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Abstract Fog is a collection of micro drops of water suspended in the air, formed as a result of cooling of moist air. In supercooled air, water droplets freeze, forming ice fog at air temperatures below − 10–15° C. As the ice drop freezes, it forms a core-shell structure. In such a particle, a high-Q Fano resonance is possible, which entails the formation of a magnetic pulse. Our theoretical calculations have predicted the time-dependent formation of Fano resonances in a freezing the outside in water droplet. Time-varying unconventional Fano resonance with magnetic field enhancement yield new method to manipulate light–matter interactions in a freezing water droplet. To the best of our knowledge this mechanism was not discussed previously.

The selective control and manipulation of nanoparticles require developing and researching new methods for designing optical tweeters, mainly based on a photonic hooks (PHs) effect. This paper first proposes a tunable PH in which a structured beam illuminates an anisotropic cutting liquid crystal microcylinder based on the Finite-DifferenceTime-Domain (FDTD) method. The PHs generated by plane wave, Gaussian, and Bessel beam are analyzed and compared. The impact of beams and LC particle parameters on the PHs are discussed. Where the influence of the extraordinary refractive index (ne) on PHs is emphasized. Our results reveal that introducing birefringence can change the bending direction of PH. Besides, the maximum intensity of the PHs increases as ne increases regardless of the beam type. The PH generated by a plane wave has a higher maximum intensity and smaller FWHM than that generated by the Gaussian and Bessel beams. The smallest FWHM and maximum intensity of the PHs generated by the Gaussian falls between that generated by the plane wave and the Bessel beam. The PH generated by a Bessel beam has the minor maximum intensity and the largest FWHM. Still, it exceeds the diffraction limit and exhibits bending twice due to its self-recovery property. This paper provides a new way to modulate PH. This work offers novel theoretical models and the degree of freedom for the design of PHs, which is beneficial for the selective manipulation of nanoparticles. It has promising applications in Mesotronics and biomedicine.

Abstract Tunable optical devices are of great interest as they offer adjustability to their functions. Temporal optics is a fast-evolving field, which may be useful both for revolutionizing basic research of time-dependent phenomena and for developing full optical devices. With increasing focus on ecological compatibility, bio-friendly alternatives are a key subject matter. Water in its various forms can open up new physical phenomena and unique applications in photonics and modern electronics. Water droplets freezing on cold surfaces are ubiquitous in nature. We propose and demonstrate the effectual generation of time domain self-bending photonic hook (time–PH) beams by using mesoscale freezing water droplet. The PH light bends near the shadow surface of the droplet into large curvature and angles superior to a conventional Airy beam. The key properties of the time–PH (length, curvature, beam waist) can be modified flexibly by changing the positions and curvature of the water–ice interface inside the droplet. Due to the modifying internal structure of freezing water droplets in real time, we showcase the dynamical curvature and trajectory control of the time–PH beams. Compared with the traditional methods, our phase-change- based materials (water and ice) of the mesoscale droplet have advantages of easy fabrication, natural materials, compact structure and low cost. Such PHs may have applications in many fields, including temporal optics and optical switching, microscopy, sensors, materials processing, nonlinear optics, biomedicine, and so on.

Abstract Adipose tissue (AT) optical properties for physiological temperatures and in vivo conditions are still insufficiently studied. The AT is composed mainly of packed cells close to spherical shape. It is a possible reason that AT demonstrates a very complicated spatial structure of reflected or transmitted light. It was shown with a cellular tissue phantom, is split into a fan of narrow tracks, originating from the insertion point and representing filament-like light distribution. The development of suitable approaches for describing light propagation in a AT is urgently needed. A mathematical model of the propagation of light through the layers of fat cells is proposed. It has been shown that the sharp local focusing of optical radiation (light localized near the shadow surface of the cells) and its cleavage by coupling whispering gallery modes depends on the optical thickness of the cell layer. The optical coherence tomography numerical simulation and experimental studies results demonstrate the importance of sharp local focusing in AT for understanding its optical properties for physiological conditions and at AT heating.

