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.     

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.     

A surface-plasmon-polariton wavelength splitter based on a metal–insulator–metal waveguide

A surface-plasmon-polariton (SPP) wavelength splitter based on a metal–insulator–metal waveguide with multiple teeth is proposed. Using the transfer-matrix method, a plasmonic band gap is identified in the multiple-toothed structure, and the splitting wavelength of the SPP splitter can be easily adapted by adjusting the widths of the teeth and the gaps. The proposed wavelength splitter is further verified through finite-difference time-domain (FDTD) simulations, in which SPPs with incident wavelengths of 756 nm and 892 nm are successfully split and guided in opposite directions in the waveguide, with extinction ratios of 30 dB and 29 dB, respectively.     

A wire-form emitter metal–insulator–semiconductor tunnel junction

Physical effects arising due to change of configuration of a MIS system from planar to cylindrical, are theoretically analyzed. Attention is paid to the voltage partitioning and all the components of tunneling current. A simple simulation model is developed enabling prediction of the band diagram details and calculation of the currents. The trends expected with decreasing system radius are elucidated. Cylindrical geometry can be faced with when quantum wire is used as an electron emitter. Similar form may also be roughly attributed to an edge region of conventional MIS capacitors.     

Hydrogen interaction with GaN metal–insulator–semiconductor diodes

Interaction mechanism of hydrogen with GaN metal–insulator–semiconductor (MIS) diodes is investigated, focusing on the metal/semiconductor interfaces. For MIS Pt-GaN diodes with a SiO₂ dielectric, the current–voltage (I–V) characteristics reveal that hydrogen changes the conduction mechanisms from Fowler–Nordheim tunneling to Poole–Frenkel emission. In sharp contrast, Pt-Si_xN_y-GaN diodes exhibit Poole–Frenkel emission in nitrogen and do not show any change in the conduction mechanism upon exposure to hydrogen. The capacitance–voltage (C–V) study suggests that the work function change of the Schottky metal is not responsible mechanism for the hydrogen sensitivity.     

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.     

Photon-assisted tunneling versus tunneling of excited electrons in metal–insulator–metal junctions

Photocurrent measurements in Ag–Al₂O₃–Al metal–insulator–metal junctions under illumination with ultra-short laser pulses reveal that tunneling and internal photoemission of excited electrons are the dominating transport mechanisms. Photon-assisted tunneling is observed under rare conditions that depend critically on the preparation of the interface. The comparison of time-resolved two-pulse correlation measurements with model calculations shows that the photon-induced transport of excited electrons is well described using a one-dimensional many-particle model for two coupled metallic leads, whereas a single-particle model for nonresonant excitation in a rectangular double-minimum potential reveals the signature of photon-assisted tunneling. PACS 73.40.-c; 72.10.-d; 73.40.Gk; 73.50.Pz     

Flexible metal–insulator–metal (MIM) devices for plastic film AM-LCD

We developed new flexible metal–insulator–metal (MIM) devices subject to plastic film substrate. The structure of the MIM device is that a Ta₂O₅ insulator is covered with two flexible Al electrodes on both sides. The flexible structure of the MIM device was successfully fabricated applying our own etch-free process.     

Reflective plasmonic waveplates based on metal–insulator–metal subwavelength rectangular annular arrays

We propose and present a quarter-wave plate using metal–insulator–metal (MIM) structure with sub-wavelength rectangular annular arrays (RAA) patterned in the upper Au film. It is found that by manipulating asymmetric width of the annular gaps along two orthogonal directions, the reflected amplitude and phase of the two orthogonal components can be well controlled via the RAA metasurface tuned by the MIM cavity effect, in which the localized surface plasmon resonance dip can be flattened with the cavity length. A quarter-wave plate has been realized through an optimized design at 1.55 μm, in which the phase difference variation of less than 2% of the π/2 between the two orthogonal components can be obtained in an ultra-wide wavelength range of about 130 nm, and the reflectivity is up to ∼90% within the whole working wavelength band. It provides a great potential for applications in advanced nanophotonic devices and integrated photonic systems.     

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.     

