Dispersionless slow light in MIM waveguide based on a plasmonic analogue of electromagnetically induced transparency

Refractive Index Sensor Based on the Fano Resonance in Metal-Insulator-Metal Waveguides Coupled with a Whistle-Shaped Cavity

A plasmonic refractive index sensor based on surface plasmon polaritons (SPPs) that consist of metal-insulator-metal (MIM) waveguides and a whistle-shaped cavity is proposed. The transmission properties were simulated numerically by using the finite element method. The Fano resonance phenomenon can be observed in their transmission spectra, which is due to the coupling of SPPs between the transmission along the clockwise and anticlockwise directions. The refractive index-sensing properties based on the Fano resonance were investigated by changing the refractive index of the insulator of the MIM waveguide. Modulation of the structural parameters on the Fano resonance and the optics transmission properties of the coupled structure of two MIM waveguides with a whistle-shaped cavity were designed and evaluated. The results of this study will help in the design of new photonic devices and micro-sensors with high sensitivity, and can serve as a guide for future application of this structure.

All-optical tunable slow-light based on an analogue of electromagnetically induced transparency in a hybrid metamaterial

We demonstrate and analyze the use of metamaterials featuring an analogue of electromagnetically induced transparency (EIT) in slow light technology. For most metamaterials, EIT-like effects suffer from intrinsic ohmic loss, and the metamaterial-based slow-light effect can only be tuned passively, which limits their application in slow light devices. We propose a hybrid metamaterial with a unit cell composed of a ring resonator formed from photoactive silicon (Si) and a rectangular bar formed from metallic silver (Ag). Based on an analogue of EIT in the designed hybrid metamaterial, we theoretically demonstrate an all-optical tunable slow-light effect in the telecommunication window. We successfully demonstrate the possibility of designing novel all-optical tunable chip-scale slow-light devices that could be used in optical buffering.

Plasmonic-induced absorption based on an end-coupled combined resonance system of a semiannular cavity and rectangular cavity

A semiannular rectangular composite cavity structure based on metal-insulator-metal waveguides is proposed. By adding a rectangular cavity at a suitable position around the semiannular cavity (SAC), a single analogous plasmonic-induced absorption (PIA) effect is achieved at the expected mode of the SAC structure. After adding two rectangular cavities together in the SAC system, dual analogous PIA effects for both modes can be realized simultaneously. In addition, the phase response is also studied, and abnormal dispersions are achieved in the PIA windows, which can be used in the integrated optics for fast/slow light. The performances of the proposed two-dimensional structure are analyzed and studied by the coupled-mode theory and the finite-difference time-domain method, respectively.

Wavelength-sensitive PIT-like double-layer graphene-based metal-dielectric-metal waveguide

A wavelength-sensitive plasmonically induced transparency-like (PIT-like) device consisting of a double-layer graphene-based metal-dielectric-metal (MDM) waveguide is proposed. We initially investigate monolayer graphene sandwiched in the MDM waveguide and utilize the phase-matching equation to explain the reflected resonant wavelength of the transmission spectra. The PIT-like windows in the transmission spectra of double-layer graphene can be achieved by tuning the applied bias voltage on the graphene layer and the distance between the graphene layer and metal substrate. We can obtain the high-performance PIT-like devices with a flexible on-off-on effect. We use a finite element method to do all related simulations.

Chirality-dependent electromagnetically induced transparency based on a double semi-periodic helix metastructure

A chiral metastructure composed of spatially separated double semi-periodic helices is proposed and investigated theoretically and experimentally in this Letter. Chirality-dependent electromagnetically induced transparency (EIT) and a slow light effect in the microwave region are observed from a numerical parameter study, while experimental results from the 3D printing sample yield good agreement with the theoretical findings. The studied EIT phenomenon arises as a result of destructive interference by coupled resonances, and the proposed chiral metastructure can be applied in areas such as polarization communication, pump-probe characterization, and quantum computing areas.

Slowing down light using terahertz semiconductor metamaterial for dual-band thermally tunable modulator applications

Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental technological advances. We have designed a terahertz (THz) semiconductor metamaterial (MM) waveguide system, which exhibits a significant slow-light effect, based on a classical electromagnetically induced transparency phenomenon. The potential of MMs for THz radiation originates from a resonant electromagnetic response that can be tailored for specific applications. By appropriately adjusting the distance between the two radiative and nonradiative modes, a flat band corresponding to a nearly constant group index (of the order of 4924) in the THz regime can be achieved. Finite-difference time-domain simulations show that the incident pulse can be slowed down. The proposed device from a paucity of naturally occurring materials has useful applications in electronic or photonic properties at terahertz frequencies. This proposed compact configuration may find potential applications in plasmonic slow-light systems, optical buffers, and thermal and electromagnetic modulating applications and temperature sensors.

