Metal-dielectric Waveguide Research Articles

Imaging optical fields below metal films and metal-dielectric waveguides by a scanning microscope.

Laser scanning confocal fluorescence microscopy (LSCM) is now an important method for tissue and cell imaging when the samples are located on the surfaces of glass slides. In the past decade, there has been extensive development of nano-optical structures that display unique effects on incident and transmitted light, which will be used with novel configurations for medical and consumer products. For these applications, it is necessary to characterize the light distribution within short distances from the structures for efficient detection and elimination of bulky optical components. These devices will minimize or possibly eliminate the need for free-space light propagation outside of the device itself. We describe the use of the scanning function of a LSCM to obtain 3D images of the light intensities below the surface of nano-optical structures. More specifically, we image the spatial distributions inside the substrate of fluorescence emission coupled to waveguide modes after it leaks through thin metal films or dielectric-coated metal films. The observed spatial distribution were in general agreement with far-field calculations, but the scanning images also revealed light intensities at angles not observed with classical back focal plane imaging. Knowledge of the subsurface optical intensities will be crucial in the combination of nano-optical structures with rapidly evolving imaging detectors.

Multiple plasmon-induced transparency effects in a multimode-cavity-coupled metal–dielectric–metal waveguide

We numerically and theoretically investigate multiple plasmon-induced transparency (PIT) effects in a multimode-cavity-coupled metal–dielectric–metal (MDM) waveguide system. The introduced multimode coupled-radiating oscillator theory (MC-ROT) gives a clear understanding of multiple PIT effects in the proposed system. Two and three PIT peaks appear in the transmission spectra corresponding to the symmetrical and asymmetrical structures, respectively. Evolution of the PIT peaks can be effectively tuned by adjusting the geometric dimensions and asymmetry of the structure. The ultra-compact plasmonic waveguide structure may have important applications for multichannel filters, optical switches, and other devices in integrated optical circuits.

Active Control of Slow Light in a Gain-Assisted Plasmon-Induced Transparency Structure

Plasmon-induced transparency (PIT) in a compact plasmonic resonator structure, which consists of a metal– dielectric–metal waveguide with an aperture-coupled square cavity and a slot resonator, is investigated theoretically and numerically. We find the transparency window and the delay of slow light in this structure can be controlled at a fixed wavelength by carefully adjusting the geometrical parameters of the aperture-coupled square cavity. By introducing gain medium into the slot resonator, the transmittance and the group index in transparency window both are dramatically enhanced with optically pumping the medium, and the group delay time can reach to 0.93 ps. In addition, triple PIT effects are also achieved by adding another aperture-coupled cavity and slot cavity, and corresponding slow light also can be dynamically tuned. This study paves a new route toward the realization of highly integrated all-optical circuits and networks, especially for ultrafast switches, optical buffers, lasers, and nanosensors.

Enhancement of intensity in a periodically layered metal-dielectric waveguide with magnetized plasma

Recently, a periodically layered metal-dielectric structure was introduced as a new type of tunable radiation source by Adamo et al. [Phys. Rev. Lett. 103, 113901 (2009); J. Opt. 12, 024012 (2010)]. It is anticipated that the proposed structure forms an important part of future THz systems. In this paper, the effect of plasma, as an extra controlling parameter, on this radiation source has been investigated. The results of the numerical calculations show that the presence of plasma can lead to a significant increase in the output power, and therefore, the device can be tuned over a wide frequency range by changing the plasma density.

Tunable Fano Resonance in Asymmetric MIM Waveguide Structure.

A plasmonic waveguide coupled system that uses a metal-insulator-metal (MIM) waveguide with two silver baffles and a coupled ring cavity is proposed in this study. The transmission properties of the plasmonic system were investigated using the finite element method. The simulation results show a Fano profile in the transmission spectrum, which was caused by the interaction of the broadband resonance of the Fabry-Perot (F-P) cavity and the narrow band resonance of the ring cavity. The Fabry-Perot (F-P) cavity in this case was formed by two silver baffles dividing the MIM waveguide. The maximum sensitivity of 718 nm/RIU and the maximum figure of merit of 4354 were achieved. Furthermore, the effects of the structural parameters of the F-P cavity and the ring cavity on the transmission properties of the plasmonic system were analyzed. The results can provide a guide for designing highly sensitive on-chip sensors based on surface plasmon polaritons.

