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We propose a universal strategy to realize a broadband control on arbitrary scatterers, through multiple coherent beams. By engineering the phases and amplitudes of incident beams, one can suppress the dominant scattering partial waves, making the obstacle lose its intrinsic responses in a broadband spectrum. The associated coherent beams generate a finite and static region, inside which the corresponding electric field intensity and Poynting vector vanish. As a solution to go beyond the sum-rule limit, our methodology is also irrespective of inherent system properties, as well as extrinsic operating wavelength, providing a non-invasive control on the wave-obstacles interaction for any kinds of shape.

Optical nanocircuits, inspired by electrical nanocircuits, provide a versatile platform for tailoring and manipulating optical fields at the subwavelength scale, which is vital for developing various innovative optical nanodevices and integrated nanosystems. Plasmonic nanoparticles can be employed as promising building blocks for optical nanocircuits with unprecedentedly high integration capacity. Among various plasmonic systems, aggregated metallic nanoparticle, known as oligomers, possess great potential in constructing functional metatronic circuits. Here, the optical nanocircuits comprising special plasmonic oligomers, such as trimers with D3h symmetry, quadrumers with D2h symmetry, and their variants with reduced symmetry, are systematically investigated in the metatronic paradigm, both theoretically and experimentally. Our proposed circuit models, based on the displacement current in the oligomers, not only reproduce the resonance spectral details, but also retrieve many hidden physical quantities associated with their optical responses. Guided by the metatronic circuits, the spectral engineering of the oligomers with reduced geometric symmetry is predicted, and subgroup decomposition of several plasmonic quadrumers is examined. Our investigation has revealed a close correlation between the metatronic circuitry and strongly coupled plasmonic oligomers. The observed correlation of spatial arrangement and frequency response in oligomers provides a metatronic guide to modulate plasmonic responses via geometric variation.

Fast detection of near-infrared (NIR) photons with high responsivity remains a challenge for photodetectors. Germanium (Ge) photodetectors are widely used for near-infrared wavelengths but suffer from a trade-off between the speed of photodetection and quantum efficiency (or responsivity). To realize a high-speed detector with high quantum efficiency, a small-sized photodetector efficiently absorbing light is required. In this paper, we suggest a realization of a dielectric metasurface made of an array of subwavelength germanium PIN photodetectors. Due to the subwavelength size of each pixel, a high-speed photodetector with a bandwidth of 65 GHz has been achieved. At the same time, high quantum efficiency for near-infrared illumination can be obtained by the engineering of optical resonant modes to localize optical energy inside the intrinsic Ge disks. Furthermore, small junction capacitance and the possibility of zero/low bias operation have been shown. Our results show that all-dielectric metasurfaces can improve the performance of photodetectors.

We study the near-field enhancement of oligomers composed of high-refractive-index spherical particles. The huge enhancement of the electromagnetic field in the gap between the particles is observed. We reveal that this phenomenon is associated with the Fano-like resonances of both electric and magnetic multipole contributions inside each particle. We also predict that electric and magnetic Purcell factors can be strongly enhanced, being larger than 103. The obtained results represent an important guideline for the realization of dielectric structures to enhance the spontaneous emission rate of quantum sources and various two-level systems.

Metasurfaces provide a compact, flexible, and efficient platform to manipulate the electromagnetic waves. However, chromatic aberration imposes severe restrictions on their applications in broadband polarization control. Here, we propose a broadband achromatic methodology to implement polarization-controlled multifunctional metadevices in mid-wavelength infrared with birefringent meta-atoms. We demonstrate the generation of polarization-controlled and achromatically on-axis focused optical vortex beams with diffraction-limited focal spots and switchable topological charge (L = 0 and L = 2). Besides, we further implement broadband achromatic polarization beamsplitter with high polarization isolation (extinction ratio up to 21). The adoption of all-silicon configuration not only facilitates the integration with CMOS technology but also endows the polarization multiplexing meta-atoms with broad phase dispersion coverage, ensuring the large size and high performance of the metadevices. Compared with the state-of-the-art chromatic aberration-restricted polarization-controlled metadevices, our work represents a substantial advance and a step toward practical applications. ∥ ⊥

The shift of Fano resonances has been widely exploited to realize biomedical sensing, optical filtering, and optical switching. Here, the formation of Fano resonances in the scattering spectra of silicon nanoparticles excited by surface plasmon polaritons is reported and the modification of Fano lineshapes is demonstrated. It is revealed that the subradiant and superradiant modes responsible for the formation of Fano resonance originate from the coherent interaction of the electric and magnetic dipoles excited in the silicon nanoparticle with their mirror images induced by a thin metal film. The modification of the asymmetry parameter q can be realized by simply adjusting the angle of the incident light or by slightly varying the environment refractive index. The tuning of the q value is accompanied by the variation of the field enhancement factors and manifested in the color change of the scattered light. The strongest enhancement of both electric and magnetic fields can be achieved at the point of contact between the silicon nanoparticle and the metal film for the symmetric Fano lineshape (q ≈ 0). These findings indicate the potential applications of such a hybrid metal–dielectric system in sensing, color display, and strong light–matter interaction.

