Subwavelength Region Research Articles

Graphene pixel based broadband polarization insensitive THz absorber

Abstract Suppressing undesired radiation and eliminating stray radiation in a high-frequency circuit might be challenging. Ultra-high frequency (UHF) electromagnetic wave absorbers are finding new applications in both the military and civilian domains. Obtaining an ultra-wide absorption band in a thin, single-layer Terahertz absorber becomes difficult. Also, achieving perfect absorption bandwidth on transverse electric (TE) and transverse magnetic (TM) mode is very sensitive. This paper presents a polarization-independent graphene-based absorbers with a near unity absorption band of 2.32 THz and 3.78 THz in a lower mid-infrared regime. The structure’s unit cell consists of cross-slot circular patches with Au as a ground separated by SiO2. To improve the absorption band another graphene layer was placed between the top and dielectric material. The absorber is polarization independent under normal incidence because of the deposited graphene structure’s fourfold symmetry. Both TE and TM polarizations were investigated, with wide-band absorption observed up to 60° incident angles in both cases. In terms of the lowest frequency of absorption, the prototype is deemed to be as thin as ∼ 0.1 λ max for both absorbers. Similarly, the effective homogeneity criteria in the subwavelength region are supported by the unit cell’s period of ∼ 0.1 λ max . The proposed absorber-2 shows 10 % higher FBW than existing work.

Formation of subwavelength tunable flat-top focus with double foci characteristic by tightly focused cylindrical vector beams

We report a numerical investigation on the generation of a subwavelength flat-top focus by tuning the polarization angle of the incident beam under tight focusing conditions. Our approach uses a binary phase mask containing two regions of opposite phases, with a variable diameter of the inner region. Properly tuning the diameter of the inner region of the mask, the flat-top profile of variable sizes (above and below the subwavelength region) is generated in a controlled manner with the various incident polarization angles of the beam and with the different numerical aperture (NA) value of the lens. Our study reveals that a particular cut-off value of polarization angle, above which the subwavelength flat-top is generated, corresponds to a specific NA that increases with the increase in the NA value. In addition, the emergence of the dual foci structure is observed in the longitudinal direction with increasing NA of the lens. The flat-top's polarization degrees-of-freedom and dual foci properties open up promising routes for potential applications in fluorescence polarization microscopy and multiplane particle trapping.

(Invited) Near Field Photocatalysis for Site-Selective Reactions By Using ZnO Nanoplates

Nanofabrication is a technology for control of the morphology, orientation, and configuration of nanomaterials. When ordinary, propagating light is employed as a processing tool, subwavelength fabrication is difficult due to the diffraction limit. However, if optical near field, which is localized in a subwavelength region is used, nanofabrication beyond the diffraction limit is possible.We have demonstrated that subwavelength nanofabrication is possible by taking advantage of the optical near field. Since hot electron-hole pairs are generated at around the plasmonic resonance sites, the energetic carriers can be used to drive oxidative dissolution of Ag,1,2 oxidative deposition of PbO2,3,4 and reductive deposition of Ag,5 in a site-selective manner. These techniques have been applied to subwavelength photochromic data storage,1,2 fabrication of chiral nanoparticles,4-7 and site-selective modification of a plasmonic photocatalyst with a co-catalyst.8 Generation of optical near field is possible even at non-plasmonic, semiconducting or dielectric nanostructures. Here we employed In-doped ZnO (In:ZnO) as a semiconductor photocatalyst. Hexagonal In:ZnO nanoplates were synthesized and adsorbed onto a glass substrate, and irradiated with linearly polarized UV light in the presence of Ag ions for reductive deposition of Ag. The optical near field generated under polarized UV light gave rise to site-selective Ag deposition. Also, the sample exhibited linear dichroism (LD) signals, indicating that the nanoparticles have optical anisotropy. On the other hand, unpolarized UV light gave no optical anisotropy. Electromagnetic simulations successfully reproduced the LD spectra. We have also performed site-selective oxidation reactions by using the hexagonal In:ZnO nanoplates. Tanabe, I.; Tatsuma, T. Nano Lett. 2012, 12, 5418–5421. Saito, K.; Tanabe, I.; Tatsuma, T. J. Phys. Chem. Lett. 2016, 7, 4363–4368.Nishi, H.; Sakamoto, M.; Tatsuma, T. Chem. Commun. 2018, 54, 11741–11744.Saito, K.; Tatsuma, T. Nano Lett. 2018, 18, 3209–3212.Ishida, T.; Isawa, A.; Kuroki, S.; Kameoka, Y.; Tatsuma, T. Appl. Phys. Lett. 2023, 123, 061111.Morisawa, K.; Ishida, T.; Tatsuma, T. ACS Nano 2020, 14, 3603–3609.Shimomura, K.; Nakane, Y.; Ishida, T.; Tatsuma, T. Appl. Phys. Lett. 2023, 122, 151109.Kim, K.; Nishi, H.; Tatsuma, T. J. Chem. Phys. 2022, 157, 111101.

