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WO3 thin films fabricated by glancing angle deposition (GLAD) are proposed as excellent electrochromic coatings with favorable ion diffusion. A = 500-nm film prepared by GLAD had a relatively large transmittance
modulation. The crystallization structure, surface morphology, chemical state, optical and electrochromic properties of WO3 thin films were systematically characterized upon annealing treatment. Compared with annealed WO3 porous nanostructured films, the amorphous as-deposited films exhibited a high coloration efficiency and stable reversibility. Furthermore, the GLAD WO3 films exhibit the tunable angular selectivity under illumination with p-polarized light because of the birefringence, which could extend the application range of nanostructured films in the electrochromic field.

Transmittance of WO3 films under ±45 p-polarized light: (a) colored and bleached state; (b) optical contrasts (%DT). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)

WO3 thin films were fabricated by glancing angle deposition technique (GLAD) and annealed at different temperatures. The WO3 films begin to crystallize at around 300 C. Comparing with the annealed films, the as-grown GLAD WO3 film exhibits relatively optimum electrochromic properties, along with a satisfactory cycling stability and large optical modulation (42.2%, 633 nm), because the loose structure facilitated fast ion diffusion and rapid color alteration. Furthermore, a noteworthy feature of tilted columnar structured films is that under illumination with p polarized light, the transmittance modulation contrast between the +45 and -45 incidence can be up to 11%. The angular selectivity of the colored and bleached states of GLAD WO3 films illuminated with p-polarized light can be tuned by applying different voltages, which provides new application features and development potential.

For more information: https://doi.org/10.1016/j.apsusc.2018.03.248

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Many species of octopus and squid rapidly alter their skin colors and patterns to camouflage themselves from both predators and prey. Scientists in the United Kingdom have developed a new type of pixel—reportedly, the smallest ever—that could make large-scale displays cheap enough to wrap around entire buildings and achieve similar color-changing effects. The researchers already knew that the combination of plasmonic metasurfaces and certain types of electrically conducting polymers can function as optical switches or pixels. By altering the refractive index of the polymer medium surrounding the plasmonic material, scientists can change the peak of the local surface plasmon resonance and the corresponding emitted color. However, the group faced two challenges: speeding up the slow refresh rate of these devices and ensuring that they emit in the visible instead of the infrared range. To solve these problems, Nanophotonics Center director Jeremy Baumberg and his colleagues manufactured large quantities of “electrochromic nanoparticle-on-mirror constructs,” or eNPoMs. These tiny objects are 80-nm-wide gold spheres coated with a shell, between 11 and 20 nm thick, of a conductive polymer called polyaniline. The thickness of that polymer shell crucially determines the gap between the gold core and the planar mirror—a gold sheet supported by flexible plastic—to which they are attached by simply spraying them on. “It is hard to get nano-assembly of tiny gaps to work on roll-to-roll processes, but we found a way to use solution processing to control the gaps simply,” Baumberg says.


Electrochromic nanoparticle-on-mirror constructs (eNPoMs) formed from gold nanoparticles encapsulated in a conductive polymer shell. [Image: NanoPhotonics Cambridge/Hyeon-Ho Jeong, Jialong Peng]

When voltage is applied to the mirror, the eNPoMs give off light, with shorter wavelengths emitted from the particles with thicker shells. The authors claim that their proof-of-concept display operated for more than three months at power densities below 300 μW per square centimeter (using 9 fJ of energy per pixel), which is a factor of 10 times less than the conventional e-paper of today. The display also had more than 109 pixels per inch and optical contrast of better than 50 percent. For a follow-up, the Cambridge team is optimizing the blue end of the generated visible spectrum of the nanopixels and making flexible array demonstrators to show to potential partners.

