High-channel-count plasmonic filter with the metal-insulator-metal Fibonacci-sequence gratings

Ultrafast and low-power multichannel all-optical switcher based on multilayer graphene

A metal-insulator-metal waveguide structure composed of a hexagonal resonator cavity and a ring with a slit is proposed. By using the finite difference time domain method, the transmission properties of the structure were studied. It was found that three distinct plasmon-induced transparency peaks appear in the visible and near-infrared bands, and the transmissivity of the three peaks is more than 80%. By tuning the structure size, the positions of the peaks can be adjusted. Then we introduced graphene, covering the surface of the cavity. By adjusting the refraction index of the graphene using light, the position of the three transmission peaks can be changed correspondingly. Based on the effect, we designed an all-optical switcher with ultrafast optical response time (about 2 ps) and low light absorption (about 2.3%). The proposed waveguide structure provides a way for the development of visible and near-infrared filters and all-optical switchers.

Tunable Coupled-Resonator-Induced Transparency in a Photonic Crystal System Based on a Multilayer-Insulator Graphene Stack

We achieve the effective modulation of coupled-resonator-induced transparency (CRIT) in a photonic crystal system which consists of photonic crystal waveguide (PCW), defect cavities, and a multilayer graphene-insulator stack (MGIS). Simulation results show that the wavelength of transparency window can be effectively tuned through varying the chemical potential of graphene in MGIS. The peak value of the CRIT effect is closely related to the structural parameters of our proposed system. Tunable Multipeak CRIT is also realized in the four-resonator-coupled photonic crystal system by modulating the chemical potentials of MGISs in different cavity units. This system paves a novel way toward multichannel-selective filters, optical sensors, and nonlinear devices.

Tunable high-channel-count bandstop graphene plasmonic filters based on plasmon induced transparency

A high-channel-count bandstop graphene plasmonic filter based on ultracompact plasmonic structure is proposed in this paper. It consists of graphene waveguide side-coupled with a series of graphene filtering units. The study shows that the waveguide-resonator system performs a multiple plasmon induced transparency (PIT) phenomenon. By carefully adjusting the Fermi level of the filtering units, any two adjacent transmitted dips which belong to different PIT units can produce coherent coupling superposition enhancement. This property prevents the attenuation of the high-frequency transmission dips of multiple PIT and leads to an excellent bandstop filter with multiple channels. Specifically, the bandwidth and modulation depth of the filters can be flexibly adjusted by tuning the Fermi energy of the graphene waveguide. This ultracompact plasmonic structure contributes to the achievement of frequency division multiplexing systems for optical computing and communications in highly integrated optical circuits.

Experimental and theoretical investigations of an air-slot coupler between dielectric and plasmonic waveguides

In this paper, we experimentally show an effective method of coupling light between dielectric waveguides and metal-dielectric-metal plasmonic waveguides using an air-slot waveguide that extends into both types of waveguides. Our experimental results validate the theoretical calculation results of the proposed coupler. In addition, we investigated the sensitivity of our design to different fabrication challenges that may result in changing the width and length of the targeted optimum values in our design. Numerical simulation results show that the cut-off wavelength can be shifted by either changing the width of the dielectric or slot waveguide. The shift occurs because, as the waveguide's width changes, the mode size changes and consequently the impedance mismatch between the dielectric and slot waveguide changes. We also found that changing the position of the air-slot waveguide with respect to the center of the dielectric waveguide resulted in a reduction in the coupling efficiency due to the reduction in the overlapped area between the mode supported by the slot waveguide and that of the dielectric waveguide.

Tunable all-optical plasmonic diode based on Fano resonance in nonlinear waveguide coupled with cavities

Tunable all-optical plasmonic diode is proposed based on the Fano resonance in an asymmetric and nonlinear system, comprising metal-insulator-metal waveguides coupled with nanocavities. The spatial asymmetry of the system gives rise to the nonreciprocity of the field localizations at the nonlinear gap between the coupled cavities and to the nonreciprocal nonlinear response. Nonlinear Fano resonance, originating from the interference between the discrete cavity mode and the continuum traveling mode, is observed and effectively tuned by changing the input power. By combining the unidirectional nonlinear response with the steep dispersion of the Fano asymmetric line shape, a transmission contrast ratio up to 41.46 dB can be achieved between forward and backward transmission. Our all-optical plasmonic diode with compact structure can find important applications in integrated optical nanocircuits.