Parallel illumination for depletion microscopy through acousto-optic spatial light modulation

Several types of super-resolution microscopy techniques, such as Stimulated Emission Depletion (STED), Reversible Saturable Optical Fluorescence Transitions (RESOLFT) or Switching Laser Mode (SLAM) microscopies, employ Laguerre-Gaussian beams (also called vortex or doughnut beams) to obtain fluorescence information within a sub-wavelength region of the specimen under observation, thus breaking the diffraction limit and producing images of greatly improved quality. However, in general, these techniques operate on a point-by-point basis, so scanning the sample in order to build a full, meaningful image, takes time. Parallelization of the illumination is the only way to make these microscopy techniques more suitable for real-time live cell imaging applications. Here, we demonstrate the parallel production of arbitrary arrays of Gaussian and Laguerre-Gaussian lasers foci suitable for super-resolution microscopy, together with the possibility to fast scan through the sample, by means of acousto-optic spatial light modulation, a technique that we have pioneered in the past in several other fields. Parallelized illumination with both Gaussian and doughnut beams will be then used to acquire super-resolution images.

Multifrequency topological interface modes in a nonlocal acoustic system with large winding numbers

Topological interface states have wide applications for their robustness and energy localization. Multiple topological interface states in 1D acoustic systems are preferred for signal enhancement at specific frequencies. However, previous 1D topological systems with local interactions can realize only one topological interface state in a bandgap. Here, we find that connected acoustic Su–Schrieffer–Heeger (SSH) chains with nonlocal interactions can achieve two or more topological interface states at different frequencies in a single bandgap, even in the subwavelength region. The difference in frequency can be modulated by merely adjusting the structural design at the interface. A mechanical metamaterial with a large winding number is designed to realize multiple nondegenerate topological interface states. This metamaterial can enhance weak acoustic signals at multiple specific frequencies simultaneously and has applications in the fields of acoustic detecting and color imaging.

A lightweight waterborne acoustic meta-absorber with low characteristic impedance rods

We investigate a meta-absorber composed of impedance-matched rods for the broadband acoustic absorption. However, with a slow longitudinal wave speed, the absorber’s density is high to keep the characteristic impedance matching with that of water. To obtain a lightweight high efficiency absorber, we analyze the influence of characteristic impedance and loss factor on absorption behaviors and find that the characteristic impedance matching is not necessary and the low characteristic impedance structure can also attain the efficient absorption performance, and the high dissipation can reduce the density required for an efficient absorption. Based on this, we propose a meta-absorber composed of low characteristic impedance rubber rods in parallel coated by a rubber cover layer and backed with a stepped resin base. By the combination of the transfer matrix method and transmission line impedance transfer equation, the acoustic performance of meta-absorber is analyzed. Compared with the characteristic impedance matching rods, the low characteristic impedance rods can release restrictions on design to attain better acoustic behaviors with a lower density. Further, the integration of rods with designed lengths can motivate coherently coupled weak resonances to obtain a broadband absorption. Finally, an optimized meta-absorber is tested in a water-filled impedance tube and the experimental results agree well with the theoretical results which can achieve a broadband low frequency quasi-perfect absorption (α>0.9) in a deep subwavelength region (λ/158 at 950 Hz) with the density close to that of water. Our work finds a new mechanism that low characteristic impedance absorbing materials can achieve the high efficiency absorption with a slow longitudinal wave speed and low density and promise broad applications in the sound stealth of marine vehicles.

