Infrared routing and switching with tunable spectral bandwidth using arrays of metallic nanoantennas.

We study infrared routing and switching with tunable spectral bandwidth using in-plane scattering of light by flat Au nanoantenna arrays. The base dimensions of these nanoantennas are approximately 250 by 850 nm, while their heights vary from 20 to 150 nm. Our results show that, with the increase in height, the arrays become more efficient scatterers while their spectra broaden within the 1-1.6µm range. Our findings demonstrate that such processes strongly depend on the incident light polarization. For a given polarization, the incident light is efficiently scattered in only two opposite directions along the plane of the arrays, with insignificant transmission. Switching such a polarization by 90∘, however, suppresses this process, allowing the light to mostly pass through the arrays with minimal scattering. These unique characteristics suggest a tunable beam splitter application in the 1-1.6µm range and even longer wavelengths.

Spin-polarization control of in-plane scattering in arrays of asymmetric U-shaped nanoantennas.

We study projection-enabled enhancement of asymmetric optical responses of plasmonic metasurfaces for photon-spin control of their far field scattering. Such a process occurs by detecting the light scattered by arrays of asymmetric U-shaped nanoantennas along their planes (in-plane scattering). The nanoantennas are considered to have relatively long bases and two unequal arms. Therefore, as their view angles along the planes of the arrays are changed, they offer an extensive range of shape and size projections, providing a wide control over the contributions of plasmonic near fields and multipolar resonances to the far field scattering of the arrays. We show that this increases the degree of the asymmetric spin-polarization responses of the arrays to circularly polarized light, offering a large amount of chirality. In particular, our results show the in-plane scattering of such metasurfaces can support opposite handedness, offering the possibility of photon spin-dependent directional control of energy routing.

MoS2 Nanodonuts for High-Sensitivity Surface-Enhanced Raman Spectroscopy.

Nanohybrids of graphene and two-dimensional (2D) layered transition metal dichalcogenides (TMD) nanostructures can provide a promising substrate for extraordinary surface-enhanced Raman spectroscopy (SERS) due to the combined electromagnetic enhancement on TMD nanostructures via localized surface plasmonic resonance (LSPR) and chemical enhancement on graphene. In these nanohybrid SERS substrates, the LSPR on TMD nanostructures is affected by the TMD morphology. Herein, we report the first successful growth of MoS2 nanodonuts (N-donuts) on graphene using a vapor transport process on graphene. Using Rhodamine 6G (R6G) as a probe, SERS spectra were compared on MoS2 N-donuts/graphene nanohybrids substrates. A remarkably high R6G SERS sensitivity up to 2 × 10-12 M has been obtained, which can be attributed to the more robust LSPR effect than in other TMD nanostructures such as nanodiscs as suggested by the finite-difference time-domain simulation. This result demonstrates that non-metallic TMD/graphene nanohybrids substrates can have SERS sensitivity up to one order of magnitude higher than that reported on the plasmonic metal nanostructures/2D materials SERS substrates, providing a promising scheme for high-sensitivity, low-cost applications for biosensing.

Localized Surface Plasmon Resonance Enhanced Light Absorption in AuCu/CsPbCl3 Core/Shell Nanocrystals.

Localized surface plasmon resonance (LSPR) is shown to be effective in trapping light for enhanced light absorption and hence performance in photonic and optoelectronic devices. Implementation of LSPR in all-inorganic perovskite nanocrystals (PNCs) is particularly important considering their unique advantages in optoelectronics. Motivated by this, the first success in colloidal synthesis of AuCu/CsPbCl3 core/shell PNCs and observation of enhanced light absorption by the perovskite CsPbCl3 shell of thickness in the range of 2-4 nm, enabled by the LSPR AuCu core of an average diameter of 7.1 nm, is reported. This enhanced light absorption leads to a remarkably enhanced photoresponse in PNCs/graphene nanohybrid photodetectors using the AuCu/CsPbCl3 core/shell PNCs, by more than 30 times as compared to the counterparts with CsPbCl3 PNCs only (8-12 nm in dimension). This result illustrates the feasibility in implementation of LSPR light trapping directly in core/shell PNCs for high-performance optoelectronics.

Ultrahigh Brightening of Infrared PbS Quantum Dots via Collective Energy Transfer Induced by a Metal-Oxide Plasmonic Metastructure.

