Waveguiding effect in 2D metal–dielectric–metal grating structure

We have studied the waveguiding effect in a 2D metal–dielectric–metal (MDM) grating structure formed on a quartz substrate. The grating was first formed via e-beam lithography and subsequently covered by Ag/MgF₂/Ag MDM films. At a pitch of 300 nm in both x- and y-directions, low reflectance and transmittance were observed in the UV–VIS range, indicating efficient coupling of normal incident light into waveguiding modes. As evidence, we measured the spectrum of the waveguide from the edge, and the bandwidth of the spectrum was as narrow as ∼74 nm. The bandwidth of the waveguide can be further improved by increasing the MDM stack number. In addition, the bandwidth can also be widened by increasing the pitch of the structure. The physical mechanism underlying the phenomena was analyzed and experimentally confirmed. Such effect could be useful in many applications, such as DFB lasers, solar cells, waveguides, and light emitting devices.     

Spectral properties of a metal–dielectric sensor structure

Numerical analysis of the spectral characteristics of a two-layer structure is performed in the processes of fabrication, measurement, and application of this structure as a sensor of the refractive index of the external medium. The effect of environmental conditions and parameters of the metal and dielectric layers on the sensor sensitivity is discussed. The results of model calculations are confirmed by experimental studies.     

Multiple Fano resonances in metal–insulator–metal waveguide with umbrella resonator coupled with metal baffle for refractive index sensing

A single baffle metal–insulator–metal(MIM) waveguide coupled with a semi-circular cavity and a cross-shaped cavity is proposed based on the multiple Fano resonance characteristics of surface plasmon polaritons(SPPs) subwavelength structure. The isolated state formed by two resonators interferes with the wider continuous state mode formed by the metal baffle, forming Fano resonance that can independently be tuned into five different modes. The formation mechanism of Fano resonance is analyzed based on the multimode interference coupled mode theory(MICMT). The finite element method(FEM) and MICMT are used to simulate the transmission spectra of this structure and analyze the influence of structural parameters on the refractive index sensing characteristics. And the transmission responses calculated by the FEM simulation are consistent with the MICMT theoretical results very well. The results show that the figure of merit(FOM)can reach 193 and the ultra-high sensitivity is 1600 nm/RIU after the structure parameters have been optimized, and can provide theoretical basis for designing the high sensitive refractive index sensors based on SPPs waveguide for high-density photonic integration with excellent performance in the near future.     

Properties of metal–insulator–metal waveguide loop reflector

A new type and easy-to-fabricate metal–insulator–metal(MIM) waveguide reflector based on Sagnac loop is designed and investigated.The transfer matrix theoretical model for the transmission of electric fields in the reflector is established,and the properties of the reflector are studied and analyzed.The simulation results indicate that the reflectivity strongly depends on the coupling splitting ratio determined by the coupling length.Accordingly, different reflectivities can be realized by varying the coupling length.For an optimum coupling length of 750 nm, the 3-dB reflection bandwidth of the MIM waveguide reflector is as wide as 1.5 μm at a wavelength of 1550 nm, and the peak reflectivity and isolation are 78%and 23 dB, respectively.     

Electromagnetic response of a compound plasmonic–dielectric system with coupled-grating-induced transparency

We propose a compound plasmonic–dielectric system consisting of one-dimension metallic gratings made of core–shell membranes and a Si grating waveguide with periodic grooves on one side, to investigate the coupled-grating-induced transparency (CGIT) effect. Both elements of the system can support certain resonant modes respectively, which have almost identical resonant frequencies but highly different quality factors, which are demonstrated by a theory model and the coupled mode theory. The electromagnetic response of the compound plasmonic–dielectric system induces coupling between these two types of gratings resonators and causes a transparency phenomenon due to the destructive interference of the resonant modes. The results show that the CGIT effect is associated with remarkable improvement of the group index corresponding to high transmission efficiency.     

The effect of the metal cylinder in the slot on the transmission properties of the metal–insulator–metal waveguide

The effects of the size, the position and the shape of the metal cylinder in the slot waveguide on the transmittance properties at the communication wavelength of 1.55 μm are investigated using the finite difference time domain method. Since the surface plasmon polartions excites the local surface plasmon resonance of the metal cylinder, the attenuation in the metal–insulator–metal waveguide is enhanced. Those results provide us with the theoretical foundation for the prediction of the effect of the imperfection in the preparation process on the transmittance properties of the metal–insulator–metal waveguides.     

