Achieve Subwavelength Focusing Research Articles

The sub-wavelength focusing of flexural waves achieved by splitting one plane lens into two halves with a controllable angle

The flexural wave focusing in plates and shells is of great importance for energy harvesting and vibration control. Traditional plane lens based on gradient index and metasurface usually exhibits normal focusing, i.e., the focusing spot size is larger than half wavelength. In order to improve its working performance, increase energy at the focal point, reduce the focusing spot size and further achieve subwavelength focusing, a novel design scheme is proposed, i.e., splitting one traditional plane lens into two halves with a controllable inner angle. Firstly, the transmission and phase are calculated by using the Timoshenko beam theory, utilizing which the lens design is finalized via changing beam thickness perpendicular to wave propagation. After that, the working performance of two sub-lenses is compared with that of one single lens via numerical simulations, including the focal position, energy distribution, and spot size, which are further validated through experimental measurements. It is demonstrated that the flexural wave can exhibit subwavelength focusing after traveling across two adjacent halves, with the focusing spot size smaller than half wavelength. Specifically, the subwavelength focusing is broadband, and still occurs in a frequency region centered at the designed frequency. The proposed design scheme is expected to provide an innovative approach for the lens design in acoustics and optics.

Ultraviolet/Visible Quasicylindrical Waves on Semimetal Cd3As2 Nanoplates

Resolving the photonic modes in real space is essential to understand the fundamental process and control of photonic behavior in optoelectric devices. However, the understanding of the photonic modes of semimetal Cd3As2 is still lacking. Herein, the quasicylindrical waves (QCW) on Cd3As2 nanoplates using photoemission electron microscopy (PEEM) are found out. The QCW is due to the optical field scattered by subwavelength indentations, with wavelength being the same as the incident light, and their amplitude decays as r−1/2 from the edges. The transverse magnetic (TM) mode dominates the observed QCW of Cd3As2 nanoplates, which is affected by the edge structure. In broadband from UV to visible, the QCWs on Cd3As2 nanoplates are observed. Further, nanostructures to achieve subwavelength focusing and interference lattice of the Cd3As2 optical QCW are achieved. These findings demonstrate the regulation of QCW, which benefits optimizing the optoelectronic performances of semimetal materials in the future.

Infrared subwavelength focusing metasurfaces for harvesting heat from the Earth’s back radiation

In this paper, we introduce two different metasurfaces that can achieve subwavelength focusing of the electromagnetic radiation in the infrared range with transmission efficiencies reaching 46% and 74.33% in silicon and thus they can be integrated in heat harvesting applications. We made our designs at the wavelength of 10 μm which is the peak of the back radiation of the Earth. In designing our metasurfaces in the infrared range, we used the phase modulation techniques that proved to be useful in the visible domain. The best focusing resolution achieved in our metasurfaces is 0.26λ (super-focusing). In addition to heat harvesting, the two proposed metasurfaces can be useful in other compact applications that require manipulating the mid-infrared radiation on a very small scale. We used copper and silicon in our metasurfaces which are specifically suitable for the mid-infrared range besides being CMOS compatible. In this paper, we also explained how heat can be harvested from the introduced metasurfaces using the Seebeck effect.

Subwavelength focusing in the infrared range using different metasurfaces

In this paper, we introduce three different metasurfaces that can achieve subwavelength focusing of the radiation in the infrared domain. The structures presented achieve high focusing resolution, high transmission efficiency or high mid-infrared bandwidth so they can satisfy the different critical requirements of different infrared applications. We chose the wavelength of 9 μm in the mid-infrared as an example in our designs, yet we showed how the performance of our structures varies across other infrared wavelengths. In building our metasurfaces in the infrared range, we used the phase modulation techniques that proved to be useful in the visible domain. We showed that by choosing the suitable materials and design parameters, the same phase modulation concepts that were useful in the visible range can also be used to achieve focusing in the infrared range. In our infrared designs and for the wavelength of 9 μm, the best focusing resolution achieved is 0.5λ in the first introduced metasurface while the best transmission efficiency achieved exceeds 60% in the third metasurface. The highest bandwidth attained among the introduced metasurfaces is 7–20 μm in case of the second structure. For infrared wavelengths other than 9 μm, the focusing resolution could reach down to 0.49λ at the wavelength of 20 μm in case of the second structure and the transmission efficiency could reach up to roughly 74% at all wavelengths between 17 μm and 20 μm in case of the third structure. The used materials are copper and silicon, which are specifically suitable for the infrared range besides being CMOS compatible and relatively inexpensive. The proposed metasurfaces can be integrated in infrared applications such as biosensors, infrared miniaturized spectrometers, thermal photovoltaic cells, and heat harvesting. Each proposed metasurface can serve in a different infrared application according to the critical requirement of the application itself whether it is a better focusing performance or a high transmission efficiency or even a wider mid-infrared bandwidth.

Subwavelength focusing in visible light band by a Fibonacci photonic quasi-crystal plano-concave lens

In this paper, we describe a one-dimensional photonic quasi-crystal (1D PQC) plano-concave lens. The simulation results show that there is a relationship between the focusing band and the photonic band structure: at the frequency ranging from the left edge of the second large bandgap to a certain frequency in the passband between the first large bandgap and the second large bandgap, the plano-concave lens can achieve subwavelength focusing. The influence of the change of the refractive index of material on the focusing characteristics of the lens is studied, and subwavelength focusing in the visible light band is achieved using common dielectric materials. The results are expected to provide a reference for the design and application of the 1D PQC plano-concave lenses.