For more information: doi: 10.1126/sciadv.aaw2205

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Congratulations to our new paper “Structural Stability and Electron Density Analysis of Doped Germanene: A First-Principles Study” by
Arash Karaei Shiraz, Arash Yazdanpanah Goharrizi, and Seyedeh Mehri Hamidi

The optimized geometry and electronic properties of doped germanene are studied by first-principles calculations. The band structure and density of states of germanene with dopants of group III (Al, Ga and In), IV (Si and Sn), and V (P, As and Sb) are investigated. The results show that group III dopants change the intrinsic behavior of germanene to p-type semiconductor, whereas group V dopants make germanene n-type semiconductor. Moreover, the pristine behavior of germanene remains unchanged by dopants of group IV. The stability of doped optimized supercells and the stability of dopants are obtained by different thermodynamic parameters such as cohesive, and relative binding energies. The binding energies are related to the localization of electrons and shown by electron localization function plots. The n-type, p-type, and intrinsic characteristics are studied by charge transfer calculation and electron difference density plots, to show how electron or hole is injected to the lattice. We found that how the stability features and the electronic properties of doped structures are related to the changes in electron density with doping. In addition, we study the charge transfer and stability of adatoms on germanene and dopants in bulk germanium. Adatoms have lower charge transfer than doped germanene, while dopants in bulk germanium have higher charge transfer values. Based on the calculated results of the present work, the adatoms are more stable than the inserted dopants in germanene.

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Metasurfaces, kinds of planar ultrathin metamaterials, are able to modify the polarization, phase, and amplitude of physical fields of optical light by designed periodic subwavelength structures, attracting great interest in recent years. Based on the different type of the material, optical metasurfaces can be separated in two categories by the materials: one is metal and the other is dielectric. Metal metasurfaces rely on the surface plasma oscillations of subwavelength metal particles. Nevertheless, the loss caused by the metal structures has been a trouble, especially for devices working in transmit modes. The dielectric metasurfaces are based on the Faraday-Tyndall scattering of high-index dielectric light scattering particles. By reasonably designing the relevant parameters of the unit structure such as the size, direction, and shape, different functions of metasurfaces can realize and bring a wide range of applications. This article focuses on the metasurface concepts such as anomalous reflections and refractions and the working principle of different types of metasurfaces. Here, we briefly review the progress in developing optical over past few years and look
into the near future.

Metasurface prism’s convergence effect. (a) Conventional optical device. (b) Metasurface prisms.

Experiments of Figure show that the prism can focus incident light. The gradient metasurface prism thickness is much smaller than the wavelength (approximately 𝜆/20) and all electromagnetic waves can be reflected and focused at the focal point.Therefore, with nearly 100% operating efficiency,
it has important application value in the flat antenna.

(a) All-media gradient metasurface lens. (b)-(e) High numerical aperture lenses; (f)-(g) visible light lenses.

When the incident light wavelength is about 500 nm, the focusing efficiency of the lens reaches 70%. In 2015, Faraon et al. of California Institute of Technology designed a high numerical aperture lens with
a round silicon column. The lens achieves 82% focusing efficiency at the 1550 nm communication wavelength. The microstructure is shown in Figures (b)–(e). The circular silicon column has a high degree of rotational symmetry, so the designed lens is polarization-independent. The height of the silicon pillar is close to 1𝜇m, the aspect ratio is relatively large, and the processing difficulty is also great. Although the proposal of dielectric metasurface is expected to solve the problem of plasmonic metasurface loss, the efficiency of the imaging lens designed in the visible light band is still limited, especially when the wavelength is 500 nm. As shown in Figures (f) and (g), the designed lens consists of a chloro-oxy dielectric rod and a glass substrate. A low loss medium material with smooth surface and high refractive index is used to solve the problem of material selection in visible band.

