Strong Optical Interference Research Articles

Dynamically transflective multicolor modulation via single metal-dielectric inorganic electrochromic electrode

Transflective electrochromic (EC) materials with the ability to dynamically adjust color in both reflection and transmission mode are extremely attractive for tunable filter, smart windows, and ultralow-power displays due to their wide applicability in dark and bright environment and unique tunability of optical properties. However, simultaneously brilliant multicolor modulation of both reflection and transmission in one EC film is challenging. Here, integrated and metal/dielectric structured EC electrode composed of three-layered Cr/Ag/WO3 (CAW) films are demonstrated. Benefiting from the synergistic effect of the good conductivity, strong optical interference, and excellent substrate-film adhesion, these well-designed CAW films successfully demonstrate dynamic variegated modulation of both reflective and transmissive colors. This modulation encompasses a broad spectrum of hues, boasts high saturation, rapid response times, and exhibits impressive cycling stability. The flexible CAW film with superior bending stability is also achieved for the successful construction of the patterned flexible EC device.

Enhanced Broadband Light Harvesting in Ultrathin Absorbers Enabled by Epitaxial Stabilization of Silver Thin Film Mirrors.

Silver thin film mirrors are attractive candidates for use as specular back reflectors to enhance broadband light absorption via strong optical interference in ultrathin film semiconductor photoabsorbers. However, deposition of metal-oxide absorbers often requires exposure to high temperature in an oxygen atmosphere, conditions that cause thermal etching and degrade the specular reflectance of silver films. Here, we overcome this challenge and demonstrate that epitaxial growth of silver mitigates thermal etching under the high-temperature oxygen-containing environments that cause polycrystalline films to degrade. The degree of thermal etching resistance is related to the epitaxial film structure, where high-quality films completely prevent thermal etching, allowing for direct deposition of metal-oxide thin film photoabsorbers at elevated temperatures without any degradation of the optical properties of the silver layer. As a proof of concept for device applications, a metal-oxide photoanode for photoelectrochemical water splitting is fabricated by directly growing epitaxial SnO2 and Ti-doped α-Fe2O3 (hematite) thin films onto stabilized silver reflectors by pulsed laser deposition. The photoanode displays enhanced broadband light absorption due to strong interference effects enabled by the highly reflective silver film and demonstrates stable operation in a photoelectrochemical cell under conditions of water photo-oxidation in alkaline electrolyte.

Plasmonics and Antenna Effect Mediated Super Absorption in Ultrathin Film via Core/Shell Design

The conventionally low-absorbing ultrathin film is recently shown to obtain a high absorption via a strong optical interference effect by coating the highly absorbing film onto a metal layer. In this letter, through finite-element simulation and modal analysis, we report that the optical performance of an ultrathin semiconductor film can be significantly improved by further nanostructuring the ultrathin semiconductor film. Through wrapping the film onto a metallic nanowire in either lying or vertical configuration, a high dielectric resonance antenna effect and a strong plasmonic resonance can be excited simultaneously, which enables an extremely high and broadband optical absorption. Especially, with introducing the vertical metal-core/semiconductor-shell design, the peak absorption can be up to >300% under an extremely high field-confinement effect, leading to an integrated photocurrent density, which can be 25 times higher than that of the planar system. The proposed scheme is useful for the realization of compact, low-cost, and highly absorbing systems for photon absorbing, solar-energy harvesting, and photodetection applications.

Magneto-optical Kerr rotation and color in ultrathin lossy dielectric

Ultra-thin optical coating comprising nanometer-thick silicon absorbing films on iron substrates can display strong optical interference effects. A resonance peak of longitudinal Kerr rotation with the silicon thickness of was found at the wavelength of 660 nm. The optical properties of silicon thin films were well controlled by the sputtering power. Non-iridescence color exhibition and Kerr rotation enhancement can be manipulated and encoded individually.

