Our new paper in journal of optical and quantum electronics

Congratulations to our new paper “Role of higher order plasmonic modes in one-dimensional nanogratings ” by Foozieh Sohrabi, Seyedeh Mehri Hamidi, Ershad Mohammadi

By theoretically investigating the optical behavior of one-dimensional gold nanogratings using Fourier Modal Method, we have shown that both integer and non-integer multiples of surface plasmon polariton wavelengths should be taken into consideration in special optical contrast ratio for highly sensitive sensing. The emergence of higher modes is the key factor for the formation of observed plasmonic band gap. Through considering the significant role of grating period and thickness respectively in horizontal and vertical surface resonances, it was demonstrated that for gold thicknesses below 100 nm, the dominant phenomenon is horizontal surface resonances while for increased thicknesses both horizontal and vertical surface resonances mediate. The transmission minima are insensitive to the grating thickness, which confirms that their origins are not vertical surface resonances. This study can open an avenue towards designing highly sensitive sensors with focus not only on the plasmonic resonance wavelength but also on its integer and non-integer multiples whose origins should be investigated in both horizontal and vertical surface resonances.

Researchers design superhydrophobic ‘nanoflower’ for biomedical applications

Plant leaves have a natural superpower—they’re designed with water repelling characteristics. Called a superhydrophobic surface, this trait allows leaves to cleanse themselves from dust particles. Inspired by such natural designs, a team of researchers at Texas A&M University has developed an innovative way to control the hydrophobicity of a surface to benefit to the biomedical field. applications in the biomedical field including biosensing, lab-on-a-chip, blood-repellent, anti-fouling and self-cleaning applications. Superhydrophobic materials are used extensively for self-cleaning characteristic of devices. However, current materials require alteration to the chemistry or topography of the surface to work. This limits the use of superhydrophobic materials. “Designing hydrophobic surfaces and controlling the wetting behavior has long been of great interest, as it plays crucial role in accomplishing self-cleaning ability,” Gaharwar said. “However, there are limited biocompatible approach to control the wetting behavior of the surface as desired in several biomedical and biotechnological applications.” The Texas A&M design adopts a ‘nanoflower-like’ assembly of two-dimensional (2-D) atomic layers to protect the surface from wetting. The team recently released a study published in Chemical Communications. 2-D nanomaterials are an ultrathin class of nanomaterials and have received considerable attention in research. Gaharwar’s lab used 2-D molybdenum disulfide (MoS2), a new class of 2-D nanomaterials that has shown enormous potential in nanoelectronics, optical sensors, renewable energy sources, catalysis and lubrication, but has not been investigated for biomedical applications. This innovative approach demonstrates applications of this unique class of materials to the biomedical industry.

This innovative technique opens many doors for expanded applications in several scientific and technological areas. The superhydrophobic coating can be easily applied over various substrates such as glass, tissue paper, rubber or silica using the solvent evaporation method. These superhydrophobic coatings have wide-spread applications, not only in developing self-cleaning surfaces in nanoelectronics devices, but also for biomedical applications. Specifically, the study demonstrated that blood and cell culture media containing proteins do not adhere to the surface, which is very promising. In addition, the team is currently exploring the potential applications of controlled hydrophobicity in stem cell fate.

For more information:

Our new paper in international journal of optics and photonics

Congratulations to our new paper “Adjustable plasmonic bandgap in one-dimensional nanograting based on localized and propagating surface plasmons” by Foozieh Sohrabi and Seyedeh Mehri Hamidi.