Pure magnetic hotspots via hollow silicon nanoparticles illuminated by cylindrical vector beams

Overshadowed by the electric dipole ones, the magnetic dipole transitions of a quantum emitter are hard to probe. This can be remedied by using pure magnetic hotspots, which host strong magnetic but quite weak electric fields in a subwavelength region. Utilizing the multiparticle Mie theory, we demonstrate pure magnetic hotspots via hollow silicon nanoparticles arranged in a ring configuration and illuminated by Bessel cylindrical vector beams. Normalized local field admittances of about 105–107 and magnetic field enhancements of about 10–30 are achievable. Moreover, the peak of the magnetic field enhancement can be shifted about 370 nm via the hollow size. In other words, the pure magnetic hotspot can be optimized for a particular magnetic dipole transition.

Toward Efficient Two-Photon Circularly Polarized Light Detection through Cooperative Strategies in Chiral Quasi-2D Perovskites.

Organic-inorganic hybrid perovskites carry unique semiconducting properties and advanced flexible crystal structures. These characteristics of organic-inorganic hybrid perovskites create a promising candidacy for circularly polarized light (CPL) detection. However, CPL detections based on chiral perovskites are limited to UV and visible wavelengths. The natural quantum well structures of layered hybrid perovskites generate strong light-matter interactions. This makes it possible to achieve near-infrared (NIR) CPL detection via two-photon absorption in the sub-wavelength region. In this study, cooperative strategies of dimension increase and mixed spacer cations are used to obtain a pair of chiral multilayered perovskites (R-β-MPA)EA2 Pb2 Br7 and (S-β-MPA)EA2 Pb2 Br7 (MPA = methylphenethylammonium and EA = ethylammonium). The distinctive bi-cations interlayer and multilayered inorganic skeletons provide enhanced photoconduction. Moreover, superior photoconduction leads to the prominent NIR CPL response with a responsivity up to 8.1 × 10-5 A W-1 . It is anticipated that this work can serve as a benchmark for the fabrication and optimization of efficient NIR CPL detection by simple chemical design.

Parallel illumination for depletion microscopy through acousto-optic spatial light modulation

Several types of super-resolution microscopy, such as Stimulated Emission Depletion (STED), Reversible Saturable Optical Fluorescence Transitions (RESOLFT) or Switching Laser Mode (SLAM) microscopies, employ Laguerre-Gaussian beams (also called vortex or doughnut beams) to obtain fluorescence information within a sub-wavelength region of the specimen under observation, thus breaking the diffraction limit and producing images of greatly improved quality. However, in general, these techniques operate on a point-by-point basis, so we need to raster scan the sample in order to build a full, meaningful image, which takes time. Parallelization of the illumination is the only way to make these microscopies more suitable for live cell imaging applications. Here, we demonstrate the parallel production of arbitrary arrays of Gaussian and Laguerre-Gaussian lasers foci suitable for super-resolution microscopy, together with the possibility to fast scan through the sample, by means of acousto-optic spatial light modulation, a technique that we have pioneered in the past in several other fields.

Nanoscale Electric Field Probing in a Single Nanowire with Raman Spectroscopy and Elastic Strain.

