Metal-dielectric Films Research Articles

Ultrafast optical nonlinearity enhancement in metallodielectric multilayer stacks

Metal-dielectric multilayer films show high transmission at some specific wavelengths of light due to multiple Bragg reflections. By designing the multilayer structure, the high transmission position can be tuned to be on resonance with the laser wavelength at which the light can penetrate into the highly nonlinear metallic layers, leading to an enhanced nonlinear optical response. By employing a femtosecond optical Kerr technique, we experimentally investigated Ag-TiO2 multilayer stacks, and an enhanced nonlinear optical response was observed.

Optical Near-Field Generation and Detection By Using a Metal–Dielectric Film Fabricated From Silver Paste and Spherical Fused Silica

A novel optical near-field generator and detector are fabricated by using a metal-dielectric film composed of silver metal and spherical fused silica. The generators formed on the facets of laser diodes convert lasing light into the optical near-field. The detectors formed on InGaAs and InP epitaxial layers clearly respond to the optical near-field converted from propagating light. This shows the applicability of metal-dielectric film to optical near-field generators and detectors.

Strong-interaction—mediated critical coupling at two distinct frequencies

I study a multilayered medium consisting of a metal-dielectric composite film, a spacer layer, and a dielectric Bragg reflector. I demonstrate a greater flexibility over the critical coupling phenomenon [Tischler et al., Opt. Lett. 31, 2045 (2006)], whereby nearly all the incident light energy is absorbed by the composite film through suppression of both transmission and reflection from the structure. For a larger volume fraction of the metal inclusions, strong light-matter coupling is shown to lead to almost total absorption at two distinct frequencies.

Influences of annealing in reducing and oxidizing ambients on flatband voltage properties of HfO2 gate stack structures

We have applied a combinatorial technique to fabricate work function (WF) tuned Pt-W alloy films and used the films as metal electrodes for HfO2∕SiO2∕Si capacitors. As the ratio, RPt, of Pt to W changes from 0 to 1, the WF value varies continuously from 4.7 to 5.5 eV. This tunability enables us to systematically investigate the effect of WF variation on electrical properties. After a forming gas annealing process, the values of flatband voltage (Vfb) from capacitance-voltage properties are almost constant, regardless of the WF variation, because of oxygen vacancy formation that results in Fermi level pinning. On additional oxidizing gas annealing (OGA), the effect of WF value on Vfb becomes dominant. However, the difference in Vfb between W and Pt is 0.34 V, which is much smaller than the observed WF difference of 0.8 eV. We attribute this phenomenon to the lowering of the effective WF due to an electric dipole, induced by oxygen vacancy formation at the metal/HfO2 interface. Moreover, a decrease in Vfb in W-rich regions was observed following the OGA, suggesting the formation of a W-O bond at the interface. These results clearly indicate that the control of bonding states at the metal/HfO2 interfaces on an atomic scale is essential for the realization of a combination of metal and high-k dielectric films in future complementary metal-oxide-semiconductor devices.

Theory of enhanced second-harmonic generation in some artificial materials

We review the recent theoretical investigation on enhanced second-harmonic generation (SHG) in soft nonlinear optical materials based on ferrofluids, graded metallic films, and graded metal-dielectric films of anisotropic particles. The SHG of soft ferrofluid-based nonlinear optical materials possess magnetic-field controllabilities, i.e., magnetic-field-controllable anisotropy, red-shift and enhancement, which are caused to appear by the shift of a resonant plasmon frequency due to the formation of the chains of the coated nanoparticles. Both graded metallic films and graded metal-dielectric films of anisotropic particles can serve as a novel optical material for producing a broad structure in both the linear and SHG response and an enhancement in the SHG signal, due to the local field effects.

Magnetic and relaxation properties of thin composite films (Co 45Fe 45Zr 10) x(Al 2O 3) 1−x

The work is devoted to the research of the magnetic and relaxation properties of metal-dielectric (composite) films and revealing their relationship with the nanostructure characteristics. The nanosize of ferromagnetic particles in the composite films results in unusual magnetic properties including an abnormal magnetization relaxation. Correlation of the magnetic relaxation frequencies and specific electric resistance with the nanostructure characteristics are discovered. The FMR line shape for films with a small percentage of the metal phase transforms from a resonant line shape to a relaxation line shape if the film is annealed. This transition is connected to the changes in the array of metal and dielectric particles and its boundary properties. The significant changes in the shape of the FMR spectra lines are connected to nanostructural transformations in the ferromagnetic granules topology and distance between the granules.