We demonstrate that a solution-processed heterojunction interface formed via the addition of a thin buffer layer of CdSe/ZnS quantum dots (QDs) to a functional metal oxide plasmonic metastructure (FMOP) can set up a collective interquantum dot energy-transport process, significantly enhancing the emission of infrared PbS quantum dots. The FMOP includes a Schottky junction, formed via deposition of a Si layer on arrays of Au nanoantennas and a Si/Al oxide charge barrier. We show when these two junctions are separated from each other by about 15 nm and the CdSe/ZnS quantum dot buffer layer is placed in touch with the Si/Al oxide junction, the quantum efficiency of an upper layer of PbS quantum dots can increase by about 1 order of magnitude. These results highlight a unique energy circuit formed via collective coupling of the CdSe/ZnS quantum dots with the hybridized states of plasmons and diffraction modes of the arrays (surface lattice resonances) and coupling between such resonances with PbS QDs via lattice-induced photonic modes.

Synthesis of water-soluble Ni(II) complexes and their role in photo-induced electron transfer with MPA-CdTe quantum dots.

Photocatalytic water splitting using solar energy for hydrogen production offers a promising alternative form of storable and clean energy for the future. To design an artificial photosynthesis system that is cost-effective and scalable, earth abundant elements must be used to develop each of the components of the assembly. To develop artificial photosynthetic systems, we need to couple a catalyst for proton reduction to a photosensitizer and understand the mechanism of photo-induced electron transfer from the photosensitizer to the catalyst that serves as the fundamental step for photocatalysis. Therefore, our work is focused on the study of light driven electron transfer kinetics from the quantum dot systems made with inorganic chalcogenides in the presence of Ni-based reduction catalysts. Herein, we report the synthesis and characterization of four Ni(II) complexes of tetradentate ligands with amine and pyridine functionalities (N2/Py2) and their interactions with CdTe quantum dots stabilized by 3-mercaptopropionic acid. The lifetime of the quantum dots was investigated in the presence of the Ni complexes and absorbance, emission and electrochemical measurements were performed to gain a deeper understanding of the photo-induced electron transfer process.

Using Silver Nanoparticles-Embedded Silica Metafilms as Substrates to Enhance the Performance of Perovskite Photodetectors.

Plasmonic metal nanostructures provide a promising strategy for light trapping and therefore can dramatically enhance photocurrent in optoelectronics only if the trapped light can be coupled effectively from plasmons to excitons, whereas the reverse transfer of energy, charge, and heat from excitons to plasmons can be suppressed. Motivated by this, this work develops a scheme to implement a metafilm with Ag nanoparticles (NPs) embedded in 10 nm thick silica (Ag NPs-silica metafilm) to the active device channel of a hybrid perovskite film/graphene photodetector. Remarkably, an enhancement factor of 7.45 in photoresponsivity, the highest so far among all the reports adopting plasmonic metal NPs in perovskite photodetectors, has been achieved on the photodetectors with the Ag NPs-silica metafilms. Considering that the synthesis of the Ag NPs-silica metafilms can be readily scaled up to coat both rigid and flexible substrates, this result provides a low-cost metaplatform for a variety of high-performance optoelectronic device applications.

Infrared plasmonic meta-modes via near-field coupling of metallic nanorods with split-ring resonators.

We study the anomalous optical properties of lattices formed via periodic arrangement of a plasmonic unit structure consisting of a metallic nanorod and U-shape split-ring resonator. When the units are closely packed, i.e., small lattice constants, and the incident light is polarized along the transverse axis of the nanorods, our results show that the near-field plasmonic coupling of these units leads to a lattice-induced meta-mode. Such a meta-mode is not an intrinsic mode of these units or their constituents (nanorods and split-ring resonator), rather it is formed via capacitive coupling of the split-ring resonator of one unit with the nanorod of another unit. This leads to a unique charge distribution, generating a strong field accumulation at the center of the nanorod. We show that this assimilates a plasmon field profile similar to that of the intrinsic quadrupole mode of the nanorods, although it occurs at wavelengths longer than their dipole modes. Our results show that such a meta-mode generates a narrow dominant optical feature in the infrared range (∼1.5 µm) with significant immunity against the rotation of the lattices.