Unusual metal–insulator transition in disordered ferromagnetic films

We present a theoretical interpretation of recent data on the conductance near and farther away from the metal–insulator transition in thin ferromagnetic Gd films of thickness b≈2$b\approx 2$–10 nm. For increasing sheet resistances a dimensional crossover takes place from d=2 to d  =3 dimensions, since the large phase relaxation rate caused by scattering of quasiparticles off spin wave excitations renders the dephasing length L_??b$L_{?}?b$ at strong disorder. The conductivity data in the various regimes obey fractional power-law or logarithmic temperature dependence. One observes weak localization and interaction induced corrections at weaker disorder. At strong disorder, near the metal–insulator transition, the data show scaling and collapse onto two scaling curves for the metallic and insulating regimes. We interpret this unusual behavior as proof of two distinctly different correlation length exponents on both sides of the transition.     

The Andreev conductance in superconductor–insulator–normal metal structures

The Andreev subgap conductance at 0.08–0.2 K in thin-film superconductor (aluminum)–insulator–normal metal (copper, hafnium, or aluminum with iron-sublayer-suppressed superconductivity) structures is studied. The measurements are performed in a magnetic field oriented either along the normal or in the plane of the structure. The dc current–voltage (I–U) characteristics of samples are described using a sum of the Andreev subgap current dominating in the absence of the field at bias voltages U < (0.2–0.4)Δ_c/e (where Δ_c is the energy gap of the superconductor) and the single-carrier tunneling current that predominates at large voltages. To within the measurement accuracy of 1–2%, the Andreev current corresponds to the formula \({I_n} + {I_s} = {K_n}\tanh \left( {{{eU} \mathord{\left/ {\vphantom {{eU} {2k{T_{eff}}}}} \right. \kern-\nulldelimiterspace} {2k{T_{eff}}}}} \right) + {K_s}{{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}} \right. \kern-\nulldelimiterspace} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}\) following from a theory that takes into account mesoscopic phenomena with properly selected effective temperature T _eff and the temperature- and fieldindependent parameters K _n and K _s (characterizing the diffusion of electrons in the normal metal and superconductor, respectively). The experimental value of K _n agrees in order of magnitude with the theoretical prediction, while K _s is several dozen times larger than the theoretical value. The values of T _eff in the absence of the field for the structures with copper and hafnium are close to the sample temperature, while the value for aluminum with an iron sublayer is several times greater than this temperature. For the structure with copper at T = 0.08–0.1 K in the magnetic field B_|| = 200–300 G oriented in the plane of the sample, the effective temperature T _eff increases to 0.4 K, while that in the perpendicular (normal) field B _⊥ ≈ 30 G increases to 0.17 K. In large fields, the Andreev conductance cannot be reliably recognized against the background of single- carrier tunneling current. In the structures with hafnium and in those with aluminum on an iron sublayer, the influence of the magnetic field is not observed.     

On the metal–insulator transition in vanadium dioxide

The conductivity σ of vanadium dioxide (VO₂) drops at a metal–insulator transition by four orders of magnitude due to the structural change between tetragonal and monoclinic crystals. In order to elucidate this conductivity drop, we introduce the semiclassical equation of motion to describe the dynamics of the conduction electron (wave packet), where the existence of a k-vector k is prerequisite for the conduction. We showed that the periodicity using the non-orthogonal bases does not legitimize the electron dynamics in solids. The theory suggests that the decrease in the dimensionality of the k-vectors due to the structural change is the cause of the conductivity drop.     

Surface modes in metal–insulator composites with strong interaction of metal particles

We theoretically examine plasmonic resonance excited between two close metallic grains embedded into a dielectric matrix. The grains sizes are assumed to be much less than the wavelength of the electromagnetic wave in the dielectric medium and the grain’s separation is assumed to be much smaller than the grains sizes. A qualitative scheme is developed that enables one to estimate frequency of the plasmonic resonance and value of the field enhancement inside the gap. Our general arguments are confirmed by rigorous analytic solution of the problem for simplest geometry—two identical spherical grains.     