Corrugated-enhanced second harmonic generation in metal–insulator–metal plasmonic waveguides

Nonlinear effects such as second-harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, the generation of second harmonic in metal–insulator–metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. Symmetric means that the metals at the top and bottom of the dielectric layer are the same and asymmetric means that the metals at the top and bottom of the dielectric layer are different. Two different structures are considered here as plasmonic waveguide for generation of second harmonic and analyzed using finite-difference time domain method. Besides the structure has grating on both sides for more coupling between photons and plasmons. The wavelength duration of grating per length unit (number of grooves) will be optimized to reach the highest second harmonic generation. To perform this optimization, the wavelength of operation λ = 458 nm is considered. It was shown that field enhancement in symmetric MIM waveguides can result in enhancement of SHG magnitude compared to the literature values and asymmetric device results in more than two orders of magnitude enhancement in SHG compared to symmetric structure. It is also shown that the electric field of second harmonic depends on the thickness of crystal (insulator). So, its thickness is optimized to achieve the highest electric field.

All-optical bit magnitude comparator device using metal–insulator–metal plasmonic waveguide

A plasmonic metal–insulator–metal (MIM) waveguide has great success in confining the surface plasmon up to a deep subwavelength scale. The structure of a nonlinear Mach–Zehnder interferometer (MZI) using a plasmonic MIM waveguide has been analyzed. A one-bit magnitude comparator has been designed using an MZI and two linear control waveguides. The device works on the Kerr effect inside the plasmonics waveguide. The mathematical description of the device is explained. The simulation of the device is done using MATLAB® and the finite-difference time-domain (FDTD) method.

Method proposing a slow light ring resonator structure coupled with a metal-dielectric-metal waveguide system based on plasmonic induced transparency.

We demonstrate the analogue of electromagnetically induced transparency (EIT) in a metal-dielectric-metal (MDM) plasmonic waveguide. Plasmonic induced transparency is a method similar to EIT. In this paper, a plasmonic MDM waveguide is proposed by using an ellipse shaped side-coupled ring resonator and simulated by finite difference time domain. Plasmonics as a new field of chip-scale technology is an interesting substrate, which is used to propose and numerically investigate a novel MDM structure. The aforementioned framework is a 2×2 plasmonic ring resonator, employing gold as a metal and polymethyl methacrylate as a dielectric. Simulations show that a transparent window is located at 1550 nm and signal wavelength is assumed to be 860 nm, which is the phenomenon of plasmonic induced transparency. The velocity of the plasmonic mode can be considerably slowed while propagating along the MDM bends. Our proposed configuration may thus be applied to storing and stopping light in plasmonic waveguide bends. This plasmonic waveguide system may find important applications for multichannel plasmonic filters, nano-scale optical switching, delay time devices, and slow-light devices in highly integrated optical circuits and networks. In comparison with our previous theoretical work based on circular shaped ring resonators, it is shown that ellipse shaped ring resonators demonstrate better specifications with a slow down factor estimated to be more than 30.

The sensing characteristics based on electromagnetically-induced transparency-like response in double-sided stub and a nanodisk waveguide system

We propose a novel metal–insulator–metal (MIM) waveguide structure consisting of a stub and a double side-coupled nanodisk. Its electromagnetically-induced transparency (EIT)-like response and transmission properties are numerically investigated by the finite element method (FEM). Numerical simulation results show that the EIT-like phenomenon appears in the transmission spectra. The transparency window can not only be manipulated by changing the radius of the nanodisk and the height of the stub, but also appear redshifted with the increasing of the refractive index n. Furthermore, the MIM waveguide structure is easy to yield a high sensitivity of 1200 nm/RIU with a figure of merit about [Formula: see text], which may be applied to nanosensors. These results would help us design the high-performance plasmonic devices based on the EIT-like response.

Refractive Index Sensor Based on Fano Resonances in Metal-Insulator-Metal Waveguides Coupled with Resonators.

A surface plasmon polariton refractive index sensor based on Fano resonances in metal–insulator–metal (MIM) waveguides coupled with rectangular and ring resonators is proposed and numerically investigated using a finite element method. Fano resonances are observed in the transmission spectra, which result from the coupling between the narrow-band spectral response in the ring resonator and the broadband spectral response in the rectangular resonator. Results are analyzed using coupled-mode theory based on transmission line theory. The coupled mode theory is employed to explain the Fano resonance effect, and the analytical result is in good agreement with the simulation result. The results show that with an increase in the refractive index of the fill dielectric material in the slot of the system, the Fano resonance peak exhibits a remarkable red shift, and the highest value of sensitivity (S) is 1125 nm/RIU, RIU means refractive index unit. Furthermore, the coupled MIM waveguide structure can be integrated with other photonic devices at the chip scale. The results can provide a guide for future applications of this structure.