Optical thermometry for high temperature multiphase environments under high-flux irradiation

A non-intrusive optical method to measure gas phase temperature in strongly scattering multiphase environments under high-flux, broad-band irradiation, relevant to conditions in high temperature solar reactors was developed and demonstrated. The high-flux irradiation with a peak flux of 450kW/m2 was provided by a 6kW metal-halide lamp coupled with a reflector and two concentrators. An ethylene/air diffusion flame, which contains fine soot particles, was employed to provide a high temperature reacting flow (approximately 1800K) with strong optical interference from nano particles having a peak soot volume fractions of ∼16ppm (with irradiation) under conditions of relevance to solar reactors. Under this environment, the proposed laser-based thermometry technique, line-wise two-line atomic fluorescence (TLAF) has been successfully demonstrated to measure flame temperature with good spatial resolution of ∼1mm. It was found that the measurement accuracy in the presence of particle and the high-flux external radiation is 65K at a typical flame temperature of ∼1800K, while the measurement precision is 38K. Results reveal that the presence of high-flux irradiation increases the flame temperature by typically 50–100K. This paper presents a thermometry technique that is suitable for temperature measurement within solar reactors, particularly in hybrid solar-thermal receiver-combustor systems. The experimental setup, measurement methodology and data processing are discussed, followed by the temperature measurements.

Large area Germanium Tin nanometer optical film coatings on highly flexible aluminum substrates.

Germanium Tin (GeSn) films have drawn great interest for their visible and near-infrared optoelectronics properties. Here, we demonstrate large area Germanium Tin nanometer thin films grown on highly flexible aluminum foil substrates using low-temperature molecular beam epitaxy (MBE). Ultra-thin (10–180 nm) GeSn film-coated aluminum foils display a wide color spectra with an absorption wavelength ranging from 400–1800 nm due to its strong optical interference effect. The light absorption ratio for nanometer GeSn/Al foil heterostructures can be enhanced up to 85%. Moreover, the structure exhibits excellent mechanical flexibility and can be cut or bent into many shapes, which facilitates a wide range of flexible photonics. Micro-Raman studies reveal a large tensile strain change with GeSn thickness, which arises from lattice deformations. In particular, nano-sized Sn-enriched GeSn dots appeared in the GeSn coatings that had a thickness greater than 50 nm, which induced an additional light absorption depression around 13.89 μm wavelength. These findings are promising for practical flexible photovoltaic and photodetector applications ranging from the visible to near-infrared wavelengths.

Effect of hole-transport-layer thickness on deep-blue emission in top-emitting cavity organic light-emitting diodes

The effect of hole-transport-layer thickness on deep-blue emission was experimentally and theoretically elucidated for top-emitting micro-cavity organic light-emitting diodes. As a result of strong optical interference in the micro-cavity, the emission spectrum and intensity from the top-emitting organic light-emitting diodes was strongly dependent on the viewing angle. Optimizing hole-transport-layer thickness lead to deep-blue emissions at a wavelength of about 460 nm with a current efficiency of 5 cd/A at 10,000 cd/m2 luminance. Light out-coupling in the top-emitting organic light-emitting diodes was modeled and calculated to show the power dissipation, and the model explained the experimental results well.

Bright visible light emission from graphene.

Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficiency emitter in the mid-infrared range. However, emission in the visible range has remained elusive. Here, we report the observation of bright visible light emission from electrically biased suspended graphene devices. In these devices, heat transport is greatly reduced. Hot electrons (∼2,800 K) therefore become spatially localized at the centre of the graphene layer, resulting in a 1,000-fold enhancement in thermal radiation efficiency. Moreover, strong optical interference between the suspended graphene and substrate can be used to tune the emission spectrum. We also demonstrate the scalability of this technique by realizing arrays of chemical-vapour-deposited graphene light emitters. These results pave the way towards the realization of commercially viable large-scale, atomically thin, flexible and transparent light emitters and displays with low operation voltage and graphene-based on-chip ultrafast optical communications.

Magneto-optical response of ferrimagnetic Tb-Fe thin films in the visible and ultraviolet range

We present wavelength-dependent measurements of Kerr rotation and ellipticity for ferrimagnetic Tb30Fe70 thin films in the optical range of 225 nm–726 nm (1.7 eV to 5.5 eV). The thicknesses of the magnetic layers vary from 19 nm to 83 nm. Strong optical interference effects in the magneto-optical spectra are observed for films grown on commonly used SiO2/Si substrates, which may easily be misinterpreted as spectral signatures of Tb. This strong optical interference feature can be fully suppressed by depositing magnetic Tb-Fe films on glass substrates.