The strongly localized electric field achieved at the Mie resonances of a silicon nanoparticle enables the generation of a large carrier density, which offers us the opportunity to manipulate the linear and nonlinear optical properties of silicon nanoparticles by optically injecting a dense electron-hole plasma. Here, we show that the dense electron-hole plasma created in a silicon nanoparticle significantly modifies the complex dielectric constant of silicon, which in turn leads to wavelength shift and amplitude change in the magnetic dipole resonance. We demonstrate that the maximum wavelength shift of the magnetic dipole resonance can be revealed by exploiting the hot-electron luminescence emitted by the silicon nanoparticle, which acts as a built-in light source with a broad bandwidth and a short lifetime. We demonstrate that the quantum efficiency of the hot-electron luminescence of silicon nanoparticles can be enhanced a factor of more than 5 through the injection of a dense electron-hole plasma. More interestingly, an acceleration of the radiative recombination process is found at high carrier densities. Our findings are helpful for understanding the modification of Mie resonances in silicon nanoparticles induced by the dense electron-hole plasmas and useful for designing silicon-based photonic devices.

Recently developed general constraint polynomial approach is shown to replace a set of algebraic equations of the functional Bethe Ansatz method by a single polynomial constraint. As the proof of principle, the usefulness of the method is demonstrated for a number of quasi-exactly solvable (QES) potentials of the Schrödinger equation, such as two different sets of modified Manning potentials with three parameters, an electron in Coulomb and magnetic fields and relative motion of two electrons in an external oscillator potential, the hyperbolic Razavy potential, and a (perturbed) double sinh-Gordon system. The approach enables one to straightforwardly determine eigenvalues and wave functions. Odd parity solutions for the modified Manning potentials are also determined. For the QES examples considered here, constraint polynomials terminate a finite chain of orthogonal polynomials in an independent variable that need not to be necessarily energy. In the majority of cases the finite chain of orthogonal polynomials is characterized by a positive-definite moment functional L, implying that a corresponding constraint polynomial has only real and simple zeros. Constraint polynomials are shown to be different from the weak orthogonal Bender–Dunne polynomials. At the same time the QES examples considered elucidate essential difference with various generalizations of the Rabi model. Whereas in the former case there are n+ 1 polynomial solutions at each point of a nth baseline, in the latter case there are at most n+ 1 polynomial solutions on entire nth baseline.

In this review, the nexus of nanophotonics and quantum science is considered. This nexus offers a wealth of applications and novel fundamental effects. Such an interdisciplinary approach may serve as a starting point for developing groundbreaking technologies. Several new directions in quantum nanophotonics are addressed, including strong light–matter coupling, bound states in the continuum, topological photonics, metasurfaces and photon sources, and levitated optomechanics. This review hopefully will foster the enhanced interaction between the nanophotonics and quantum science communities. Such a synergy will lead to the realization of novel communication, sensing, and information processing systems.

Increasing demand for higher resolution of miniaturized displays requires techniques achieving high contrast tunability of the images. Employing metasurfaces for image contrast manipulation is a new and rapidly growing field of research aiming to address this need. Here, a new technique to achieve image tuning in a reversible fashion is demonstrated by dielectric metasurfaces composed of subwavelength resonators. It is demonstrated that by controlling the temperature of a metasurface the encoded transmission pattern can be tuned. To this end, two sets of nanoresonators composed of nonconcentric silicon disks with a hole that exhibit spectrally sharp Fano resonances and forming a Yin-Yang pattern are designed and fabricated. Through exploitation of the thermo-optical properties of silicon, full control of the contrast of the Yin-Yang image is demonstrated by altering the metasurface temperature by ΔT ≈ 100 °C. This is the first demonstrated technique to control an image contrast by temperature. Importantly, the turning technique does not require manipulating the external stimulus, such as polarization or angle of the illumination and/or the refractive index of this environment. These results open many opportunities for transparent displays, optical switches, and tunable illumination systems.

The development of a miniaturised device that provides efficient beam manipulation with high transmittance is extremely desirable for the broad range of applications including holography, metalens, and imaging. Recently, the potential of dielectric metasurfaces has been unleashed to efficiently manipulate the beam with full 2π-phase control by overlapping the electric and magnetic dipole resonances. However, in the visible range for available materials, it comes with the price of higher absorption that reduces efficiency. Here, we have considered dielectric amorphous silicon (a-Si) nanodisk and engineered them in such a way which provides minimal absorption loss in the visible range. We have experimentally demonstrated meta-deflector with high transmittance which operates in the visible wavelengths. The supercell of proposed meta-deflector consists of 15 amorphous silicon nanodisks numerically shows the transmission efficiency of 95% and deflection efficiency of 95% at operating wavelength of 715 nm. However, experimentally measured transmission and deflection efficiencies are 83% and 71%, respectively, having the experimental deflection angle of 8.40°. Nevertheless, by reducing the supercell length, the deflection angle can be controlled, and the value 15.50° was experimentally achieved using eight disks supercell. Our results suggest a new way to realise the highly transmittance metadevice with full 2π-phase control operating with the visible light which could be applicable in the imaging, metalens, holography, and display applications.