In this paper, we demonstrate a novel photonic hook being initiated using an air–liquid interface (ALI). This bent light focus is produced by immersing a dielectric micro-cylinder partially at the edge of a thin liquid film whose thickness is smaller than the diameter of the micro-cylinder. Unlike the well-known properties of normal near-field focuses, this photonic hook propagates horizontally in the liquid along the ALI at specific depths and does not require the material processing of microscopic particles or the modulation of light irradiation for initiation. A morphological analysis indicates that the contrast in the refractive indexes of the ALI causes this phenomenon at the shadow end of the micro-cylinder with a transverse dimension smaller than the diffraction limit. Compared to previously discovered photonic hooks, the unique setup of this photonic hook can generate a force field that enables optical trapping in the region slightly beneath the ALI, and the related optical pressures have been simulated.

We show that electric field on the plane surface of truncated sphere or cylinders (so called Janus particles) have sharp resonances versus the depth of removed segment of a sphere or cylinder. These resonances are related to the excited whispering gallery waves caused by truncation. It is a new mechanism of the field localization. Optimization of this effect for cylinders permits to reach a super resolution in the line thickness, which can be used for contact optical lithography.

Abstract Photonic hooks have demonstrated to be great candidates for multiple applications ranging from sensing up to optical trapping. In this work, we propose a mechanism to produce such bent structured light beams by exploiting the diffraction and scattering generated by a pair of dielectric rectangles immersed in free space. It is shown how the photonic hooks are generated away from the output surface of the dielectrics by correctly engineering each individual dielectric structure to generate minimum diffraction and maximum scattering along the propagation axis. Different scenarios are studied such as dual-dielectric structures having different lateral dimensions and refractive index as well as cases when both dielectrics have the same lateral dimensions. The results are evaluated both numerically and theoretically demonstrating an excellent agreement between them. These results may open new avenues for optical trapping, focusing and sensing devices via compact and simple dual-dielectric structures.

This paper pays attention to the broader interest of freezing water droplets in mesotronics, particularly to their use as a new all-optical device platform. Here, we show that a freezing mesoscale water droplet with a low Bond number can behave as fully biocompatible natural microlense to form a photonic hook for application in a tunable temperature-controlled optical switch. We first introduced and demonstrated the basic concepts of an optical switch without changes in the wavelength of illumination of a particle or any moving parts being involved. The principle of the operation of the switch is based on the temperature-induced phase change inside the water droplet’s refractive index. The simulation results show that the optical isolation of switched channels for an optical switch with linear dimensions of about 15 λ3 based on a freezing water droplet can reach 10 dB in the process of temperature variation at a fixed wavelength. The use of freezing mesoscale droplets acting as a time-domain photonic hook generator open an intriguing route for optical switching in multifunctional green electronics tools for sensing, integrated optics and optical computers.

Abstract In this study, we report the characteristics of acoustic jets obtained through a mesoscale (radius less than 5 wavelengths) ABS cylinder made with a 3D printer. We have analyzed the influence of cylinder size on the characteristic parameters of an acoustic jet, such as maximum acoustic intensity at focus, Full Width at Half Maximum and length of Acoustic Jet. FWHM below 0.5 wavelength in AJ was experimentally obtained. It has been observed that there are two operating regimes depending on the cylinder radius: the resonant and the non-resonant. In the resonant regime, the excitation of Whispering Gallery Modes results in optimal parameter values of the acoustic jet. However, as it is a resonant regime, any minimal variation in cylinder size, working frequency or refractive index would make resonance disappear. In non-resonant mode, a phononic crystal has been embedded inside the cylinder and the characteristic parameters of the acoustic jet have been studied. These have been observed to improve. Finally, we have shown that curved acoustic jets can be obtained with the ABS cylinder with a phononic crystal embedded inside.

In this study, we propose a novel design of triangular mesoscale Janus prisms for the generation of the long photonic hook. Numerical simulations based on the finite-difference time-domain method are used to examine the formation mechanism of the photonic hook. The electric intensity distributions near the micro-prisms are calculated for operation at different refractive indices and spaces of the two triangular micro-prisms. The asymmetric vortices of intensity distributions result in a long photonic hook with a large bending angle. The length and the bending angle of the photonic hook are efficiently modulated by changing the space between the two triangular micro-prisms. Moreover, the narrow width of the photonic hook is achieved beyond the diffraction limit. The triangular Janus micro-prisms have high potential for practical applications in optical tweezers, nanoparticle sorting and manipulation and photonic circuits.

