Hybridized Guided-Mode Resonances via Colloidal Plasmonic Self-Assembled Grating

Tamm plasmon polariton-based planar hot-electron photodetector for the near-infrared region

Light-trapping devices have always been a topic of intense interest among researchers. One such device that has gained attention is the hot-electron photodetector with a tunable detection wavelength. Photodetectors based on plasmon nanostructures that provide excitation of surface plasmon polaritons are challenging to manufacture. To address this issue, a planar hot-electron photodetector based on a Tamm plasmon polariton localized in a metal-semiconductor-multilayer mirror structure has been proposed in this study. The parameters and materials of the structure were adjusted to ensure perfect absorption at the resonance wavelength. As a result, the photoresponsivity of the proposed device can reach 42.6 mA W-1 at 905 nm. For the first time, the photosensitivity was calculated analytically by solving the dispersion law for the Tamm plasmon polariton.

High-Sensitivity Optical Sensors Empowered by Quasi-Bound States in the Continuum in a Hybrid Metal-Dielectric Metasurface

Enhancing light-matter interaction is a key requisite in the realm of optical sensors. Bound states in the continuum (BICs), possessing high quality factors (Q factors), have shown great advantages in sensing applications. Recent theories elucidate the ability of BICs with hybrid metal-dielectric architectures to achieve high Q factors and high sensitivities. However, the experimental validation of the sensing performance in such hybrid systems remains equivocal. In this study, we propose two symmetry-protected quasi-BIC modes in a metal-dielectric metasurface. Our results demonstrate that, under the normal incidence of light, the quasi-BIC mode dominated by dielectric can achieve a high Q factor of 412 and a sensing performance with a high bulk sensitivity of 492.7 nm/RIU (refractive index unit) and a figure of merit (FOM) of 266.3 RIU-1, while the quasi-BIC mode dominated by metal exhibits a stronger surface affinity in the biotin-streptavidin bioassay. These findings offer a promising approach for implementing metasurface-based sensors, representing a paradigm for high-sensitivity biosensing platforms.

Refractiveindex.info database of optical constants

We introduce the refractiveindex.info database, a comprehensive open-source repository containing optical constants for a wide array of materials, and describe in detail the underlying dataset. This collection, derived from a meticulous compilation of data sourced from peer-reviewed publications, manufacturers' datasheets, and authoritative texts, aims to advance research in optics and photonics. The data is stored using a YAML-based format, ensuring integrity, consistency, and ease of access. Each record is accompanied by detailed metadata, facilitating a comprehensive understanding and efficient utilization of the data. In this descriptor, we outline the data curation protocols and the file format used for data records, and briefly demonstrate how the data can be organized in a user-friendly fashion akin to the books in a traditional library.

Hybridization between plasmonic and photonic modes in laser-induced self-organized quasi-random plasmonic metasurfaces

Plasmonic metasurfaces made of perfectly regular 2D lattices of metallic nanoparticles deposited on surfaces or close to waveguides can exhibit hybridized plasmonic and photonic modes. The latter arise from the excitation of surface or guided modes through the in-plane coherent scattering of periodic arrays. Recently, laser-induced self-organization of random plasmonic metasurfaces has been used to create nanoparticle gratings embedded in protective layers. Despite the broad size distribution and positional disorder of nanoparticles, the resulting nanostructures exhibit strong coupling between plasmonic and photonic modes in transverse electric polarization, leading to dichroism, which is well-reproduced from one laser printing to another. Here, we examine quantitatively the effect of inhomogeneities at the nanoscale on the hybridization between localized plasmonic modes and delocalized guided modes by considering realistic laser-induced self-organized nanoparticle arrays embedded in a two-layer system. By referring to regular samples, we describe the optical mechanisms involved in the hybridization process at characteristic wavelengths, based on far and near field simulations. Two kinds of real samples are considered, featuring different levels of coupling between the plasmonic and photonic modes. The results demonstrate that controlling the statistical properties of plasmonic metasurfaces, such as the nanoparticle size distribution and average position, over areas a few micrometers wide is enough to control in a reproducible manner the hybridization mechanisms and their resulting optical properties. Thus, this study shows that the inherent irregularities of laser-induced self-organized nanostructures are compatible with smart functionalities of nanophotonics, and confirms that laser processing has huge potential for real-world applications.

Super-Radiant SERS Enhancement by Plasmonic Particle Gratings

Despite recent developments, surface-enhanced Raman spectroscopy (SERS) applications face challenges in achieving both high sensitivity and uniform Raman signals over a large area. Using the directional self-assembly of plasmonic nanoparticles in lattice structures, we show how one can increase the SERS signal 43-fold over randomly aligned gold nanoparticles without relying on the photoluminescence of Rhodamine 6G. For this study, we have chosen the lattice constant for an off-resonant case that matches the lattice resonance and super-radiant plasmon mode along the particle chain. Supported by electromagnetic simulations, we systematically analyze the radiative components of the plasmon modes by varying the particle size while keeping the lattice periodicity constant. We perform polarization-dependent SERS measurements and compare them with other standard SERS excitation wavelengths. Using the self-assembled plasmonic particle lattice, we have developed an effective SERS substrate that provides a significantly higher signal with 73% less surface coverage. This colloidal approach enables the cost-effective and scalable fabrication of highly sensitive, uniform, and polarization-dependent SERS substrates.