Anderson localization of polar eigenmodes in random planar composites

Anderson localization of classical waves in disordered media is a fundamental physicalphenomenon that has attracted attention in the past three decades. More recently,localization of polar excitations in nanostructured metal–dielectric films (also known asrandom planar composites) has been subject of intense studies. Potential applications ofplanar composites include local near-field microscopy and spectroscopy. A numberof previous studies have relied on the quasistatic approximation and a directanalogy with localization of electrons in disordered solids. Here I consider thelocalization problem without the quasistatic approximation. I show that localization ofpolar excitations is characterized by algebraic rather than by exponential spatialconfinement. This result is also valid in two and three dimensions. I also showthat the previously used localization criterion based on the gyration radius ofeigenmodes is inconsistent with both exponential and algebraic localization. Analternative criterion based on the dipole participation number is proposed. Numericaldemonstration of a localization–delocalization transition is given. Finally, it is shown that,contrary to the previous belief, localized modes can be effectively coupled to runningwaves.

Superlens based on metal-dielectric composites

Pure noble metals are typically considered to be the materials of choice for a near-field superlens that allows subwavelength resolution by recovering both propagating and evanescent waves. However, a superlens based on bulk metal can operate only at a single frequency for a given dielectric host. In this Letter, it is shown that a composite metal-dielectric film, with an appropriate metal filling factor, can operate at practically any desired wavelength in the visible and near-infrared ranges. Theoretical analysis and simulations verify the feasibility of the proposed lens.

Enhancement of optical nonlinearity in Ag:BaTiO3 composite films by applying an electric field during growth

We prepared the Ag:BaTiO3 composite films by pulsed-laser deposition technique. The films were grown on MgO (100) substrates under a nitrogen pressure of 10.0 Pa at 600°C, and a parallel electric field was applied on the substrate during growth. The chemical natures of Ag in the films were measured by x-ray photoelectron spectroscopy. The linear optical properties of the films were studied in the wavelength range of 330 to 700 nm. The third-order nonlinear optical susceptibilities of the films were determined by z-scan method at the wavelength of 532 nm with a laser duration of 10 ns. The figure of merit (FOM) was ∼1 order of magnitude larger than that of the films that were grown without an electric field and reached 1.62×10^−10 esu cm. Our experimental results have demonstrated that an external electrical field can induce the metal-particle orientation in order and therefore enhances the FOM of the metal-dielectric composite films.

Effective nonlinear optical properties of graded metal-dielectric composite films of anisotropic particles

We study the effective third-order nonlinear susceptibility of a graded metal-dielectric composite film of anisotropic particles with weak nonlinearity by invoking the local field effects within the Maxwell-Garnett theory. We further numerically demonstrate that the film can serve as a novel optical material to produce a broad structure in both the linear and the nonlinear response and an enhancement in the nonlinear response.

Second-harmonic generation in graded metal–dielectric films of anisotropic particles

We investigate theoretically the effective second-harmonic generation (SHG) susceptibility in graded films of metal–dielectric random composites with anisotropically shaped particles by invoking the local field effects. Using numerical examples, we demonstrate that in the presence of gradation profiles and/or shape effects, graded metal–dielectric films could be used as novel optical materials for producing a broad structure in both the linear and nonlinear (SHG) response and with an enhancement in the nonlinear response.

Near-Field Intensity Correlations in Semicontinuous Metal-Dielectric Films

Spatial intensity correlation functions are obtained from near-field scanning optical microscope measurements of semicontinuous metal-dielectric films. The concentration of metal particles on a dielectric surface is varied over a wide range to control the scattering strength. At low and high metal coverages where scattering is weak, the intensity correlation functions exhibit oscillations in the direction of incident light due to excitation of propagating surface waves. In the intermediate regime of metal concentration, the oscillatory behavior is replaced by a monotonic decay as a result of strong scattering and anomalous absorption. Significant differences in the near-field intensity correlations between metallic and dielectric random systems are demonstrated.

Third order nonlinear optical susceptibility of Cu:Al2O3 nanocomposites: From spherical nanoparticles to the percolation threshold

The third order optical susceptibility of metal-dielectric nanocomposite films (Cu:Al2O3) has been determined by degenerate four wave mixing. The films have been synthesized by alternate pulsed laser deposition and consisted of Cu nanoparticles in an amorphous Al2O3 matrix. They have metal volume fractions, p, ranging from 0.07 to 0.45, and morphologies that range from spherical particles (diameter, φ∼2 nm) to a random network when close to the percolation threshold. In nanocomposites containing isolated oblate spheroids (p⩽0.17), the optical response at wavelengths close to that of the surface plasmon resonance (SPR) can be described in the frame of the Maxwell-Garnett effective medium theory. Above the particle coalescence threshold, in nanocomposites with higher Cu content (p⩾0.2), both the linear absorption in the near-infrared and the third order nonlinear optical susceptibility at the SPR are greatly enhanced, the latter achieving values as high as 1.8×10−7 esu. These results are discussed in terms of multipolar interactions among particles and giant local resonance effects that appear as a consequence of the particle coalescence and the increase in size of the nanocrystals.