For more information: https://doi.org/10.1155/2019/2329168

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Following a decade of radical advances in the areas of integrated photonics and computing architectures, we discuss the use of optics in the current computing landscape attempting to re-define and refine their role based on the progress in both research fields. We present the current set of critical challenges faced by the computing industry and provide a thorough review of photonic Network-on-Chip (pNoC) architectures and experimental demonstrations, concluding to the main obstacles that still impede the materialization of these concepts. We propose the employment of optics in chip-to-chip (C2C) computing architectures rather than on-chip layouts towards reaping their benefits while avoiding technology limitations on the way to manycore set-ups. We identify multisocket boards as the most prominent application area and present recent advances in optically enabled multisocket boards, revealing successful 40Gb/s transceiver and routing capabilities via integrated photonics. These results indicate the potential to bring energy consumption down by more than 60% compared to current QuickPath Interconnect (QPI) protocol, while turning multisocket architectures into a single-hop low-latency setup for even more than 4 interconnected sockets, which form currently the electronic baseline.We go one step further and demonstrate how optically-enabled 8-socket boards can be combined via a 256×256 Hipoλaos Optical Packet Switch into a powerful 256-node disaggregated system with less than 335nsec latency, forming a highly promising solution for the latency-critical rack-scale memory disaggregation era. Finally, we discuss the perspective for disintegrated computing via optical technologies as a means to increase the number of synergized high-performance cores overcoming die area constraints, introducing also the concept of cache disintegration via the use of future off-die ultra-fast optical cache memory chiplets.

(a) Conventional CMP architecture with on-chip Cache Memories and Electrical Bus for CPU-MM communication (b) The proposed CMP architecture with off-chip optical

Assuming, for example, an optical CMP-to-cache bus speed and optical cache operational speed of 16GHz, as has been modelled in [164], with a reasonable processing core clock speed of 2GHz, the cache access system performs 8x faster than the processing cores. This indicates that the optical cache can serve all 8 processing cores within a single 2GHz cycle. Regarding latency, every core has 8 cache clock cycles available to complete its request within a single core clock cycle, including of course optoelectronic conversion at the CMP interface, propagation in the optical bus and cache accessing. Assuming a bus length of 1cm, which can be considered as a reasonable value within a macrochip System-in-Package, the time-of-flight is just 50psec for a waveguide-based bus refractive index of 1.5. With optoelectronic conversion taking place at the bus clock speed and at the Memory Address and Memory Buffer Register (MAR and MBR, respectively) interfaces, ultra-fast cache access latency can be obviously easily retained. For detailed timing diagrams that present the optical cache circuitry operation at various stages for both Read and Write operations and the TDM-based access scheme followed in the proposed system of Fig. (b).
This has been extensively analyzed, where also the performance of the system depicted in Fig. was thoroughly investigated via detailed simulations using the gem5 simulation engine and the PARSEC benchmark suite. The main findings when comparing the system of Fig.10(a) with the system of Fig.(b) for the same amount of total cache capacity can be summarized as follows:
• The use of a shared L1 cache yields an important reduction in the cache miss rate of more than 75%, especially when executing parallel programs with high data sharing and exchange needs among their threads; the high volumes of data exchange increase the traffic and consequently the miss rate among the dedicated L1d caches in typical architectures with dedicated L1 caching.
• The shared L1 cache negates the need for cache coherency updates and cache coherency protocols, simplifying the program execution and contributing significantly in cache miss ratio reduction by cancelling all cache coherency misses.
• Cache miss ratio reduction and concurrent multiple core service translate to important execution time speed-up factors that were shown to range between 10% and 20% for computational settings that employed cache capacities equal to the Sparc T5 processor and IBM’s Power7 processor, respectively.
Extending this concept into a macrochip layout with multiple core and optical cache chiplets can bring additional benefits, since caching will be rather utilized as a pool of resources that will facilitate time and energy savings. Moreover, it can transform computing from a rigid into a versatile and flexible environment, where caching and processing resources can be exploited on demand depending on the workload requests, allowing eventually also for cache and processing power upgrades similar to the way that DRAM upgrades are currently being performed.