Plasmonic Wavelength Demultiplexer with Mode Conversion Capabilities

An efficient wavelength demultiplexer with its input as a metal-insulator-metal (MIM) waveguide mode and output an out of plane free-space mode is proposed. The proposed demultiplexer design is integrated on a MIM waveguide such that power is evanescently coupled into an array of appropriately designed cavity-groove combination. The demultiplexer design permits control of phase of the dropped wavelength to achieve a desired wavefront. We demonstrate this through generation of circular and plane wavefront. By controlling the evanescently coupled power into the cavity-groove combinations, it is possible to dramatically improve the efficiency of the proposed demultiplexer. Results are simulated using FEM technique.

Modeling of all-optical even and odd parity generator circuits using metal-insulator-metal plasmonic waveguides

Plasmonic metal-insulator-metal (MIM) waveguides sustain excellent property of confining the surface plasmons up to a deep subwavelength scale. In this paper, linear and S-shaped MIM waveguides are cascaded together to design the model of Mach-Zehnder interferometer (MZI). Nonlinear material has been used for switching of light across its output ports. The structures of even and odd parity generators are projected by cascading the MZIs. Parity generator and checker circuit are used for error correction and detection in an optical communication system. Study and analysis of proposed designs are carried out by using the MATLAB simulation and finite-differencetime-domain (FDTD) method.

Surface plasmon polariton mode in the metal-insulator-metal waveguide

Metal-insulator-metal (MIM) waveguides with subwavelength apertures exhibit unique properties that are considered breakthroughs in surface plasma technology. In this paper, we established a surface plasmon polariton (SPP) mode of MIM structure as well as a theoretical model of reflectivity and excitation coefficients to clarify the SPP mode structure. During simulation calculations, attenuation of the TM ground state mode with large-thickness dielectrics is over the oscillation mode, which is obviously different from the traditional dielectric waveguide. The TE mode exhibits characteristics of anamorphic dielectric optical waveguides, and its propagation distance is longer than that of the TM0 mode. With increasing length, the cavity quality factor Q increases, which indicates that the SPP reflection is limited. The quality factor of the cavity and the end-face have a close relationship. MIM waveguides with larger Q values can produce optical waves coupled with the SPP mode. In this paper, we analyzed SPP propagation constants and attenuation coefficients to promote the development of an integrated surface plasmon.

Enhancement of Second Harmonic Generation in Metal-Insulator-Metal Plasmonic Waveguides

Nonlinear effects such as second harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, the generation of second harmonic in metal-insulator-metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. This study considered two different structures as plasmonic waveguides for the generation of second harmonic, and analysis was performed using the finite-difference time-domain method. Besides, the structure has grating on both sides for more coupling between photons and plasmons. The wavelength duration of grating per unit length (number of grooves) was optimized to reach the highest second harmonic generation. To perform this optimization, the wavelength of operation (λ = 458 nm) was considered. It was shown that field enhancement in symmetric MIM waveguides can result in the enhancement of SHG magnitude when compared to literature values. Also, asymmetric devices result in more than two orders of magnitude enhancement in SHG, as compared to the symmetric structure. It has been shown that the electric field of the second harmonic depends on the thickness of the crystal (insulator). Hence, its thickness was optimized to achieve the highest electric field.

Plasmonic wavelength splitter based on a metal–insulator–metal waveguide with a graded grating coupler

A plasmonic wavelength splitter based on a sub-wavelength metal-insulator-metal (MIM) periodic rectangle wrinkle waveguide with a graded grating coupler is theoretically analyzed and experimentally demonstrated. The surface plasmon polaritons (SPPs), excited in the metal grating with wavelength selection, are deflected by the graded difference according to the aplanatic parametric principle. The wave vector of the deflected SPPs meets the phase-matching condition and couples into the periodic rectangle wrinkle waveguide with a plasmonic bandgap. The characteristic of the plasmonic wavelength splitter is simulated by finite difference time domain (FDTD) simulation, which agrees well with the theoretical analysis. By electron beam lithography and ion beam etching process, the plasmonic wavelength splitter was fabricated. The SPPs excited by incident 650 and 832 nm were successfully split and guided to opposite directions of the MIM waveguide with extinction ratios of 27.5 dB and 32.7 dB, respectively, which was observed under an optical microscope using a CCD camera. The proposed wavelength splitter is simple fabricated and has a large coupling aperture by utilizing the graded grating coupler.

