Transition metal dichalcogenide nanospheres for high-refractive-index nanophotonics and biomedical theranostics.

Recent developments in the area of resonant dielectric nanostructures have created attractive opportunities for concentrating and manipulating light at the nanoscale and the establishment of the new exciting field of all-dielectric nanophotonics. Transition metal dichalcogenides (TMDCs) with nanopatterned surfaces are especially promising for these tasks. Still, the fabrication of these structures requires sophisticated lithographic processes, drastically complicating application prospects. To bridge this gap and broaden the application scope of TMDC nanomaterials, we report here femtosecond laser-ablative fabrication of water-dispersed spherical TMDC (MoS2 and WS2) nanoparticles (NPs) of variable size (5 to 250 nm). Such NPs demonstrate exciting optical and electronic properties inherited from TMDC crystals, due to preserved crystalline structure, which offers a unique combination of pronounced excitonic response and high refractive index value, making possible a strong concentration of electromagnetic field in the NPs. Furthermore, such NPs offer additional tunability due to hybridization between the Mie and excitonic resonances. Such properties bring to life a number of nontrivial effects, including enhanced photoabsorption and photothermal conversion. As an illustration, we demonstrate that the NPs exhibit a very strong photothermal response, much exceeding that of conventional dielectric nanoresonators based on Si. Being in a mobile colloidal state and exhibiting superior optical properties compared to other dielectric resonant structures, the synthesized TMDC NPs offer opportunities for the development of next-generation nanophotonic and nanotheranostic platforms, including photothermal therapy and multimodal bioimaging.

Multiresonant all-dielectric metasurfaces based on high-order multipole coupling in the visible.

In many cases, optical metasurfaces are studied in the single-resonant regime. However, a multiresonant behavior can enable multiband devices with reduced footprint, and is desired for applications such as display pixels, multispectral imaging and sensing. Multiresonances are typically achieved by engineering the array lattice (e.g., to obtain several surface lattice resonances), or by adopting a unit cell hosting one (or more than one) nanostructure with some optimized geometry to support multiple resonances. Here, we present a study on how to achieve multiresonant metasurfaces in the visible spectral range by exploiting high-order multipoles in dielectric (e.g., diamond or titanium dioxide) nanostructures. We show that in a simple metasurface (for a fixed particle and lattice geometry) one can achieve triple resonance occurring nearly at RGB (red, green, and blue) wavelengths. Based on analytical and numerical analysis, we demonstrate that the physical mechanism enabling the multiresonance behavior is the lattice induced coupling (energy exchange) between high-order Mie-type multipoles moments of the metasurface's particles. We discuss the influence on the resonances of the metasurface's finite size, surrounding material, polarization, and lattice shape, and suggest control strategies to enable the optical tunability of these resonances.

Lasing Effect in Symmetrical van der Waals Heterostructured Metasurfaces Due to Lattice-Induced Multipole Coupling.

New practical ways to reach the lasing effect in symmetrical metasurfaces have been developed and theoretically demonstrated. Our approach is based on excitation of the resonance of an octupole quasi-trapped mode (OQTM) in heterostructured symmetrical metasurfaces composed of monolithic disk-shaped van der Waals meta-atoms featured by thin photoluminescent layers and placed on a substrate. We revealed that the coincidence of the photoluminescence spectrum maximum of these layers with the wavelength of high-quality OQTM resonance leads to the lasing effect. Based on the solution of laser rate equations and direct full-wave simulation, it was shown that lasing is normally oriented to the metasurface plane and occurs from the entire area of metasurface consisting of MoS2/hBN/MoTe2 disks with line width of generated emission of only about 1.4 nm near the wavelength 1140 nm. This opens up new practical possibilities for creating surface emitting laser devices in subwavelength material systems.

Trapped mode control in metasurfaces composed of particles with the form birefringence property.

Progress in developing advanced photonic devices relies on introducing new materials, discovered physical principles, and optimal designs when constructing their components. Optical systems operating on the principles of excitation of extremely high-quality factor trapped modes (also known as the bound states in the continuum, BICs) are of great interest since they allow the implementation of laser and sensor devices with outstanding characteristics. In this paper, we discuss how one can utilize the anisotropic properties of novel materials (transition metal dichalcogenides, TMDs), particularly, the bulk molybdenum disulfide (MoS2), to realize the excitation of trapped modes in dielectric metasurfaces. The bulk MoS2 is a thin-film structure in which the light wave behaves the same way as that in the uniaxial anisotropic material with the form birefringence property. Our metasurface is composed of an array of disk-shaped nanoparticles (resonators) made of the MoS2 material under the assumption that the anisotropy axis of MoS2 can be tilted to the rotation axis of the disks. We perform a detailed analysis of eigenwaves and scattering properties of such anisotropic resonators as well as the spectral features of the metasurface revealing dependence of the excitation conditions of the trapped mode on the anisotropy axis orientation of the MoS2 material used.