Acoustic field of an ultrasonic cavity resonator with two open ends: Experimental measurements and lattice Boltzmann method modeling

High-intensity focused ultrasound (HIFU) has become an attractive therapeutic tool for noninvasive tumor treatment. The key component of HIFU systems is the acoustic transducer, which generates a focal region of high-intensity focused ultrasonic energy. A key determinant of safety in HIFU treatment is the size of the focal region. To achieve subwavelength focusing, we previously investigated the feasibility of an ultrasonic spherical cavity resonator (USCR) with two open ends. To further investigate the properties of the USCR, experiments and simulations were performed to comprehensively characterize the acoustic field generated. The emphasis was on the field formation process, the pressure distribution, the frequency dependence, and the acoustic nonlinearity. As a novel simulation approach, an axisymmetric isothermal multi-relaxation-time lattice Boltzmann method (MRT-LBM) model was used to numerically analyze the acoustic field. The reliability of this model was verified by comparing the results generated with those from experiments. The MRT-LBM model gave new insight into conventional acoustic numerical simulations and provided significant indications for USCR parameter optimization. The USCR demonstrated its feasibility for application in HIFU treatment or in other fields that demand high-precision focusing.

Super-resolution ultrasonic imaging of stiffness variations on a microscale active metasurface

Surface acoustic waves propagating over an immersed membrane metasurface, such as an array of capacitive micromachined ultrasonic transducers, can be leveraged to achieve subwavelength focusing and imaging. This is demonstrated numerically and experimentally utilizing a time reversal method on a 2D membrane array at MHz frequencies. The focusing region is a dense metasurface of CMUT membranes with 6.5 MHz resonance frequency that supports a wave field that is evanescent normal to the metasurface and capable of super-resolution along the metasurface. Electrostatically actuated membranes, spatially separate from the focusing region, are used to generate the focused wave field. Subwavelength focusing is demonstrated on the metasurface with a resolution of a single membrane resonator or λ/5. Similar techniques allow for super-resolution imaging of a subwavelength defect or change in the medium of the focusing region. A subwavelength sized imaging target, obtained by altering the stiffness of a single membrane by 1.2%, is shown to be properly imaged with subwavelength resolution. These results pave the way for practical implementation of ultrasonic super-resolution imaging systems using metasurfaces.

Efficient optimization of super-oscillatory lens and transfer function analysis in confocal scanning microscopy

Super-oscillatory lens (SOL) provides a promising way to achieve subwavelength focusing in the regime of far-field optics and realize super-resolution imaging in confocal scanning microscopy (CSM). Both binary amplitude and phase SOLs are designed with an efficient optimization method using genetic algorithm and fast Hankel transform algorithm either in oil immersion medium or in air. A much brighter hotspot is readily focused by a phase SOL compared with the amplitude counterpart, e.g., 5.8 times as bright as the latter. To fundamentally interpret the super-resolution imaging mechanism by SOL in CSM, transfer function analysis is conducted compared with basic confocal imaging. The coherent transfer function (CTF) is derived and numerically calculated. The extension of the cutoff frequency and the remarkable enhancement of the magnitude of CTF in the high frequency passband account for super-resolution imaging by SOL in CSM; however, the limited extension of the frequency domain also implies that the attainable resolution is physically limited.

Super-oscillatory optical needle

Super-oscillatory optical lenses have recently been shown to achieve subwavelength focusing and have been used for super-resolution imaging. However, the subwavelength hotspots created by these lenses are always accompanied by sidebands containing a significant fraction of the optical energy and are highly localised in the axial direction. Here, we report a class of super-oscillatory lenses that form extended subwavelength optical needles on a 15λ field of view.

FDTD Analysis of Sub-Wavelength Focusing Phenomena in Plasmonic Meta-Screens

The Finite-Difference Time-Domain (FDTD) method is used to study the frequency dependence of the transmission through a plasmonic screen with slots of sizes and spacing that were chosen using the spatially shifted beam approach to achieve subwavelength focusing. It is shown that the electric field beamwidth at the focal plane exhibits a resonance-like behavior in the vicinity of the frequency for which the meta-screen was designed to work. At this frequency range, the power flow path is shown to exhibit circulation around the slotted screen. Results are also compared with those given in [1] for a similar device working at a lower frequency range where the metallic screen behaves as a perfect electric conductor.

Microtube's Tapers Affect its Subwavelength Focusing Effect

A tapered microtube fabricated from a glass capillary tube can achieve subwavelength focusing at optical frequencies. This focusing effect is influenced by many factures. The role of taper ratios is invested in this paper with theoretical analysis and numerical simulation. It revealed that when the taper ratio is larger, the focusing spot has higher intensity and smaller FWHM.

Near-field focusing properties of zone plates in visible regime - New insights

Near-field focusing properties of zone plates are investigated in the visible regime by a 3-dimensional finite-difference time-domain method. It is shown that Frensel zone plates (FZPs) with metallic coatings can achieve subwavelength focusing in the visible wavelength. The characteristics of subwavelength focusing are found to be independent of the type of metal coatings used. All the FZPs exhibit similar shift in focal length and depth of focus when compared with classical calculations. These results indicate that plasmonic waves do not contribute to subwavelength focusing. Instead the subwavelength focusing characteristic is attributed to the interference of diffracted evanescent waves from a large numerical aperture. It is found that the near-field focusing of FZPs suppresses higher order foci such that the corresponding diffraction efficiency is improved. The use of phase zone plate structured on glass without opaque coating is proposed to improve the diffraction efficiency of subwavelength focusing.