The paper under discussion has some shortcomings in regard to mathematical formulation of the problem related to acoustics, its justification, some references analyses and the text, and we feel that this study has not been finished to its full extent. From the point of view of acoustic jet formation, the mathematical model used by the authors has a number of significant limitations which are not taken into account. Moreover, the formal analogy between the parameters of electromagnetic waves and acoustic waves should be used with caution in the conditions of localized beams (acoustic jet), and the limits of their application should be justified.

In this manuscript, we demonstrate the design and experimental proof of an optical cloaking structure that multi-directionally conceals a perfectly electric conductor (PEC) object from an incident plane wave. The dielectric modulation around the highly reflective scattering PEC object is determined by an optimization process for multi-directional cloaking purposes. Additionally, to obtain the multi-directional effect of the cloaking structure, an optimized slice is mirror symmetrized through a radial perimeter. The three-dimensional (3D) finite-difference time-domain method is integrated with genetic optimization to achieve a cloaking design. In order to overcome the technological problems of the corresponding devices in the optical range and to experimentally demonstrate the proposed concept, our experiments were carried out on a scale model in the microwave range. The scaled proof-of-concept of the proposed structure is fabricated by 3D printing of polylactide material, and the brass metallic alloy is used as a perfect electrical conductor for microwave experiments. A good agreement between numerical and experimental results is achieved. The proposed design approach is not restricted only to multi-directional optical cloaking but can also be applied to different cloaking scenarios dealing with electromagnetic waves at nanoscales as well as other types such as acoustic waves. Using nanotechnology, our scale proof-of-concept research will take the next step toward the creation of “optical cloaking” devices.

The recently emerged field of Mesotronics provides novel opportunities for subwavelength magnetic and electric field localization and giant enhancement by mesoscale dielectric particles and structures from low-index to high-index materials, supported by novel optical phenomena. In this case, two regions: non-resonant and resonant, can be distinguished. In this short review, which is a direct continuation of our recently published study, we continue to present the authors’ point of view on some new optical effects in dielectric mesotronics. Among them are anomalous apodization effect in phase mesoscale gratings, new effects on high order Fano-resonances and extreme effects in field localization, mesoscale particle-based super-resolution and high-speed communications, photonic hook-based high-contrast subwavelength imaging, and reverse optical energy flow in a perforated resonant spherical particle.

Optical switches are key elements in modern network communications. We present the results of the experimental verification of a new theoretical concept proposed earlier for a full-optical wavelength-selective dual-channel switch based on the photonic hook effect, which is free from using any micro-mechanical devices or nonlinear materials. A large-scale laboratory prototype of such a device based on an off-axis Wood zone plate is considered, and its main parameters in the millimeter wavelength range are investigated. On the basis of the experiments, we show that the optical isolation of switched channels for a switch based on an off-axis zone plate can achieve 15 dB at a frequency difference of 25 GHz in a frequency range of 93 to 136 GHz. Given the scaling, these results can be transferred to another range, including the optical one.

The development of all-dielectric structures with high magnetic response at optical frequencies has become a matter of intense study in past years. However, magnetic effects are weak at optical frequencies due to the small value of the magnetic permeability of natural materials. To this end, natural dielectric materials are unemployable for practical “magnetic” applications in optics. We have shown for the first time that it is possible to induce intense magnetic concentric subwavelength “hot circles” in a dielectric mesoscale Janus particle. The basis of the Janus particle is a combination of the effects of a photonic jet, whispering-gallery waves, and the concept of solid immersion. Simulations show an (H/H0)2/(E/E0)2 contrast of more than 10, and maximal magnetic field intensity enhancement is more than 1000 for a wavelength-scaled particle with a refractive index n < 2 and a size parameter in the order of 30. This work may provide a new way to realize precise magnetic devices for integrated photonic circuits and light–matter interaction.