For the same potential as originally studied by Ma (Phys. Rev., 71 (1947) 195) we obtain analytic expressions for the Jost functions and the residue of the S-matrix of both i) redundant poles and ii) the poles corresponding to true bound states. This enables us to demonstrate that the Heisenberg condition is valid in spite of the presence of redundant poles and a singular behaviour of the S-matrix for . In addition, we analytically determine the overall contribution of redundant poles to the asymptotic completeness relation, provided that the residue theorem can be applied. The origin of redundant poles and zeros is shown to be related to peculiarities of analytic continuation of a parameter of two linearly independent analytic functions.

With the wave interferometric approach, we study how extrinsically coherent waves excitation can dramatically alter the overall scattering properties, resulting in tailoring the energy assignment between radiation and dissipation, as well as filtering multipolar resonances. As an illustration, we consider cylindrical passive systems encountered by arbitrary configurations of incident waves with various illuminating directions, phases, and intensities. With formulas for dissipation and radiation powers, we demonstrate that a coherent superposition of incident waves extrinsically interferes with the targeted channels in a desirable way. Moreover, the interferometric results can be irrespective of inherent system properties such as size, material, and structure. Our approach paves a non-invasive solution to manipulate wave–obstacle interaction at will.

Second-harmonic generation (SHG) in resonant dielectric Mie-scattering nanoparticles has been hailed as a powerful platform for nonlinear light sources. While bulk-SHG is suppressed in elemental semiconductors, for example, silicon and germanium due to their centrosymmetry, the group of zincblende III-V compound semiconductors, especially (100)-grown AlGaAs and GaAs, have recently been presented as promising alternatives. However, major obstacles to push the technology toward practical applications are the limited control over directionality of the SH emission and especially zero forward/backward radiation, resulting from the peculiar nature of the second-order nonlinear susceptibility of this otherwise highly promising group of semiconductors. Furthermore, the generated SH signal for (100)-GaAs nanoparticles depends strongly on the polarization of the pump. In this work, we provide both theoretically and experimentally a solution to these problems by presenting the first SHG nanoantennas made from (111)-GaAs embedded in a low index material. These nanoantennas show superior forward directionality compared to their (100)-counterparts. Most importantly, based on the special symmetry of the crystalline structure, it is possible to manipulate the SHG radiation pattern of the nanoantennas by changing the pump polarization without affecting the linear properties and the total nonlinear conversion efficiency, hence paving the way for efficient and flexible nonlinear beam-shaping devices.

We discuss the effect of a sharp variation in the input signal temporal envelope upon the output at destructive interference in Fano resonances. The interference takes place between the resonant and off-resonant partitions canceling each other. For high-Q resonances, the characteristic relaxation time for the former may be much larger than that for the latter. Then, if the variation of the input is faster than the relaxation of the resonant partition, it destroys the balance between the partitions, i.e., the conditions for the destructive interference. Therefore, the variation (whether an increase or a decrease) must give rise to a sharp spike in the uncompensated output. To illustrate these general arguments, we study numerically the scattering of a linearly polarized laser pulse of a rectangular envelope by a dielectric cylinder. The simulation reveals pronounced spikes in the scattered light, situated just behind the leading edge of the incident pulse and, counterintuitively, behind its trailing edge, when the incident pulse is already complete. The second spike is explained by the rapid release of the electromagnetic energy confined in the cylinder in the steady-state scattering owing to the destructive Fano interference. The spikes have narrow scattering diagrams so that the effect may be employed to design new nanodevices, e.g., a single-nanoparticle laser generating ultrashort pulses.

Nano-antennas are replicas of antennas that operate at radio-frequencies, but with considerably smaller dimensions when compared with their radio frequency counterparts. Noble metals based nano-antennas have the ability to enhance photoinduced phenomena such as localized electric fields, therefore-they have been used in various applications ranging from optical sensing and imaging to performance improvement of solar cells. However, such nano-structures can be damaged in high power applications such as heat resisted magnetic recording, solar thermo-photovoltaics and nano-scale heat transfer systems. Having a small footprint, nano-antennas cannot handle high fluences (energy density per unit area) and are subject to being damaged at adequately high power (some antennas can handle just a few milliwatts). In addition, given that nano-antennas are passive devices driven by external light sources, the potential damage of the antennas limits their use with high power lasers: this liability can be overcome by employing materials with high melting points such as chromium (Cr) and tungsten (W). In this article, we fabricate chromium and tungsten nano-antennas and demonstrate that they can handle 110 and 300 times higher fluence than that of gold (Au) counterpart, while the electric field enhancement is not significantly reduced.