Investigation of electrical properties of HfO2 metal–insulator–metal (MIM) devices

This paper is devoted to the study of the electrical properties of Au/HfO₂/TiN metal–insulator–metal (MIM) capacitors in three distinctive modes: (1) alternative mode (C–f), (2) dynamic regime [thermally stimulated currents, TSCs I(T)] and (3) static mode [I(V)]. The electrical parameters are investigated for different temperatures. It is found that capacitance frequency C–f characteristic possesses a low-frequency dispersion that arises for high temperature (T > 300 °C). Accordingly, the loss factor exhibits a dielectric relaxation (with an activation energy E _a ~ 1.13 eV) which is intrinsically related to the diffusion of oxygen vacancies. The relaxation mechanisms of electrical defects in a dynamic regime (TSCs) analysis show that defect related to the TSC peak observed at 148.5 °C (E _a ~ 1 eV) is in agreement with impedance spectroscopy (C–f). On the other hand, when the MIM structures are analyzed in static mode, the I–V plots are governed by Schottky emission. The extrapolation of the curve at zero field gives a barrier height of 1.7 eV.     

Plasmonic planar lens based on stack of metal–insulator–metal waveguides with height tuning

We introduce a technique capable of focusing electromagnetic (EM) waves through plasmonic nanoslits symmetrically arranged along the indented semi-circular surface in silver background. The EM transports through the tuning slits in the form of surface plasmon polaritons (SPPs), and gets the required phase retardations to focus at the focal plane. Due to the subwavelength nature of planar metallic lens, we present the rigorous electromagnetic analysis by using two dimensional (2D) finite difference time domain (FDTD) method. These height-modulated slits with uniform width are demonstrated to have unique advantages in beam manipulation. In combination with previous studies, it is expected that our structure with small number of slits could lead to realization of optimum designs of plasmonic nano-lens.     

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.     

Magnetic field induced insulator–metal crossover in underdoped cuprates

Within the charge-spin separation fermion-spin theory, the influence of a strong external magnetic field on the normal state transport of the underdoped cuprates is discussed. It is shown that when superconductivity in the underdoped cuprates is suppressed in the presence of a strong external magnetic field, the system reveals a low temperature normal state insulator–metal crossover. It is also shown that this striking insulator–metal crossover behavior is intriguingly related to the antiferromagnetic correlation.     

Analysis of surface and guided wave plasmon polariton modes in insulator–metal–insulator planar plasmonic waveguides

Theoretical and experimental study of the surface plasmon–polariton and guided wave plasmon polariton modes is presented for the Sapphire/Ag/Polycarbonate/Air structure. Theoretical results are obtained by solving complex multilayer eigenvalue equations as well as the reflectivity equation for this structure. It is proposed that the mode attenuation can be significantly reduced by inserting a low index dielectric buffer between the metal and the guiding dielectric layer. The dispersion and attenuation curves are generated. Both the surface plasmon and guided wave plasmon polariton modes are studied experimentally. The experimental values of the effective refractive indices agree well with the theoretical values. The electric field profiles are generated and used to examine the nature of modes. After optimization of various parameters the condition for low loss single mode guiding is obtained for the proposed structure. Effect of metal thickness on surface plasmon mode is also discussed. It is inferred that in a properly optimized plasmonic waveguide, the losses can be reduced by a factor of 4.     

Tunneling conductance oscillations in spin–orbit coupled metal–insulator–superconductor junctions

The tunneling conductance for a device consisting of a metal–insulator–superconductor (MIS) junction is studied in presence of Rashba spin–orbit coupling (RSOC) via an extended Blonder–Tinkham–Klapwijk formalism. We find that the tunneling conductance as a function of an effective barrier potential that defines the insulating layer and lies intermediate to the metallic and superconducting electrodes, displays an oscillatory behavior. The tunneling conductance shows high sensitivity to the RSOC for certain ranges of this potential, while it is insensitive to the RSOC for others. Additionally, when the period of oscillations is an odd multiple of a certain value of the effective potential, the conductance spectrum as a function of the biasing energy demonstrates a contrasting trend with RSOC, compared to when it is not an odd multiple. The explanations for the observation can be found in terms of a competition between the normal and Andreev reflections. Similar oscillatory behavior of the conductance spectrum is also seen for other superconducting pairing symmetries, thereby emphasizing that the insulating layer plays a decisive role in the conductance oscillations of a MIS junction. For a tunable Rashba coupling, the current flowing through the junction can be controlled with precision.