Balancing silicon/aluminum oxide junctions for super-plasmonic emission enhancement of quantum dots via plasmonic metafilms.

We study the impact of structural features of Si/Al oxide junctions on metal-oxide plasmonic metafilms formed via placing such junctions in close vicinity of an Au/Si Schottky barrier. The emission intensity and dynamics of colloidal semiconductor quantum dots deposited on such metafilms are investigated, while the surface morphology and structural compositions of the Si/Al oxide junction are controlled. The results show the conditions wherein the Si/Al oxide junction can reshape the impact of plasmonic effects, allowing it to increase the lifetimes of excitons. Under these conditions, the plasmonic metafilms can quarantine excitons against the fluctuating trap environments of the quantum dots, offering super-plasmonic emission enhancement that includes enhancement of the spontaneous emission decay rate combined with the suppression of Auger decay.

Semiconductor quantum dot super-emitters: spontaneous emission enhancement combined with suppression of defect environment using metal-oxide plasmonic metafilms.

We demonstrate that a metal-oxide plasmonic metafilm consisting of a Si/Al oxide junction in the vicinity of a thin gold layer can quarantine excitons in colloidal semiconductor quantum dots against their defect environments. This process happens while the plasmon fields of the gold layer enhance spontaneous emission decay rates of the quantum dots. We study the emission dynamics of such quantum dots when the distance between the Si/Al oxide junction and the gold thin layer is varied. The results show that for distances less than a critical value the lifetime of the quantum dots can be elongated while they experience intense plasmon fields. This suggests that the metal-oxide metafilm can keep photo-excited electrons in the cores of the quantum dots, suppressing their migration to the surface defect sites. This leads to suppression of Auger recombination, offering quantum dot super-emitters with emission that is enhanced not only by the plasmon fields (Purcell effect), but also by strong suppression of the non-radiative decay caused by the defect sites.

Ultrafast emission decay with high emission efficiency of quantum dots in plasmonic-dielectric metasubstrates.

We report on the ultrafast decay of quantum dots that reaches nearly 430 ps without reduction in the emission intensity when near a metasubstrate of simple geometry. By implementing a layered structure of amorphous silicon sandwiched between two gold layers, the balance between plasmonic near field enhancement and energy transfer of the quantum dots has been shifted. This is achieved by tailoring the amount of Förster resonance energy transfer (FRET) to the top Au layer and plasmonic near field enhancement by the bottom Au layer. We also study the impact of deposition of Al oxide on the top Au layer, forming a charge barrier junction that can suppress Auger recombination of quantum dots. The results show that such a barrier can increase emission efficiency of the metastructure without significant reduction in the decay rate. We study the impact of formation of small nanoislands to contiguous islands via variation of the thickness of the top Au layer, and discuss how such morphologies determine the amount of FRET, screening of plasmonic field from the bottom Au layer, and enhancement of emission due to their own plasmonic fields.

Biological sensing using hybridization phase of plasmonic resonances with photonic lattice modes in arrays of gold nanoantennas.

We study biological sensing using the hybridization phase of localized surface plasmon resonances (LSPRs) with diffraction modes (photonic lattice modes) in arrays of gold nanoantennas. We map the degree of the hybridization process using an embedding dielectric material (Si), identifying the critical thicknesses wherein the optical responses of the arrays are mainly governed by pure LSPRs (insignificant hybridization), Fano-type coupling of LSPRs with diffraction orders (hybridization state), and their intermediate state (hybridization phase). The results show that hybridization phase can occur with slight change in the refractive index (RI), leading to sudden reduction of the linewidth of the main spectral feature of the arrays by about one order of magnitude while it is shifted nearly 140 nm. These processes, which offer significant improvement in RI sensitivity and figure of merit, are utilized to detect monolayers of biological molecules and streptavidin-conjugated semiconductor quantum dots with sensitivities far higher than pure LSPRs. We further explore how these sensors can be used based on the uncoupled LSPRs by changing the polarization of the incident light.

Biological sensing and control of emission dynamics of quantum dot bioconjugates using arrays of long metallic nanorods.