Core-shell particles as efficient broadband absorbers in infrared optical range.

We demonstrate that efficient broadband absorption of infrared radiation can be obtained with deeply subwavelength spherical dielectric particles covered by a thin metal layer. Considerations based on Mie theory and the quasi-static approximation reveala wide range of configuration parameters, within which the absorption cross section reaches the geometrical one and exceeds more than by order of magnitude the scattering cross section in the infrared spectrum. We show that the absorption is not only efficient but also broadband with the spectral width being close to the resonant wavelength corresponding to the maximum of the absorption cross section. We obtain a simple analytical expression for the absorption resonance that allows one to quickly identify the configuration parameters ensuring strong infrared absorption in a given spectral range. Relation between the absorption resonance and excitation of the short-range surface palsmon modes in the metal shell of particles is demonstrated and discussed. Our results can be used as practical guidelines for realization of efficient broadband infrared absorbers of subwavelength sizes desirable in diverse applications.

Transverse Scattering and Generalized Kerker Effects in All-Dielectric Mie-Resonant Metaoptics.

All-dielectric resonant nanophotonics lies at the heart of modern optics and nanotechnology due to the unique possibilities to control scattering of light from high-index dielectric nanoparticles and metasurfaces. One of the important concepts of dielectric Mie-resonant nanophotonics is associated with the Kerker effect that drives the unidirectional scattering of light from nanoantennas and Huygens metasurfaces. Here we suggest and demonstrate experimentally a novel effect manifested in the nearly complete simultaneous suppression of both forward and backward scattered fields. This effect is governed by the Fano resonance of an electric dipole and off-resonant quadrupoles, providing necessary phases and amplitudes of the scattered fields to achieve the transverse scattering. We extend this concept to dielectric metasurfaces that demonstrate zero reflection with transverse scattering and strong field enhancement for resonant light filtering, nonlinear effects, and sensing.

Resonant suppression of light transmission in high-refractive-index nanoparticle metasurfaces.

High-refractive-index nanoparticle two-dimensional arrays have attracted a lot of interest recently, as they support both electric and magnetic resonances and can be implemented as functional metasurfaces. Here we show that under particular conditions, the all-dielectric nanoparticle metasurfaces can resonantly suppress transmission. As an important example, resonant electric and magnetic dipole (MD) responses of silicon nanoparticle arrays are considered in the air, as well as in the dielectric matrix in visible and infrared spectral ranges. We show that the wave resonantly scattered forward by one or both electric and MD moments of nanoparticles can destructively interfere with the incident wave, providing significant suppression of the transmission through the array. The reported effect can find important applications in different fields related to optics and photonics such as the development of filters, sensors, and solar cells.

Direct Amplitude-Phase Near-Field Observation of Higher-Order Anapole States.

Anapole states associated with the resonant suppression of electric-dipole scattering exhibit minimized extinction and maximized storage of electromagnetic energy inside a particle. Using numerical simulations, optical extinction spectroscopy, and amplitude-phase near-field mapping of silicon dielectric disks, we demonstrate high-order anapole states in the near-infrared wavelength range (900-1700 nm). We develop the procedure for unambiguously identifying anapole states by monitoring the normal component of the electric near-field and experimentally detect the first two anapole states as verified by far-field extinction spectroscopy and confirmed with the numerical simulations. We demonstrate that higher-order anapole states possess stronger energy concentration and narrower resonances, a remarkable feature that is advantageous for their applications in metasurfaces and nanophotonics components, such as nonlinear higher-harmonic generators and nanoscale lasers.

Resonant forward scattering of light by high-refractive-index dielectric nanoparticles with toroidal dipole contribution.

In this Letter, we demonstrate and investigate the Kerker-type effect in high-index dielectric nanoparticles for which the third-order multipoles give a considerable contribution to the light scattering process. It is shown that the Kerker-type effect (strong suppression of the backward light scattering and, simultaneously, resonant forward light scattering) can be associated with the resonant excitation of a toroidal dipole moment in the system. This effect is realized due to the interference of the scattered waves generated by electric, magnetic, and toroidal dipole moments of high-index nanoparticles.

Highly Stable Monocrystalline Silver Clusters for Plasmonic Applications.