A short-range terahertz (THz) wireless transmission in the 300 GHz band is demonstrated using low-profile wavelength-scaled dielectric transmitting and receiving cuboid antennas (DCAs). These dielectric cuboid antennas are made of polytetrafluoroethylene with dimensions of approximately 1.2 mm × 1.2 mm × 1.3 mm. The near-field pattern of a DCA at 300 GHz was measured using an electro-optic sensing technique, and its far-field pattern characterization was based on the near-field to far-field transformation. The measured antenna gain was 15.06 ± 0.06 dBi. By employing DCAs as transmitting and receiving antennas, a 17.5 Gbps data transmission rate at distances of approximately 200 and 50 mm with bit error rates of 3.31 × 10–3 and 7.51 × 10–7 respectively, is demonstrated. The proposed mesoscopic scale DCA is a promising antenna type in intra-device communications and Kiosk download applications for future mobile devices operating in the 300 GHz band.

Per the laws of geometrical optics, a plane-parallel medium does not possess focusing properties. However, in this paper, we demonstrate for the first time that an acoustic cuboid particle filled with CO2 is able to focus sound despite its flat surface. It is worth noting that traditional lenses focus sound through their curved surfaces. We report both numerically and experimentally the acoustic focusing of a cuboid of 2λ side, where λ is the wavelength in air. From these results, it can be derived that its focusing capabilities are close to the diffraction limit: at frequency of 5000 Hz, acoustic beam waist is about 0.5 wavelength and the gain is 8.8 dB. The flat structure lens proposed open new possibilities to design and built new lightweight and low cost acoustic lenses for different applications. Keywords: Sound focusing, CO2 acoustic lens, Diffraction limit

A floating zone plate (ZP) was installed in the axial vircator for the formation of VC2 by using the wall charge of drifting electrons. The material of the opaque zones was changed to aluminum, copper, and glass to observe the effects. The main frequency of the microwave increased when VC2 is formed. At new frequencies, the opaque zones with aluminum and copper behave as an obstacle for the propagation of microwave, while the glass is suitable for the power enhancement. These results have significant importance for the implementation of real experiments to form VC2 by wall charge.

Photonic hook is a high-intensity, bent light focus with a proportional curvature to the wavelength of the incident light. Based on this unique light-bending phenomenon, a novel near-field photonic switch by means of a right-trapezoid dielectric Janus particle-lens embedded in the core of a planar waveguide is proposed for switching the photonic signals at two common optical communication wavelengths, 1310 nm and 1550 nm, by using numerical simulations. The signals at these two wavelengths can be guided to different routes according to their oppositely bent photonic hooks to realise wavelength selective switching. The switching mechanism is analysed by an in-house developed three-dimensional (3D) Poynting vector visualisation technology. It demonstrates that the 3D distribution and number of Poynting vector vortexes produced by the particle highly affect the shapes and bending directions of the photonic hooks causing the near-field switching, and multiple independent high-magnitude areas matched by the regional Poynting vector streamlines can form these photonic hooks. The corresponding mechanism can only be represented by 3D Poynting vector distributions and is being reported for the first time.

In this work, we report the first experimental observation of a new type of near-field curved acoustic beam confirmed by simulations. This new curved acoustical beam is generated by asymmetric distribution of the vortices in a polymer Janus particle (particle with broken symmetry) immersed in water. The origin of the vortices is in the conversion of an incident longitudinal wave mode to a shear wave in a solid and then back to a longitudinal wave in the water and has unique features, the radius of curvature of acoustical hook is less than the wavelength. Indeed, it is the smallest radius of curvature ever recorded for any acoustical beams. These results may be potentially useful when an object, located in the path of the beam, must be avoided. It could also have potential applications in particle manipulations.

In this article, we study the optical force exerted on nanorods. In recent years, the capture of micro-nanoparticles has been a frontier topic in optics. A Photonic Jet (PJ) is an emerging subwavelength beam with excellent application prospects. This paper studies the optical force exerted by photonic jets generated by a plane wave illuminating a Generalized Luneburg Lens (GLLs) on nanorods. In the framework of the dipole approximation, the optical force on the nanorods is studied. The electric field of the photonic jet is calculated by the open-source software package DDSCAT developed based on the Discrete Dipole Approximation (DDA). In this paper, the effects of the nanorods’ orientation and dielectric constant on the transverse force Fx and longitudinal force Fy are analyzed. Numerical results show that the maximum value of the positive force and the negative force are equal and appear alternately at the position of the photonic jet. Therefore, to capture anisotropic nanoscale-geometries (nanorods), it is necessary to adjust the position of GLLs continuously. It is worth emphasizing that manipulations with nanorods will make it possible to create new materials at the nanoscale.