For more information: DOI 10.1109/JLT.2018.2875995

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The experimental demonstration of narrow resonances in arrays of metallic nanoparticles was more elusive due to limitations in the quality of samples and the use of focused beams. Kravets et al. reported ultranarrow plasmonic resonances in asymmetric (different refractive indexes in the upper and lower media) arrays of Au nanoparticles. Shortly after, Auguié and Barnes and Chu et al. reported narrow resonances in symmetric arrays. The potential of narrow plasmonic resonances in arrays of nanoparticles for modifying the emission of fluorophores was also demonstrated. The origin of the narrow resonances, which are known as surface lattice resonances (SLRs), is the diffractive coupling of LSPRs through in-plane diffraction orders in symmetric media or evanescent
diffraction orders—the so-called Rayleigh anomalies (RAs)—in asymmetric media. SLRs can be described as a driven damped coupled oscillator system in which one oscillator has the natural frequency of the LSPR while the other has the frequency of the diffraction order. Nanoparticle arrays are open cavities that are easy to fabricate and offer the possibility of integration with thin films or planar structures. The remarkable properties of SLRs have led to improved surface-enhanced Raman scattering, sensitive bio/chemical sensing, plasmonic band-edge lasing, strong light–matter coupling, Bose–Einstein condensation, and optoelectronic devices. The multidisciplinary impact of SLRs has stimulated the quest toward modes with the highest possible quality (Q) factor. One strategy to obtain narrow linewidths with SLRs is by coupling multipolar resonances with different diffraction orders. However, Q-factors by these approaches vary significantly over momentum space.

a) Figure of merit of silver films, defined as the ratio of the real and imaginary components of the permittivity. b) AFM map of the surface of silver films deposited at 10 Å s−1; the surface roughness, defined in terms of the root mean square of the surface profile, is 0.567 nm. c) AFM map of a silver film deposited at 30 Å s−1 with RMS = 3.817 nm.

We have demonstrated high quality factor plasmonic resonances in arrays of Ag nanoparticles (Q > 300). These resonances, known as surface lattice resonances, emerge from the coupling of localized surface plasmon polaritons to diffraction orders in the plane of the array. The quadratic dispersion of SLRs leads to a nearly constant Q-factor over a wide range
of wave vectors or angles of incidence. We have investigated the role of the intrinsic quality of the metal in the Q-factor of SLRs. We have also iscussed the effect of the adhesion layer used between the substrate and the metal on the SLRs. The suppression of this layer can lead to SLRs with Q-factors larger than 1500. These extremely high Q-factors render arrays of metallic nanoparticles very interesting systems for plasmonic applications such in sensors, for enhanced light–matter interaction and nonlinear phenomena.

a) Extinction spectra as a function of the wave vector parallel to the surface, showing the dispersion of the degenerate SLRs resulting from the coupling of LSPRs in Ag nanoparticles to (0, ±1) RAs. The LSPR at λ = 475 nm is independent of the wave vector, whereas the white curve corresponds to the (0, ±1) RAs. b) Extinction spectra showing the dispersion of the diagonal SLRs arising from the coupling of the LSPRs to the (±1, 0) RAs (indicated
by the white lines). c) Extinction spectra from part (a), measured at different angles. d) Extinction spectra from part (b), measured at different angles. The insets in parts (a) and (b) show a schematic representation of the angle-dependent extinction measurements, where the rectangles represent the nanorods, the orange double arrow indicates the polarization of the incident light, and the white curve illustrates the rotation direction of the sample.

For more information: DOI: 10.1002/adom.201801451

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Exhibiting a red-shifted absorption/scattering feature compared to conventional plasmonic metals, titanium nitride nanoparticles (TiN NPs) look as very promising candidates for biomedical applications, but these applications are still underexplored despite the presence of extensive data for conventional plasmonic counterparts. Here, we report the fabrication of ultrapure, size-tunable TiN NPs by methods of femtosecond laser ablation in liquids and their biological testing. We show that TiN NPs demonstrate strong and broad plasmonic peak around 640–700 nm with a significant tail over 800 nm even for small NPs sizes (<7 nm). In vitro tests of laser-synthesized TiN NPs on cellular models evidence their low cytotoxicity and excellent cell uptake. We finally demonstrate a strong photothermal therapy effect on U87–MG cancer cell cultures using TiN NPs as sensitizers of local hyperthermia under near-infrared laser excitation. Based on absorption band in the region of relative tissue transparency and acceptable biocompatibility, laser-synthesized TiN NPs promise the advancement of biomedical modalities employing plasmonic effects, including absorption/scattering contrast imaging, photothermal therapy, photoacoustic imaging and SERS.