Plasmon induced transparency effect through alternately coupled resonators in terahertz metamaterial

We analyze plasmon induced transparency (PIT) in a planar terahertz metamaterial comprising of two C-shaped resonators and a cut-wire. The two C-shaped resonators are placed alternately on both sides of the cut-wire such that it exhibits a PIT effect when coupled with the cut wire. We have further shown that the PIT window is modulated by displacing the C-shaped resonators w.r.t. the cut-wire. A lumped element equivalent circuit model is reported to explain the numerical observations for different coupling configurations. The PIT effect is further explored in a metamaterial comprising of a cross like structure and four C-shaped resonators. For this configuration, the PIT effect is studied for the incident light polarized in both x and y directions. It is observed that such a structure exhibits equally strong PIT effects for both the incident polarizations, indicating a polarization independent response to the incident terahertz radiation. Our study could be significant in the development of slow light devices and polarization independent sensing applications.

Wide band dispersionless slow light in hetero-MIM plasmonic waveguide

A flat slow-light band over a wide frequency range is obtained in the hetero-MIM (metal-insulator-metal) waveguide with zero group velocity dispersion (GVD). The zero GVD originates from dispersion compensation by the photonic mode and the plasmonic mode, the mechanism of which does not exist in the homo-MIM structure. By changing dielectric permittivity of the insulator or the difference of two different metallic plasma frequencies, the group index and the bandwidth can be tuned. The dispersionless slow light characteristic in the hetero-MIM waveguide may be useful in the new design of plasmonic devices.

Tuning Fano resonances with a nano-chamber of air

By designing a polymer-film-coated asymmetric metallic slit structure that only contains one nanocavity side-coupled with a subwavelength plasmonic waveguide, the Fano resonance is realized in the experiment. The Fano resonance originates from the interference between the narrow resonant spectra of the radiative light from the nanocavity and the broad nonresonant spectra of the directly transmitted light from the slit. The lateral dimension of the asymmetric slit is only 825 nm. Due to the presence of the soft polymer film, a nano-chamber of air is constructed. Based on the opto-thermal effect, the air volume in the nano-chamber is expanded by a laser beam, which blueshifts the Fano resonance. This tunable Fano resonance in such a submicron slit structure with a nano-chamber is of importance in the highly integrated plasmonic circuits.

Tunable Multi-switching in Plasmonic Waveguide with Kerr Nonlinear Resonator

We propose a nanoplasmonic waveguide side-coupled with bright-dark-dark resonators in our paper. A multi-oscillator theory derived from the typical two-oscillator model, is established to describe spectral features as well as slow-light effects in bright-dark-dark structures, and confirmed by the finite-difference time domain (FDTD). That a typical plasmon induced transparency (PIT) turns to double PIT spectra is observed in this waveguide structure. At the same time, multi-switching effects with obvious double slow-light bands based on double PIT are also discovered in our proposed structure. What's more, dynamically tuning the multi-switching is achieved by means of filling Fabry-Perot resonators with the Kerr nonlinear material Ag-BaO. These results may have applications in all-optical devices, moreover, the multi-oscillator theory may play a guiding role in designing plasmonic devices.

Plasmonic coaxial waveguide-cavity devices

We theoretically investigate three-dimensional plasmonic waveguide-cavity structures, built by side-coupling stub resonators that consist of plasmonic coaxial waveguides of finite length, to a plasmonic coaxial waveguide. The resonators are terminated either in a short or an open circuit. We show that the properties of these waveguide-cavity systems can be accurately described using a single-mode scattering matrix theory. We also show that, with proper choice of their design parameters, three-dimensional plasmonic coaxial waveguide-cavity devices and two-dimensional metal-dielectric-metal devices can have nearly identical transmission spectra. Thus, three-dimensional plasmonic coaxial waveguides offer a platform for practical implementation of two-dimensional metal-dielectric-metal device designs.

Optical cavity-assisted broadband optical transparency of a plasmonic metal film

We theoretically present a powerful method to achieve a continuous metal film structure with broadband optical transparency via introducing a dielectric Fabry-Pérot (FP) cavity. An incident optical field could be efficiently coupled and confined with the strong localized plasmons by the non-close-packed plasmonic crystal at the input part and could then become re-radiated output via the transmission channel supported by the dielectric cavity. The formed photonic-plasmonic system could therefore make the seamless metal film structure have a superior near-unity transparency (up to 97%) response and a broadband transparent spectrum with bandwidth >245 nm (with transmittance >90%) in the optical regime. The observed optical properties of the proposed structure can be highly tuned via varying the structural parameters. Based on the colloidal assembly method, the proposed plasmonic crystal can be fabricated in a large area. In addition, the achieved optical transparency can be retained in the extremely roughed metal film structure. Thereby, the findings could offer a feasible way to achieve a broadband transparent metal film structure and hold potential applications in transparent electrodes, touch screens and interactive electronics.