Insulator—insulator—metal plasmonic waveguide for parasitic scattering suppression in plasmonic optics

We have shown using rigorous electromagnetic simulations that a planar structure consisting of two isotropic dielectric layers can be used to reduce parasitic scattering in plasmonic elements by an order-of-magnitude (to 1–3%). The proposed approach can be used for designing various plasmonic elements such as lenses, Bragg reflectors and plasmonic crystals.     

Simulation of superconducting single photon detector coupled with metal–insulator–metal concentric ring grating

Sub-wavelength metal–insulator–metal (MIM) concentric ring grating structure has been introduced to couple with superconducting single photon detector (SSPD) to enhance its response. The effects of the coupling structure parameters on the enhancement factor have been studied systematically. Our numerical simulation results show that the optical density arriving at the detection area of SSPD can be greatly enhanced by the structure of MIM concentric ring grating, corresponding to the improvement of detection response of the detector. A high enhancement factor more than 40 times can be obtained at wavelength of \(1.55~\upmu \hbox {m}\) with proper structure parameters.     

New design of triplexer based on metal–insulator–metal plasmonic ring resonators

In this work, we propose a new design of all-optical triplexer based on of metal–insulator–metal（MIM） plasmonic waveguide structures and ring resonators. By adjusting the radii of ring resonators and the gap distance, certain wavelengths can be filtered out and the crosstalk of each channel can also be reduced. The numerical results show that the proposed MIM plasmonic waveguide structure can really function as an optical triplexer with respect to the three wavelengths, that is, 1310, 1490, and 1550 nm, respectively. It can be widely used as the fiber access network element for multiplexer–demultiplexer wavelength selective in fiber-to-the-home communication systems with transmission efficiency higher than 90%. It can also be a potential key component in the applications of the biosensing systems.     

Electro-optic metal–insulator–semiconductor–insulator–metal Mach-Zehnder plasmonic modulator

The performance of a CMOS-compatible electro-optic Mach-Zehnder plasmonic modulator is investigated using electromagnetic and carrier transport simulations. Each arm of the Mach-Zehnder device comprises a metal–insulator–semiconductor–insulator–metal (MISIM) structure on a buried oxide substrate. Quantum mechanical effects at the oxide/semiconductor interfaces were considered in the calculation of electron density profiles across the structure, in order to determine the refractive index distribution and its dependence on applied bias. This information was used in finite element simulations of the electromagnetic modes within the MISIM structure in order to determine the Mach-Zehnder arm lengths required to achieve destructive interference and the corresponding propagation loss incurred by the device. Both inversion and accumulation mode devices were investigated, and the layer thicknesses and height were adjusted to optimise the device performance. A device loss of <8 dB is predicted for a MISIM structure with a 25 nm thick silicon layer, for which the device length is <3 μm, and <5 dB loss is predicted for the limiting case of a 5 nm thick silicon layer in a 1.2 μm long device: in both cases, the maximum operating voltage is 7.5 V.     

Cross-shaped metal–semiconductor–metal plasmonic crystal for terahertz modulator

The transmission and tuning properties of a cross-shaped plasmonic crystal based on periodic metal–semiconductor–metal (MSM) structures have been investigated in the terahertz (THz) regime. According to the mode analysis, we find that the different resonance modes in the plasmonic crystal show the different changes when this device is actively controlled by the carrier injection of the MSM structures. The longitudinal modes disappear, while the horizontal mode moves to a higher frequency. The former leads to an intensity modulation at 0.5 THz and 1.1 THz when the groove depth h = 60 μm, and the later leads to a band blue-shift from 1.325 THz to 1.38 THz. These results will be applied to THz modulation and tunable filtering.     

Plasmonic finite-thickness metal–semiconductor–metal waveguide as ultra-compact modulator

We propose a plasmonic waveguide with semiconductor gain material for optoelectronic integrated circuits. We analyze properties of a finite-thickness metal–semiconductor–metal (F-MSM) waveguide to be utilized as an ultra-compact and fast plasmonic modulator. The InP-based semiconductor core allows electrical control of signal propagation. By pumping the core we can vary the gain level and thus the transmittance of the whole system. The study of the device was made using both analytical approaches for planar two-dimensional case as well as numerical simulations for finite-width waveguides. We analyze the eigenmodes of the F-MSM waveguide, propagation constant, confinement factor, Purcell factor, absorption coefficient, and extinction ratio of the structure. We show that using thin metal layers instead of thick ones we can obtain higher extinction ratio of the device.     