(a) Schematics of laser ablation setup. A laser beam is focused on the surface of the TiN target, which is placed in the vessel filled with a liquid. The vessel is mounted on a moving translation stage to avoid ablation from the same area of the target. (b) Schematic of laser fragmentation setup to minimize size dispersion of NPs. Ar bubbling used optionally to remove dissolved oxygen.

For more information: https://www.nature.com/articles/s41598-018-37519-1

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Researchers in Switzerland and Australia have brought together the physics of dielectric metasurfaces and hyperspectral imaging to create an ultrasensitive, label-free biosensing platform. The team believes that the platform—reportedly capable of detecting and analyzing samples at spatial concentrations of less than three molecules per square micron—could ultimately enable compact portable diagnostics for personalized medicine. It could also, according to the researchers, offer a route to high-throughput, high-resolution optical characterization of single-atom-thick, 2-D materials such as graphene, a key requirement for advancing the technical development of those much-ballyhooed materials.


The detection platform demonstrated by scientists in Switzerland and Australia, which combines a high-Q-factor dielectric surface and hyperspectral imaging, can reportedly detect biomolecules at a density of less than three molecules per square micron of detector.

The dielectric difference

Sensors that operate via surface plasmon resonances—the subwavelength concentration and amplification of light on surfaces decorated with nanoscale metallic antennas—already have a long pedigree in biomedical diagnostics. As cells or biomolecules bind to the nanostructures, they change the local refractive index by an infinitesimal amount, which in turn leads to sharp changes in the peak wavelength of the surface plasmon resonance. Those wavelength changes can be read to track the presence, concentration and growth of the biological agent under study.


Bound states in the continuum

Originally a concept from quantum mechanics, BICs are confined waves that remain localized within a continuous spectrum of radiating waves. (More precisely, they represent discrete solutions of the single-particle Schrödinger equation from quantum mechanics, embedded within a continuum of positive energy states.) In principle, such bound states would be perfectly localized and would have an infinite quality (Q) factor. In practice, for device design, one can mathematically engineer a subwavelength optical “supercavity” that supports a “quasi-BIC” with an extremely high Q and an extremely narrow resonance width.

High Q response with a hyperspectral kicker

The researchers behind the new work realized that, by virtue of those characteristics, a surface that exploited such supercavity-enabled BICs would be very responsive to local refractive-index changes—and, thus, could enable a very sensitive biodetection platform. So they set about building one. To do so, they used electron-beam lithography to pattern a 100-nm-thick layer of amorphous silicon (on a fused-silica substrate) with an array of tilted silicon “nanobars” around 100 nm wide and 280 nm long. The specific configuration and tilting angle of the bars had been mathematically calculated to take maximum advantage of quasi-BIC states—and the high Q factors and spectrally isolated resonances they enable—in the near-infrared region. That set up a metasurface that, when illuminated with a laser of the right wavelength, was primed to respond dramatically, via changes in resonance peak, to refractive-index changes due to individual biomolecules clinging to the surface. Next, the team supercharged this dielectric-metasurface platform by combining it with another hot technique: hyperspectral imaging. To obtain the hyperspectral data from the metasurface sensor, the team used a supercontinuum laser source to illuminate the sensor, sweeping the laser through multiple, narrow-linewidth frequencies by means of a laser line tunable filter. At each frequency of incident light, a high-resolution CMOS camera captures a new image of the resonant response of the sensor.

The team believes that the combination of high-Q resonant dielectric metasurfaces and high-throughput, imaging-based data acquisition amounts to “a superior and versatile sensing platform.” And the researchers suggest that further explorations—leveraging alternative materials, other dimensions such as incident-light polarization, and machine learning—could expand the system’s flexibility still further, potentially enabling “a field-deployable high-throughput single-molecule detector for biomedical applications.”

For more information: doi: 10.1038/s41566-019-0394-6

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Researchers in China and the United States have reported the creation of upconversion nanoparticles (UCNPs) that can latch onto retinal photoreceptors and serve as tiny antennae for otherwise invisible near-infrared (NIR) light, converting it into a visible signal .
 The research team found that mice injected with the photoreceptor-binding nanoparticles were able not only to perceive NIR radiation, but even to distinguish between different shapes in the dark based only on their NIR signals.

