Thin Film Optical Devices Research Articles

Effectivity of plasma etching on template removal of reverse micelle deposited nanoparticles

Polymer templated nanoparticles, such as with diblock copolymers, are useful for improving thin-film optical devices by the modification of interfaces between different layers. The existence of an insulating polymer is a challenge to the application of templated nanoparticles. In this paper, we address this issue by assessing the effectivity of gas plasmas in the removal of the polymer templates from a thin film of loaded diblock copolymer reverse micelles to produce iron oxide nanoparticles. Using Raman spectroscopy, we have described a method that can be used to evaluate polymer removal effectivity which can be employed equally for different systems, and was used to quantify other polymer removal methods such as UV/ozone exposure and electron bombardment. We found that a mixture of reactive ion etching and ionic bombardment is highly effective for oxide materials and can be accomplished by leaking air in pure gas plasmas.

Active Tuning of Optical Constants in the Visible–UV: Praseodymium‐Doped Ceria—a Model Mixed Ionic–Electronic Conductor

AbstractMixed ionic–electronic conductors offer chemical and electrical means for active tuning of their optical constants, e.g., with variations in oxygen non‐stoichiometry in Pr0.1Ce0.9O2–δ, enabling implementation of adaptive thin film optical devices. In situ chemo‐tuning of the extinction coefficient in Pr0.1Ce0.9O2–δ at elevated temperatures is demonstrated and a tuning model is provided that treats the interdependence of mobile oxygen vacancies and small polarons coupled to variations in optically active praseodymium ions. Furthermore, a new means for electro‐tuning of the optical constants of mixed ionic–electronic conductors is demonstrated experimentally and modeled for Pr0.1Ce0.9O2–δ thin films deposited on grid‐like electrode structures. Modeling of non‐steady‐state optical transmittance modulations in the latter allows for estimation of oxygen vacancy mobility that determines the switching speed of the device. Quenched‐in values of nr and k to room temperature become nonvolatile, providing a modulation range in the extinction coefficient of Δk ≈ 0.1 (change of ≈800%) and in the refractive index of Δnr ≈ 0.1 (relative to initial nr of ≈2.35). Key figures of merit, including transmission optical modulation of ≈0.04 per 1 mV nm–1, switching energy per area of 1.9 nJ µm–2, and switching times of seconds, are demonstrated, with further improvements possible.

Experimental Study on the Thickness-Dependent Hardness of SiO2 Thin Films Using Nanoindentation

SiO2 thin films are widely used in micro-electro-mechanical systems, integrated circuits and optical thin film devices. Tremendous efforts have been devoted to studying the preparation technology and optical properties of SiO2 thin films, but little attention has been paid to their mechanical properties. Herein, the surface morphology of the 500-nm-thick, 1000-nm-thick and 2000-nm-thick SiO2 thin films on the Si substrates was observed by atomic force microscopy. The hardnesses of the three SiO2 thin films with different thicknesses were investigated by nanoindentation technique, and the dependence of the hardness of the SiO2 thin film with its thickness was analyzed. The results showed that the average grain size of SiO2 thin film increased with increasing film thickness. For the three SiO2 thin films with different thicknesses, the same relative penetration depth range of ~0.4–0.5 existed, above which the intrinsic hardness without substrate influence can be determined. The average intrinsic hardness of the SiO2 thin film decreased with the increasing film thickness and average grain size, which showed the similar trend with the Hall-Petch type relationship.

Impact of hydrogenation and annealing on CdSe/ZnS thin film for solar cell application

This research paper reports the importance of CdSe/ZnS thin film in improving optical and electrical properties. These thin films were prepared on a deeply cleaned glass substrate using vacuum thermal evaporation under the pressure of 10–5 torr. These films were arranged into a quartz glass tube for rapid thermal annealing (RTA) at 60 and 120 s using a 500Watt halogen lamp. These films were also arranged in a hydrogen gas chamber at 100psi for hydrogenation. The Thermoelectric properties (TEP) and the Seebeck coefficient increase with increasing annealing time, also with introducing hydrogenation it was growing a lot. The UV–VIS–NIR spectrometer has been used to investigate diversity in the energy band gap by tuning annealing and hydrogenation in the range 300–850 nm. The X-ray diffraction (XRD) have been taken in scope 10° ≤ 2θ ≤ 80°, its outcome conform simple cubic structure with lattice parameter 5.4 and 5.04 A for CdSe and ZnS, respectively. The scanning electron microscope (SEM) results investigate that the crystallinity and grain size increase with annealing time. This work is essential in thin film optical devices and solar cell applications.