During the last decade, new unusual physical phenomena have been discovered in studying the optics of dielectric mesoscale particles of an arbitrary three-dimensional shape with the Mie size parameter near 10 (q~10). The paper provides a brief overview of these phenomena from optics to terahertz, plasmonic and acoustic ranges. The different particle configurations (isolated, regular or Janus) are discussed, and the possible applications of such mesoscale structures are briefly reviewed herein in relation to the field enhancement, nanoparticle manipulation and super-resolution imaging. The number of interesting applications indicates the appearance of a new promising scientific direction in optics, terahertz and acoustic ranges, and plasmonics. This paper presents the authors’ approach to these problems.

In this study, we present the simulations and experimental observations of photonic jet (PJ) shaping by control of tangential electric field components at illuminating wavelengths of 405 nm, 532 nm, and 671 nm. The PJs are generated by a single dielectric 4-micrometer cube that was fabricated from polydimethylsiloxane (PDMS). The dielectric cube is deposited on a silicon substrate and placed on two aluminum masks with a width equal to the side length of the cube. Due to the appearance of the metal masks, the focal length and decay length of the generated PJs decreased almost twice, while the PJ resolution increased 1.2 times. Thus, PJ shaping can be controlled by the presence of the metal mask along the lateral surface of the cube without changing the external shape or internal structure of the cube. This effect is based on the control of the tangential components of the electric field along the lateral surface of the cube. In the case of a one-sided metal mask, the effect of optical deflection and bending is predicted to form a photonic hook. Due to the low cost of these dielectric cubes, they have potential in far-field systems to better meet the requirements of modern optical integration circuits and switches.

An infrared (IR) pyroelectric detector was investigated for terahertz (THz) detection using the principle of the terajet effect, which focuses the beam beyond the diffraction limit. The terahertz beam was coupled to the detector’s optical window through a two-wavelength-dimension dielectric cubic particle-lens based on the terajet effect. We experimentally demonstrate an enhancement of about 6 dB in the sensitivity under excitation of 0.2 THz without degradation of the noise equivalent power value. The results show that the proposed method could be applied to increase the sensitivity of various commercial IR sensors for THz applications that do not require modification of the internal structure, and it may apply also to acoustics and plasmonic detectors.

The photonic nanojet phenomenon is commonly used both to increase the resolution of optical microscopes and to trap nanoparticles. However, such photonic nanojets are not applicable to an entire class of objects. Here we present a new type of photonic nanojet in reflection mode with the possibility to control the modulation of the photonic nanojet by a standing wave. In contrast to the known kinds of reflective photonic nanojets, the reported one occurs when the aluminum oxide hemisphere is located at a certain distance from the substrate. Under illumination, the hemisphere generates a primary photonic nanojet directed to the substrate. After reflection, the primary nanojet acts as an illumination source for the hemisphere, leading to the formation of a new reflective photonic nanojet. We show that the distance between the hemisphere and substrate affects the phase of both incident and reflected radiation, and due to constructive interference, the modulation of the reflective photonic nanojet by a standing wave can be significantly reduced. The results obtained contribute to the understanding of the processes of photonic nanojet formation in reflection mode and open new pathways for designing functional optical devices.

The study of accelerating Airy-family beams has made significant progress, not only in terms of numerical and experimental investigations, but also in conjunction with many potential applications. However, the curvature of such beams (and hence their acceleration) is usually greater than the wavelength. Relatively recently, a new type of localized wave beams with subwavelength curvature, called photonic hooks, was discovered. This paper briefly reviews the substantial literature concerning photonic jet and photonic hook phenomena, based on the photonic jet principle. Meanwhile, the photonic jet ensemble can be produced by optical wave diffraction at 2D phase diffraction gratings. The guidelines of jets’ efficient manipulation, through the variation of both the shape and spatial period of diffraction grating rulings, are considered. Amazingly, the mesoscale dielectric Janus particle, with broken shape or refractive index symmetry, is used to generate the curved photonic jet—a photonic hook—emerging from its shadow-side surface. Using the photonic hook, the resolution of optical scanning systems can be improved to develop optomechanical tweezers for moving nanoparticles, cells, bacteria and viruses along curved paths and around transparent obstacles. These unique properties of photonic jets and hooks combine to afford important applications for low-loss waveguiding, subdiffraction-resolution nanopatterning and nanolithography.