Double plasmonic nanodisks design for electromagnetically induced transparency and slow light

An analog of plasmonic system for electromagnetically induced transparency (EIT), in which a small nanodisk with a big side-coupled-nanodisk is directly coupled to the metal-insulator -metal (MIM) waveguide, has been proposed and investigated theoretically and numerically. When the resonant frequencies of the two nanodisks differ not too much, a powerful EIT-like effect can be obtained, and the transparency window can be easily tuned by adjusting the radii of the two nanodisks. The plasmonic device can be used as a high-performance EIT-like filter with transmission over 80% and full width at half-maximum (FWHM) less than 30nm, besides, the novel structure shows a high group index over 355. The system paves a new way toward highly integrated optical circuits and networks, especially for wavelength-selective, ultrafast switching, light storage and nonlinear devices.

Design of a compact and high sensitive refractive index sensor base on metal-insulator-metal plasmonic Bragg grating

A nanometric and high sensitive refractive index sensor based on the metal-insulator-metal plasmonic Bragg grating is proposed. The wavelength encoded sensing characteristics of the refractive index sensor were investigated by analyzing its transmission spectrum. The numerical results show that a good linear relationship between the Bragg wavelength and the refractive index of the sensing material can be obtained, which is in accordance with the analytical results very well. A high refractive index sensitivity of 1,488 nm/RIU around Bragg resonance wavelength of 1,550 nm was obtained. Besides, the simulation results show that the sensitivity is depended on the Bragg resonance wavelength and the longer the Bragg resonance wavelength, the higher sensitivity can be obtained. Furthermore, the figure of merit of the refractive index sensor can be greatly increased by introducing a nano-cavity in the proposed plasmonic Bragg grating structure. This work pave the way for high sensitive nanometric refractive index sensor design and application.

Plasmon-induced transparency in metal-insulator-metal waveguide side-coupled with multiple cavities

We have demonstrated the analogue of electromagnetically induced transparency (EIT) in the metal-insulator-metal plasmonic waveguide, which consists of a bus waveguide side-coupled with a series of slot cavities. By finite-difference time-domain simulations, it is found that the resonance wavelength of the slot cavity can be controlled by adjusting the length of the cavity. Moreover, the EIT-like response is strongly dependent on the coupling separation between the corresponding adjacent cavities. Multiple-peak plasmon-induced transparency can be realized by cascading multiple cavities with different lengths and suitable cavity-cavity separations. This ultracompact plasmonic waveguide system may find important applications for multichannel plasmonic filter, nanoscale optical switching, and slow-light devices in highly integrated optical circuits and networks.

Dual-channel dispersionless slow light based on plasmon-induced transparency

I have proposed a dual-channel dispersionless slow-light waveguide system based on plasmon-induced transparency. By appropriately tuning the stub depth, two transparency windows in the transmission spectrum can be achieved due to the destructive interference between the electromagnetic fields from the three stubs. Two flat bands can be achieved in the transparency windows, which have nearly constant group indices over the bandwidth of 2 THz. The analytical results show that the group velocity dispersion parameters of the two channels equal zero, which indicates that the incident pulse can be slowed down without distortion. The proposed plasmonic waveguide system can realize slow-light effect without pulse distortion, and thus can find important applications on slow-light systems, optical buffers, and all-optical signal processors in highly integrated optical circuits.

Plasmonic spectral splitting in multi-resonator-coupled waveguide systems

Spectral splitting is numerically investigated in a metal-insulator-metal plasmonic waveguide coupled with a series of disk cavities for the first time to our best knowledge. The finite-difference time-domain simulations find that, when an identical cavity is introduced into the single-cavity-coupled structure, a resonance peak emerges in reflection dip due to the plasmonic analogue of electromagnetically induced transparency. By cascading multiple cavities into the waveguide system, the resonance spectra are gradually split because of the phase-coupled effects. Particularly, the quality factors of splitting resonance spectra can be rapidly improved with increasing the number of coupled cavities. The proposed plasmonic systems may find potential applications in highly integrated optical circuits, especially for multichannel filtering, all-optical switching, and slow-light devices.

Control of density and LSPR of Au nanoparticles on graphene

In this study, we introduce the tunable density and localized surface plasmon resonance (LSPR) of plasmonic gold (Au) nanoparticles which were formed on monolayer graphene at room temperature, based on the difference of the reduction potential between graphene and the Au(3+) precursor. The size of the Au nanoparticles was ~40 nm, which is very desirable to provide an optical enhancement effect by LSPR in the full visible range. It is demonstrated that the density of the Au nanoparticles was modulated by the surface energy of the graphene on the substrate as well as the concentration of the Au(3+) precursor. Furthermore, the cycle number of the reduction process strongly affected the distribution of the nanoparticle size and their optical properties. The LSPR of the plasmonic Au nanoparticles was red-shifted from 560 to 620 nm and its full width at half maximum broadened as the Au(3+) precursor concentration was increased and the cyclic reduction process progressed. Based on the optical enhancement of the plasmonic Au nanoparticles and the extraordinary physical characteristics of graphene, the Au/graphene assembly may offer a promising optoelectronic platform for next-generation flexible optical electronics or biosensors.

