Semi-transparent Metal Film Research Articles

Computational visualization of semi-transparent metallic thin films with roughness.

We model the visual appearance of thin, semi-transparent metallic films coated on arbitrary three-dimensional substrates, incorporating effects including nanoscale film roughness, microscale substrate roughness, and source of light. Film reflectance is modeled by combining electrodynamic simulations with the Schlick approximation, which is adapted and validated to describe the color appearance of thin semi-transparent metallic films with nanoscale, subwavelength roughness. Diffuse scattering originating from microscale roughness of the substrate and partial reflectance is described by a microfacet model. Photorealistic rendered images generated by our approach are qualitatively compared to photographs of fabricated thin-film samples under similar lighting conditions. We render images of semi-transparent metallic films as a function of film thickness, multilayer composition, substrate type, nanoscale film roughness, microscale substrate roughness, and environmental lighting, yielding physically plausible results consistent with previously reported observations.

Ultrathin Metal Crystals: Growth on Supported Graphene Surfaces and Applications.

Controlled nucleation and growth of metal clusters in metal deposition processes is a long-standing issue for thin-film-based electronic devices. When metal atoms are deposited on solid surfaces, unintended defects sites always lead to a heterogeneous nucleation, resulting in a spatially nonuniform nucleation with irregular growth rates for individual nuclei, resulting in a rough film that requires a thicker film to be deposited to reach the percolation threshold. In the present study, it is shown that substrate-supported graphene promotes the lateral 2D growth of metal atoms on the graphene. Transmission electron microscopy reveals that 2D metallic single crystals are grown epitaxially on supported graphene surfaces while a pristine graphene layer hardly yields any metal nucleation. A surface energy barrier calculation based on density functional theory predicts a suppression of diffusion of metal atoms on electronically perturbed graphene (supported graphene). 2D single Au crystals grown on supported graphene surfaces exhibit unusual near-infrared plasmonic resonance, and the unique 2D growth of metal crystals and self-healing nature of graphene lead to the formation of ultrathin, semitransparent, and biodegradable metallic thin films that could be utilized in various biomedical applications.

Enhanced Directed Radiative Surface Plasmon Emission from Periodically Corrugated Noble Metal Films

Massively enhanced directed radiation emission from periodically corrugated thin gold and silver films at the surface plasmon resonance is reported. The optical setup comprises attenuated total reflection in a modified Kretschmann configuration, with the semi-transparent metal films placed at the base plate of a polymeric semi-cylinder. Experiments performed at a fixed wavelength confirm earlier observations on appearance of two symmetrically arranged first-order diffraction beams. These emanate under resonance conditions into the upper and lower half-spaces, the latter due to coupling of the non-radiating surface plasmon to an angularly directed radiative mode. Collimated incident and spectrally broadband radiation under resonance conditions likewise split into two symmetrically arranged first-order, but spectrally dispersed continuous, diffraction beams. For silver, unlike smooth metal films, the non-radiative surface plasmon (SP) also splits into two spectrally separated resonance dips at 550 and 700 nm. The spectral continuum in the SP-mediated radiative beam in the lower half-space resembles the complementary color feature of the absorbing SP-mode. Its intensity exceeds the isotropic radiative plasmon emission from smooth, weakly roughened SP-active gold metal layers by at least two orders of magnitude.

Narrowband thermal radiation from closed-end microcavities

High spectral selectivity of thermal radiation is important for achieving high-efficiency energy systems. In this study, intense, narrowband, and low directional absorption/radiation were observed in closed-end microcavity which is a conventional open-end microcavity covered by a semi-transparent thin metal film. The quality factor (Q factor) of optical absorption band strongly depended on the film electrical conductivity. Asymmetric and narrow absorption band with a Q factor of 25 at 1.28 μm was obtained for a 6-nm-thick Au film. Numerical simulations suggest that the formation of a fixed-end mode at the cavity aperture contributes to the narrowband optical absorption. The closed-end microcavity filled with SiO2 exhibits intense and isotropic thermal radiation over a wide solid angle according to numerical simulation. The narrow and asymmetric absorption spectrum was experimentally confirmed in a model of closed-end microcavity.