Plasmonic circular resonators for refractive index sensors and filters

A plasmonic refractive index sensor based on a circular resonator is proposed. With all three dimensions below 1 μm, the sensor has a compact and simple structure granting it ease-of-fabrication and ease-of-use. It is capable of sensing trace amounts of liquid or gas samples. The sensing properties are investigated using finite elements method. The results demonstrate that the plasmonic sensor has a relatively high sensitivity of 1,010 nm/RIU, and the corresponding sensing resolution is 9.9 × 10(-5) RIU. The sensor has a relatively high quality factor of 35, which is beneficial for identifying each transmission spectrum. More importantly, the sensitivity is not sensitive to changes of structure parameters, which means that the sensitivity of the sensor is immune to the fabrication deviation. In addition, with a transmittance of 5% at the resonant wavelength, this plasmonic structure can also be employed as a filter. In addition, by filling material like LiNbO3 or liquid crystal in the circular resonator, this filter can realize an adjustable wavelength-selective characteristic in a wide band.

Electromagnetically induced absorption in detuned stub waveguides: a simple analytical and experimental model

We give an analytical and experimental demonstration of a classical analogue of the electromagnetic induced absorption (EIA) in a simple photonic device consisting of two stubs of lengths d1 and d2 grafted at the same site along a waveguide. By detuning the lengths of the two stubs (i.e. δ = d(2) - d(1)) we show that: (i) the amplitudes of the electromagnetic waves in the two stubs can be written following the two resonators model where each stub plays the role of a radiative resonator with low Q factor. The destructive interference between the waves in the two stubs may give rise to a sharp resonance peak with high Q factor in the transmission as well as in the absorption. (ii) The transmission coefficient around the resonance induced by the stubs can be written following a Fano-like form. In particular, we give an explicit expression of the position, width and Fano parameter of the resonances as a function of δ. (iii) By taking into account the loss in the waveguides, we show that at the transmission resonance, the transmission (reflection) increases (decreases) as a function of δ. Whereas the absorption goes through a maximum around 0.5 for a threshold value δth which depends on the attenuation in the system and then falls to zero. (iv) We give a comparison between the phase of the determinant of the scattering matrix, the so-called Friedel phase and the phase of the transmission amplitude. (v) The effect of the boundary conditions at the end of the resonators on the EIA resonance is also discussed. The analytical results are obtained by means of the Green's function method, whereas the experiments are carried out using coaxial cables in the radio-frequency regime. These results should have important consequences for designing integrated devices such as narrow-frequency optical or microwave filters and high-speed switches.

Combined theoretical analysis for plasmon-induced transparency in waveguide systems

We propose a novel combination of a radiation field model and the transfer matrix method (TMM) to demonstrate plasmon-induced transparency (PIT) in bright-dark mode waveguide structures. This radiation field model is more effective and convenient for describing direct coupling in bright-dark mode resonators, and is promoted to describe transmission spectra and scattering parameters quantitatively in infinite element structures by combining it with the TMM. We verify the correctness of this novel combined method through numerical simulation of the metal-dielectric-metal (MDM) waveguide side-coupled with typical bright-dark mode, H-shaped resonators; the large group index can be achieved in these periodic H-shaped resonators. These results may provide a guideline for the control of light in highly integrated optical circuits.

A 4-way wavelength demultiplexer based on the plasmonic broadband slow wave system

We propose a broadband slow wave system based on the thin metal-insulator-metal (MIM) graded grating structure composed of two corrugated metal strips with periodic array of grooves on a thin dielectric substrate. The guided spoof surface plasmon polaritons (SSPPs) at different frequencies can be localized at different positions along the ultrathin MIM grating. By introducing specially designed non-corrugated MIM branches with specific lengths at the locations where the EM waves are trapped, the trapped EM waves can be released and propagate along these branches. A 4-way wavelength demultiplexer based on such plasmonic broadband slow wave system is then demonstrated and fabricated. To improve the isolations between different branches at lower frequencies, band-reject filters are inserted at the front of some MIM branches. The measurements and the simulation results have shown very good agreements, which validate the feasibility of the 4-way wavelength demultiplexer.

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.

