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.

Broadband optical properties of Ti3C2 MXene revisited.

The exceptional optical, electrical, and mechanical capabilities of layered transition metal carbides, nitrides, and carbonitrides, called MXenes, revolutionized materials science. Among them, Ti3C2 received the most attention owing to the developed synthesis and processing methods, high conductivity, and pronounced plasmonic response. The latter, however, remains controversial with the open question of whether the peak around 800 nm has plasmonic or interband transition origin. To address this issue, we combine spectroscopic ellipsometry and transmittance results with first-principle computations. Their combination reveals that although Ti3C2 is a metal, its optical response becomes plasmonic (Re ε < 0) above 1415 nm, in contrast to the previous understanding. In addition to fundamental significance, this dual dielectric/plasmonic optical response opens a path for theranostic applications, as we demonstrated on the example of Ti3C2 nanospheres. Thus, our study revisits broadband (300-3300 nm) optical constants of Ti3C2 and broadens its application scope in photonics.

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.

Optical Nanoimaging of Surface Plasmon Polaritons Supported by Ultrathin Metal Films.

The physics of electrons, photons, and their plasmonic interactions change dramatically when one or more dimensions are reduced to atomic-level thicknesses. For example, graphene exhibits unique electrical, plasmonic, and optical properties. Likewise, atomic-thick metal films are expected to exhibit extraordinary quantum optical properties. Several methods of growing ultrathin metal films were demonstrated, but the quality of the obtained films was much worse compared to bulk films. In this work, we propose a new method of making ultrathin gold films that are close in their properties to bulk gold films. Excellent plasmonic properties are revealed by directly observing quasi-short- and quasi-long-range plasmons in such a film via scanning near-field optical microscopy. The results pave the way for the use of ultrathin gold films in flexible and transparent nanophotonics and optoelectronic applications.

Dual-Band Coupling of Phonon and Surface Plasmon Polaritons with Vibrational and Electronic Excitations in Molecules.

Strong coupling (SC) between light and matter excitations bears intriguing potential for manipulating material properties. Typically, SC has been achieved between mid-infrared (mid-IR) light and molecular vibrations or between visible light and excitons. However, simultaneously achieving SC in both frequency bands remains unexplored. Here, we introduce polaritonic nanoresonators (formed by h-BN layers on Al ribbons) hosting surface plasmon polaritons (SPPs) at visible frequencies and phonon polaritons (PhPs) at mid-IR frequencies, which simultaneously couple to excitons and molecular vibrations in an adjacent layer of CoPc molecules, respectively. Employing near-field optical nanoscopy, we demonstrate the colocalization of near fields at both visible and mid-IR frequencies. Far-field transmission spectroscopy of the nanoresonator structure covered with a layer of CoPc molecules shows clear mode splittings in both frequency ranges, revealing simultaneous SPP-exciton and PhP-vibron coupling. Dual-band SC may offer potential for manipulating coupling between exciton and molecular vibration in future optoelectronics, nanophotonics, and quantum information applications.

Giant and Tunable Excitonic Optical Anisotropy in Single-Crystal Halide Perovskites.

During the last years, giant optical anisotropy has demonstrated its paramount importance for light manipulation. In spite of recent advances in the field, the achievement of continuous tunability of optical anisotropy remains an outstanding challenge. Here, we present a solution to the problem through the chemical alteration of halogen atoms in single-crystal halide perovskites. As a result, we manage to continually modify the optical anisotropy by 0.14. We also discover that the halide perovskite can demonstrate optical anisotropy up to 0.6 in the visible range─the largest value among non-van der Waals materials. Moreover, our results reveal that this anisotropy could be in-plane and out-of-plane depending on perovskite shape─rectangular and square. As a practical demonstration, we have created perovskite anisotropic nanowaveguides and shown a significant impact of anisotropy on high-order guiding modes. These findings pave the way for halide perovskites as a next-generation platform for tunable anisotropic photonics.

van der Waals Materials for Overcoming Fundamental Limitations in Photonic Integrated Circuitry.