Absorption enhancement in GaAs based quantum dot solar cells using double-sided nanopyramid arrays

Quantum dot solar cells (QDSCs) are regarded as one of the most efficient devices due to their intermediate band structures. A suitable light-trapping (LT) strategy matching the absorption spectrum is important to improve the photocurrent conversion efficiency of QDSCs. In this paper, we have proposed a design of the periodically patterned top and bottom dielectric nanopyramid arrays for highly efficient light trapping in GaAs-based QDSCs. The dielectric nanopyramid arrays significantly improve the light absorption of QDSCs in the longer wavelength between 0.8 µm and 1.2 µm. In addition, this LT structure ensures a completely flat window layer and back surface field layer while passivating these semiconductor surfaces. For the optimized double-sided structure, the short-circuit current generated by QDSC is 34.32m A/c m 2, where the photocurrent from the quantum dots (QDs) is 5.17m A/c m 2. Compared to the photocurrent of the QDSC without an LT structure, the photocurrent of the double-sided structure is increased by 84%. The QD photocurrent of the double-sided structure is increased by 570% compared to that of the QDSC without the LT structure.

Nanoscale modeling of dynamically tunable planar optical absorbers utilizing InAs and InSb in metal-oxide-semiconductor-metal configurations

The attainment of dynamic tunability in spectrally selective optical absorption has been a longstanding objective in modern optics. Typically, Fabry-Perot resonators comprising metal and semiconductor thin films have been employed for spectrally selective light absorption. In such resonators, the resonance wavelength can be altered via structural modifications. The research has progressed further with the advent of specialized patterning of thin films and the utilization of metasurfaces. Nonetheless, achieving dynamic tuning of the absorption wavelength without altering the geometry of the thin film or without resorting to lithographic fabrication still poses a challenge. In this study, the incorporation of a metal-oxide-semiconductor (MOS) architecture into the Fabry-Perot nanocavity is shown to yield dynamic spectral tuning in a perfect narrowband light absorber within the visible range. Such spectral tuning is achieved using n-type-doped indium antimonide and n-type-doped indium arsenide as semiconductors in a MOS-type structure. These semiconductors offer significant tuning of their optical properties via electrically induced carrier accumulation. The planar structure of the absorber models presented facilitates simple thin-film fabrication. With judicious material selection and appropriate bias voltage, a spectral shift of 47 nm can be achieved within the visible range, thus producing a discernible color change.

Optical gratings fabricated using the capillary-assisted self-assembly of nanoparticles on a flexible substrate

We demonstrate a cost-effective and high-throughput fabrication technique to deposit colloidal nanoparticles on a patterned polymer substrate using a capillary-assisted self-assembly method over a large area. In particular, we fabricate optical gratings using gold nanoparticles and a polymer substrate. We show the versatility of the technique over different nanoparticle diameters and grating periodicities. Through both experiments and simulations, we show enhanced transmission in the first-order diffraction of the gold-polymer grating as compared to the air-polymer grating. Our fabrication technique also enables the transfer of the nanoparticle pattern from the polymer substrate to any desired surface. Here we demonstrate the transfer of the nanoparticle grating structure to the tip of optical fibers.

Numerical Simulation of Integrated Generation and Shaping of Airy and Bessel Vortex Beams Based on All-Dielectric Metasurface

Integrating multiple independent functions into a single optical component is one of the most important topics in research on photoelectric systems. In this paper, we propose a multifunctional all-dielectric metasurface that can achieve a variety of non-diffractive beams depending on the polarization state of the incident light. Using the anisotropic TiO₂ rectangular column as the unit structure, the three functions of generating polygonal Bessel vortex beams under left-handed circularly polarized incidence, Airy vortex beams under right-handed circularly polarized incidence and polygonal Airy vortex-like beams under linearly polarized incidence are realized. In addition, the number of polygonal beam sides and the position of focal plane can be adjusted. The device could facilitate further developments in scaling complex integrated optical systems and fabricating efficient multifunctional components.

Modified Distributed Bragg Reflectors for Color Stability in InGaN Red Micro-LEDs

The monolithic integration of InGaN-based micro-LEDs is being of interest toward developing full-color micro-displays. However, the color stability in InGaN red micro-LED is an issue that needs to be addressed. In this study, the modified distributed Bragg reflectors (DBRs) were designed to reduce the transmission of undesired spectra. The calculated optical properties of the InGaN red micro-LEDs with conventional and modified DBRs have been analyzed, respectively. The CIE 1931 color space and the encoded 8-bit RGB values are exhibited for the quantitative assessment of color stability. The results suggest the modified DBRs can effectively reduce the color shift, paving the way for developing full-color InGaN-based micro-LED displays.

Label-Free Bound-States-in-the-Continuum Biosensors

Bound states in the continuum (BICs) have attracted considerable attentions for biological and chemical sensing due to their infinite quality (Q)-factors in theory. Such high-Q devices with enhanced light-matter interaction ability are very sensitive to the local refractive index changes, opening a new horizon for advanced biosensing. In this review, we focus on the latest developments of label-free optical biosensors governed by BICs. These BICs biosensors are summarized from the perspective of constituent materials (i.e., dielectric, metal, and hybrid) and structures (i.e., grating, metasurfaces, and photonic crystals). Finally, the current challenges are discussed and an outlook is also presented for BICs inspired biosensors.