Uniform theoretical description of plasmon-induced transparency in plasmonic stub waveguide

We investigate a classic analog of electromagnetically induced transparency (EIT) in a metal-dielectric-metal (MDM) bus waveguide coupled to two stub resonators. A uniform theoretical model, for both direct and indirect couplings between the two stubs, is established to study spectral features in the plasmonic stub waveguide, and the theoretical results agree well with the finite difference time domain simulations. Adjusting phase difference and coupling strength of the interaction, one can realize the EIT-like phenomena and achieve the required slow light effect. The theoretical results may provide a guideline for the control of light in highly integrated optical circuits.

Fano resonances in a single defect nanocavity coupled with a plasmonic waveguide

Two Fano resonances are theoretically predicted in a single defect nanocavity, consisting of a rectangular cavity with a small stub defect, side-coupled with a plasmonic waveguide. These two Fano resonances are found to originate from two different mechanisms. One is caused by the excitation of a high-order resonant mode in the rectangular cavity owing to the structural breaking, and the other is attributed to the inherent resonant mode in the small stub defect. The narrow high-order mode and inherent mode couple with the broad low-order resonant mode in the rectangular cavity, giving rise to two Fano resonances. Because of the different origins, these two Fano resonances exhibit quite different responses to the variations of the structural dimensions. This has important applications in highly sensitive and multiparameter sensing in the complicated environments.

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.

A novel planar metamaterial design for electromagnetically induced transparency and slow light

A novel planar plasmonic metamaterial for electromagnetically induced transparency and slow light characteristic is presented in this paper, which consists of nanoring and nanorod compound structures. Two bright modes in the metamaterial are induced by the electric dipole resonance inside nanoring and nanorod, respectively. The coupling between two bright modes introduces transparency window and large group index. By adjusting the geometric parameters of metamaterial structure, the transmittance of EIT window at 385 THz is about 60%, and the corresponding group index and Q factor can reach up to 1.2 × 10³ and 97, respectively, which has an important application in slow-light device, active plasmonic switch, SERS and optical sensing.

Lasing in plasmon-induced transparency nanocavity

We propose a plasmon-induced transparency (PIT) nanocavity for achieving nanoscopic coherent light source. The compact cavity is constructed by a pair of detuned nano-stubs incorporated with four-level gain medium. The PIT response enables the reduction of the coupling loss from cavity to waveguide while keeping the cavity size unchanged, different from the end-facet Fabry-Pérot cavity in which the radiation loss decreases at the cost of size increment. In order to study the lasing behavior of surface plasmon wave in the PIT cavity, the self-consistent finite element method is employed to model the interactions between gain and propagating surface plasmons. The dynamics of the whole lasing process is observed, and the linear output-input relation is obtained for the single mode plasmon lasing. It is demonstrated that smaller stub-pair detuning provides stronger feedback inside the cavity. Consequently, the lasing threshold of pumping rate decreases quadratically with the decreasing of detuning. However, the output-input extraction efficiency will improve when the detuning is not so small. One of the advantages for the proposal is that the lasing output power from the cavity can directly couple towards the metal-dielectric-metal waveguide platform, facilitating the field of integrated plasmonic circuits and molecular-scale coherent light source.

Formation and evolution mechanisms of plasmon-induced transparency in MDM waveguide with two stub resonators

We demonstrate the realization of plasmonic analog of electromagnetically induced transparency (EIT) in a system composing of two stub resonators side-coupled to metal-dielectric-metal (MDM) waveguide. Based on the coupled mode theory (CMT) and Fabry-Perot (FP) model, respectively, the formation and evolution mechanisms of plasmon-induced transparency by direct and indirect couplings are exactly analyzed. For the direct coupling between the two stub resonators, the FWHM and group index of transparent window to the inter-space are more sensitive than to the width of one cut, and the high group index of up to 60 can be achieved. For the indirect coupling, the formation of transparency window is determined by the resonance detuning, but the evolution of transparency is mainly attributed to the change of coupling distance. The consistence between the analytical solution and finite-difference time-domain (FDTD) simulations verifies the feasibility of the plasmon-induced transparency system. It is also interesting to notice that the scheme is easy to be fabricated and may pave the way to highly integrated optical circuits.

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.

Gain-assisted trapping of light in tapered plasmonic waveguide

We have investigated the slow light and trapping effects in tapered metal-insulator-metal plasmonic waveguides. It is found that a significant reduction of group velocity (<0.01c) can be obtained when considering the intrinsic loss of realistic metal. The theoretical analysis shows that the group velocity can be further decreased, even approach zero in the lossless metallic waveguides. The perfect trapping of light is realized when an appropriate gain material is incorporated in the core layer to compensate metallic loss. The proposed ultracompact configuration may find excellent applications on nanoscale optical storages.

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.