A mammalian limitation

Like other mammals, including humans, mice can see only in the visible part of the spectrum, ranging from wavelengths of 400 to 700 nm. They perceive the visible world through light-absorbing pigments in retinal photoreceptors—the well-known rods, which handle low-light, monochrome vision, and the cones, which are generally responsible for color vision in comparatively brightly lit conditions.
The researchers behind the new work, led by Tian Xue and Jin Bao at the University of Science and Technology of China and Gang Han at the University of Massachusetts Medical School, USA, hit upon a different approach. Instead of using wearable goggles to convert NIR light to visible light, one could do the same thing by adding a tiny NIR antenna to the photoreceptor itself.


Upconversion nanoparticles were coated with a polymer and attached to proteins that bound to retinal photoreceptors. Tests showed that the nanoparticles, injected into the retinas of lab mice, allowed the mice to see infrared radiation as green light.


The team began by chemically creating NIR-sensitive UCNPs—core-shell nanoparticles tuned to have an excitation peak at 980 nm, firmly in the NIR, and to re-emit light at 535 nm, in the green part of the visible-light spectrum. Next, they coated the UCNPs with poly-acrylic acid, to which they then tied a protein strand, concanavalin A (ConA). The researchers chose ConA specifically for its known ability to grab onto sugar residues in the outer segments of the mouse photoreceptors. Spectroscopy confirmed that the ConA strands had bound tightly to the nanoparticles.

Next, the researchers put the mice, with their souped-up retinas, through a number of tests, to see what impact the nanoparticles had on their vision. First, they found that the pupils of mice with the nanoparticle-treated retinas contracted when illuminated with 980-nm light, while the pupils of control mice showed no reaction. Next, they trained both UCNP-treated and control mice to expect a mild shock when confronted with a 20-second green-light pulse at 535 nm, the emission wavelength of the UCNPs, and thus to show a “freezing” behavior upon such a light flash. The treated mice showed the freezing behavior when stimulated with 980-nm (NIR) light or with 535-nm (visible) light, whereas the control mice froze only when stimulated with the 535 light. That clearly indicated that the UCNPs were absorbing the 980-nm light, and converting it to a visible-light signal that could be sopped up by the attached photoreceptors and sent to the mouse brain to interpret.


In a maze test, mice treated with the nanoparticles were able to distinguish between different shapes projected in only infrared light.

The team found that one injection of the nanoparticles gave the mice some level of NIR vision for up to ten weeks after the treatment. And tests months after the treatment, according to the team, suggested that the injections did not cause any long-term damage to the animals’ retinas. While much additional work clearly remains to be done, the research team is bullish about the prospects for its technology. In addition to creating a potential alternative to night-vision goggles for some military, encryption and security applications, the team points out that, by using different kinds of UCNPs, the treatment could help repair certain vision problems, such as red color-vision defects. And for other eye diseases, the nanoparticles could, the team believes, serve as targeted drug-delivery systems that could release small molecules locally to photoreceptors on stimulation with light of a specific wavelength. Co-P.I. Geng Han, in a press release accompanying the research, pitched the potential even further. “It is very likely that the sky may look very differently both at night and in daytime,” he said. “We may have the capability to view all the hidden information from NIR and IR radiation in the universe which is invisible to our naked eyes.”

For more information: doi: 10.1016/j.cell.2019.01.038

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Enhancing magneto-optical effects is crucial for size reduction of key photonic devices based on non-reciprocal propagation of light and to enable active nanophotonics. We disclose a so far unexplored approach that exploits dark plasmons to produce an unprecedented amplification of magneto-optical activity. We designed and fabricated non-concentric magnetoplasmonic-disk/plasmonic-ring-resonator nanocavities supporting multipolar dark modes. The broken geometrical symmetry of the design enables coupling with free-space light and hybridization of dark modes of the ring nanoresonator with the dipolar localized plasmon resonance of the magnetoplasmonic disk. Such hybridization generates a multipolar resonance that amplifies the magneto-optical response of the nanocavity by ~1-order of magnitude with respect to the maximum enhancement achievable by localized plasmons in bare magnetoplasmonic nanoantennas. This large amplification results from the peculiar and enhanced electrodynamic response of the nanocavity, yielding an intense magnetically-activated radiant magneto-optical dipole driven by the low-radiant multipolar resonance. The concept proposed is general and, therefore, our results open a new path that can revitalize research and applications of magnetoplasmonics to active nanophotonics and flat optics.