High-performance parallel tandem MoTe2/perovskite solar cell based on reduced graphene oxide as hole transport layer

Recently, the impressive achievements accomplished in multijunction (tandem) perovskite solar cells have triggered a huge research effort to boost their performance. Here, using a three-dimensional (3D) finite element method (FEM) technique, we propose and investigate a parallel tandem PSCs consisting of two absorbing layers of MoTe2 and CH3NH3PbI3 with cascaded bandgaps to more efficiently use the near-infrared (NIR) solar spectrum. Endowed with a bandgap of about 1 eV, the MoTe2 layer in conjunction with a CH3NH3PbI3 layer is able to broaden the light absorption range of structure beyond the wavelength of 800 nm, up to 1200 nm. In addition to this, the MoTe2 material can not only appreciably harvest light even with a thickness as low as 20 nm due to their high absorption coefficient, but also make a perfect band alignment with the CH3NH3PbI3 layer. As a result, the proposed multijunction PCS yields a high power conversion efficiency (PCE) of 18.52% with a VOC of 0.83 V, Jsc of 26.25 mA/cm2, and FF of 0.84, which is considerably greater than its corresponding single-junction PSCs with PCE, VOC, Jsc, and FF of, 14.01%, 1.14 V, 15.20 mA/cm2, and 0.81, respectively. Furthermore, to mitigate the VOC loss caused by the low bandgap of MoTe2, we demonstrate an increase in VOC from 0.84 to 0.928 V and in PCE from 18.52% to 20.32%, when we replace a reduced graphene oxide (rGO) layer with Spiro-OMeTAD layer as a hole transport layer (HTL).

Flexible hyperspectral surface plasmon resonance microscopy

Optical techniques for visualization and quantification of chemical and biological analytes are always highly desirable. Here we show a hyperspectral surface plasmon resonance microscopy (HSPRM) system that uses a hyperspectral microscope to analyze the selected area of SPR image produced by a prism-based spectral SPR sensor. The HSPRM system enables monochromatic and polychromatic SPR imaging and single-pixel spectral SPR sensing, as well as two-dimensional quantification of thin films with the measured resonance-wavelength images. We performed pixel-by-pixel calibration of the incident angle to remove pixel-to-pixel differences in SPR sensitivity, and demonstrated the HSPRM's capabilities by using it to quantify monolayer graphene thickness distribution, inhomogeneous protein adsorption and single-cell adhesion. The HSPRM system has a wide spectral range from 400 nm to 1000 nm, an optional field of view from 0.884 mm² to 0.003 mm² and a high lateral resolution of 1.2 μm, demonstrating an innovative breakthrough in SPR sensor technology.

Bayesian optimization of nanophotonic electromagnetic shielding with very high visible transparency

Transparent electromagnetic interference (EMI) shielding is needed in many optoelectronic applications to protect electronic devices from surrounding radiation while allowing for high visible light transmission. However, very high transmission (over 92.5%), high EMI shielding efficiency (over 30 dB) structures have yet to be achieved in the literature. Bayesian optimization is used to optimize different nanophotonic structures for high EMI shielding efficiency (SE) and high visible light transmission (T¯ _{v i s} ). Below 90% average visible light transmission, sandwich structures consisting of high index dielectric/silver/high index dielectric films are determined to be optimal, where they are able to achieve 43.1 dB SE and 90.0% T¯ _{v i s} . The high index of refraction dielectric layers reduce absorption losses in the silver and can be engineered to provide for antireflection through destructive interference. However, for optimal EMI shielding with T¯ _{v i s} above 90%, the reflection losses at the air/dielectric interfaces need to be further reduced. Optimized double sided nanocone sandwich structures are determined to be best where they can achieve 41.2 dB SE and 90.8% T¯ _{v i s} as well as 35.6 dB SE and 95.1% T¯ _{v i s} . K-means clustering is utilized to show the performance of characteristic near-Pareto optimal structures. Double sided nanocone structures are shown to exhibit omnidirectional visible transmission with SE = 35.6 dB and over 85% T¯ _{v i s} at incidence angles of 70 ^∘.

Molybdenum Oxide Functional Passivation of Aluminum Dimers for Enhancing Optical-Field and Environmental Stability

In this contribution, we present an experimental and numerical study on the coating of Al plasmonic nanostructures through a conformal layer of high-refractive-index molybdenum oxide. The investigated structures are closely coupled nanodisks where we observe that the effect of the thin coating is to help gap narrowing down to the sub-5-nm range, where a large electromagnetic field enhancement and confinement can be achieved. The solution represents an alternative to more complex and challenging lithographic approaches, and results are also advantageous for enhancing the long-term stability of aluminum nanostructures.

Robustness Analysis of Metasurfaces: Perfect Structures Are Not Always the Best

Optical metasurfaces rely on subwavelength scale nanostructures, which puts significant constraints on nanofabrication accuracies. These constraints are becoming increasingly important, as metasurfaces are maturing toward real applications that require the fabrication of very large area samples. Here, we focus on beam steering gradient metasurfaces and show that perfect nanofabrication does not necessarily equate with best performances: metasurfaces with missing elements can actually be more efficient than intact metasurfaces. Both plasmonic metasurfaces in reflection and dielectric metasurfaces in transmission are investigated. These findings are substantiated by experiments on purposely misfabricated metasurfaces and full-wave calculations. A very efficient quasi-analytical model is also introduced for the design and simulations of metasurfaces; it agrees very well with full-wave calculations. Our findings indicate that the substrate properties play a key role in the robustness of a metasurface and the smoothness of the approximated phase gradient controls the device efficiency.