Optical performance improvement of semi-transparent metal film electrodes with biomimetic subwavelength gratings for high-performance optoelectronic device applications

Semi-transparent metal electrodes consisting of silver-coated subwavelength gratings on polymer film/polyethylene terephthalate substrates for high-performance optoelectronic device applications.

The effect of holes in the dispersion relation of propagative surface plasmon modes of nanoperforated semitransparent metallic films

We have analysed the effect that holes have on the properties of propagative surface plasmon modes in semitransparent nanoperforated Au films. The modes have been excited in Kretschmann configuration. Contrary to continuous films, where only one mode is excited, two modes are observed in Au nanohole array. The origin of this different behavior is discussed using effective optical properties for the nanoperforated films. The presence of the holes affects the effective optical constants of the membranes in two ways: it changes the contribution of the free electrons, and it gives rise to a localized transition due to a hole induced plasmon resonance. This localized transition interacts with the propagative surface plasmon modes, originating the two detected modes.

Observation of hybrid state of Tamm and surface plasmon-polaritons in one-dimensional photonic crystals

Experimental observation of hybrid mode of Tamm plasmon-polariton and surface plasmon-polariton is reported. The hybrid state is excited in one-dimensional photonic crystal terminated by semitransparent metal film under conditions of total internal reflection for transverse-magnetic-polarized light. Coupling between Tamm and surface plasmon-polaritons leads to repulsion of their dispersion curves controlled by metal film thickness.

Lambertian white top-emitting organic light emitting device with carbon nanotube cathode

We demonstrate that white organic light emitting devices (OLEDs) with top carbon nanotube (CNT) electrodes show almost no microcavity effect and exhibit essentially Lambertian emission. CNT top electrodes were applied by direct lamination of multiwall CNT sheets onto white small molecule OLED stack. The devices show an external quantum efficiency of 1.5% and high color rendering index of 70. Due to elimination of the cavity effect, the devices show good color stability for different viewing angles. Thus, CNT electrodes are a viable alternative to thin semitransparent metallic films, where the strong cavity effect causes spectral shift and non-Lambertian angular dependence. Our method of the device fabrication is simple yet effective and compatible with virtually any small molecule organic semiconductor stack. It is also compatible with flexible substrates and roll-to-roll fabrication.

Enhancement of the photoluminescence of porous silicon by sputter deposition of semitransparent metal films

Optoelectronic devices based on porous silicon (PS) undergo substantial degradation in luminescence with aging due to atmospheric oxidation. The passivation of PS has been reported with a transparent conducting material or a semi-transparent metal. In this paper, we report enhancement of the photoluminescence (PL) of PS by the passivation of PS with ultrathin metal films such as silver (Ag), aluminum (Al), and gold (Au). It has been found that Ag and Au are respectively most and least effective in enhancing the PL of PS among those three different metals. The highest PL enhancing effect of Ag is mostly attributed to the high electrical conductivity of Ag, whereas the lowest PL enhancing effect of Au is due to the lowest optical transmittance of Au. The details of the PL enhancing effect of metal passivation are discussed with the aid of FTIR analysis results.

Light trapping cavity enhanced light transmission through a single sub-wavelength aperture in a metal film

We demonstrate that optical transmission of a normally incident, monochromatic plane wave through a single sub-wavelength aperture in an opaque metal film can be substantially enhanced by a thin, semitransparent metal film placed parallel to the opaque metal film in front of the aperture. When the semi-transparent and the opaque metal film are separated by a proper distance, a light trapping cavity is formed and the sub-wavelength aperture exhibits a transmission maximum. An enhancement factor of approximately 40 is demonstrated for a cylindrical 100 nm diameter hole in a silver film.