Compared to the long history of plasmonic gratings, there
are only a few studies regarding the bandgap in the propagation of plasmonic
surface waves. Considering the previous studies on interpretation of plasmonic
bandgap formation, we discuss this phenomenon using the effect of both surface
plasmon polariton (SPP) and localized surface plasmon (LSP) for our fabricated
one-dimensional metallic-polymeric grating. This structure is composed of
metallic grating on the surface of PDMS with different concentration of
embedded gold nanoparticles. By sweeping the incident angles, we have seen that
the SPP, LSP and their coupling cause two gaps in reflection regime which are
originated from SPP supported by thin film gold film and LSP supported by gold
nanoparticles. The first gap is attributed to the patterned metallic film
because it vanishes by increasing the nanoparticles which may destroy the
pattern while the second gap can be formed by embedded nanoparticles because it
becomes more considerable by raising the incubation time.  Therefore, the drowning time of patterned
samples (e.g. 24h. 48h and 72h) in HAuCl4 plays the key role in adjustability
of plasmonic bandgap. Notably, the interaction between SPP and LSP can be the
origin of the shift in gap center from 300 to 550. To best of over knowledge,
this study is the first study on the plasmonic band gap as a function of both
SPP and LSP.

Light-Emitting Plexciton: Exploiting Plasmon-Exciton Interaction in the Intermediate Coupling Regime

The interaction between plasmons in metal nanostructures and excitons in layered materials attracts recent interests due to its fascinating properties inherited from the two constituents, e.g., the high tunability on its spectral or spatial properties from the plasmonic component, and the large optical
nonlinearity or light emitting properties from the excitonic counterpart. Here, we demonstrate the light-emitting plexcitons from the coupling between the neutral excitons in monolayer WSe2 and highly-confined nanocavity plasmons in nanocube-over-mirror system. We observe, simultaneously, an anti-crossing dispersion curve of the hybrid system in the dark-field scattering spectrum and a 1700 times enhancement in the photoluminescence. We attribute the large photoluminescence enhancement to the increased local density of states by both the plasmonic and excitonic constituents in the intermediate coupling regime. What’s more, increasing the confinement of the hybrid systems is achieved by shrinking down the size of hot spot within the gap between the nanocube and the metal film. Numerical calculations reproduce the experimental observations and provide the effective number of excitons taking part in the interaction. This highly compact system provides a room temperature testing platform for quantum cavity electromagnetics at the deep subwavelength scale.

Our hybrid NCOM sample consists of a monolayer WSe2 inserted into the gap between a silver nanocube and a gold film (Figure 1a). Interestingly, the emission of this mode is shown to be highly directional along the film normal, making it addressable by free space radiation.30 Under the illumination of a normal incident x-polarized plane wave (as shown in Figure 1b inset), Figures 1c-e show the scattered electric field of x, y, z- component of the M mode normalized to the incident electric field E0.

For more information: DOI: 10.1021/acsnano.8b05880

Femtosecond Light Transmission and Subradiant Damping in Plasmonic Crystal

They report the first observation of subradiance in plasmonic nanocrystals. Amplitude- and phaseresolved ultrafast transmission experiments directly reveal the coherent coupling between surface plasmon polaritons (SPPs) induced by periodic variations in the dielectric function. This interaction results in the formation of plasmonic band gaps and coupled SPP eigenmodes with different symmetries, as directly shown by near-field imaging. In antisymmetric modes, radiative SPP damping is strongly suppressed, increasing the SPP lifetime from 30 fs to more than 200 fs. The findings are analyzed within a coupled resonance model.

FIG. 1 (color online). (a) Time structure of the electric field Ein(t) of the incident 11 fs pulses and of the pulses Eout(t) transmitted through a nanoslit array with a0=650 nm at an angle θ of 8°. (b) Representative transmission spectrum T(ω)=│Eout(ω)/Ein(ω)│2 and (c) spectral phase φ(ω)=arg[Eout(ω)/Ein(ω)] (open circles) near the SM[ _ 2] resonance. Solid lines: Fit to Eq. (1) with parameters indicated in (b).

For more information: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.94.113901

Electrochromic behavior of WO3 thin films prepared by GLAD

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

Tiny Plasmonic Pixels, Big Potential

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

Our new paper in Materials Research Express

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.

Two-Dimensional Optical Metasurfaces: From Plasmons to Dielectrics

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

Optics in Computing: from Photonic Network-on-Chip to Chip-to-Chip Interconnects and Disintegrated Architectures

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