Role of Substrate Type in the Process of Polyelectrolyte Multilayer Formation

Polyelectrolyte multilayers are coatings formed by the alternate deposition of polycations and polyanions on a charged surface. In this study we examined how the type of substrate affects a multilayer prepared from poly(allylamine hydrochloride) and poly(acrylic acid). Silicon and titanium wafers were used as substrates. Their properties were systematically studied using ellipsometry, tensiometry, atomic force microscopy and streaming potential measurements. Multilayers were built up at pH = 7 with tetramethylammonium chloride as the background salt. The growth of films was monitored by ellipsometry, while the morphology and surface roughness were determined by atomic force microscopy. It was found that the thickness of multilayers containing 10 layers on silicon is 10 nm, whereas the thickness of the same film on titanium is three times higher. It was shown that multilayers formed on silicon display a grain-like structure, which was not the case for a film formed on titanium. Such morphological properties are also reflected in the surface roughness. Finally, it was shown that, in addition to the electrostatic interactions, the hydrophobicity of the substrate also plays an important role in the polyelectrolyte multilayer formation process and influences its thickness and properties.

Mild Sol-Gel Conditions and High Dielectric Contrast: A Facile Processing toward Large-Scale Hybrid Photonic Crystals for Sensing and Photocatalysis

Solution processing of highly performing photonic crystals has been a towering ambition for making them technologically relevant in applications requiring mass and large-area production. It would indeed represent a paradigm changer for the fabrication of sensors and for light management nanostructures meant for photonics and advanced photocatalytic systems. On the other hand, solution-processed structures often suffer from low dielectric contrast and poor optical quality or require complex deposition procedures due to the intrinsic properties of components treatable from solution. This work reports on a low-temperature sol-gel route between the alkoxides of Si and Ti and poly(acrylic acid), leading to stable polymer-inorganic hybrid materials with tunable refractive index and, in the case of titania hybrid, photoactive properties. Alternating thin films of the two hybrids allows planar photonic crystals with high optical quality and dielectric contrast as large as 0.64. Moreover, low-temperature treatments also allow coupling the titania hybrids with several temperature-sensitive materials including dielectric and semiconducting polymers to fabricate photonic structures. These findings open new perspectives in several fields; preliminary results demonstrate that the hybrid structures are suitable for sensing and the enhancement of the catalytic activity of photoactive media and light emission control.

1D Colloidal chains: recent progress from formation to emergent properties and applications

Integrating nanoscale building blocks of low dimensionality (0D; i.e., spheres) into higher dimensional structures endows them and their corresponding materials with emergent properties non-existent or only weakly existent in the individual building blocks. Constructing 1D chains, 2D arrays and 3D superlattices using nanoparticles and colloids therefore continues to be one of the grand goals in colloid and nanomaterial science. Amongst these higher order structures, 1D colloidal chains are of particular interest, as they possess unique anisotropic properties. In recent years, the most relevant advances in 1D colloidal chain research have been made in novel synthetic methodologies and applications. In this review, we first address a comprehensive description of the research progress concerning various synthetic strategies developed to construct 1D colloidal chains. Following this, we highlight the amplified and emergent properties of the resulting materials, originating from the assembly of the individual building blocks and their collective behavior, and discuss relevant applications in advanced materials. In the discussion of synthetic strategies, properties, and applications, particular attention will be paid to overarching concepts, fresh trends, and potential areas of future research. We believe that this comprehensive review will be a driver to guide the interdisciplinary field of 1D colloidal chains, where nanomaterial synthesis, self-assembly, physical property studies, and material applications meet, to a higher level, and open up new research opportunities at the interface of classical disciplines.

Concentration Quantification of TiO2 Nanoparticles Synthesized by Laser Ablation of a Ti Target in Water

In this work, we present a quantitative method for determining the concentration of metal oxide nanoparticles (NP) synthesized by laser ablation in liquid. The case study was performed with titanium dioxide nanoparticles (TiO₂ NP), which were synthesized by laser ablation of a Ti target in water. After synthesis, a colloidal solution was analyzed with UV-Vis spectroscopy. At the same time, the craters that remained on the Ti target after ablation were evaluated with an optical microscope to determine the volume of the ablated material. SEM microscopy was used to determine the TiO₂ NP size distribution. It was found that synthesized TiO₂ NP followed a Log-Normal diameter distribution with a maximum at about 64 nm. From the volume of ablated material and NP size distribution, under the assumption that most of the ablated material is consumed to form nanoparticles, a concentration of nanoparticles can be determined. The proposed method is verified by comparing the calculated concentrations to the values obtained from the Beer–Lambert law using the Mie scattering theory for the NP cross-section calculation.

