A Facile Strategy for All-Optical Controlling Platform by Using Plasmonic Perfect Absorbers

Dual-band light absorption with the maximal absorptivity up to 99.7% and the minimal spectral bandwidth down to 3 nm is obtained in the plasmonic absorbers consisting of triple-layer plasmonic crystal-nonlinear medium cavity-metal substrate structure, where the intercalated dielectric material is chosen to be a Kerr medium cavity. Efficient all-optical controlling with high spectral intensity change ratios and detecting signal-to-noise is achieved for the system after a slight increase of pumping intensity. These impressive results mainly result from the strong plasmonic resonant field confinement in the middle nonlinear Kerr medium cavity and the near-perfect relative intensity change response by the ultra-sharp anti-reflection spectrum. This work can lay a foundation for advanced all-optical devices by exploiting light perfect absorption behavior and resonant optical field enhancement.     

Nanoscale Optical Directional Coupler

Ultracompact optical directional coupler is one of the key elements for nanoscale optical networks and highly integrated optical circuits. Although the transverse size has been reduced down to subwavelength by exploiting plasmonic waveguides, the longitudinal size has remained yet on the micrometer-scale, which seems to be a fundamental limitation by the conventional configuration based on cross-talk coupling between two neighboring waveguides. We have proposed a novel conception of optical directional coupler based on loss-overcompensated resonant coupling between two plasmonic waveguides via an in-between gain-assisted nanocavity. The loss-overcompensated state can be achieved by adjusting pumping rate in the nanocavity. The validity of the proposed conception is confirmed by numerical simulations of a physical model with the deep-subwavelength planar footprint of 300 nm × 300 nm, THz bandwidth, and an exceptionally low energy consumption on the order of 0.1 fJ per signal pulse. To our knowledge, it is the first proposed ultrafast nanoscale four-port directional coupler.     

All-Optical Plasmonic Switches Based on Coupled Nano-disk Cavity Structures Containing Nonlinear Material

All-optical plasmonic switches based on a novel coupled nano-disk cavity configuration containing nonlinear material are proposed and numerically investigated. The finite difference time domain simulation results reveal that the single-disk plasmonic structure can operate as an “on–off” switch with the presence/absence of pumping light. We also demonstrate that the proposed T-shaped plasmonic structure with two disk cavities can switch signal light from one port to another under an optical pumping light, functioning as a bidirectional switch. The proposed nano-disk cavity plasmonic switches have many advantages such as compact size, requirement of low pumping light intensity, and ultra-fast switching time at a femto-second scale, which are promising for future integrated plasmonic devices for applications such as communications, signal processing, and sensing.     

Coupled Plasmonic Cavities on Moire Surfaces

Surface plasmon polariton (SPP) waveguides formed by coupled plasmonic cavities on metallic Moire surfaces have been investigated both experimentally and numerically. The Moire surface, fabricated by interference lithography, contains periodic arrays of one-dimensional cavities. The coupling strength between the cavities has been controlled by changing the periodicities of the Moire surface. The ability to control the coupling strength allows us to tune the dispersion and the group velocity of the plasmonic coupled cavity mode. Reflection measurements and numerical simulation of the array of SPP cavities have shown a coupled resonator type plasmonic waveguide band formation within the band gap. Coupling coefficients of cavities and group velocities of SPPs are calculated for a range of cavity sizes from weakly coupled regime to strongly coupled regime.     

Large and Ultrafast Optical Response of a One-Dimensional Plasmonic–Photonic Cavity

The resonant coupling of a localized surface plasmon mode and a cavity mode in a photonic crystal has been recently shown to strongly tailor the stationary optical response of gold nanoparticles. Here, we demonstrate that this can be further exploited for controlling light on an ultrashort time scale. The stationary and ultrafast optical responses of such a plasmonic–photonic cavity are investigated numerically. We show that the transient photo-induced change of the optical transmittance of a bare nanocomposite thin film can be amplified up to 60 times once resonantly coupled to the cavity mode in the hybrid device, despite the degradation of this mode due to absorption losses. In addition, different all-optical, ultrafast, efficient, and reversible photonic functions (increase or decrease of the signal intensity, transient spectral shift of the cavity mode) can be achieved depending on the spectral position of the transmitted mode tuned by varying the angle of incidence. The transient modification of the signal intensity is predicted to reach about 300 % after a subpicosecond rise time when the defect mode matches the plasmon resonance.     