Local field distribution in random metal–dielectric films; theory and experiment

The local field properties of metal–dielectric films are investigated in terms of the probability distribution function (PDF). For metal coverage p, close to the percolation threshold we obtain a wide, lognormal PDF which for low metal concentrations gradually transforms into a scaling, power-law distribution function. Strong enhancement of the local fields in a broad spectral range is verified by an exact numerical procedure. In experimental studies, metal–dielectric films were synthesized by a pulsed laser deposition technique, and their optical properties were investigated using near-field optical microscopy. The experimentally and numerically obtained probability functions are found to be in good agreement.

Magnetorefractive effect in (Co50Fe50)x(Al2O3)1−x granular films

The reflectivity spectra and the magnetorefractive effect (MRE) of (Co50Fe50)x(Al2O3)1−x metal-dielectric granular films (0.07<x<0.52) are analyzed in the IR spectral range λ=2.5–25 µm. It is revealed that the specific features observed in the spectra at λ≈8.5 and 20 µm are associated with the excitation of phonon modes in the dielectric matrix. The magnetorefractive effect in the films is observed below the percolation thresh-old only in p-polarized light and above the percolation threshold for both the p and s polarizations. It is demonstrated that the optical properties of (Co50Fe50)x(Al2O3)1−x films in the IR spectral range, to a first approximation, can be interpreted in the framework of the effective-medium theory and the magnetorefractive effect can be explained in terms of the modified Hagen-Rubens relation.

Vanishing loss effect on the effective ac conductivity behaviour for two-dimensional composite metal–dielectric films at the percolation threshold

We study numerically the imaginary part of the effective ac conductivity as well as its probability distribution in the limit of vanishing losses in two-dimensional composites. The results show, in this limit, strong fluctuations with a symmetric probability distribution centred at zero. Since the real part of the ac conductivity was also found to vanish in this limit (no dissipation), further discussions are provided on the interpretation of such fluctuations in terms of the field energy in the sample.

Plasmon localization and local field distribution in metal-dielectric films.

An exact and very efficient numerical method for calculating the effective conductivity and local-field distributions in random R-L-C networks is developed. Using this method, the local-field properties of random metal-dielectric films are investigated in a wide spectral range and for a variety of metal concentrations p. It is shown that for metal concentrations close to the percolation threshold (p=p(c)) and frequencies close to the resonance, the local-field intensity is characterized by a non-Gaussian, exponentially broad distribution. For low and high metal concentrations a scaling region is formed that is due to the increasing number of noninteracting dipoles. The local electric fields are studied in terms of characteristic length parameters. The roles of both localized and extended eigenmodes in Kirchhoff's Hamiltonian are investigated.

Inhomogeneous SiOx〈Fe〉 metal-dielectric films as a material for infrared thermal radiation detectors

A vacuum deposition method for producing SiOx〈Fe〉 composite metal-dielectric films in which the metal concentration in the SiOx dielectric matrix varies across thickness is described. The reflection and transmission of the films in the 2–12 µm IR spectral range are studied and their optical properties are simulated. In the temperature range 283–390 K, the temperature-sensitive properties of the SiOx〈Fe〉 films with a phase volume ratio of 23% (Fe): 77% (SiO) is investigated. For these films, the temperature coefficient of resistance is found. The feasibility of these films as a sensitive layer in microbolometers is demonstrated.

Growth, morphology, and optical and electrical properties of semicontinuous metallic films

The growth and optical properties of semicontinuous silver films on insulator substrates were studied experimentally and theoretically. In the experimental studies, films were synthesized by the pulsed-laser deposition technique, characterized by electron microscopy and in situ optical and dc electrical resistance measurements and studied using near-field optical microscopy. The percolation threshold of the films was found to be at \ensuremath{\sim}65% metal filling fraction, higher than the 50%--60% range of values predicted for two-dimensional (2D) bond or site percolation films and suggesting the importance of grain coalescence and 3D growth in our system. Local optical properties measured by near-field optical microscopy were compared with theoretical results obtained using the block-elimination method, with good agreement. Local-field distributions were found to depend strongly on the metal concentration and wavelength of illumination. The degree of localization was found to increase at metal concentrations both above and below the percolation threshold. At a very high metal coverage, very strong local fields were observed in submicron voids of a metal dielectric film. These fields are likely due to localized surface plasmon polaritons.

LOCAL ARRANGEMENT EFFECTS ON THE STATISTICAL PROPERTIES OF THE EFFECTIVE CONDUCTIVITY AND FIELD IN 2D COMPOSITES

We study the effect of the local arrangement of the metallic grains in metal-dielectric composite films on the statistical properties of the effective conductivity and the local field at the percolation threshold and for a characteristic frequency (where the conductivities of the two components are of the same magnitude). It is found that the segregation enhances sensitively the effective conductivity of the system as well as the local field. Further discussions on the field localization and other phenomena are provided.