Design and Modelling of Metal-Oxide Nanodisk Arrays for Structural Colors and UV-Blocking Functions in Solar Cell Glass Covers

We present a multifunctional structural coloration strategy for solar cell glass covers based on all-dielectric nanoscatterer arrays. Titanium dioxide (TiO2) nanostructures are designed to efficiently scatter in the visible and absorb in the UV region, making them suitable candidates as UV absorptive color coatings. Results from finite difference time domain (FDTD) simulations on a square lattice of TiO2 nanocylinders show that a rich palette in the reflected colors can be obtained by varying the period of the lattice. The reflected colors are narrow-banded, with a typical FWHM ~11–17 nm, leading to a minimal penalty on the amount of transmitted light. This narrow band reflectance is attributed to the interaction of Mie resonances between individual scatterers with their neighbors in the lattice. The color appearance, with viewing angles of ~45°, is maintained for incidence angles up to ~70°. With TiO2 being transparent for a major part of silicon solar cells spectral response (400–1100 nm), a loss of ~4.5–9.2% in the short-circuit current has been estimated in the specified wavelength range, primarily due to the loss of photons in the reflected light. Furthermore, due to the inherent UV-absorption properties of TiO2, the proposed color-cover designs reduce the transmittance of UV radiation (320–400 nm) by up to ~63.70%, potentially preventing the degradation of the encapsulation materials and thus increasing the lifetime expectancy of a solar panel.

Multifunctional Optical Vortex Beam Generator via Cross-Phase Based on Metasurface

We propose a multifunctional optical vortex beam (OVB) generator via cross-phase based on a metasurface. Accordingly, we separately investigate the two different propagation characteristics of OVB modulated by the low-order cross-phase (LOCP) and the high-order cross-phase (HOCP) in a self-selected area. When LOCP modulation is added to OVB, topological charges can be measured for any order of OVB. Moreover, we achieve the rotation tunable performance successfully by adding the rotation component. Then, we realize the function of polygonal beam generation and singularities regulation with the HOCP. The order of the HOCP is exactly equal to the number of a polygon OVB’s sides. The waist radius and usable width of the beam lengthens as the distance of the self-selected area increases. When the conversion rate is doubled, the distance between singularities widens by about 0.5 μm. The proposed OVB generator provides a simple strategy for detecting the value of topological charges and achieving OVB shaping and singularity manipulation simultaneously. We hope this can open new horizons for promoting the development of photon manipulation, optical communication, and vortex beam modulation.

A versatile technology for colloidal crystal transfer using parylene coatings and hydrosoluble polymers

We propose a novel versatile colloidal crystal transfer technique compatible with a wide range of water-insoluble substrates regardless of their size, material, and wettability. There are no inherent limitations on colloidal particles material and size. The method possibilities are demonstrated via the colloidal transfer on quartz, glass substrates with a flat and curved surface, and via the fabrication of 3D colloidal structure with 5 overlaid colloidal monolayers. The process occurs at a room temperature in water and is independent from the illumination conditions, which makes it ideal for experimental manipulations with sensitive functional substrates. We performed the nanosphere photolithography process on a photosensitive substrate with a transferred colloidal monolayer. The metallized hexagonal arrays of nanopores demonstrated a clear resonant plasmonic behavior. We believe that due to its high integration possibilities the proposed transfer technique will find applications in a large-area surface nanotexturing, plasmonics, and will speed up a device fabrication process.

Quasi-Guided Modes in Titanium Dioxide Arrays Fabricated via Soft Nanoimprint Lithography

Reversible quasi-guided modes (QGMs) are observed in titanium dioxide (TiO2) metasurface arrays fabricated via soft nanoimprint lithography. A TiO2 layer between the nanopillar array and the substrate can facilitate the propagation of QGMs. This layer is porous, allowing for the tuning of the layer properties by incorporating another material. The presence of the QGMs is strongly dependent on the refractive index of the TiO2 layer. QGMs are not supported if the refractive index of the porous TiO2 is too low. It is demonstrated that after depositing R6G on the array QGMs can be observed as very strong and narrow reflectance peaks and transmittance dips. Furthermore, as the second material can penetrate through the pores into the layer it can experience the regions of high field enhancement associated with the QGMs. These results are of interest for a wide range of applications including but not limited to sensing, nonlinear optics, and emission control.

Waveguide efficient directional coupling and decoupling via an integrated plasmonic nanoantenna

The development of integrated photonic devices has led to important advancements in the field of light-matter interaction at the nanoscale. One of the main focal points is the coupling between single photon emitters and optical waveguides aiming to achieve efficient optical confinement and propagation. In this work, we focus on the characterization of a hybrid dielectric/plasmonic waveguide consisting of a gold triangular nanoantenna placed on top of a TiO₂ waveguide. The strong directionality of the device is experimentally demonstrated by comparing the intensity scattered by the nanotriangle to the one scattered by a SNOM tip for different illumination geometries. The ability of the plasmonic antenna to generate powerful coupling between a single emitter and the waveguide will also be highlighted through numerical simulations.

Electrostatically Tuned Optical Filters Based on Hybrid Plasmonic-Dielectric Thin Films for Hyperspectral Imaging

Hyperspectral imaging has a wide range of uses, from medical diagnostics to crop monitoring; however, conventional hyperspectral imaging systems are relatively slow, bulky, and rather costly. In this paper, we present an inexpensive, compact tunable optical filter for hyperspectral applications. The filter is based on a Fabry-Pérot interferometer utilizing hybrid metallic-dielectric mirrors and actuated using a MEMS electrostatic actuator. The optical filter is designed using the transfer matrix method; then, the results were verified by an electromagnetic wave simulator. The actuator is based on a ring-shaped parallel plate capacitor and is designed using COMSOL Multiphysics. An actuation displacement of 170 nm was used, which is the required distance to tune the filter over the whole visible range (400-700 nm). There are two designs proposed for the optical filter: the first was optimized to provide maximum transmission and the other is optimized to have minimum full-width-half-maximum (FWHM) value. The first design has a maximum transmission percentage of 94.45% and a minimum transmission of 86.34%; while the minimum FWHM design had an average FWHM value of 7.267 nm. The results showed improvements over the current commercial filters both in transmission and in bandwidth.