All-dielectric nanophotonics attracts ever increasing attention nowadays due to the possibility of controlling and configuring light scattering on high-index semiconductor nanoparticles. It opens a room of opportunities for designing novel types of nanoscale elements and devices, and paves the way for advanced technologies of light energy manipulation. One of the exciting and promising prospects is associated with utilizing the so-called toroidal moment, being the result of poloidal currents excitation, and anapole states, corresponding to the interference of dipole and toroidal electric moments. Here, higher-order toroidal moments of both types (up to the electric octupole toroidal moment) are presented and investigated in detail via the direct Cartesian multipole decomposition allowing new near- and far-field configurations to be obtained. Poloidal currents can be associated with vortex-like distributions of the displacement currents inside nanoparticles, revealing the physical meaning of the high-order toroidal moments and the convenience of the Cartesian multipoles as an auxiliary tool for analysis. High-order nonradiating anapole states accompanied by the excitation of intense near-fields are demonstrated. It is believed that the results are of high importance for both the fundamental understanding of light scattering by high-index particles and a variety of nanophotonics applications and light governing on nanoscale.

Dynamical tuning of the nonlinear optical wavefront allows for a specific spectral response of predefined profiles, enabling various applications of nonlinear nanophotonics. This study experimentally demonstrates the dynamical switching of images generated by an ultrathin silicon nonlinear metasurface supporting a high-quality leaky mode, which is formed by partially breaking a bound-state-in-the-continuum (BIC) generated by the collective magnetic dipole (MD) resonance excited in the subdiffractive periodic systems. Such a quasi-BIC MD state can be excited directly under normal plane wave incidence and leads to a strong near-field enhancement to further boost the nonlinear process, resulting in a 500-fold enhancement of the third-harmonic emission experimentally. Due to sharp spectral features and asymmetry of the unit cell, it allows for effective tailoring of the nonlinear emissions over spectral or polarization responses. Dynamical nonlinear image tuning is experimentally demonstarted via polarization and wavelength control. The results pave the way for nanophotonics applications such as tunable displays, nonlinear holograms, tunable nanolaser, and ultrathin nonlinear nanodevices with various functionalities.

The spin–orbit interaction (SOI) of light is usually weak in dielectric particles and recently is enhanced in plasmonic structures. This Letter Article shows that strong SOI can arise in dielectric particles with dual symmetry, resulting in enhanced spin Hall effect of scattered light. It reveals the relationship between spin Hall shift and dual symmetry. The interference between electric and magnetic dipolar modes plays an important role in enhancing the spin Hall shift. Strong SOI is demonstrated by the optical singularity of the orbital angular momentum. The results can find potential applications in dielectric-based photonic devices, such as optical sensing and manipulation of subwavelength nanoparticles.

As one of the most important semiconductors, silicon has been used to fabricate electronic devices, waveguides, detectors, solar cells, etc. However, the indirect bandgap and low quantum efficiency (10−7) hinder the use of silicon for making good emitters. For integrated photonic circuits, silicon-based emitters with sizes in the range of 100−300 nm are highly desirable. Here, we show the use of the electric and magnetic resonances in silicon nanoparticles to enhance the quantum efficiency and demonstrate the white-light emission from silicon nanoparticles with feature sizes of ~200 nm. The magnetic and electric dipole resonances are employed to dramatically increase the relaxation time of hot carriers, while the magnetic and electric quadrupole resonances are utilized to reduce the radiative recombination lifetime of hot carriers. This strategy leads to an enhancement in the quantum efficiency of silicon nanoparticles by nearly five orders of magnitude as compared with bulk silicon, taking the three-photon-induced absorption into account.

A novel type of all-dielectric metalattice is proposed and experimentally demonstrated, which supports broadband dominant toroidal dipole response. The unit cell consists of four rectangular bars made of dielectric materials of moderate contrast. By performing multipole decomposition it is found that toroidal dipole response is dominant over a wide frequency range. The correlation between the toroidal dipole response and longitudinal electric field component is also shown. By using this feature, as well as direct near-field mapping, the broadband toroidal dipole response of such metalattice is experimentally verified. Since the unit cell exhibits C4 rotational symmetry this response is shown to be polarization independent.

We demonstrate that a dielectric anapole resonator on a metallic mirror can enhance the third harmonic emission by two orders of magnitude compared to a typical anapole resonator on an insulator substrate. By employing a gold mirror under a silicon nanodisk, we introduce a novel characteristic of the anapole mode through the spatial overlap of resonantly excited Cartesian electric and toroidal dipole modes. This is a remarkable improvement on the early demonstrations of the anapole mode in which the electric and toroidal modes interfere off-resonantly. Therefore, our system produces a significant near-field enhancement, facilitating the nonlinear process. Moreover, the mirror surface boosts the nonlinear emission via the free-charge oscillations within the interface, equivalent to producing a mirror image of the nonlinear source and the pump beneath the interface. We found that these improvements result in an extremely high experimentally obtained efficiency of 0.01%.