Temperature-dependent sensitivity of surface plasmon resonance sensors at the gold–water interface

Instrumental performance and sensitivity of surface plasmon polariton or resonance (SPR) sensors are crucially affected by ambient temperature fluctuations. Apart from temperature-induced opto-mechanical displacements, resonance conditions and associated output quantities vary, due to thermo-physical properties of optical components, like prism, semi-transparent metal-film and aqueous analyte solution. The variation of instrumental sensitivity S as function of environmental temperature has been exploited experimentally in the temperature region 275K<T<320K. A compact, robust commercial SPR device has been used, operating in the angular interrogation mode. At temperature <300 K, Sθ steadily deteriorates from 120°/RIU to 30°/RIU. For comparison, the thermal device characteristics also has been evaluated theoretically within the range 273K<T<370K for both, angular interrogation mode (AIM) and wavelength interrogation mode (WIM). Fresnel’s equation system and established analytical sensitivity expressions have been elaborated, and differences assigned to simplifications in the analytical form. An appropriate thermo-physical data set of experimentally verified optical materials parameters n(λ,T) and ɛ(λ,T) for all, BK7 glass prism, gold metal film, and aqueous analyte has been used. A singularity has been identified in the simulations near the freezing point of water, while a soft transition appears in the experiment. Towards higher device temperatures at 300K<T<370K,Sθ,λ decreases weakly. The effect is more pronounced at shorter wavelength ≤550 nm, and in the wavelength interrogation mode. A steady increase towards higher T was observed for the SPR-line broadening parameter in the AIM, but a distinct maximum resolved at 305 K for wavelength interrogation.

The efficiency of surface-plasmon coupled emission for sensitive fluorescence detection

Surface-plasmon coupled emission (SPCE) has emerged as a new and potentially powerful tool for highly sensitive fluorescence detection. In the case of SPCE, the fluorescence is collected through a semi-transparent thin metal film deposited on glass. We present a theoretical analysis of SPCE, studying the potential enhancement of the fluorescence collection efficiency, brightness, quantum-yield, and photostability. The results are compared with fluorescence detection on a pure glass surface. It is shown that SPCE does not lead to any improvement, but that the metal film actually reduces the sensitivity of fluorescence detection.

Total internal reflection ellipsometry: principles and applications.

A concept for a measurement technique based on ellipsometry in conditions of total internal reflection is presented. When combined with surface plasmon resonance (SPR) effects, this technique becomes powerful for monitoring and analyzing adsorption and desorption on thin semitransparent metal films as well as for analyzing the semitransparent films themselves. We call this technique total internal reflection ellipsometry (TIRE). The theory of ellipsometry under total internal reflection combined with SPR is discussed for some simple cases. For more advanced cases and to prove the concept, simulations are performed with the Fresnel formalism. The use of TIRE is exemplified by applications in protein adsorption, corrosion monitoring, and adsorption from opaque liquids on metal surfaces. Simulations and experiments show greatly enhanced thin-film sensitivity compared with ordinary ellipsometry.

Method for determination of the dielectric function of a thin absorbing film on variable substrates from transmission spectra.

Transmission spectra of bilayers of a strongly absorbing dye molecule on thin semitransparent metallic films show a pronounced variation of the shape as a function of the thickness of the metal film. The shape changes with increasing thickness of the metal film from the form of an absorption spectrum as determined by the imaginary part of the dielectric function to an antisymmetric shape characteristic of the dispersion of the real part of the dielectric function in the vicinity of a resonance. These different spectra shapes were exploited to derive the complex dielectric function of a dye layer from transmission spectra of the layer on metal films of a different thickness. This method proved to be a simple alternative to determination of the dielectric function of a thin film of a dye by spectroscopic ellipsometry.