Plasmonic sensor configurations utilizing localized plasmon resonances in silver nanostructures typically suffer from the rapid degradation of silver under ambient atmospheric conditions. In this work, we report on the fabrication and detailed characterization of ensembles of monocrystalline silver nanoparticles (NPs), which exhibit a long-term stability of optical properties under ambient conditions without any protective treatments. Ensembles with different densities (surface coverages) of size-selected NPs (mean diameters of 12.5 and 24 nm) on quartz substrates are fabricated using the cluster-beam technique and characterized by linear spectroscopy, two-photon-excited photoluminescence, surface-enhanced Raman scattering microscopy, and transmission electron, helium ion, and atomic force microscopies. It is found that the fabricated ensembles of monocrystalline silver NPs preserve their plasmonic properties (monitored with optical spectroscopy) and strong field enhancements (revealed by surface-enhanced Raman spectroscopy) at least 5 times longer as compared to chemically synthesized silver NPs with similar sizes. The obtained results are of high practical relevance for the further development of sensors, resonators, and metamaterials utilizing the plasmonic properties of silver NPs.

Spatiotemporal characterization of SPP pulse propagation in two-dimensional plasmonic focusing devices.

The spatiotemporal evolution of a SPP wave packet with femtosecond duration is experimentally investigated in two different plasmonic focusing structures. A two-dimensional reconstruction of the plasmonic field in space and time is possible by the numerical analysis of interferometric time-resolved photoemission electron microscopy data. We show that the time-integrated and time-resolved view onto the wave packet dynamics allow one to characterize and compare the capabilities of two-dimensional components for use in plasmonic devices operating with ultrafast pulses.

Demonstration of magnetic dipole resonances of dielectric nanospheres in the visible region.

Strong resonant light scattering by individual spherical Si nanoparticles is experimentally demonstrated, revealing pronounced resonances associated with the excitation of magnetic and electric modes in these nanoparticles. It is shown that the low-frequency resonance corresponds to the magnetic dipole excitation. Due to high permittivity, the magnetic dipole resonance is observed in the visible spectral range for Si nanoparticles with diameters of ∼200 nm, thereby opening a way to the realization of isotropic optical metamaterials with strong magnetic responses in the visible region.

Low-frequency magnetic response of gold nanoparticles.

Gold nanoparticles (AuNPs) exposed to low frequency magnetic fields have shown promise in enhancing biological processes, such as cellular reprogramming. Despite the experimental evidence, a comprehensive understanding of the underlying physical principles and the corresponding theory remains elusive. The most common hypothesis is that functionalized nanoparticles transiently amplify magnetic fields, leading to improved cellular reprogramming efficiency. However, a detailed investigation on this topic is lacking. This paper bridges this knowledge gap by conducting a comprehensive investigation on the magnetic response of surface-modified AuNPs exposed to magnetic fields with frequencies up to hundreds of MHz. Starting with the inherent properties of bulk gold material, we explore a wide range of magnetic susceptibilities that might result from the redistribution of charge carriers due to bond molecules on the particle surfaces. Through analytical models and numerical electromagnetic simulations, we examine various geometric factors that can enhance the magnetic response, including the number of particles, spatial distribution, size, and shape. Our broad investigation provides researchers with analytical and numerical estimates of the magnetic response of nanoparticles, and the associated limits that can be expected. We found that a magnetic field enhancement comparable to the incident field requires very high magnetic susceptibilities, well beyond the values measured in functionalized gold nanoparticles thus far.

SERS uncovers the link between conformation of cytochrome c heme and mitochondrial membrane potential.

The balance between the mitochondrial respiratory chain activity and the cell's needs in ATP ensures optimal cellular function. Cytochrome c is an essential component of the electron transport chain (ETC), which regulates ETC activity, oxygen consumption, ATP synthesis and can initiate apoptosis. The impact of conformational changes in cytochrome c on its function is not understood for the lack of access to these changes in intact mitochondria. We have developed a novel sensor that uses unique properties of label-free surface-enhanced Raman spectroscopy (SERS) to identify conformational changes in heme of cytochrome c and to elucidate their role in functioning mitochondria. We have verified that molecule bond vibrations assessed by SERS are a reliable indicator of the heme conformation during changes in the inner mitochondrial membrane potential and ETC activity. We have demonstrated that cytochrome c heme reversibly switches between planar and ruffled conformations in response to the inner mitochondrial membrane potential (ΔΨ) and H+ concentration in the intermembrane space. This regulates the efficiency of the mitochondrial respiratory chain, thus, adjusting the mitochondrial respiration to the cell's consumption of ATP and the overall activity. We have found that under hypertensive conditions cytochrome c heme loses its sensitivity to ΔΨ that can affect the regulation of ETC activity. The ability of the proposed SERS-based sensor to track mitochondrial function opens broad perspectives in cell bioenergetics.

Photonic bandgap plasmonic waveguides.