Simple structure, quick response, and highly sensitive miniaturized sensors are highly desirable for the broadrange of sensing applications. In this work, we numerically investigated a highly sensitive photonic-crystalfiber (PCF)-based plasmonic sensor for refractive index sensing. We consider gold as the plasmonic material,which is used outside the fiber structure to exhibit the plasmonic phenomena and to help detect the surroundingmedium refractive index. The proposed PCF is designed to enable the evanescent field to interact with an externalsensing medium leading to a highly sensitive response. The sensor performance has been investigated by wavelength and amplitude interrogation methods. The proposed sensor exhibits the maximum amplitude sensitivity of2,843 RIU-1 with the sensor resolution of 3.5 × 10-6 RIU, which is the highest among the reported PCF SPRsensors, to the best of our knowledge. It also shows wavelength sensitivity of 18,000 nm/RIU and sensor resolution of 5.6 × 10-6 RIU. The figure of merit of the proposed sensor is about 400. The sensor response alsoallows us to detect the refractive index variation in the range of 1.33 to 1.41. Such promising results and broadsensing range ensure that the proposed sensor will be a suitable candidate for biological analytes and biochemicaland organic chemical detections.

Based on fundamental properties of the light scattering by a particle under a plane, linearly polarized wave illumination, we rigorously prove the existence of the ultimate upper limit for the light absorption by any partial mode and calculate this limit explicitly. The limit is a certain simple universal function of the incident light wave number, and the multipolarity of the corresponding partial mode solely. It does not depend on the optical constants of the scatterer, its size, or even its shape. First, we obtain this result for the scattering by a spherical particle. Then, we generalize it to an arbitrary finite obstacle. The results are valid for any polarization of the incident wave, any angle of its incidence, and any type of the scatterer (homogeneous, stratified, or with smoothly variable refractive index). We also prove that the maximal partial absorption cross section for any finite scatterer cannot exceed the corresponding value for a homogeneous sphere in 3D and circular cylinder in 2D. As an example, the results are applied to maximize the absorption cross section of a spherical core-shell structure.

Kepler's observation of comet tails initiated the research on the radiation pressure of celestial objects and 250 years later they found new incarnation after the Maxwell's equations were formulated to describe a plethora of light-matter coupling phenomena. Further, quantum mechanics gave birth to the photon drag effect. Here, we develop a microscopic theory of this effect which can occur in a general system containing Bose-Einstein-condensed particles, which possess an internal structure of quantum states. By analyzing the response of the system to an external electromagnetic field we find that such a drag results in a flux of particles constituting both the condensate and the excited states. We show that in the presence of the condensed phase, the response of the system acquires steplike behavior as a function of the electromagnetic field frequency with the elementary step determined by the internal energy structure of the particles.

Advances in the field of plasmonics, i.e., nanophotonics exploiting the optical properties of metal nanostructures, paved the way for the development of ultrasensitive devices, including biological and various other sensors whose operation is based on the nanoscale localization of the electromagnetic field. However, the high dissipation loss of metal nanostructures limits their use in many current applications, including metasurfaces, metamaterials, and nanowaveguides, thus requiring the development of new devices that combine metal nanostructures with highly refractive dielectric nanoparticles. The resulting metal-dielectric (hybrid) nanostructures have demonstrated many interesting properties from the practical application standpoint (moderate dissipation loss, resonant magnetooptical response, strong nonlinear optical properties, etc.), thus placing this field at the vanguard of the modern science of light. We review the current state of theoretical and experimental research into hybrid metal-dielectric nanoantennas and their derivative nanostructures capable of selectively scattering light waves, directionally amplifying and transmitting optical signals, controlling the propagation of such signals, and generating optical harmonics.

Toroidal multipole moments are usually underestimated as they are quite weak in most cases of light-matter interaction. Herein, we reveal a strong link between the toroidal dipole resonance and the bound state in the continuum in the context of all-dielectric metasurfaces. We introduce the concept of toroidal dipole bound states in the continuum, in which two eigenmodes of the silicon metasurface exhibit an intrinsic toroidal dipolar character and have an infinite lifetime. They can be classified as transverse (trivial) and longitudinal (nontrivial) toroidal dipole modes, which correspond to symmetry unprotected and protected bound states in the continuum, respectively. We demonstrate that such toroidal bound states in the continuum supported by the symmetry metasurface can be turned into ultrahigh-Q resonances with a dominant toroidal dipole excitation, which validates their physical origin associated with the ultrahigh-Q toroidal dipole leaky resonances. A full multipole decomposition with dispersive, lossy, and substrate effects further validates that the proposed concept is general and can also be generalized to other structures.

