsic low loss and low crosstalk. In this study, we focus on constructing negative capacitors and inductors for waveguide metatronics, effectively expanding the operating frequency range of waveguide integrated circuits. The incorporation of negative elements enables wideband impedance matching in waveguide, which have been both theoretically explored and experimentally validated within the waveguide metatronics paradigm. Furthermore, we have demonstrated that the negative elements can also be realized in the optical domain through the utilization of a silicon waveguide with photonic crystal cladding, indicating the feasibility and universality of wideband waveguide metatronics. The negative lumped elements could boost the progress of the waveguide metatronic technique, achieving superior performance on the conventional lumped circuits within waveguides that solely rely on positive elements.

Epitaxially Grown Silicon Nanowires with a Gold Molecular Adhesion Layer for Core/Shell Structures with Compact Mie and Plasmon Resonances.

Noble-metal plasmonic nanostructures have attracted much attention because they can support deep-subwavelength optical resonances, yet their performance tends to be limited by high Ohmic absorption losses. In comparison, high-index dielectric materials can support low-loss optical resonances but do not tend to yield the same subwavelength optical confinement. Here, we combine these two approaches and examine the dielectric-plasmonic resonances in dielectric/metal core/shell nanowires. Si nanowires were grown epitaxially from (111) substrates, and direct deposition of Au on these structures by physical vapor deposition yielded nonconformal Au islands. However, by introduction of a molecular adhesion layer prior to deposition, cylindrical Si/Au core/shell nanostructures with conformal metal shells were successfully fabricated. Examining these structures as optical cavities using both optical simulations and experimental extinction measurements, we found that the structures support Mie resonances with quality factors enhanced up to ∼30 times compared with pure dielectric structures and plasmon resonances with optical confinement enhanced up to ∼5 times compared with pure metallic structures. Interestingly, extinction spectra of both Mie and plasmon resonances yield Fano line shapes, whose manifestation can be attributed to the combination of high quality factor resonances, Mie-plasmon coupling, and phase delay of the background optical field. This work demonstrates a bottom-up synthetic method for the production of freestanding, cylindrically symmetric semiconductor/metal core/shell nanowires that enables the efficient trapping of light on deep-subwavelength length scales for varied applications in photonics and optoelectronics.

Sound absorption characteristics of a compact meta-structure under grazing flow

This study proposes a compact meta-structure for broadband and low-frequency sound absorption under grazing flow. It comprises a microperforated panel (MPP) and resonators with embedded tunable orifices and C-shaped cavities. The sound absorption characteristics of the proposed absorber are investigated via theoretical model (acoustic impedance circuit) and numerical model (finite element method). Consistent predicted results from both models manifest that the proposed meta-structure possesses good absorption capacity under a deep-subwavelength scale, compared to the conventional MPP absorber. To simultaneously achieve better low-frequency and broadband noise damping, a parallel configuration of the meta-structure and an MPP absorber is studied numerically and experimentally. Besides, notable variation on the acoustic performance of the proposed absorber is observed in case of grazing flow, which leads to the decline of the absorption peak and makes the peak wider than that without flow. The practical significance of this study is that it proposes a promising meta-structure for wide frequency sound attenuation applications; furthermore, it elucidates the flow effects on the acoustic performance of the meta-structure, providing a route to reduce flow-related noise in a broad frequency range.

Gain-assisted magnetoplasmonic switching in metal–dielectric–metal plasmonic waveguides

Optical switches in metal–dielectric–metal (MIM) structures attracted much interest for chip-scale plasmonic modulator merging nanoelectronics and ultrafast photonics. The absorption switches employed gain modulations; thus, their switching speed is limited to the sub-gigahertz range because of the spontaneous lifetime of the transition. In this paper, we theoretically predicted that the function of the absorption switches is achieved employing magnetic field reversal and constant gain instead of gain modulations. We investigated analytically and numerically transmission in the presence of both gain and gyration and revealed that the transmission shift by the external magnetic field is maximized at a resonance value of gain. The switching speed can reach the THz range thanks to the large optical bandwidth in the MIM stub structures and the advanced magnetization switching technology. The MIM structure with combined gain and magnetoplasmonic properties enhances interaction between light and magnetic field on nanoscale and enables the device footprint down to the deep subwavelength scale λ2/50.

Photoluminescence imaging of single photon emitters within nanoscale strain profiles in monolayer WSe2

Local deformation of atomically thin van der Waals materials provides a powerful approach to create site-controlled chip-compatible single-photon emitters (SPEs). However, the microscopic mechanisms underlying the formation of such strain-induced SPEs are still not fully clear, which hinders further efforts in their deterministic integration with nanophotonic structures for developing practical on-chip sources of quantum light. Here we investigate SPEs with single-photon purity up to 98% created in monolayer WSe2 via nanoindentation. Using photoluminescence imaging in combination with atomic force microscopy, we locate single-photon emitting sites on a deep sub-wavelength spatial scale and reconstruct the details of the surrounding local strain potential. The obtained results suggest that the origin of the observed single-photon emission is likely related to strain-induced spectral shift of dark excitonic states and their hybridization with localized states of individual defects.

Topological robust corner states of a two-dimensional square lattice with C4 symmetry in fully coupled dipolar arrays

A profound insight into topological photonic systems will empower us to harness their maximum potential and discover uncharted topological phenomena. For the square quadripartite lattice with only nearest-neighbor reciprocal couplings, in the out-of-plane mode, sublattice symmetry makes the eigenvalues of the Hamiltonian symmetric around zero energy, while C 4 symmetry closes the gap between the central bands. Meanwhile, topological corner states (CSs) are fixed at zero energy due to chiral symmetry. Thereby, the CSs cannot appear in a gap, but are embedded in the bulk. In this paper, the full coupling between dipoles is considered in the Hamiltonian, i.e., not only the near-field of nearest-neighbor, but also the far-field dipole-dipole interactions are taken into account to investigate the new potential topological properties. The results show that the sublattice symmetry of the system will be broken, leading to the appearance of a zero-energy band gap. Nevertheless, the generalized chiral symmetry ensures that the CSs are still pinned to zero energy. That is, in-gap CSs are provided. Additionally, the silicon carbide materials used in this paper can confine light to the deep subwavelength scale, which has great potential in enhancing light-matter interactions in the terahertz range.

Broadband acoustic absorbers based on double split-ring resonators at deep subwavelength scale

We report both experimentally and numerically that a low-frequency acoustic absorber is realized by double split-ring resonators backed with a rigid wall. This absorber leverages the impedance matching and dissipation effect, which arises due to the thermal-viscous loss within the dual channels. As a result, this absorber achieves near-perfect sound absorption (the absorption coefficient α = 0.99) at a subwavelength thickness of around λ/23. By assembling six unit cells with distinct structure parameters to form a supercell, the fractional bandwidth (the ratio of the bandwidth to the center frequency) is increased to 40% with an average α of 0.86. Acoustic experiment results validate this exceptional performance, which is also in agreement with the simulation results. Moreover, by employing the supercell, we create an anechoic room demonstrating broadband sound absorption in a wide range of incident angles while occupying significantly less space than traditional sound-absorbing porous materials. Our double split-ring composite design paves the way for broadband acoustic absorbers at the deep subwavelength scale