Microstructure of pulsed laser deposited ceramic–metal and polymer–metal nanocomposite thin films

Ceramic–metal (MgO combined with Fe, Ti and Ni₈₀Nb₂₀) and polymer–metal (polycarbonate combined with Ag and Pd) nanocomposite multilayers were deposited at room temperature by laser ablation (at 248 nm). The multilayers were characterized by X-ray reflectometry, infrared spectroscopy and transmission electron microscopy. In the case of MgO/metal multilayers, well-layered structures are produced down to layer periodicities of 1.2 nm, necessary for tunneling magnetoresistance devices and X-ray mirrors in the water window. The interface roughness in the case of polymer/metal multilayers is found to be a strong function of the metal layer thickness and also the nature of the metal. PACS 68.55.-a; 81.15.Fg     

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.     

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.     

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.     

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.     

Analysis of interface states and series resistance for Al/PVA:n-CdS nanocomposite metal–semiconductor and metal–insulator–semiconductor diode structures

This paper presents the fabrication and characterization of Al/PVA:n-CdS (MS) and Al/Al₂O₃/PVA:n-CdS (MIS) diode. The effects of interfacial insulator layer, interface states (N _ss ) and series resistance (R _s ) on the electrical characteristics of Al/PVA:n-CdS structures have been investigated using forward and reverse bias I–V, C–V, and G/w–V characteristics at room temperature. Al/PVA:n-CdS diode is fabricated with and without insulator Al₂O₃ layer to explain the effect of insulator layer on main electrical parameters. The values of the ideality factor (n), series resistance (R _s ) and barrier height (? _b ) are calculated from ln(I) vs. V plots, by the Cheung and Norde methods. The energy density distribution profile of the interface states is obtained from the forward bias I–V data by taking into account the bias dependence ideality factor (n(V)) and effective barrier height (? _e ) for MS and MIS diode. The N _ss values increase from mid-gap energy of CdS to the bottom of the conductance band edge for both MS and MIS diode.     

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.     

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.     

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.     

Synaptic behaviors of a single metal–oxide–metal resistive device

The mammalian brain is far superior to today’s electronic circuits in intelligence and efficiency. Its functions are realized by the network of neurons connected via synapses. Much effort has been extended in finding satisfactory electronic neural networks that act like brains, i.e., especially the electronic version of synapse that is capable of the weight control and is independent of the external data storage. We demonstrate experimentally that a single metal–oxide–metal structure successfully stores the biological synaptic weight variations (synaptic plasticity) without any external storage node or circuit. Our device also demonstrates the reliability of plasticity experimentally with the model considering the time dependence of spikes. All these properties are embodied by the change of resistance level corresponding to the history of injected voltage-pulse signals. Moreover, we prove the capability of second-order learning of the multi-resistive device by applying it to the circuit composed of transistors. We anticipate our demonstration will invigorate the study of electronic neural networks using non-volatile multi-resistive device, which is simpler and superior compared to other storage devices.     

Tunable 2H–TaSe_2 room-temperature terahertz photodetector

Two-dimensional transition metal dichalcogenides(TMDs) provide fertile ground to study the interplay between dimensionality and electronic properties because they exhibit a variety of electronic phases, such as semiconducting, superconducting, charge density waves(CDW) states, and other unconventional physical properties. Compared with other classical TMDs, such as Mott insulator 1T–TaS_2 or superconducting 2H–NbSe_2, bulk 2H–TaSe_2 has been a canonical system and a touchstone for modeling the CDW measurement with a less complex phase diagram. In contrast to ordinary semiconductors that have only single-particle excitations, CDW can have collective excitation and carry current in a collective fashion. However, manipulating this collective condensation of these intriguing systems for device applications has not been explored. Here, the CDW-induced collective driven of non-equilibrium carriers in a field-effect transistor has been demonstrated for the sensitive photodetection at the highly-pursuit terahertz band. We show that the 2H–TaSe_2-based photodetector exhibits a fast photoresponse, as short as 14 μs, and a responsivity of over 27 V/W at room temperature. The fast response time, relative high responsivity and ease of fabrication of these devices yields a new prospect of exploring CDW condensate in TMDs with the aim of overcoming the existing limitations for a variety of practical applications at THz spectral range.     

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.     

Wave classification and resonant excitations in lossy metal–dielectric multilayers

Loss-induced optical Bloch waves are investigated in a metal–dielectric semi-infinite multilayer placed over a substrate. In addition to the well-known classification of the electromagnetic waves into forward and backward waves, the electrostatics-based sub-classification elucidates one-level finer details of the wave nature. For this purpose, a hypothetical process is devised by continuously varying the signal velocity. As a result, a given electromagnetic wave turns out to be related to a certain electrostatic state. By this way, the metal's material loss is found to play an essential role in inducing depthwise energy flows, although it has the usual detrimental role in wave attenuation. Depending on the wave direction, normal and inverted reflection peaks on resonant excitations are illustrated.     

Fullerene-based electrode interlayers for bandgap tunable organometal perovskite metal–semiconductor–metal photodetectors

Perovskite photoconductor-type photodetector with metal–semiconductor–metal(MSM) structure is a basic device for photodetection applications. However, the role of electrode interlayer in MSM-type perovskite devices is less investigated compared to that of the pin diode structure. Here, a systematic investigation on the influence of phenyl-C_(61)-butyric acid methyl ester(PCBM) and indene-C_(60) bisadduct(ICBA) interfacial layers for MSM perovskite photodetectors is reported.It is found that the fullerene-based interlayer significantly enhances the photocurrent of the MSM photodetectors. On one hand, the PCBM interlayer is more suitable for CH_3 NH_3 PbI_3 photodetector, with the responsivity two times higher than that of the device with ICBA interlayer. The ICBA layer, on the other hand, becomes more effective when the band gap of perovskite is enlarged with bromine composition, denoted as CH_3 NH_3 Pb(I_(1-x)Br_x)_3(0 ≤ x ≤1). It is further found that the specific detectivity of photodetectors with ICBA interlayer becomes even higher than those with PCBM when the bromine compositional percentage reaches 0.6(x 0.6).     

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.