We propose a hybrid metasurface-based perfect absorber which shows the near-unity absorbance and facilities to work as a refractive index sensor. We have used the gold mirror to prevent the transmission and used the amorphous silicon (a-Si) nanodisk arrays on top of the gold mirror which helps to excite the surface plasmon by scattering light through it at the normal incident. We numerically investigated the guiding performance. The proposed absorber is polarization independent and shows a maximum absorption of 99.8% at a 932 nm wavelength in the air medium. Considering the real applications, by varying the environments refractive indices from 1.33 to 1.41, the proposed absorber can maintain absorption at more than 99.7%, with a red shift of the resonant wavelength. Due to impedance matching of the electric and magnetic dipoles, the proposed absorber shows near-unity absorbance over the refractive indices range of 1.33 to 1.41, with a zero-reflectance property at a certain wavelength. This feature could be utilized as a plasmonic sensor in detecting the refractive index of the surrounding medium. The proposed plasmonic sensor shows an average sensitivity of 325 nm/RIU and a maximum sensitivity of 350 nm/RIU over the sensing range of 1.33 to 1.41. The proposed metadevice possesses potential applications in solar photovoltaic and photodetectors, as well as in organic and bio-chemical detection.

We experimentally demonstrate that an enhanced THz magnetic source can be achieved by placing a subwavelength fiber in front of a sub-wavelength aperture illuminated by a THz plane wave. This hybrid system can be considered as a unit cell of metasurfaces for THz wave manipulation and can also be scaled to optical frequencies opening up avenues for developing fiber based optical devices such as nanoantenna and laser.

This book provides a comprehensive overview of the photonic sensing field by covering plasmonics, photonic crystal, and SOI techniques from theory to real sensing applications.

We study the third-harmonic generation by structured light in subwavelength silicon nanoparticles which support both electric and magnetic multipolar Mie resonances. By tailoring the vectorial structure of the pumping light, we may control both strength and polarization composition of the excited harmonic fields. In this way, we generate nonlinear fields with radial or azimuthal polarizations by addressing selectively a different type of multipolar Mie resonances, also enhancing or suppressing the optically induced nonlinear magnetic response.

Highly sensitive and miniaturized sensors are highly desirable for real-time analyte/sample detection. In this Letter, we propose a highly sensitive plasmonic sensing scheme with the miniaturized photonic crystal fiber (PCF) attributes. A large cavity is introduced in the first ring of the PCFs for the efficient field excitation of the surface plasmon polariton mode and proficient infiltration of the sensing elements. Due to the irregular air-hole diameter in the first ring, the cavity exhibits the birefringence behavior which enhances the sensing performance. The novel plasmonic material gold has been used considering the chemical stability in an aqueous environment. The guiding properties and the effects of the sensing performance with different parameters have been investigated by the finite element method, and the proposed PCFs have been fabricated using the stack-and-draw fiber drawing method. The proposed sensor performance was investigated based on the wavelength and amplitude sensing techniques and shows the maximum sensitivities of 11,000 nm/RIU and 1;420 RIU−1, respectively. It also shows the maximum sensor resolutions of 9.1 × 10−6 and 7 × 10−6 RIU for the wavelength and amplitude sensing schemes, respectively, and the maximum figure of merits of 407. Furthermore, the proposed sensor is able to detect the analyte refractive indices in the range of 1.33–1.42; as a result, it will find the possible applications in the medical diagnostics, biomolecules, organic chemical, and chemical analyte detection.

Nonradiating current configurations have been drawing the attention of the physics community for many years. It has been demonstrated recently that dielectric nanoparticles provide a unique platform to host such nonradiating modes, called "anapoles". Here we study theoretically the excitation of such exotic anapole modes in silicon nanoparticles using structured light. Alternative illumination configurations, properly designed, are able to unlock hidden behavior of scatterers. Particularly, azimuthally polarized focused beams enable us to excite ideal anapole modes of magnetic type in dielectric nanoparticles. Firstly, we perform the decomposition of this type of excitation into its multipolar content and then we employ the T-matrix method to calculate the far-field scattering properties of nanoparticles illuminated by such beams. We propose several configuration schemes where magnetic anapole modes of simple or hybrid nature can be detected in silicon nanospheres, nanodisks and nanopillars.

Novel types of optical hybrid metasurfaces consist of metallic and dielectric elements are designed and proposed for controlling the interference between magnetic and electric modes of the system, in a reversible manner. By employing the thermo-optical effect of silicon and gold nanoantennas we demonstrate an active control on the excitation and interference between electric and magnetic modes, and subsequently, the Kerker condition, as a directive radiation pattern with zero backscattering, via temperature control as a versatile tool. This control allows precise tuning optical properties of the system and stimulating switchable sharp spectral Fanolike resonance. Furthermore, it is shown that by adjusting the intermediate distance between metallic and dielectric elements, opposite scattering directionalities are achievable in an arbitrary wavelength. Interestingly, this effect is shown to have a direct influence on nonlinear properties, too, where 10-fold enhancement in the intensity of third harmonic light can be obtained for this system, via heating. This hybrid metasurface can find a wide range of applications in slow light, nonlinear optics and bio-chemical sensing.