In this work, the fishnet metamaterial is applied to several converging metalenses by combining the zoning, reference phase, and phase reversal techniques. First, the zoning and reference phase techniques are implemented in several metalenses at 55 GHz (λ0=5.45 mm) with a short focal length of 1.5 λ0. Then, the phase reversal technique is applied to these metalenses by switching from a concave to a convex profile in order to change the phase distribution inside of them. The designs are evaluated both numerically and experimentally demonstrating that chromatic dispersion (the shift of the position of the focus at different frequencies) is reduced when using the phase-reversed profiles. It is shown how the position of the focus remains at the same location within a relatively broadband frequency range of ~4% around the design frequency without affecting the overall behaviour of the metalenses. The best performance is achieved with the design that combines both reference phase and phase reversal techniques, with an experimental position of the focus of 1.75 λ0, reduced side lobes, and a power enhancement of 6.5 dB. The metalenses designed here may find applications in situations where a wideband response and low side lobes are required because of the reduced chromatic aberrations of the focus.

Currently, phased arrays have found wide application in ultrasonic nondestructive testing. Volumetric results provided by the

A method of detecting dielectric objects hidden behind an opaque barrier located on a reflective background, based on the distortion of interference fringes, is proposed in this article. Experiments conducted in the millimeter wavelength range demonstrated the effectiveness of the method under consideration, which does not require a holographic image reconstruction. Such a system can be classified as contour imaging.

One of the most captivating properties of dielectric mesoscale particles is their ability to form a sub-diffraction limited-field localization region, near their shadow surfaces. However, the transverse size of the field localization region of a dielectric mesoscale particle is usually larger than λ/3. In this present paper, for the first time, we present numerical simulations to demonstrate that the size of the electromagnetic field that forms in the localized region of the dielectric mesoscale sphere can be significantly reduced by introducing a nanohole structure at its shadow surface, which improves the spatial resolution up to λ/40 and beyond the solid immersion diffraction limit of λ/2n. The proposed nanohole-structured microparticles can be made from common natural optical materials, such as glass, and are important for advancing the particle-lens-based super-resolution technologies, including sub-diffraction imaging, interferometry, surface fabrication, enhanced Raman scattering, nanoparticles synthesis, optical tweezer, etc.

The image performance of acoustic and ultrasound sensors depends on several fundamental parameters such as depth of focus or lateral resolution. There are currently two different types of acoustic diffractive lenses: those that form a diffraction-limited spot with a shallow depth of focus (zone plates) and lenses that form an extended focus (quasi-Bessel beams). In this paper, we investigate a pupil-masked Soret zone plate, which allows the tunability of a normalized angular spectrum. It is shown that the depth of focus and the lateral resolution can be modified, without changing the lens structure, by choosing the size of the pupil mask. This effect is based on the transformation of spherically-converging waves into quasi-conical waves, due to the apodization of the central part of the zone plate. The theoretical analysis is verified with both numerical simulations and experimental measurements. A Soret zone plate immersed in water with D/2F = 2.5 and F = 4.5 λ changes its depth of focus from 2.84 λ to 5.9 λ and the lateral resolution increases from 0.81 λ to 0.64 λ at a frequency of 250 kHz, by modifying the pupil mask dimensions of the Soret zone plate.

A structure based on periodically arranged 3D dielectric cuboids connected by photonic nanojets (PNJs) is proposed with the aim of increasing the propagation distance of surface plasmon polaritons (SPPs) at the telecom wavelength of 1550 nm. The performance of the structure is evaluated and compared with the case without the cuboids demonstrating that the SPPs propagation length is enhanced by a factor greater than 2, reaching a value of approximately 19λ0, when the gap between the cuboids is 2.5λ0. Also, the dependence of the propagation length with the height of the cubes is evaluated, showing that this parameter is critical for a good performance of the chain. A subwavelength resolution is obtained for all the jets generated at the output of the cuboids.