A type of a plasmonic waveguide has been proposed featuring an "open" design that is easy to manufacture, simple to excite and offers convenient access to a plasmonic mode. Optical properties of photonic bandgap (PBG) plasmonic waveguides are investigated experimentally by leakage radiation microscopy and numerically using the finite element method confirming photonic bandgap guidance in a broad spectral range. Propagation and localization characteristics of a PBG plasmonic waveguide have been discussed as a function of the wavelength of operation, waveguide core size, and the number of ridges in the periodic reflector for fundamental and higher order plasmonic modes of the waveguide.

Detuned electrical dipoles for plasmonic sensing.

We demonstrate that a pair of electrical dipolar scatterers resonating at different frequencies, i.e., detuned electrical dipoles, can be advantageously employed for plasmonic sensing of the environment, both as an individual subwavelength-sized sensor and as a unit cell of a periodic array. It is shown that the usage of the ratio between the powers of light scattered into opposite directions (or into different diffraction orders), which peaks at the intermediate frequency, allows one to reach a sensitivity of ≈ 400 nm/RIU with record high levels of figure of merit exceeding 200. Qualitative considerations are supported with detailed simulations and proof-of-principle experiments using lithographically fabricated gold nanorods with resonances at 800 nm.

Enhanced absorption in all-dielectric metasurfaces due to magnetic dipole excitation.

All-dielectric nanophotonics lies at a forefront of nanoscience and technology as it allows to control light at the nanoscale using its electric and magnetic components. Bulk silicon does not experience any magnetic response, nevertheless, we demonstrate that the metasurface made of silicon parallelepipeds allows to excite the magnetic dipole moment leading to the broadening and enhancement of the absorption. Our investigations are underpinned by the numerical predictions and the experimental verifications. Also surprisingly we found that the resonant electric quadrupole moment leads to the enhancement of reflection. Our results can be applied for a development of absorption based devices from miniature dielectric absorbers, filters to solar cells and energy harvesting devices.

Holographic evanescent-wave focusing with nanoparticle arrays.

Three-dimensional focusing of evanescent waves by specially configured surface arrays of nanoparticles emulating near-field optical holograms of dipole sources (located close to the surface) is suggested and analyzed. The idea is to place chains of nanoparticles along bright fringes of calculated (holographic) interference patterns so that the local nanoparticle density along these chains would be proportional to the local intensity contrast in the interference patterns. Three different configurations are considered: a holographic scheme with totally internally reflected reference and reconstructing waves, a modified scheme with the reconstructing wave being represented by a suitable surface plasmon polariton (SPP) plane wave, and a SPP holographic scheme with reference and reconstructing waves both being (phase-conjugated) laterally-confined (Gaussian) SPP beams. Our numerical approach is based on the Greens function technique with the point-dipole approximation for radiation scattering by nanoparicles. We demonstrate that a nanoparticle array configured in accordance with the intensity interference pattern formed by a dipole field and a reference wave allows one to efficiently focus the (phase-conjugated) reconstructing wave (via its scattering by the nanoparticle array) at the site of the dipole. Influence of the polarization and wavelength of the reconstructing wave on the resulting intensity distribution is also considered. Fabrication of suitable nanoparticle arrays is discussed along with their potential applications.

Refracting surface plasmon polaritons with nanoparticle arrays.

Refraction of surface plasmon polaritons (SPPs) by various structures formed by a 100-nm-period square lattice of gold nanoparticles on top of a gold film is studied by leakage radiation microscopy. SPP refraction by a triangular-shaped nanoparticle array indicates that the SPP effective refractive index increases inside the array by a factor of approximately 1.08 (for the wavelength 800 nm) with respect to the SPP index at a flat surface. Observations of SPP focusing and deflection by circularly shaped areas as well as SPP waveguiding inside rectangular arrays are consistent with the SPP index increase deduced from the SPP refraction by triangular arrays. The SPP refractive index is found to decrease slightly for longer wavelengths within the wavelength range of 700-860 nm. Modeling based on the Green's tensor formalism is in a good agreement with the experimental results, opening the possibility to design nanoparticle arrays for specific applications requiring in-plane SPP manipulation.

Surface plasmon polariton beam focusing with parabolic nanoparticle chains.

We report on the focusing of surface plasmon polariton (SPP) beams with parabolic chains of gold nanoparticles fabricated on thin gold films. SPP focusing with different parabolic chains is investigated in the wavelength range of 700-860 nm, both experimentally and theoretically. Mapping of SPP fields is accomplished by making use of leakage radiation microscopy, demonstrating robust and efficient SPP focusing into submicron spots. Numerical simulations based on the Green's tensor formalism show very good agreement with the experimental results, suggesting the usage of elliptical corrections for parabolic structures to improve their focusing of slightly divergent SPP beams.