Enhancement of hole injection by metal anode in organic light-emitting diodes

We studied the new transparent anode to replace indium tin oxide (ITO). We used a semi-transparent metal film for an anode of organic light-emitting diodes (OLEDs). The transparent anode is composed of a low-resistivity metal and an oxide semiconductor with a high refractive index. The OLED using the metal anode has almost identical performance to the OLED using ITO. Luminance of the OLED using the metal anode was 10 000 cd/m 2, luminance per current density was 2.5 cd/A at 11 V. Moreover, the OLED using the metal anode showed a lager current density than OLEDs using ITO. It is inferred that a metal anode has superior hole injection into the hole transport layer in comparison with ITO.

Index determination of opaque and semitransparent metallic films: application to light absorbers.

Index-determination methods based on reflectance and transmittance measurements are developed for both opaque and semitransparent metallic films. Results are given concerning chromium and nickel layers manufactured by electron-beam deposition. To take account of the evolution of the optical constants versus layer thickness, an inhomogeneous layer model is used, which permits us to obtain a good agreement between measurements and calculations. Results are applied to the design and manufacture of light absorbers for which accurate index knowledge is required. Measured absorption is higher than 0.999 on both broadband and monochromatic components.

Surface-plasmon–polariton excitation of optical microcavities and second-harmonic emission

We have created microscopic Fabry–Perot optical resonator cavities between the flattened end of a tunneling microscope tip and a semitransparent metal film. Power is transferred to cavity modes from a laser beam by means of surface-plasmon–polariton generation and subsequent radiative decay. We compare the dependence of the axial emission from these modes on cavity length and on the angle of incidence with the predictions of a stratified-medium model. The mode structure has a finesse that approaches the theoretical limit. At high incident power, second-harmonic radiation produced by weakly localized surface-plasmon–polaritons is also coupled into the cavity and detected in axial emission.

Electrostatic force-feedback force sensor incorporated in an ultrahigh vacuum force microscope

A force sensor based on a fiber-optic interferometric displacement transducer incorporated in an ultrahigh vacuum atomic force microscope is described. The operation of the sensor is based on balancing the tip-sample interfacial force using an electrostatic actuator. The electrodes of the actuator are formed by the grounded W cantilever and the metallized end facet of the optical fiber used by the interferometer. Chemical reduction of Ag by a wet chemical method is used for metal coating of the fiber end. A special masking procedure is used to obtain a window hole in the metal coating at the position of the fiber core to allow for optical beam output. Using a window instead of a semitransparent metal film allows us to save the low-finesse characteristics of the interferometer which facilitates the calibration of cantilever displacement. The performance of the sensor is discussed and exemplified by experimental results from force-separation measurements on the W–Au system in ultrahigh vacuum.

Field-induced functions of porous Si as a confined system

Electroluminescent porous Si (PS) diodes exhibit various useful functions under a high-electric field. The experimental PS diodes are composed of thin semitransparent metal films, PS layers (about 500 nm thick in minimum), p- or n-type Si substrates and ohmic back contacts. Definite nonlinear electrical behavior (negative resistance and nonvolatile bistable memory effects) and cold electron emission phenomena appear in these PS diodes associated with the EL emission. Both the negative resistance and memory effects are related to the charging of Si nanocrystallites by field-induced carrier injection. The electron emission observed in the PS diodes formed on n+–Si substrates is caused by hot electrons tunnelling through the top contact. By an appropriate structural control of PS, the effective drift length under a high-field conduction is significantly increased, and then electrons are emitted ballistically. These functions reflect the activity of PS as a nanocrystalline confined system.

Vertical-cavity surface-emitting laser with a thin metal mirror fabricated by double implantation using a tungsten wire mask

We have devised a novel vertical-cavity surface-emitting laser structure. The epitaxial layers have been grown by molecular beam epitaxy (MBE). The bottom mirror consists of 30.5 pairs of multilayer distributed Bragg reflector (DBR). The top mirror is a hybrid reflector consisting of 8.5 pairs of multilayer and a semi-transparent metal film. The structure is obtained by two He ion implantations using tungsten wires as an implant mask. The lowest threshold current is 8.2 mA, and the highest peak light output power is 1.8 mW under room-temperature cw conditions.