Highly Sensitive Plasmonic Temperature Sensor Based on Photonic Crystal Surface Plasmon Waveguide

We propose a highly sensitive temperature sensor based on photonic crystal surface plasmon waveguides comprising different plasmonic active metals such as gold, silver, and aluminum, utilizing surface plasmon resonance phenomenon. We found that the resonance wavelength can be easily and substantially tuned over a broad spectral range by changing the temperature and also by judiciously choosing the different plasmonic metals. Employing coupled mode theory, we found that the proposed sensor can be used in harsh environment with sensitivity as high as ～70 pm/K around telecommunication window.     

Ultra-high Sensitivity Plasmonic Nanosensor Based on Multiple Fano Resonance in the MDM Side-Coupled Cavities

We propose a compact plasmonic structure comprising a metal-dielectric-metal (MDM) waveguide coupled with a side cavity and groove resonators. The proposed system is investigated by the finite element method. Simulation results show that the side-coupled cavity supports a local discrete state and the groove provides a continuous spectrum, the interaction between them, gives rise to the Fano resonance. The asymmetrical line shape and the resonant wavelength can be easily tuned by changing the geometrical parameters of the structure. Moreover, we can extend this plasmonic structure by the double side-coupled cavities to gain the multiple Fano resonances. The proposed structure can serve as an excellent plasmonic sensor with a sensitivity of ～1900 nm/RIU and a figure of merit of about ～3.8?×?10⁴, which can find wide applications for nanosensors.     

Realization of Graphene-Based Tunable Plasmon-Induced Transparency by the Dipole-Dipole Coupling

A numerical and theoretical study is presented on the realization of tunable plasmon-induced transparency (PIT) phenomenon in the three-dimensional patterned graphene nanostrips. The simulation results reveal that the PIT effect is generated due to the excitation of dark mode which can be considered a dipole. The three-level plasmonic system is employed to explain the physical mechanism of the PIT effect. Different from previous reported form (dipole-quadrupole coupling), the proposed is attributed to the dipole-dipole coupling. The PIT effect can be tuned by changing the coupling length between bright and dark mode as well as the Fermi energy of graphene. Our studies provide guidance for fabricating ultra-compact devices in practical application.     

Tunable Plasmonic Dispersion and Strong Coupling in Graphene Ribbon and Double Layer Sheets Structure

Based on the interplay between propagating surface plasmon polaritons (PSPs) in graphene ribbon and double layer sheets structure, we theoretically demonstrate a tunable strong coupling mechanism significantly different from reported conventional noble metal nanostructures. The strong electromagnetic coupling between the low order antisymmetric and high order symmetric PSPs modes occurs due to the intersections of dispersion curves, which leads to a modification of plasmonic dispersion and multiple significant anti-crossing regions. Of particular, this strong coupling is controllable through external gate voltage of graphene sheets or ribbon. The results offer an effective regime to dynamically tune the interaction of graphene PSPs, which may find applications in the field of nanophotonic devices in the mid-infrared range.     

Plasmon–Molecule Coupling with Directional Absorption Features: A First-Principles Study

Plasmonics - The coupling between plasmonic nanocavity and quantum emitters has been a major focus of quantum optics and material science research over the last few years. In this work, using...     

Noise Induces Oscillation and Synchronization of the Circadian Neurons

The principle clock of mammals, named suprachiasmatic nucleus (SCN), coordinates the circadian rhythms of behavioral and physiological activity to the external 24 h light-dark cycle. In the absence of the daily cycle, the SCN acts as an endogenous clock that regulates the ~24h rhythm of activity. Experimental and theoretical studies usually take the light-dark cycle as a main external influence, and often ignore light pollution as an external influence. However, in modern society, the light pollution such as induced by electrical lighting influences the circadian clock. In the present study, we examined the effect of external noise (light pollution) on the collective behavior of coupled circadian oscillators under constant darkness using a Goodwin model. We found that the external noise plays distinct roles in the network behavior of neurons for weak or strong coupling between the neurons. In the case of strong coupling, the noise reduces the synchronization and the period of the SCN network. Interestingly, in the case of weak coupling, the noise induces a circadian rhythm in the SCN network which is absent in noise-free condition. In addition, the noise increases the synchronization and decreases the period of the SCN network. Our findings may shed new light on the impact of the external noise on the collective behavior of SCN neurons.     