Refractive index sensor based on a Tamm Fabry-Perot hybrid resonance

A highly sensitive refractive index sensor is proposed by using the resonant coupling of a Tamm state to a Fabry-Perot resonance in slits periodically pierced in a metal film. This new hybrid resonance exists at the interface between a dielectric Bragg mirror and a subwavelength metal grating. Contrary to a classic Tamm plasmon, it is sensitive to the ambient media refractive index due to its confinement inside the grating slits. The proposed sensor output can be either the reflected intensity or preferably the wavelength, and its sensitivity and figure of merit are numerically investigated with the help of rigorous coupled wave analysis. The sensitivity of the studied sensor is 87 nm/RIU for a refractive index range from 1.25 to 1.38, and, at the expense of the resolution, it can reach up to 160 nm/RIU for a grating duty cycle of 50%. The figure of merit is around ${{7.5}}\;{{\rm{RIU}}^{- 1}}$ with a large measuring range. The index sensor performances and operating resonance wavelength can be modified by adjusting the grating geometric parameters (height and duty cycle). The possibility to achieve wavelength modulation in any spectral range makes the proposed grating Tamm structure an attractive candidate to design refractive index sensors and photonic components in the infrared and terahertz domains.

Dielectric optical nanoantennas

Nanophotonics allows the manipulation of light on the subwavelength scale. Optical nanoantennas are nanoscale elements that enable increased resolution in bioimaging, novel photon sources, solar cells with higher absorption, and the detection of fluorescence from a single molecule. While plasmonic nanoantennas have been extensively explored in the literature, dielectric nanoantennas have several advantages over their plasmonic counterparts, including low dissipative losses and near-field enhancement of both electric and magnetic fields. Nanoantennas increase the optical density of states, which increase the rate of spontaneous emission due to the Purcell effect. The increase is quantified by the Purcell factor, which depends on the mode volume and the quality factor. It is one of the main performance parameters for nanoantennas. One particularly interesting feature of dielectric nanoantennas is the possibility of integrating them into optical resonators with a high quality-factor, further improving the performance of the nanoantennas and giving very high Purcell factors. This review introduces the properties and parameters of dielectric optical nanoantennas, and gives a classification of the nanoantennas based on the number and shape of the nanoantenna elements. An overview of recent progress in the field is provided, and a simulation is included as an example. The simulated nanoantenna, a dimer consisting of two silicon nanospheres separated by a gap, is shown to have a very small mode volume, but a low quality-factor. Some recent works on photonic crystal resonators are reviewed, including one that includes a nanoantenna in the bowtie unit-cell. This results in an enormous increase in the calculated Purcell factor, from 200 for the example dimer, to 8 × 10⁶ for the photonic crystal resonator. Some applications of dielectric nanoantennas are described. With current progress in the field, it is expected that the number of applications will grow and that nanoantennas will be incorporated into new commercial products. A list of relevant materials with high refractive indexes and low losses is presented and discussed. Finally, prospects and major challenges for dielectric nanoantennas are addressed.

Dynamically tunable ultra-narrowband perfect absorbers for the visible-to-infrared range based on a microcavity integrated graphene pair

Dynamically tunable ultra-narrowband perfect absorbers are important to next-generation active photonic devices. A composite structure of a graphene pair and a microcavity with Bragg mirrors are proposed for this purpose. Based on the electrically controllable doping of graphene and critical coupling of the incident light, the microcavity-graphene composite structure achieves peak absorptance higher than 99.5%, a relative peak width ($\Delta \lambda /{\lambda _0}$) smaller than 1.1%, and a modulation depth larger than 92.0% throughout the visible-to-mid-infrared range, surpassing other structures in comprehensive performance. By changing the number of the dielectric pairs in the Bragg mirrors, the device can become an amplitude or a spectral modulator. The results are based on the optical constants from experiment data, including the surface conductivity of graphene with relatively low mobility, so they are more useful in practical situations.

Changes in Optical Parameters of SiO2:TiO2 Films Obtained by Sol-Gel Method Observed as a Result of Thermal Treatment

The research focused on materials having potential applications in technology of planar evanescent wave sensors. Four samples of binary SiO₂:TiO₂ thin films having different titania content were manufactured through the sol-gel method and dip-coating technique on polished silicon substrates. The samples were subjected to repeated heating/cooling protocols. Simultaneously, their optical parameters were monitored by spectroscopic ellipsometry as they evolved under varying temperature. Subsequent analysis confirmed linear dependence of refractive index on titania content, at least in vis-NIR wavelengths, as well as a low value of the thermal expansion coefficient. It was shown that the thickness of SiO₂:TiO₂ films decreased as a result of annealing processes, which may be a consequence of reduced porosity.