Emerging high-index all-dielectric nanostructures, capable of manipulating light on the subwavelength scale, empower designing and implementing novel antireflection and light-trapping layers in many photonic and optoelectronic devices. However, their performance and practicality are compromised by relatively narrow bandwidths and highly sophisticated fabrications. In this paper, we demonstrate an ultra-broadband (300-1200 nm) directional light scattering strategy using high-index surface silicon oligomer resonators fabricated by a facile, scalable, and low-cost colloidal lithography technique. The exceptional broadband forward scattering stems from a combined effect of strongly intercoupled Mie resonances within the oligomers composed of randomly positioned nanodisks in the visible region and a strong electric mode coupling between the oligomers and the high-index substrate in the red-to-near-infrared region. By implementing this efficient approach in silicon solar cells, the integrated optical reflection loss across the wavelength range 300-1200 nm can be as low as 7%. Consequently, the short-circuit current density determined from the external quantum efficiency of solar cells can be increased to 35.1 from 25.1 mA/cm2, representing an enhancement of 40%, with a demonstrated energy conversion efficiency exceeding 15.0%. The insights in this paper hold great potentials for new classes of light management and steering photonic devices with drastically improved practicality.

We study optical wave manipulations through high-index dielectric metalattices in both diffractionless metasurface and diffractive metagrating regimes. It is shown that the collective lattice couplings can be employed to tune the excitation efficiencies of all electric and magnetic multipoles of various orders supported by each particle within the metalattice. The interferences of those adjusted multipoles lead to highly asymmetric angular scattering patterns that are totally different from those of isolated particles, which subsequently enables flexible beam manipulations, including perfect reflection, perfect transmission, and efficient large-angle beam steering. The revealed functioning mechanism of manipulated interplays between lattice couplings and multipolar interferences can shed new light on both photonic branches of metasurfaces and metagratings, which can potentially inspire many advanced applications related to optical beam controls.

Manipulating the spontaneous emission rate of optical emitters with all-dielectric nanoparticles benefits from their low-loss nature and thus provides a relatively large extrinsic quantum yield. However, such Purcell effect greatly depends on the orientation of the dipole emitter. Here, we introduce the concept of isotropic magnetic Purcell effect with Purcell factors of about 300 and large extrinsic quantum yield (more than 80%) for a magnetic dipole emitter of arbitrary orientation in an asymmetric silicon nanocavity. The extrinsic quantum yield can even be boosted to nearly 100% by utilizing a GaP nanocavity. Isotropy of the Purcell factor is manifested via the orientation-independent emission of the magnetic dipole source. This isotropic Purcell effect is robust against small displacement of the emitter on the order of 10 nm, releasing the requirement of precise alignment in experiments.

Dielectric sub-wavelength particles have opened up a new platform for realization of magnetic light. Recently, we have demonstrated that a dipole emitter by a sub-wavelength fiber leads to an enhanced magnetic response. Here, we experimentally demonstrate an enhanced magnetic dipole source in the terahertz frequency range. By placing the fiber next to the hole in a metal screen, we find that the radiation power can be enhanced more than one order of magnitude. The enhancement is due to the excitation of the Mie-type resonances in the fiber. We demonstrate that such a system is equivalent to a double-fiber system excited by a magnetic source. This coupled magnetic dipole and optical fiber system can be considered a unit cell of metasurfaces for manipulation of terahertz radiation and is a proof-of-concept of a possibility to achieve enhanced radiation of a dipole source in proximity of a sub-wavelength fiber. It can also be scaled down to optical frequencies opening up promising avenues for developing integrated nanophotonic devices such as nanoantennas or lasers on fibers.

We experimentally and numerically study the angularly resolved transmission properties of dielectric metasurfaces consisting of silicon nanodisks which support electric and magnetic dipolar Mie-type resonances in the near-infrared spectral range. First, we concentrate on Huygens metasurfaces which are characterised by a spectral overlap of the fundamental electric and magnetic dipole resonances of the silicon nanodisks at normal incidence. Huygens metasurfaces exhibit a high transmitted intensity over the spectral width of the resonances due to impedance matching, while the transmitted phase shows a variation of 2π as the wavelength is swept across the width of the resonances. We observe that the transmittance of the Huygens metasurfaces depends on the incidence angle and is sensitive to polarization for non-normal incidence. As the incidence angle is increased starting from normal incidence, the two dipole resonances are shifted out of the spectral overlap and the resonant features appear as pronounced transmittance minima. Next, we consider a metasurface with an increased nanodisk radius as compared to the Huygens metasurface, which supports spectrally separate electric and magnetic dipole resonances at normal incidence. We show that for TM polarisation, we can shift the resonances of this metasurface into spectral overlap and regain the high resonant transmittance characteristic of Huygens metasurfaces at a particular incidence angle. Furthermore, both metasurfaces are demonstrated to reject all TM polarized light incident under angles other than the design overlap angle at their respective operation frequency. Our experimental observations are in good qualitative agreement with numerical calculations.

We fabricate AlGaAs nanoantennas on a glass substrate and demonstrate the highest nonlinear conversion efficiency of 10-4 with the capability for shaping the radiation patterns and polarization of the second harmonic emission in both forward and backward directions. We also decode dynamic multipolar contributions to the second harmonic generation within such nanoantennas.