Ultra-High Sensitivity Nanosensor Based on Multiple Fano Resonance in the MIM Coupled Plasmonic Resonator

This paper proposes a compact plasmonic structure that is composed of a metal-insulator-metal (MIM) waveguide coupled with a groove and stub resonators, and then investigates it by utilizing the finite element method (FEM). Simulation results show that the interaction between the local discrete state caused by the stub resonator and the continuous spectrum caused by the groove resonator gives rise to one of the two Fano resonances, while the generation of the other resonance relies only on the groove. Meanwhile, the asymmetrical linear shape and the resonant wavelength can be easily tuned by changing the parameters of the structure. By adding stubs on the groove, we excited multiple Fano resonances. The proposed structure can serve as an excellent plasmonic sensor with a sensitivity of 2000 nm/RIU and a figure of merit of about 3.04?×?10³, which can find extensive applications for nanosensors.     

Synthesis,Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.     

Active Focal Length Control of Terahertz Slitted Plane Lenses by Magnetoplasmons

Active plasmonic devices are mostly designed at visible frequencies. Here, we propose an active terahertz (THz) plasmonic lens tuned by an external magnetic field. Unlike other tunable devices where the tuning is achieved by changing the plasma frequency of materials, the proposed active lens is tuned by changing the cyclotron frequency through manipulating magnetoplasmons (MPs). We have theoretically investigated the dispersion relation of MPs of a semiconductor?Cinsulator?Csemiconductor structure in the Voigt configuration and systematically designed several lenses realized with a doped semiconductor slab perforated with sub-wavelength slits. It is shown through finite?Cdifference time?Cdomain simulations that THz wave propagating through the designed structure can be focused to a small size spot via the control of MPs. The tuning range of the focal length under the applied magnetic field (up to 1?T) is ??3??, about 50% of the original focal length. Various lenses, including one with two focal spots and a tunable lens for dipole source imaging, are realized for the proposed structure, demonstrating the flexibility of the design approach. The proposed tunable THz plasmonic lenses may find applications in THz science and technology such as THz imaging.     

Tunable Multi-Fano Resonances in MDM-Based Side-Coupled Resonator System and its Application in Nanosensor

In this paper, two Fano resonances are achieved in the proposed plasmonic system. Theoretical analysis and simulation results show that two discrete states coupled with a continua lead to these Fano resonances. The discrete states are provided by the side-coupled square cavity, and a baffle plate placed in metal-dielectric-metal waveguide is used to produce a continuous transmission spectrum. The resonant wavelengths and the linewidth of these Fano resonances can be easily tuned by adjusting the parameters of system. This system exhibits high sensitivities as high as 850 and 1120 nm/RIU corresponding to two Fano resonances, and the figure of merit can reach to 1.7 × 10⁵ by optimizing the system. By introducing another square cavity, four Fano resonances are obtained which originate from four discrete states coupled with continua, and they can be tuned independently. The flexible multi-Fano resonances system has significant application bio-nanosensor, nonlinear, and slow light devices.     

Influence of Anisotropy on Optical Bistability in Plasmonic Nanoparticles with Cylindrical Symmetry

In this paper, the effect of radial anisotropy on optical bistability in the cylindrical nanoshells is theoretically investigated within the quasi-static approximation. We consider two cases: when the shell is anisotropic and the core is nonlinear metal and when the core is anisotropic and the shell is a nonlinear metal. The dependence of optical bistability on the size of the nonlinear/anisotropic shell or core, the embedding medium, the anisotropy parameter, and the type of noble metals as candidates for plasmonics is studied and demonstrated. We show that by changing the type of the plasmonic metal, the switching threshold field changes can be used to design nanoparticle-based all-optical sensors. It is also shown that significant optical bistability and all-optical switching behavior can be obtained in the cylindrical nanoshells due to nonlinearity enhancement via the plasmonic structure.     