High efficient unidirectional surface plasmon excitation utilizing coupling between metal-insulator-metal waveguide and metal-insulator interface

A new structure is proposed, which can realize parallel coupling between metal-insulator-metal (MIM) waveguide and plasmon on metal-insulator (MI) interface. An example for wavelength of 680nm shows the coupling efficiency can be high as 82%, with short coupling length of 1.2µm. By using MIM waveguide with proper length, a unidirectional plasmon generator is realized. The generator shows excitation efficiency as high as 78%, with high extinction ratio as 1:170. It also shows a good tolerance for the wavelength. The results are of vital importance for optical integration and unidirectional plasmon excitation.

Tunable band (pass and stop) filters based on plasmonic structures using Kerr-type nonlinear rectangular nanocavity

We have proposed and numerically investigated two plasmonic structures for bandpass and band-stop filters. The bandpass filter is composed of two metal–insulator–metal (MIM) waveguides coupled to each other by a nonlinear rectangular nanocavity. The band-stop filter consists of an MIM waveguide side coupled to a Kerr-type nonlinear rectangular nanocavity. The optical filtering effect is verified by two-dimensional (2-D) finite-difference time-domain (FDTD) simulations. It is demonstrated that based on optical nonlinearity we can easily make the proposed filters tunable by properly adjusting the intensity of incident light without changing the dimensions of the structures. The simulation results revealed that within the transmission spectrum, the selected central wavelength and the bandwidth of the filter can be tuned by the input signal intensity. The proposed structures are suitable to be used as highly dense integrated optical circuits, where limitations on the dimensions of the filter structure are vital.

Fano Resonance Based on End-Coupled Cascaded-Ring MIM Waveguides Structure

Though adding a groove to a plasmonic end-coupled perfect ring (PR) resonator, two additional resonance modes, which can be controlled by the length of the groove, will arise in this proposed ring-groove (RG) joint metal-insulator-metal (MIM) waveguide. By further cascading, the PR resonator and the RG joint resonator, single and dual Fano resonances with asymmetric line shapes are obtained due to the interference effects between the dark modes and the bright modes. High figure of merit and high refractive-index sensitivity are achieved, and thus this structure is suitable for the biochemistry sensing area. Interestingly, normal and abnormal dispersions are also investigated for the Fano peaks and dips, respectively. The performances of the proposed structure are investigated by using the finite-difference time-domain method.

Analysis of asymmetry of Fano resonance in plasmonic metal-insulator-metal waveguide

A Fano resonance has stimulated interests due to its asymmetrical resonance profile. Fano resonance generally emerges due to coupling and destructive interference between two resonating modes having disparate quality factors. This coupling between modes leads to an asymmetrical resonance profile which makes Fano resonance quite distinct from the traditional Lorentzian profile. Asymmetry is a key feature of Fano resonance. It is examined using Complex plane analysis and redefined using Reflection Argand diagram. The newly defined parameter named as Argand Asymmetric ratio (ρ) is in line with the previously defined asymmetrical factor of Fano resonance. This newly defined parameter (ρ) is also studied against coupling and frequency detuning between resonating modes. The parameter is also numerically investigated by exciting Fano resonance in a pair of resonators coupled MIM (Metal-Insulator-Metal) waveguide using the OptiFDTD tool. The newly defined parameter can find its way in analyzing different Fano resonance based applications like as optical buffers, sign reversal Fano tuning, switches, modulators, etc.

Attenuated total reflection response to wavelength tuning of plasmon-induced transparency in a metal-insulator-metal structure.

We experimentally demonstrated a plasmon-induced transparency in a metal-insulator-metal (MIM) structure based on the attenuated total reflection (ATR) response. Here, the MIM waveguide (MIMWG) mode and the surface plasmon polariton (SPP) resonance mode acted as low- and high-Q resonance modes, respectively. The dependence of the resonance angles of SPP and MIMWG mode resonances on the incident wavelength differed, which allowed the coupling condition between the two modes to be tuned via the wavelength. When the resonance angles of the two modes coincided, the ATR response showed a symmetric plasmon-induced transparency spectrum; in contrast, when the resonance angles were detuned, the ATR exhibited a sharp asymmetric spectrum characteristic to off-resonance Fano interference.