We study biological sensing using plasmonic and photonic-plasmonic resonances of arrays of ultralong metallic nanorods and analyze the impact of these resonances on emission dynamics of quantum dot bioconjugates. We demonstrate that the LSPRs and plasmonic lattice modes of such array can be used to detect a single self-assembled monolayer of alkanethiol at the visible (550 nm) and near infrared (770 nm) range with well resolved shifts. We study adsorption of streptavidin-quantum dot conjugates to this monolayer, demonstrating that formation of nearly two dimensional arrays of quantum dots with limited emission blinking can lead to extra well-defined wavelength shifts in these modes. Using spectrally-resolved lifetime measurements we study the emission dynamics of such quantum dot bioconjugates within their monodispersed size distribution. We show that, despite their close vicinity to the nanorods, the rate of energy transfer from these quantum dots to nanorods is rather weak, while the plasmon field enhancement can be strong. Our results reveal that the nanorods present a strongly wavelength or size-dependent non-radiative decay channel to the quantum dot bioconjugates.

Turning on plasmonic lattice modes in metallic nanoantenna arrays via silicon thin films.

We study control of optical coupling of plasmon resonances in metallic nanoantenna arrays using ultrathin layers of silicon. This technique allows one to establish and tune plasmonic lattice modes of such arrays, demonstrating a controlled transformation from the localized surface plasmon resonances of individual nanoantennas to their optimized collective lattice modes. Depending on the polarization and incident angle of light, our results support two different types of the silicon-induced plasmonic lattice resonances. For s-polarization these resonances follow the Rayleigh anomaly, while for p-polarization an increase in the incident angle makes the lattice resonances significantly narrower and slightly blueshifted.

Metallic nanoparticle shape and size effects on aluminum oxide-induced enhancement of exciton-plasmon coupling and quantum dot emission.

We investigate the shape and size effects of gold metallic nanoparticles on the enhancement of exciton-plasmon coupling and emission of semiconductor quantum dots induced via the simultaneous impact of metal-oxide and plasmonic effects. This enhancement occurs when metallic nanoparticle arrays are separated from the quantum dots by a layered thin film consisting of a high index dielectric material (silicon) and aluminum oxide. Our results show that adding the aluminum oxide layer can increase the degree of polarization of quantum dot emission induced by metallic nanorods by nearly two times, when these nanorods have large aspect ratios. We show when the aspect ratio of these nanorods is reduced to half, the aluminum oxide loses its impact, leading to no improvement in the degree of polarization. These results suggest that a silicon/aluminum oxide layer can significantly enhance exciton-plasmon coupling when quantum dots are in the vicinity of metallic nanoantennas with high aspect ratios.

Competition between rice (Oryza sativa L.) and (barnyardgrass (Echinochloa crus-galli (L.) P. Beauv.) as affected by methanol foliar application.

Pot experiment was conducted in Iran, to evaluate the effect of methanol on competition between rice (Oryza sativa) and barnyardgrass (Echinochloa crus-galli). The experiment was conducted as a randomized complete block design with a factorial treatment arrangement and three replicates. Factors were two aqueous methanol foliar applications (0, and 14% v/v) and five rice: barnyardgrass ratios (100:0, 75:25, 50:50, 25:6, and 0:100). Replacement series diagrams for aboveground dry weight illustrated that 'Shiroudi' was more competitive than barnyardgrass as averaged across methanol foliar applications. When methanol was not sprayed, the lines for 'Shiroudi' and barnyardgrass intersected at 75:25 rice: barnyardgrass ratio, but when methanol was sprayed at 14% v/v, the lines for 'Shiroudi' and barnyardgrass intersect at the left of the 75:25 rice: barnyardgrass mixture proportion. These indicate that methanol application reduced competitive ability of 'Shiroudi' against barnyardgrass for aboveground biomass accumulation. At the same time, Methanol foliar application significantly reduced the relative crowding coefficient of 'Shiroudi' while simultaneously it significantly increased the relative crowding coefficient of barnyard grass. This indicates that methanol foliar application reduced the competitive ability of 'Shiroudi' against barnyardgrass for shoot biomass accumulation. This experiment illustrated that foliar spray of aqueous methanol can not be recommended for rice under weedy conditions.

Probing the structural dependency of photoinduced properties of colloidal quantum dots using metal-oxide photo-active substrates.