With the advance of on-chip nanophotonics, there is a high demand for high-refractive-index and low-loss materials. Currently, this technology is dominated by silicon, but van der Waals (vdW) materials with a high refractive index can offer a very advanced alternative. Still, up to now, it was not clear if the optical anisotropy perpendicular to the layers might be a hindering factor for the development of vdW nanophotonics. Here, we studied WS2-based waveguides in terms of their optical properties and, particularly, in terms of possible crosstalk distance. Surprisingly, we discovered that the low refractive index in the direction perpendicular to the atomic layers improves the characteristics of such devices, mainly due to expanding the range of parameters at which single-mode propagation can be achieved. Thus, using anisotropic materials offers new opportunities and novel control knobs when designing nanophotonic devices.

Elastic Gallium Phosphide Nanowire Optical Waveguides-Versatile Subwavelength Platform for Integrated Photonics.

Emerging technologies for integrated optical circuits demand novel approaches and materials. This includes a search for nanoscale waveguides that should satisfy criteria of high optical density, small cross-section, technological feasibility and structural perfection. All these criteria are met with self-assembled gallium phosphide (GaP) epitaxial nanowires. In this work, the effects of the nanowire geometry on their waveguiding properties are studied both experimentally and numerically. Cut-off wavelength dependence on the nanowire diameter is analyzed to demonstrate the pathways for fabrication of low-loss and subwavelength cross-section waveguides for visible and near-infrared (IR) ranges. Probing the waveguides with a supercontinuum laser unveils the filtering properties of the nanowires due to their resonant action. The nanowires exhibit perfect elasticity allowing fabrication of curved waveguides. It is demonstrated that for the nanowire diameters exceeding the cut-off value, the bending does not sufficiently reduce the field confinement promoting applicability of the approach for the development of nanoscale waveguides with a preassigned geometry. Optical X-coupler made of two GaP nanowires allowing for spectral separation of the signal is fabricated. The results of this work open new ways for the utilization of GaP nanowires as elements of advanced photonic logic circuits and nanoscale interferometers.

TiS3 Nanoribbons: A Novel Material for Ultra-Sensitive Photodetection across Extreme Temperature Ranges.

Photodetectors that can operate over a wide range of temperatures, from cryogenic to elevated temperatures, are crucial for a variety of modern scientific fields, including aerospace, high-energy science, and astro-particle science. In this study, we investigate the temperature-dependent photodetection properties of titanium trisulfide (TiS3)- in order to develop high-performance photodetectors that can operate across a wide range of temperatures (77 K-543 K). We fabricate a solid-state photodetector using the dielectrophoresis technique, which demonstrates a quick response (response/recovery time ~0.093 s) and high performance over a wide range of temperatures. Specifically, the photodetector exhibits a very high photocurrent (6.95 × 10-5 A), photoresponsivity (1.624 × 108 A/W), quantum efficiency (3.3 × 108 A/W·nm), and detectivity (4.328 × 1015 Jones) for a 617 nm wavelength of light with a very weak intensity (~1.0 × 10-5 W/cm2). The developed photodetector also shows a very high device ON/OFF ratio (~32). Prior to fabrication, the TiS3 nanoribbons were synthesized using the chemical vapor technique and characterized according to their morphology, structure, stability, and electronic and optoelectronic properties; this was performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and a UV-Visible-NIR spectrophotometer. We anticipate that this novel solid-state photodetector will have broad applications in modern optoelectronic devices.

Plasmonic nanojet: an experimental demonstration: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 3244 (2020)OPLEDP0146-959210.1364/OL.391861.

Plasmonic nanojet: an experimental demonstration.

We propose and study a microstructure based on a dielectric cuboid placed on a thin metal film that can act as an efficient plasmonic lens allowing the focusing of surface plasmons at the subwavelength scale. Using numerical simulations of surface plasmon polariton (SPP) field intensity distributions, we observe high-intensity subwavelength spots and formation of the plasmonic nanojet (PJ) at the telecommunication wavelength of 1530 nm. The fabricated microstructure was characterized using amplitude and phase-resolved scattering-type scanning near-field optical microscopy. We show the first experimental observation of the PJ effect for the SPP waves. Such a novel, to the best of our knowledge, and simple platform can provide new pathways for plasmonics, high-resolution imaging, and biophotonics, as well as optical data storage.

Vertically Coupled Plasmonic Racetrack Ring Resonator for Biosensor Applications.