Nanoimprinted multifunctional nanoprobes for a homogeneous immunoassay in a top-down fabrication approach

Multifunctional nanoparticles are discussed as versatile probes for homogeneous immunoassays for in-vitro diagnostics. Top-down fabrication allows to combine and tailor magnetic and plasmonic anisotropic properties. The combination of nanoimprint lithography, thin film deposition, and lift-off processing provides a top-down fabrication platform, which is both flexible and reliable. Here, we discuss the material compositions and geometrical designs of monodisperse multicomponent nanoparticles and their consequences on optical and magnetic properties. The rotational hydrodynamics of nanoparticles is measured and considered under the influence of magnetic shape anisotropy in the framework of the Stoner-Wohlfarth theory. The plasmon-optical properties are explained by discrete-dipole finite-element simulations. Rotational dynamical measurements of imprinted nanoprobes for two test proteins demonstrate the applicability as highly sensitive biomolecular nanoprobes.

Strain sensing based on resonant states in 2D dielectric photonic quasicrystals

This paper reports the numerical and experimental study of the strain sensing effect of bidimensional quasiperiodic structures made with dielectric cylinders. Structures of around 100 cylinders arranged following a Penrose quasiperiodic disposition were simulated, built and measured, in different states of deformation. The selected quasiperiodic structure contains a symmetric decagonal ring resonator that shows two states in its photonic band gap. The frequency of these states varies linearly in opposite directions as the structure is axially deformed, becoming an interesting sensing principle that can be exploited to build optical strain gauges. As a proof of concept, centimeter-scale glass cylinder (ε_r=4.5) structures were fabricated and their transmission spectra were measured in the microwave range. The same structures were simulated using finite integration time domain showing a good agreement with the measurements. The sensitivity of the prototype built was 12.4 kHz/µε, very linear in a wide range. Therefore, we conclude that the states in the gap of the resonator rings of 2D quasicrystals can find an interesting application in optical strain gauge construction.

Dependence of the damage in optical metal/dielectric coatings on the energy of ions in irradiation experiments for space qualification

Terrestrial accelerator facilities can generate ion beams which enable the testing of the resistance of materials and thin film coatings to be used in the space environment. In this work, a [Formula: see text]/Al bi-layer coating has been irradiated with a [Formula: see text] beam at three different energies. The same flux and dose have been used in order to investigate the damage dependence on the energy. The energies were selected to be in the range 4-100 keV, in order to consider those associated to the quiet solar wind and to the particles present in the near-Earth space environment. The optical, morphological and structural modifications have been investigated by using various techniques. Surprisingly, the most damaged sample is the one irradiated at the intermediate energy, which, on the other hand, corresponds to the case in which the interface between the two layers is more stressed. Results demonstrate that ion energies for irradiation tests must be carefully selected to properly qualify space components.

Layer-selective magnetization switching in the chirped photonic crystal with GdFeCo

Here we propose a magnetophotonic structure for the layer-selective magnetization switching with femtosecond laser pulses of different wavelengths. It is based on a chirped magnetophotonic crystal (MPC) containing magnetic GdFeCo and nonmagnetic dielectric layers. At each operating wavelength the laser pulses heat up to necessary level only one GdFeCo layer that leads to its magnetization reversal without any impact on the magnetization of the other layers. Moreover, magneto-optical reading of the MPC magnetization state is discussed. Lateral dimensions of the considered MPC can be made small enough to operate as a unit cell for data storage.

Prospects of Coupled Organic-Inorganic Nanostructures for Charge and Energy Transfer Applications

We review the field of organic-inorganic nanocomposites with a focus on materials that exhibit a significant degree of electronic coupling across the hybrid interface. These nanocomposites undergo a variety of charge and energy transfer processes, enabling optoelectronic applications in devices which exploit singlet fission, triplet energy harvesting, photon upconversion or hot charge carrier transfer. We discuss the physical chemistry of the most common organic and inorganic components. Based on those we derive synthesis and assembly strategies and design criteria on material and device level with a focus on photovoltaics, spin memories or optical upconverters. We conclude that future research in the field should be directed towards an improved understanding of the binding motif and molecular orientation at the hybrid interface.

Light Scattering from Rough Silver Surfaces: Modeling of Absorption Loss Measurements

We consider two series of experimental setups of multilayered Ag/ZnO thin films with varying surface morphologies given by atomic force microscopy images. The absorption loss under diffuse scattering is studied theoretically by applying a combination of the scattering matrix approach with diffraction theory for randomly nanotextured interfaces. Our modeling is in excellent agreement with the respective measurements. The theoretical approach is applicable to a wide range of wavelengths, surface morphologies, and materials for both measured and computed rough surface morphologies.

Broadband absorption using all-graphene grating-coupled nanoparticles on a reflector

In this paper, the hybridized localized surface plasmon resonances (LSPRs) of a periodic assembly of graphene-wrapped nanoparticles are used to design a nanoparticle assisted optical absorber. Bandwidth enhancement of this structure via providing multiple types of plasmonic resonances in the associated unit cell using two densely packed crossly stacked graphene strips is proposed. The designed graphene strips support fundamental propagating surface plasmons on the ribbons, and gap plasmons in the cavity constructed by the adjacent sections. Graphene strips exhibit a hyperbolic dispersion region in the operating spectrum and assist in the bandwidth enhancement. Moreover, since the nanoparticles are deposited on the top strips, real-time biasing of them can be easily conducted by exciting the surface plasmons of the strip without the necessity to electrically connect the adjacent nanoparticles. The overall dynamic bandwidth of the structure, using a two-state biasing scheme, covers the frequencies of 18.16–40.47 THz with 90% efficiency. Due to the symmetry of the structure, the device performs similarly for both transverse electric (TE) and transverse magnetic (TM) waves and it has a high broadband absorption rate regarding different incident angles up to 40°. Due to the presence of 2D graphene material and also using hollow spherical particles, our proposed absorber is also lightweight and it is suitable for novel compact optoelectronic devices due to its sub-wavelength dimensions.