Magnetoplasmonic NCRD ferromagnetic-nanoantenna/gold-nanocavity and parent Py-DI and Au-RI nanostructures. a Schematic of the NCRD hybrid structure with its four geometric characteristic parameters. b Atomic force and c SEM images of the individual NCRD nanocavity and array. SEM images of the parent single d Py-DI, e Au-RI constituents and arrays. The scale bars in panels c-e correspond to 5 μm and those in their insets to 100 nm.

Both experimental and simulated spectra of the NCRD array display two strongly marked dips located at 600 and 1650 nm and a weaker dip at 820 nm. A comparison with the spectra (simulated and experimental) of the array of bare Au-RI and a close inspection of the spectral dependence of calculated surface charge distribution maps in Fig. 2c, reveal that the two most prominent dips in the NCRD correspond to the excitation of the so-called antibonding and bonding plasmonic resonances in the Au ring portion of the nanocavity at 600 nm and 1650, respectively.

Au-RI and NCRD optical properties and electrodynamics. a Simulated and b experimental transmittance spectra for the NCRD, Py-DI and Au-RI structures. Dashed lines mark the major features in the spectra at 600, 820 and 1600 nm. The small black arrow in panel a highlights a minor feature due to the weak far-field diffractive coupling in a simulated periodic array of defect-less structures. c Surface charge density maps (see Methods) for the Au-RI and NCRD structures at 600, 820 and 1600 nm, normalized to the map at 820 nm for the NCRD for direct comparison. Simulations in panel c are carried out using linearly polarized electromagnetic radiation as indicated by the black arrow (Ei = 1V/m). The surface charge density for the Au-RI at 820 nm has been multiplied by a factor 10 for visualization purposes.

We have demonstrated that high-order multi-polar dark plasmon resonances in magnetoplasmonic nanocavities can be utilized to achieve unprecedented enhancement of the magneto-activated optical response, beyond the present limitations of magnetoplasmonic nanoantennas, enabling a far more efficient active control of the light polarization under weak magnetic fields. The superior behavior of geometrical symmetry broken magnetoplasmonic nanocavities, as compared to corresponding nanoantennas, is explained by the generation of largely enhanced magnetic-field-induced radiant dipole in the magnetoplasmonic nanoantenna driven by a hybrid low-radiant multipolar Fano resonance mode. Therefore, in this novel design, a large enhancement of the magneto-optical response, i.e., the magneto-activated electrical dipole inducing the light polarization modification, is achieved without a significant increase of the pure optical response thanks to the low-radiant character of the hybrid mode. As a result, in the NCRD magnetoplasmonic nanocavity the MOA is additionally amplified by ~1-order of magnitude with respect to the parent Py-DI structure. The novel concept unveiled here opens a fresh path towards applications of magnetoplasmonics to a variety of fields ranging from flat and active nanophotonics to sensing. Therefore, this exploratory work should catalyze future research. Tuning of dark and bright plasmon modes can be achieved by varying the design and the materials to boost both plasmonics (e.g. using silver instead of gold) and intrinsic MO activity (e.g., employing multilayers Au/Co), as well as tuning the relative spectral position and sharpness of the dark and dipolar modes, and thus of the Fano resonance line shape and intensity. Finally, this mechanism might have a huge impact on forthcoming photonic nanotechnologies based on plasmon-mediated local enhanced manipulation of electronic spin-currents opening excellent perspectives in disclosing novel opto-electronic phenomena.

For more information: https://arxiv.org/abs/1903.08392v1


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