In this work we investigate the Raman response of extremely strained gallium phosphide nanowires. We analyze new strain-induced spectral phenomena such as 2-fold and 3-fold phonon peak splitting which arise due to nontrivial internal electric field distribution coupled with inhomogeneous strain. We show that high bending strain acts as a probe allowing us to define the electric field distribution with deep subwavelength resolution using the corresponding changes of the Raman spectra. We investigate the nature of the localization with respect to nanowire diameter, excitation spot position, and light polarization, supporting the experiment with 3D numerical modeling. Based on our findings we propose a research tool allowing to precisely localize the electric field in a certain subwavelength region of the nanophotonic resonator.

Imaging ultrafast evolution of subwavelength-sized topography using single-probe structured light microscopy

Imaging ultrafast processes in femtosecond (fs) laser–material interactions such as fs laser ablation is very important to understand the physical mechanisms involved. To achieve this goal with high resolutions in both spatial and temporal domains, a combination of optical pump–probe microscopy and structured illumination microscopy can be a promising approach, but suffers from the multiple-frame method with a phase shift that is inapplicable to irreversible ultrafast processes such as ablation. Here, we propose and build a wide-field single-probe structured light microscopy (SPSLM) to image the ultrafast three-dimensional topography evolution induced by fs lasers, where only a single imaging frame with a single structured probe pulse is required for topography reconstruction, benefiting from Fourier transform profilometry. The second harmonic of the fs laser is used as the structured probe light to improve spatial lateral resolution into the subwavelength region of ∼ 478 nm , and the spatial axial and temporal resolutions are estimated to be ∼ 22 nm and ∼ 256 fs , respectively. With SPSLM, we successfully image the ultrafast topography evolution of a silicon wafer surface impacted by single and multiple fs pulses. The variable formation and evolution of the laser induced periodic surface structures during an ultrashort time are visualized and analyzed. We believe that SPSLM will be a significant approach for revealing and understanding various ultrafast dynamics, especially in fs laser ablation and material science.

Enhanced quadrupole effects for atoms in surface spiral beams with a thin dielectric waveguide

Recently, some physical research has recognized the coupling of optical phase singularity light with quadrupole-active atomic transitions. We show that a Laguerre-Gaussian beam, when completely internally reflected at the dielectric/vacuum interface, can generate a well-defined surface spiral beam with a two-dimensional intensity distribution confined to a sub-wavelength region near the interface outside of the dielectric. The quadrupole interaction has been shown to be significantly enhanced in the case of coating the planar surface of a dielectric prism using multilayer dielectric thin films of different thicknesses. The mechanical action on the atom due to the optical forces characterized by optical quadrupole transitions could greatly facilitate the revolution of optical manipulation of neutral atoms along the dielectric/vacuum interface. The typical optical quadrupole potential, evaluated for the atomic transition, corresponds to λ=675.0nm with ΓQ=7.8×105s-1, in Cesium atoms, which is a dipole-forbidden but quadrupole-allowed transition. The parameters used to compute the optical quadrupole potential are similar to those in recent experiments. The factors controlling the general enhancement are pointed out and discussed.

Acoustic Subwavelength Manipulation of Particles with a Quasiperiodic Plate

Acoustic manipulation has been a topic of significant research in recent years. However, the trapping scale in the radiative acoustic field domain is severely limited by the diffraction limit of the classic wave. Here, we report on a simple strategy to create multiple acoustic diffraction-limit-broken spots at different spatial positions in the far-field region for a wide frequency range. The subwavelength acoustic field comes from the interference of the plentiful diffractive beams specific to a quasiperiodic structure plate. We analyze the acoustic radiation force of the gradient field at multiheight plane above the plate for different frequencies and further demonstrate acoustic pushing and ring-shaped trapping of polystyrene particles within this subwavelength region. This work could be envisaged to pave a way for high-resolution manipulation of tiny particles in biomedical and cell research fields.