Plasmonic modulator based on gain-assisted metal–semiconductor–metal waveguide

We investigate plasmonic modulators with gain material to be implemented as ultra-compact and ultra-fast active nanodevices in photonic integrated circuits. We analyze metal–semiconductor–metal (MSM) waveguides with InGaAsP-based active material layers as ultra-compact plasmonic modulators. The modulation is performed by changing the gain of the core, that results in different transmittance through the waveguides. A MSM waveguide enables high field localization and therefore high modulation speed. Bulk semiconductor, quantum wells and quantum dots, arranged in either horizontal or vertical layout, are considered as the core of the MSM waveguide. Dependences on the waveguide core size and gain values of various active materials are studied. The designs consider also practical aspects like n- and p-doped layers and barriers in order to obtain close to reality results. The effective propagation constants in the MSM waveguides are calculated numerically. Their changes in the switching process are considered as a figure of merit. We show that a MSM waveguide with electrical current control of the gain incorporates compactness and deep modulation along with having a reasonable level of transmittance.     

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.     

Surface plasmon polaritons maskless lithography of metal–dielectric multilayers with tunable resolution

We report the design of surface plasmon polaritons maskless lithography based on metal–dielectric multilayers. The interference pattern is generated by the interference of the counter-propagating bulk plasmon polariton (BPP) modes, which are excited by the attenuated total reflection method. The resolution of the interference fringes can be tuned with the incident angles, because different BPP modes are excited in the metal–dielectric multilayers. The feature size as small as 34 nm (about λ/13) can be realized under the 436 nm TM-polarized illumination, which is suitable for deep-subwavelength lithography. The exposure depth decreases with increasing the resonant angle. Electromagnetic simulations by a finite difference time-domain method on the two-dimensional and three-dimensional examples have been performed to validate the designs.     

Toward superlensing with metal–dielectric composites and multilayers

We report on the fabrication of two types of adjustable, near-field superlens designs: metal–dielectric composites and metal–dielectric multilayer films. We fabricated a variety of films with different materials, thicknesses and compositions. These samples were characterized physically and optically to determine their film composition, quality, and optical responses. Our results on metal–dielectric composites indicate that although the real part of the effective permittivity generally follows effective medium theory predictions, the imaginary part does not and substantially higher losses are observed. Going forward, it appears that multilayer metal–dielectric designs are more suitable for sub-diffraction imaging applications because they could provide both tunability and low loss.     

Tunable plasmonic filter with circular metal–insulator– metal ring resonator containing double narrow gaps

Tunable filter based on two metal–insulator–metal (MIM) waveguides coupled to each other by a ring resonator with double narrow gaps is designed and numerically investigated by finite-difference time-domain (FDTD) simulations. The propagating modes of surface plasmon polaritons (SPPs) are studied. By introducing narrow gaps in ring resonators, the transmission in different resonance modes can be effectively adjusted by changing the gap width (g), and the transmitted peak wavelength has a nonlinear relationship with g. Another structure consisting two cascading ring resonators and regular MIM waveguide have also been proposed. The mechanism based on circular ring resonators with narrow gaps may provide a novel method for designing all-optical integrated components in optical communication and computing.     

Band-stop optical nanofilters with split-ring resonators based on metal–insulator–metal structure

Novel band-stop filters with circular split-ring resonators based on the metal–insulator–metal(MIM) structure are presented, with their transmission properties of SPPs propagating through the filter simulated by the finite-difference timedomain(FDTD) method. The variation of the gap of the split ring can affect the transmission characteristics, i.e., the transmission spectrum of SPPs exhibiting a shift, which is useful for modulating the filter. Linear and nonlinear media are used in the resonator respectively. By varying the refractive index of the linear medium, the transmission properties can be changed obviously, and the effect caused by changing the incident intensity with a nonlinear medium is similar.Several resonant modes that are applicable can be enhanced by changing the position of the gap of the split ring. Thus, the transmission properties can be modulated by adjusting the size of the gap, varying the refractive index, and changing the incident intensity of the input light. These methods may play significant roles in applications of optical integrated circuits and nanostructural devices.