Ultra-thin optical interference coatings on rough and flexible substrates

Recently demonstrated ultra-thin optical coatings comprising nanometer-thick highly absorbing films on top of reflecting substrates can display strong optical interference effects, resulting in structural colors and absorption enhancement. Here, we demonstrate that these optical interference effects persist when the films are deposited on substrates that have a large degree of roughness and inhomogeneity on micro- and nano-scales. In particular, we deposited films of gold and amorphous germanium onto paper which serves as a rough and flexible substrate and observe matte interference colors that vary as a function of the germanium thickness.

Minimizing optical losses in bulk heterojunction polymer solar cells

The efficiency that a solar cell can reach is ultimately limited by the number of photons absorbed in its active layer. Bulk heterojunction polymer solar cells are fabricated from a stack of thin film layers, each of which is thinner than a single wavelength from an incident photon within its absorption band. One consequence of this thin film layer stack is a strong optical interference between the various layers that can change the quantity of light dissipated in the active layer by 50%. Here we use optical modeling to quantitatively calculate the dissipation in each of the various layers as functions of wavelength and layer thickness. Using this information the loss free short circuit current density can be calculated (Jscmax). Optimization of Jscmax leads to direct improvements in the efficiency of the solar cell through improved light dissipation in the active layer. The optical properties for a P3HT:PCBM active layer and a model Lorentzian low band gap spectrum are optimized and ideal fabrication conditions are reported for these materials.

Optical constants of conjugated polymer/fullerene based bulk-heterojunction organic solar cells

The performance of organic solar cells consisting of multiple layers, which are a few hundred nanometers thick, is determined by strong optical interference effects. In order to model their optical and photoelectrical behavior, we determined the optical constants of all components of this system. This was done by fitting model dielectric functions to reflection and transmission spectra of all layers in the solar cell device. We put a special emphasis on understanding the optical behavior of the photoactive bulk heterojunction film, which consists of a composite of semiconducting polymers with fullerenes.

In situ reflectance anisotropy studies of AlxGa1−xAs layers grown on GaAs(001) by molecular beam epitaxy

Reflectance anisotropy spectroscopy was used to examine the surfaces of AlxGa1−xAs layers grown on GaAs(001) by molecular beam epitaxy, where the Al mole fraction was varied across the whole composition range x=(0.0,0.25,0.50,0.75,1.0). All surfaces were also independently characterized using reflection high-energy electron diffraction, and were found to exhibit a c(4×4) reconstruction. After initial changes in the spectra were observed on depositing very thin layers (≤20 monolayers), in the intermediate thickness range a regime was entered in which strong optical interference effects appeared. These effects are accurately accounted for using a four-media model. For thicker layers (≥8000 monolayers), interference effects were seen to diminish and spectra representative of the surfaces of bulk AlxGa1−xAs were obtained.

Production of optically thin free-standing oil films from the edge of a rotating disc

Abstract A method is described for forming thin free-standing oil films which are spun from the edge of a sharp-edged rotating disc. The films can be made thin enough to show strong optical interference colors when viewed in white light. The thinnest films have areal densities down to about 10–20 μg/cm 2 . A stable roughly triangular film with an area of about 10 cm 2 and fairly uniform thickness can be readily produced. Much larger films having either greater thickness or less stability are also possible. Films have been produced both in air and in vacuum.

Interference effects in laser micromachining of thin films on silicon

Some recent designs of new electronic circuits on silicon have been implemented by laser removal of thin metallic film from the composite structure. The presence of an insulating dielectric region between the thin metallic film and the silicon substrate gives rise to a complex laser-micromachining process whose understanding is crucial to successful applications. In this paper we explore the laser-machining process at 1.06 μm for a tantalum nitride thin-film resistors material on top of a dielectric film on silicon. We experimentally verify the existence of strong optical interference effects in laser machining of these resistors and determine their influence on various laser-trimming parameters. Good correlation is obtained between an analytical model for these interference effects and the experimental data. For structures containing a single SiO2 layer with thickness up to 1 μm between the tantalum film and silicon, it is observed that from a laser-trimming standpoint, a SiO2 thickness of 9000 or 5400 Å is preferred to all others. A selection of either one of these thickness will lead to a wider laser-trimming-process window. These results could, therefore, be viewed as structure design guidelines for laser trimming by material removal at 1.06 μm.