We used photoactive substrates consisting of about 1 nm coating of a metal oxide on glass substrates to investigate the impact of the structures of colloidal quantum dots on their photophysical and photochemical properties. We showed during irradiation these substrates can interact uniquely with such quantum dots, inducing distinct forms of photo-induced processes when they have different cores, shells, or ligands. In particular, our results showed that for certain types of core-shell quantum dot structures an ultrathin layer of a metal oxide can reduce suppression of quantum efficiency of the quantum dots happening when they undergo extensive photo-oxidation. This suggests the possibility of shrinking the sizes of quantum dots without significant enhancement of their non-radiative decay rates. We show that such quantum dots are not influenced significantly by Coulomb blockade or photoionization, while those without a shell can undergo a large amount of photo-induced fluorescence enhancement via such blockade when they are in touch with the metal oxide.

Coherently-enabled environmental control of optics and energy transfer pathways of hybrid quantum dot-metallic nanoparticle systems.

It is well-known that optical properties of semiconductor quantum dots can be controlled using optical cavities or near fields of localized surface plasmon resonances (LSPRs) of metallic nanoparticles. In this paper we study the optics, energy transfer pathways, and exciton states of quantum dots when they are influenced by the near fields associated with plasmonic meta-resonances. Such resonances are formed via coherent coupling of excitons and LSPRs when the quantum dots are close to metallic nanorods and driven by a laser beam. Our results suggest an unprecedented sensitivity to the refractive index of the environment, causing significant spectral changes in the Förster resonance energy transfer from the quantum dots to the nanorods and in exciton transition energies. We demonstrate that when a quantum dot-metallic nanorod system is close to its plasmonic meta-resonance, we can adjust the refractive index to: (i) control the frequency range where the energy transfer from the quantum dot to the metallic nanorod is inhibited, (ii) manipulate the exciton transition energy shift of the quantum dot, and (iii) disengage the quantum dot from the metallic nanoparticle and laser field. Our results show that near meta-resonances the spectral forms of energy transfer and exciton energy shifts are strongly correlated to each other.

Quantum dot-metallic nanorod sensors via exciton-plasmon interaction.

We investigate quantum nanosensors based on hybrid systems consisting of semiconductor quantum dots and metallic nanorods in the near-infrared regime. These sensors can detect biological and chemical substances based on their impact on the coherent exciton-plasmon coupling and molecular resonances supported by such systems when they interact with a laser field. We demonstrate that the ultrahigh sensitivity of such molecular resonances on environmental conditions allows dramatic and nearly instantaneous changes in the total field experienced by the semiconductor quantum dot via minuscule variations of the local refractive indices of the quantum dot or nanorod. The proposed nanosensors can utilize quantum effects to control the sense (or direction) of the changes in the quantum dot emission, allowing us to have bistable switching from dark to bright states or vice versa via adsorption (or detachment) of biomolecules. These sensors can also offer detection of ultra-small variations in the local dielectric constant of the quantum dots or metallic nanorods via coherent induction of time delays in the effective field experienced by the quantum dots when the hybrid systems interact with time-dependent laser fields. This leads to unprecedented bulk refractive index sensitivities. Our results show that one can utilize quantum phase to control the coherent exciton-plasmon dynamics in these sensors such that introduction of a biomolecule can increase or decrease the time delay. These results offer novel ways to detect single biomolecules via application of quantum coherence to convert their impact into spectacular optical events.

Optical response of a quantum dot-metal nanoparticle hybrid interacting with a weak probe field.

We study optical effects in a hybrid system composed of a semiconductor quantum dot and a spherical metal nanoparticle that interacts with a weak probe electromagnetic field. We use modified nonlinear density matrix equations for the description of the optical properties of the system and obtain a closed-form expression for the linear susceptibilities of the quantum dot, the metal nanoparticle, and the total system. We then investigate the dependence of the susceptibility on the interparticle distance as well as on the material parameters of the hybrid system. We find that the susceptibility of the quantum dot exhibits optical transparency for specific frequencies. In addition, we show that there is a range of frequencies of the applied field for which the susceptibility of the semiconductor quantum dot leads to gain. This suggests that in such a hybrid system quantum coherence can reverse the course of energy transfer, allowing flow of energy from the metallic nanoparticle to the quantum dot. We also explore the susceptibility of the metal nanoparticle and show that it is strongly influenced by the presence of the quantum dot.