High-uniformity multichannel plasmonic filter using linearly lengthened insulators in metal-insulator-metal waveguide

A high-uniformity multichannel plasmonic filter based on metal-insulator-metal (MIM) waveguide is proposed. It consists of two metal layers and sandwiched multiple insulator super units structured by alternately stacking two insulators with different refractive indices. By linearly lengthening the high refractive index insulators in each super unit, the dispersion-induced loss coefficient can be reduced and flattened, leading to a high uniformity among multiple transmission channels of the plasmonic filter. The corresponding spectral characteristics are numerically investigated by using the finite-difference time-domain method. Fourteen transmission channels with an excellent channel uniformity of ±0.2 dB and a peak-to-notch contrast ratio greater than 14.8 dB in the range of 1-2 μm, have been confirmed.

Slow light engineering in periodic-stub-assisted plasmonic waveguide

We investigate the slow light engineering in periodic-stub-assisted plasmonic waveguide based on transmission line theory. It is found that the dispersion relationship of the proposed waveguide can be easily modified by tuning the stub depth and the period. The theoretical results show that a large normalized delay bandwidth product of 0.65 can be achieved at 1550 nm, meanwhile maintaining the group index of 35. In addition, the proposed waveguide shows "S-shaped" dispersion curve, which implies that the group velocity dispersion parameter at the inflection point equals zero and a dispersion-free slow light waveguide can be realized. Due to the excellent buffering capacity, the proposed compact configuration can find important applications on optical buffers in highly integrated optical circuits.

Analysis of subwavelength bandpass plasmonic filters based on single and coupled slot nanocavities

Subwavelength surface plasmon polariton optical filters based on metal-insulator-metal slot nanocavites are proposed and analyzed by using coupled mode theory and a finite element method. Simulation results reveal that a single slot cavity coupled with two access waveguides possesses a bandpass-filtering characteristic with its performance affected by its geometric parameters. To further improve the filtering performance, we explore coupled slot cavities as high-order plasmonic filters. When the slot cavities are side-coupled, the bandpass filtering spectrum is dependent on the positions of the access waveguides. The two slot cavities can also be set orthogonal, leading to strong mutual coupling. With careful tuning of the relative length between the two cavities, improved filtering spectrum can be obtained. Given the subwavelength footprint of the proposed plasmonic filters, they can be used in an ultradense plasmonic integrated circuit for optical signal processing.

Numerical investigation of an all-optical switch in a graded nonlinear plasmonic grating

We have proposed and numerically investigated an all-optical switch based on a metal-insulator-metal waveguide with graded nonlinear plasmonic gratings. The influences of grating depth and refractive index of a Kerr nonlinear medium on the transmission of the switch are exactly analyzed by utilizing transmission line theory. The finite-difference time-domain simulation results show that the highly compact structure possesses excellent switch function by tuning the incident electric field intensity. In addition, the simulation results show that this all-optical switch has an ultrawide operating frequency regime and femtosecond-scale response time (~130 fs). Such a switch can find potential applications for all-optical signal processing and optical communication.

Tunable high-channel-count bandpass plasmonic filters based on an analogue of electromagnetically induced transparency

We have proposed a novel type of bandpass plasmonic filter consisting of metal-insulator-metal bus waveguides coupled with a series of side-coupled cavities and stub waveguides. The theoretical modeling demonstrates that our waveguide-resonator system performs a plasmonic analogue of electromagnetically induced transparency (EIT) in atomic systems, as is confirmed by numerical experiments. The plasmonic EIT-like response enables the realization of nanoscale bandpass filters with multiple channels. Additionally, the operating wavelengths and bandwidths of our filters can be efficiently tuned by adjusting the geometric parameters such as the lengths of stub waveguides and the coupling distances between the cavities and stub waveguides. The ultracompact configurations contribute to the achievement of wavelength division multiplexing systems for optical computing and communications in highly integrated optical circuits.