Grating-Coupled Surface Plasmon-Polariton Sensing at a Flat Metal-Analyte Interface in a Hybrid-Configuration

Surface plasmon resonance-based sensors have emerged as commercially fostering portable biodetectors. The scientific community is engaged in extensive research to improve their performance in terms of sensitivity, selectivity, and reproducibility for the recognition of specific biomolecules. Essentially, there is a need for miniaturizing the size of existing sensors with innovative designs without compromising their bioaffinity and sensitivity performance. In this work, we propose and demonstrate a grating-coupled surface plasmon polariton (SPP) sensor on a thin flat gold layer using a hybrid configuration. The proof of concept of the grating architecture has been realized through an innovative fabrication procedure, with experimental verification of its bulk sensitivity. The geometry is identical to the prism-coupling configuration, yet with miniaturization and compactness. Characteristics of the excited modes in the spectral regime of interest are investigated using the finite-difference time-domain simulations. The effective index calculation of the resonance conditions and the accompanying field distribution can identify the excited SPP and metal-assisted guided-mode resonance modes. Detailed probing of the electric field distribution of the desired SPP mode reveals an extended evanescent decay length of 1284 nm, close to the theoretical limit, and an extended propagation length of 270 μm. The experimental demonstration of the reflectance dip with two different analyte media perceived an increased bulk sensitivity of 1133 nm/RIU. Remarkably, this resonant mode exhibits sensing capabilities for a wide range of analyte refractive indexes. We believe that the fabricated configuration with observed high sensitivity and calculated ultradeep evanescent field penetration depth along with extended propagation length can lead to the designing of a hands-on biochip for detecting large biomolecules.

Rapid large-scale fabrication of multipart unit cell metasurfaces

Periodic diffractive elements known as metasurfaces constitute platform technology whereby exceptional optical properties, not attainable by conventional means, are attained. Generally, with increasing unit-cell complexity, there emerges a wider design space and bolstered functional capability. Advanced devices deploying elaborate unit cells are typically generated by electron-beam patterning which is a tedious, slow process not suitable for large surfaces and quick turnaround. Ameliorating this condition, we present a novel route towards facile fabrication of complex periodic metasurfaces based on sequential exposures by laser interference lithography. Our method is fast, cost-effective, and can be applied to large surface areas. It is enabled by precise control over periodicity and exposure energy. With it we have successfully patterned and fabricated one-dimensional (1D) and two-dimensional (2D) multipart unit cell devices as demonstrated here. Thus, zero-order transmission spectra of an etched four-part 1D grating device are simulated and measured for both transverse-electric (TE) and transverse-magnetic (TM) polarization states of normally incident light. We confirm non-resonant wideband antireflection (∼800 nm) for TM-polarized light and resonance response for TE-polarized light in the near-IR band spanning 1400-2200 nm in a ∼100 mm² device. Furthermore, it is shown that this method of fabrication can be implemented not only to pattern periodic symmetric/asymmetric designs but also to realize non-periodic metasurfaces. The method will be useful in production of large-area photonic devices in the realm of nanophotonics and microphotonics.

Method for single-shot fabrication of chiral woodpile photonic structures using phase-controlled interference lithography

In this report, we propose a large-area, scalable and reconfigurable single-shot optical fabrication method using phase-controlled interference lithography (PCIL) to realize submicrometer chiral woodpile photonic structures. This proposed technique involves a 3 + 3 double-cone geometry with beams originated from a computed phase mask displayed on a single spatial light modulator. Simulation studies show the filtering response of such structures for linearly polarized plane wave illumination, with structural features tunable through a single parameter of interference angle. Further, these single chiral woodpile structures show dual chirality on illumination with both right circularly and left circularly polarized light through simulation. Experimentally fabricated patterns on photoresist show resemblance to the desired chiral woodpile structures.

Mechanotunable Surface Lattice Resonances in the Visible Optical Range by Soft Lithography Templates and Directed Self-Assembly

We demonstrate a novel colloidal self-assembly approach toward obtaining mechanically tunable, cost-efficient, and low-loss plasmonic nanostructures that show pronounced optical anisotropy upon mechanical deformation. Soft lithography and template-assisted colloidal self-assembly are used to fabricate a stretchable periodic square lattice of gold nanoparticles on macroscopic areas. We stress the impact of particle size distribution on the resulting optical properties. To this end, lattices of narrowly distributed particles (∼2% standard deviation in diameter) are compared with those composed of polydisperse ones (∼14% standard deviation). The enhanced particle quality sharpens the collective surface lattice resonances by 40% to achieve a full width at half-maximum as low as 16 nm. This high optical quality approaches the theoretical limit for this system, as revealed by electromagnetic simulations. One hundred stretching cycles demonstrate a reversible transformation from a square to a rectangular lattice, accompanied by polarization-dependent optical properties. On the basis of these findings we envisage the potential applications as strain sensors and mechanically tunable filters.