Plasmonic chemical and biological sensors offer significant advantages such as really compact sizes and extremely high sensitivity. Biosensors based on plasmonic waveguides and resonators are some of the most attractive candidates for mobile and wearable devices. However, high losses in the metal and complicated schemes for practical implementation make it challenging to find the optimal configuration of a compact plasmon biosensor. Here, we propose a novel plasmonic refractive index sensor based on a metal strip waveguide placed under a waveguide-based racetrack ring resonator made of the same metal. This scheme guarantees effective coupling between the waveguide and resonator and low loss light transmittance through the long-range waveguide. The proposed device can be easily fabricated (e.g., using optical lithography) and integrated with materials like graphene oxide for providing adsorption of the biomolecules on the sensitive part of the optical elements. To analyze the properties of the designed sensing system, we performed numerical simulations along with some analytical estimations. There is one other interesting general feature of this sensing scheme that is worth pointing out before looking at its details. The sensitivity of the considered device can be significantly increased by surrounding the resonator with media of slightly different refractive indices, which allows sensitivity to reach a value of more than 1 µm per refractive index unit.

Superior Sensitivity of Copper-Based Plasmonic Biosensors.

Plasmonic biosensing has been demonstrated to be a powerful technique for quantitative determination of molecular analytes and kinetic analysis of biochemical reactions. However, interfaces of most plasmonic biosensors are made of noble metals, such as gold and silver, which are not compatible with industrial production technologies. This greatly limits biosensing applications beyond biochemical and pharmaceutical research. Here, we propose and investigate copper-based biosensor chips fully fabricated with a standard complementary metal-oxide-semiconductor (CMOS) process. The protection of thin copper films from oxidation is achieved with SiO2 and Al2O3 dielectric films deposited onto the metal surface. In addition, the deposition of dielectric films with thicknesses of only several tens of nanometers significantly improves the biosensing sensitivity, owing to better localization of electromagnetic field above the biosensing surface. According to surface plasmon resonance (SPR) measurements, the copper biosensor chips coated with thin films of SiO2 (25 nm) and Al2O3 (15 nm) show 55% and 75% higher sensitivity to refractive index changes, respectively, in comparison to pure gold sensor chips. To test biomolecule immobilization, the copper-dielectric biosensor chips are coated with graphene oxide linking layers and used for the selective analysis of oligonucleotide hybridization. The proposed plasmonic biosensors make SPR technology more affordable for various applications and provide the basis for compact biosensors integrated with modern electronic devices.

Optical constants and structural properties of thin gold films.

We report a comprehensive experimental study of optical and electrical properties of thin polycrystalline gold films in a wide range of film thicknesses (from 20 to 200 nm). Our experimental results are supported by theoretical calculations based on the measured morphology of the fabricated gold films. We demonstrate that the dielectric function of the metal is determined by its structural morphology. Although the fabrication process can be absolutely the same for different films, the dielectric function can strongly depend on the film thickness. Our studies show that the imaginary part of the dielectric function of gold, which is responsible for optical losses, rapidly increases as the film thickness decreases for thicknesses below 80 nm. At the same time, we do not observe a noticeable dependence of optical constants on the film thickness for thicker samples. These findings establish design rules for thin-film plasmonic and nanophotonic devices.

Ultralow-Loss CMOS Copper Plasmonic Waveguides.

Surface plasmon polaritons can give a unique opportunity to manipulate light at a scale well below the diffraction limit reducing the size of optical components down to that of nanoelectronic circuits. At the same time, plasmonics is mostly based on noble metals, which are not compatible with microelectronics manufacturing technologies. This prevents plasmonic components from integration with both silicon photonics and silicon microelectronics. Here, we demonstrate ultralow-loss copper plasmonic waveguides fabricated in a simple complementary metal-oxide semiconductor (CMOS) compatible process, which can outperform gold plasmonic waveguides simultaneously providing long (>40 µm) propagation length and deep subwavelength (∼λ(2)/50, where λ is the free-space wavelength) mode confinement in the telecommunication spectral range. These results create the backbone for the development of a CMOS plasmonic platform and its integration in future electronic chips.

Boosting Local Field Enhancement by on-Chip Nanofocusing and Impedance-Matched Plasmonic Antennas.