Tunable Hyperbolic Phonon Polaritons in a Suspended van der Waals α‐MoO3 with Gradient Gaps

AbstractHighly confined and low‐loss hyperbolic phonon polaritons (HPhPs) sustained in van der Waals crystals exhibit outstanding potential to concentrate the long‐wave electromagnetic fields deep into the subwavelength region. However, precise tuning on the HPhP propagation characteristics is a critical challenge to facilitate its practical applications in nanophotonic devices and circuits. This study, by taking advantage of the varying air gaps in a suspended van der Waals α‐MoO3 crystal, shows the feasibility to tune wavelength and damping rate of the HPhPs propagating inside the α‐MoO3. The results indicate that the dependence of polariton wavelength on the gap distance for HPhPs in lower and upper Reststrahlen bands contradict each other. Most interestingly, the tuning range of the polariton wavelengths for HPhPs in the lower band, which exhibits in‐plane hyperbolicity, is wider than that for HPhPs in the upper band which exhibits out‐of‐plane hyperbolicity. A polariton wavelength elongation of up to 160% and a reduction in the damping rate up to 35% is recorded. These findings can not only provide fundamental insight into the nanoscale manipulation of light by polaritonic crystals, but also open up new opportunities for tunable nanophotonic applications.

Fourier-component engineering to control light diffraction beyond subwavelength limit

Abstract Resonant physical phenomena in planar photonic lattices, such as bound states in the continuum (BICs) and Fano resonances with 100% diffraction efficiency, have garnered significant scientific interest in recent years owing to their great ability to manipulate electromagnetic waves. In conventional diffraction theory, a subwavelength period is considered a prerequisite to achieving the highly efficient resonant physical phenomena. Indeed, most of the previous studies, that treat anomalous resonance effects, utilize quasiguided Bloch modes at the second stop bands open in the subwavelength region. Higher (beyond the second) stop bands open beyond the subwavelength limit have attracted little attention thus far. In principle, resonant diffraction phenomena are governed by the superposition of scattering processes, owing to higher Fourier harmonic components of periodic modulations in lattice parameters. But only some of Fourier components are dominant at band edges with Bragg conditions. Here, we present new principles of light diffraction, that enable identification of the dominant Fourier components causing multiple diffraction orders at the higher stopbands, and show that unwanted diffraction orders can be suppressed by engineering the dominant Fourier components. Based on the new diffraction principles, novel Fourier-component-engineered (FCE) metasurfaces are introduced and analyzed. It is demonstrated that these FCE metasurfaces with appropriately engineered spatial dielectric functions can exhibit BICs and highly efficient Fano resonances even beyond the subwavelength limit.

Wide Rayleigh waves bandgap engineered metabarriers for ground born vibration attenuation

The present study proposes two types of engineered resonant metabarrier designs for ground born vibration attenuation at subwavelength frequency region. By establishing analytical and full-scale 3D numerical models, the effectiveness of metabarrier in attenuating incoming Rayleigh surface waves in the earthquake frequency range of interest is investigated. The metabarrier designs are comprised of a cylindrical steel mass encapsulated inside a hollow concrete setup mounted with low-stiffness rubber bearings. The interaction of surface ground motion caused by the propagation of Rayleigh waves with the vertical resonant mode of metabarriers result in the trapping and/or mode conversion of Rayleigh waves into bulk shear waves that propagate deep into the ground. This so-called wave hybridization or the local resonance phenomena induces an extremely wide low frequency bandgap from both designs that is found effective in attenuating incoming Rayleigh waves at the deep-subwavelength scale. By varying the height and arrangement types of resonant metabarriers, the seismic metawedge phenomena are also investigated. In the bandgap frequency range, the Rayleigh waves attenuation is validated by performing a full-scale 3D numerical frequency response analysis including an actual earthquake record (Imperial Valle-1940 El-Centro) based time history analysis. The findings reported indicate such metabarriers can be embedded into the ground surrounding civil infrastructures to be protected from earthquake hazards.