Optical bistability based on an analog of electromagnetically induced transparency in plasmonic waveguide-coupled resonators

We have investigated numerically an optical bistability effect based on an analog of electromagnetically induced transparency (EIT) in a nanoscale plasmonic waveguide-coupled resonator system. The system consists of a metal-insulator-metal waveguide side-coupled with a slot cavity and a nanodisk cavity containing Kerr nonlinear material. By finite-difference time-domain simulations, the EIT-like spectral peak has a redshift with an increase of the dielectric constant of the nanodisk cavity. More importantly, we have achieved an optical bistability with threshold intensity about three times lower than that of recent literature [Appl. Opt.50, 5287 (2011)]. The results show that our plasmonic structure can find more excellent application in highly integrated optical circuits, especially all-optical switching.

Exploiting aperiodic designs in nanophotonic devices

In this work we consider the role of aperiodic order-order without periodicity-in the design of different optical devices in one, two and three dimensions. To this end, we will first study devices based on aperiodic multilayered structures. In many instances the recourse to Fibonacci, Thue-Morse or fractal arrangements of layers results in improved optical properties compared with their periodic counterparts. On this basis, the possibility of constructing optical devices based on a modular design of the multilayered structure, where periodic and quasiperiodic subunits are properly mixed, is analyzed, illustrating how this additional degree of freedom enhances the optical performance in some specific applications. This line of thought can be naturally extended to aperiodic arrangements of optical elements, such as nanospheres or dielectric rods in the plane, as well as to three-dimensional photonic quasicrystals based on polymer materials. In this way, plentiful possibilities for new tailored materials naturally appear, generally following suitable optimization algorithms. Then, we present a detailed discussion on the physical properties supporting the preferential use of aperiodic devices in a number of optical applications, opening new avenues for technological innovation. Finally we suggest some related emerging topics that deserve some attention in the years to come.

Optical bistability in metal-insulator-metal plasmonic waveguide with nanodisk resonator containing Kerr nonlinear medium

We numerically investigate the optical bistability effect in the metal-insulator-metal waveguide with a nanodisk resonator containing a Kerr nonlinear medium. It is found that the increase of the refractive index, which is induced by enhancing the incident intensity, can cause a redshift for the resonance wavelength. The local resonant field excited in the nanodisk cavity can significantly increase the Kerr nonlinear effect. In addition, an obvious bistability loop is observed in the proposed structure. This nonlinear structure can find important applications for all-optical switching in highly integrated optical circuits.

Multiple responses of TPP-assisted near-perfect absorption in metal/Fibonacci quasiperiodic photonic crystal

Absorption properties in one-dimensional quasiperiodic photonic crystal composed of a thin metallic layer and dielectric Fibonacci multilayers are investigated. It is found that a large number of photonic stopbands can occur at the dielectric Fibonacci multilayers. Tamm plasmon polaritons (TPPs) with the frequencies locating at each photonic stopband are excited at the interface between the metallic layer and the dielectric layer, leading to almost perfect absorption for the energy of incident wave. By adjusting the length of dielectric layer with higher refractive-index or the Fibonacci order, the number of absorption peaks can be tuned effectively and enlarged significantly.

Wavelength demultiplexing structure based on arrayed plasmonic slot cavities

A compact wavelength demultiplexing structure based on arrayed metal-insulator-metal (MIM) slot cavities is proposed and demonstrated numerically. The structure consists of a bus waveguide perpendicularly coupled with a series of slot cavities, each of which captures SPPs at the resonance frequency from the bus waveguide and tunes the transmission wavelength by changing its geometrical parameters. A cavity theory model is used to design the operating wavelengths of the structure. Moreover, single band transmission of each channel and the adjustable transmission bandwidth can be obtained by altering the drop waveguide positions and the coupling distance. The proposed arrayed slot cavity-based structure could be utilized to develop ultracompact optical wavelength demultiplexing device for large-scale photonic integration.

Tunable wavelength-division multiplexing based on metallic nanoparticle arrays

We report a tunable wavelength-division multiplexing (WDM) structure based on two-dimensional silver nanoparticle arrays. The linewidth of the multiple geometric resonances of the arrays is of the order of several nanometers generally, which guarantees high wavelength selectivity. Optical channels can be selectively activated by setting the polarization of the incident wave. The operation wavelength can be tuned from the visible to the near infrared, and the free spectral range can be adjusted from hundreds to tens of nanometers by varying the size of the constituent particles and the interparticle distances. The proposed structure can provide an extinction ratio of ~10 and a quality factor of ~700. This tunable, easy-to-produce, and subwavelength WDM structure is desirable for plasmonic integrated circuits.