A dielectric metasurface at the variation of the refractive index of the environment has been numerically simulated. The optical response of the metasurface contacting both a homogeneous medium with different refractive indices and a liquid crystal controlled by the temperature and applied electric field has been considered. The results can be used to produce optical devices for various aims. Numerical simulations have been performed for the parameters of the liquid crystal E7 widely used in industry.

Simultaneous scattering invisibility and free-space field enhancement have been achieved based on multipolar interferences among all-dielectric nanoparticles. The scattering properties of all-dielectric nanowire quadrumers are investigated and two sorts of scattering invisibilities have been identified: the trivial invisibility where the individual nanowires are not effectively excited; and the nontrivial invisibility with strong multipolar excitations within each nanowire, which results in free-space field enhancement outside the particles. It is revealed that such nontrivial invisibility originates from not only the simultaneous excitations of both electric and magnetic resonances, but also their significant magnetoelectric cross-interactions. We further show that the invisibility obtained is both polarization and direction selective, which can probably play a significant role in various applications including non-invasive detection, sensing, and non-disturbing medical diagnosis with high sensitivity and precision.

For passive electromagnetic scatterers, we explore a variety of extreme limits on directional scattering patterns in a phase diagram, regardless of the details on the geometric configurations and material properties. By demonstrating the extinction cross sections with the power conservation intrinsically embedded in the phase diagram, we give an alternative interpretation for Kerker's first and second conditions, associated with zero backward scattering (ZBS) and nearly zero forward scattering (NZFS). Physical boundaries and limitations for these directional radiations are illustrated along with a generalized Kerker's condition with implicit parameters. By taking the dispersion relations of gold silicon core-shell nanoparticles into account, we reveal the realistic parameters to experimentally implement ZBS and NZFS at optical frequencies by means of a phase diagram.

For an isotropic quantum resonant scatterer, such as a quantum dot embedded in host semiconductors, we propose a method to achieve resonant electron scattering by taking physical quantities into account through the second-order expansion. All needed physical information for spherical harmonic channels in anomalous quantum resonant scattering is revealed in a parameter space as the size of quantum scatterer is comparable to the de Broglie wavelength of incoming matter wave. Our results provide the guideline to realize quantum resonant scatterers with state-of-the-art semiconductor heterostructure technology.

All-dielectric metasurfaces provide a powerful platform for a new generation of flat optical devices, in particular, for applications in telecommunication systems, due to their low losses and high transparency in the infrared. However, active and reversible tuning of such metasurfaces remains a challenge. This study experimentally demonstrates and theoretically justifies a novel scenario of the dynamical reversible tuning of all-dielectric metasurfaces based on the temperature-dependent change of the refractive index of silicon. How to design an all-dielectric metasurface with sharp resonances by achieving interference between magnetic dipole and electric quadrupole modes of constituted nanoparticles arranged in a 2D lattice is shown. Thermal tuning of these resonances can cause drastic but reciprocal changes in the directional scattering of the metasurface in a spectral window of 75 nm. This change can result in a 50-fold enhancement of the radiation directionality. This type of reversible tuning can play a significant role in novel flat optical devices including the metalenses and metaholograms.

We reveal that an isotropic, homogeneous, subwavelength particle with high refractive index can produce ultra-small total scattering. This effect, which follows from the inhibition of the electric dipole radiation, can be identified as a Fano resonance in the scattering efficiency and is associated with the excitation of an anapole mode in the particle. This anapole mode is non-radiative and emerges from the destructive interference of electric and toroidal dipoles. The invisibility effect could be useful for the design of highly transparent optical materials. This article is part of the themed issue 'New horizons for nanophotonics'.

Nanophotonics is a rapidly developing field of research with many suggestions for a design of nanoantennas, sensors and miniature metadevices. Despite many proposals for passive nanophotonic devices, the efficient coupling of light to nanoscale optical structures remains a major challenge. In this article, we propose a nanoscale laser based on a tightly confined anapole mode. By harnessing the non-radiating nature of the anapole state, we show how to engineer nanolasers based on InGaAs nanodisks as on-chip sources with unique optical properties. Leveraging on the near-field character of anapole modes, we demonstrate a spontaneously polarized nanolaser able to couple light into waveguide channels with four orders of magnitude intensity than classical nanolasers, as well as the generation of ultrafast (of 100 fs) pulses via spontaneous mode locking of several anapoles. Anapole nanolasers offer an attractive platform for monolithically integrated, silicon photonics sources for advanced and efficient nanoscale circuitry.

We demonstrate the active tuning of all-dielectric metasurfaces exhibiting high-quality factor (high-Q) resonances. The active control is provided by embedding the asymmetric silicon meta-atoms with liquid crystals, which allows the relative index of refraction to be controlled through heating. It is found that high quality factor resonances (Q = 270 ± 30) can be tuned over more than three resonance widths. Our results demonstrate the feasibility of using all-dielectric metasurfaces to construct tunable narrow-band filters.