Multi-Band Ultra-Sharp Transmission Response in All-Dielectric Resonant Structures Containing Kerr Nonlinear Media

All-dielectric resonant structure (ADRS) consisting of high-index nonlinear dielectrics has been theoretically and numerically demonstrated with multi-band ultra-sharp transmission response in this work. Bandwidth down to sub-nanometer and spectral Q-factor up to 920 are achieved in this ADRS-based metamaterial-like platform. Strong resonant electric field distributions by the high-index dielectric resonators and efficient coupling between the layered dielectric particles and the cavity mainly contribute to the multiple narrowband light transmission filtering. By using a Kerr nonlinear medium as the resonant dielectric, the positions of the transmission dips in the spectrum can be actively tuned by the incident light intensity. Due to the ultra-narrow spectral feature and the strong electric field distribution by the resonators, an efficient all-optical switching behavior with high spectral difference intensity and contrast ratio is obtained. Further study presents the observed multi-band transmission with high scalability by tuning the structural parameters. These optical features hold the predicted ADRS be potentially applied to constructing dielectric metamaterial-based all-optical switching or active subtractive transmission filtering with low power threshold at sub-diffraction scale.     

Phase Resetting Curves Allow for Simple and Accurate Prediction of Robust N:1 Phase Locking for Strongly Coupled Neural Oscillators

Existence and stability criteria for harmonic locking modes were derived for two reciprocally pulse coupled oscillators based on their first and second order phase resetting curves. Our theoretical methods are general in the sense that no assumptions about the strength of coupling, type of synaptic coupling, and model are made. These methods were then tested using two reciprocally inhibitory Wang and Buzsáki model neurons. The existence of bands of 2:1, 3:1, 4:1, and 5:1 phase locking in the relative frequency parameter space was predicted correctly, as was the phase of the slow neuron's spike within the cycle of the fast neuron in which it occurred. For weak coupling the bands are very narrow, but strong coupling broadens the bands. The predictions of the pulse coupled method agreed with weak coupling methods in the weak coupling regime, but extended predictability into the strong coupling regime. We show that our prediction method generalizes to pairs of neural oscillators coupled through excitatory synapses, and to networks of multiple oscillatory neurons. The main limitation of the method is the central assumption that the effect of each input dies out before the next input is received.     

Dynamically Tunable by Kerr Effect Multichannel Filter Based on Plasmon Induced Transparencies at Optical Communication Range

Dynamically tunable multichannel filter based on plasmon-induced transparencies (PITs) is proposed in a plasmonic waveguide side-coupled to slot and rectangle resonators system at optical communication range. The slot and rectangle resonators in this system can be regarded as radiative or dark resonators as same as the radiative or dark elements in the metamaterial structure with the help of the evanescent coupling. The multiple PIT responses which can enable the realization of nanoscale filter with four channels are originated from the direct near-field coupling and indirect phase couple through a plasmonic waveguide simultaneously. Moreover, the magnitudes and bandwidths of the filter can be efficiently tuned by controlling of the geometric parameters such as the coupling distances and the pump light-induced refractive index change of the Kerr material which is embedded into the metal-dielectric-metal waveguide between the radiative resonators.     

A Tunable Slow Light Device with Multiple Channels Based on Plasmon-Induced Transparency

Slow light devices with buffering capability play a critical role in all-optical signal processing. In this paper, multiple slow light phenomena are implemented based on plasmon-induced transparency (PIT) in our device. The device mainly consists of dual tooth cavities coupled with stub resonators, respectively. Temporal coupled-mode theory model illustrates that the triple PIT phenomena can be achieved based on different formation mechanisms. The simulation results calculated by the finite-difference time-domain method reveal that significant slow light response occurs at two wavelength regions. In addition, the parameters of structure have an important influence on PIT response and slow light characteristics. Moreover, the separate manipulation of wavelength, transmission and group index at transparency peak can be achieved in different slow light channels by adjusting the structural parameters. This plasmonic device is of great significance for the design of optical networks on chips.