Tunable control of subwavelength topological interface modes in locally resonance piezoelectric metamaterials

This paper reports the evidence of the tunable sub-wavelength topological interface state in local resonance piezoelectric metamaterials. An elastic beam serves as the host medium with periodically arranged local resonators and attached piezoelectric layers. Thus, the shunt circuits connected with the piezoelectric layers can easily change the electromechanical stiffness in any unit cell. By inspired the band-folding mechanism, doubling the primitive unit cell generates two Dirac points, whose one lower point falls below the locally resonant bandgap and is in the sub-wavelength region. Then, employing the shunt circuit’s tunability opens band-folding points to develop two bandgaps associated with the Bragg scattering effect. Band inversion and topological transition exist during the negative capacitance parameter variation. Numerical simulations demonstrate interface wave propagation for excitation at the interface frequency in the heterostructure, composted of two media with different topological invariants. With the assistance of the piezoelectric shunting circuit, our proposed design can induce wave localization over multiple frequency bands, which has potential for applications such as signal filters, energy harvesting and vibration isolation. Finally, we investigate the relationship between the interface frequency and capacitance and consider the local resonance effect on topological interface modes.

Low frequency topologically protected wave transport in sinusoidal lightweight acoustic metamaterials

Topological phononic crystals and acoustic metamaterials have attracted enormous research attention in recent years due to the presence of robust and disorder-immune wave propagation. In this study, a sinusoidal lightweight elastic topological insulator with protected interface modes is investigated at a subwavelength frequency region. By a wave dispersion study, the dual Dirac cones are observed at a subwavelength frequency region due to the employment of two distinct cylinders connected with sinusoidal ligaments. Both cylindrical masses and sinusoidal ligaments are found responsible for opening low-frequency bandgaps that manipulate elastic wave wavelengths almost 30 times larger than the lattice size. Consequently, the subwavelength bandgap closing-and-reopening phenomenon with phase transitions is further captured and opposite signs of the valley Chern numbers are obtained for different structural parameters. A supercell structure is constructed based on the phase transition, and dual topologically protected interface modes (TPIMs) are captured with different quality factors. The comparison of topologically protected interface modes shows that TPIM I is in a higher and wider frequency range, while TPIM II is positioned in a comparatively narrow and extremely low-frequency range. Finally, the robust elastic wave propagation along various designated paths is demonstrated. The proposed lightweight topologically protected phononic lattice may spark future investigation of topological edge states in metadevices at a subwavelength frequency region.

Tailoring the band-pass properties of FSS-based THz filters

High-performance THz band-pass filters are highly demanded for THz spectroscopy and imaging applications. In this work, we have analyzed the spectral features of a THz band-pass filter and divided the spectrum into three spectral regions, including a pure waveguide region, a subwavelength region and a cut-off region. Based on the understanding of the spectrum, we have proposed and fabricated two kinds of THz band-pass filters, which were characterized by time-domain THz spectroscopy and show narrow bandwidths and high stopband suppression.

Frequency-Tunable Terahertz Plasmonic Structure Based on the Solid Immersed Method for Sensing.

Terahertz waves are located in the frequency band between radio waves and light, and they are being considered for various applications as a light source. Generally, the use of light requires focusing; however, when a terahertz wave is irradiated onto a small detector or a small measurement sample, its wavelength, which is much longer than that of visible light, causes problems. The diffraction limit may make it impossible to focus the terahertz light down to the desired range by using common lenses. The Bull’s Eye structure, which is a plasmonic structure, is a promising tool for focusing the terahertz light beyond the diffraction limit and into the sub-wavelength region. By utilizing the surface plasmon propagation, the electric field intensity and transmission coefficient can be enhanced. In this study, we improved the electric field intensity and light focusing in a small region by adapting the solid immersion method (SIM) from our previous study, which had a frequency-tunable nonconcentric Bull’s Eye structure. Through electromagnetic field analysis, the electric field intensity was confirmed to be approximately 20 times higher than that of the case without the SIM, and the transmission measurements confirmed that the transmission through an aperture had a gap of 1/20 that of the wavelength. This fabricated device can be used in imaging and sensing applications because of the close contact between the transmission aperture and the measurement sample.