Strongly confined surface plasmon-polariton modes can be used for efficiently delivering the electromagnetic energy to nanosized volumes by reducing the cross sections of propagating modes far beyond the diffraction limit, that is, by nanofocusing. This process results in significant local-field enhancement that can advantageously be exploited in modern optical nanotechnologies, including signal processing, biochemical sensing, imaging, and spectroscopy. Here, we propose, analyze, and experimentally demonstrate on-chip nanofocusing followed by impedance-matched nanowire antenna excitation in the end-fire geometry at telecom wavelengths. Numerical and experimental evidence of the efficient excitation of dipole and quadrupole (dark) antenna modes are provided, revealing underlying physical mechanisms and analogies with the operation of plane-wave Fabry-Pérot interferometers. The unique combination of efficient nanofocusing and nanoantenna resonant excitation realized in our experiments offers a major boost to the field intensity enhancement up to ∼12000, with the enhanced field being evenly distributed over the gap volume of 30 × 30 × 10 nm(3), and promises thereby a variety of useful on-chip functionalities within sensing, nonlinear spectroscopy and signal processing.

Direct characterization of plasmonic slot waveguides and nanocouplers.

We demonstrate the use of amplitude- and phase-resolved near-field mapping for direct characterization of plasmonic slot waveguide mode propagation and excitation with nanocouplers in the telecom wavelength range. We measure mode's propagation length, effective index and field distribution and directly evaluate the relative coupling efficiencies for various couplers configurations. We report 26- and 15-fold improvements in the coupling efficiency with two serially connected dipole and modified bow-tie antennas, respectively, as compared to that of the short-circuited waveguide termination.

Nanofocusing in circular sector-like nanoantennas.

Gold circular sector-like nanoantennas (with a radius of 500 nm and a taper angle of 60°, 90°, and 120°) on glass are investigated in a near-infrared wavelength range (900 - 2100 nm). Amplitude- and phase-resolved near-field images of circular sector-like antenna modes at telecom wavelength feature a concentric circular line of phase contrast, demonstrating resonant excitation of a standing wave of counter-propagating surface plasmons, travelling between a tip and opposite circular edge of the antenna. Transmission spectra obtained in the range 900 - 2100 nm are in good agreement with numerical simulations, revealing the main feature of this antenna configuration, viz., the resonance wavelength, in contrast to triangular antennas, does not depend on the taper angle and is determined only by the sector radius. This feature together with a robust and easily predictable frequency response makes circular sector-like nanoantennas very promising for implementing bowtie antennas and attractive for many applications.

Crystallinity as a factor of SERS stability of silver nanoparticles formed by Ar+ irradiation.

The plasmonic sensors based on silver nanoparticles are limited in application due to their relatively fast degradation in the ambient atmosphere. The technology of ion-beam modification for the creation of monocrystalline silver nanoparticles (NPs) with stable plasmonic properties will expand the application of silver nanostructures. In the present study, highly-stable monocrystalline NPs were formed on the basis of a thin silver film by low-energy ion irradiation. Combined with lithography, this technique allows the creation of nanoparticle ensembles in variant forms. The characterization of the nanoparticles formed by ion-beam modification showed long-term outstanding for Ag nanoparticles stability of their plasmonic properties due to their monocrystalline structure. According to optical spectroscopy data, the reliable plasmonic properties in the ambient atmosphere are preserved for up to 39 days. The mapping of crystal violet dye via surface-enhanced Raman spectroscopy (SERS) revealed a strong amplification factor sustaining at least thrice as long as the one of similarly sized polycrystalline silver NPs formed by annealing. The plasmonic properties sustain more than a month of storage in the ambient atmosphere. Thus, ion-beam modification of silver film makes it possible to fabricate NPs with stable plasmonic properties and form clusters of NPs for sensor technology and SERS applications.

Wandering principal optical axes in van der Waals triclinic materials.

Nature is abundant in material platforms with anisotropic permittivities arising from symmetry reduction that feature a variety of extraordinary optical effects. Principal optical axes are essential characteristics for these effects that define light-matter interaction. Their orientation - an orthogonal Cartesian basis that diagonalizes the permittivity tensor, is often assumed stationary. Here, we show that the low-symmetry triclinic crystalline structure of van der Waals rhenium disulfide and rhenium diselenide is characterized by wandering principal optical axes in the space-wavelength domain with above π/2 degree of rotation for in-plane components. In turn, this leads to wavelength-switchable propagation directions of their waveguide modes. The physical origin of wandering principal optical axes is explained using a multi-exciton phenomenological model and ab initio calculations. We envision that the wandering principal optical axes of the investigated low-symmetry triclinic van der Waals crystals offer a platform for unexplored anisotropic phenomena and nanophotonic applications.