Thin-Film Optical Devices Based on Transparent Conducting Oxides: Physical Mechanisms and Applications

This paper provides a review of optical devices based on a wide band-gap transparent conducting oxide (TCO) while discussing related physical mechanisms and potential applications. Intentionally using a light-induced metastability mechanism of oxygen defects in TCOs, it is allowed to detect even visible lights, eluding to a persistent photoconductivity (PPC) as an optical memory action. So, this PPC phenomenon is naturally useful for TCO-based optical memory applications, e.g., optical synaptic transistors, as well as photo-sensors along with an electrical controllability of a recovery speed with gate pulse or bias. Besides the role of TCO channel layer in thin-film transistor structure, a defective gate insulator can be another approach for a memory operation with assistance for gate bias and illuminations. In this respect, TCOs can be promising materials for a low-cost transparent optoelectronic application.

Liquid-Phase Epitaxial Growth and Characterization of Nd:YAl3(BO3)4 Optical Waveguides

We investigated the fabrication of neodymium doped thin film optical waveguide-based devices as potential active sources for planar integrated optics. Liquid-phase epitaxial growth was used to fabricate neodymium-doped yttrium aluminum borate films on compatible lattice-matched, un-doped yttrium aluminum borate substrates. We observed the refractive index contrast of the doped and un-doped crystal layers via differential interference contrast microscopy. In addition, characterization by X-ray powder diffraction, optical absorption and luminescence spectra demonstrated the crystal quality, uniformity and optical guiding of the resulting thin films.

Depletion-Mediated Interfacial Assembly of Semiconductor Nanorods.

Electronic devices comprised of nanocrystal (NC) thin film are projected to demonstrate enhanced figure of merit if NC building blocks self-assemble into highly uniform, 2-dimensional (2-D) superstructures with long-range order. Despite intensive research efforts and remarkable progress, long-range assembly of colloidal anisotropic NCs into thin films with orientational and positional order has remained to be addressed. One of the most promising approaches is to dissolve excess free molecules into NC solution, which has enabled the formation of NC monolayers with exceptional quality at air/solution interface. Nevertheless, the assembly mechanism and the role of free molecules have not been comprehensively elucidated, restricting the use of the approach. Here, we find that the interfacial assembly of CdSe/CdS core/shell nanorods (NRs) results in various ordered structures in the presence of free oleic acid molecules. The structures include a bundle of standing NRs, a belt of multilayered lying NRs, and a monolayer smectic phase, obtained by simple change in density of surface ligands on the NRs. Experimental observation and theoretical calculation reveal that the assembly is initiated at the air/solution interface due to the preferential depletion attraction of NRs to the interface. However, subsequent growth is significantly altered depending on the ligand density that determines the relative magnitude of interface-NR depletion attraction to inter-NR attraction. Highly ordered structures of NRs, especially for the monolayer smectic phase, are promising as a polarized light-emitting layer for thin-film optical devices. In addition, our findings on the depletion-mediated NR assembly provide important and universal design criteria for 2-D structuring of NCs with diverse geometries and compositions.

Robust design of topology-optimized metasurfaces

Topology-optimized metasurfaces are thin film optical devices that have received much interest because they support ultra-high diffraction efficiencies. An important design consideration is ensuring that devices are insensitive to imperfections arising from realistic fabrication processing. We show that topology-optimized metasurfaces can be made robust by incorporating the performance of geometrically eroded and dilated devices directly into the iterative optimization algorithm. We additionally apply topology optimization to refine conventional designs and make them more robust. Unexpectedly, we find that devices robust to systematic erosion and dilation variations are also robust to random periodic perturbations. An analysis of the optical modes of robust devices indicates that robustness is enforced via highly complex and non-intuitive interactions between these modes and cannot be enforced using simple design rules. These concepts can more generally address other fabrication imperfections, such as thickness and refractive index variation, and can extend to other diffractive and nanophotonic platforms.

Epitaxial ferroelectric oxide thin films for optical applications

Ferroelectrics are non-centrosymmetric crystalline materials that possess a spontaneous polarization that can be switched by an electric field. The electric-field-dependent optical response of these materials makes them important for optical devices, such as modulators or beam deflectors. In the inexorable drive to miniaturization, the concept of integrated thin film optical devices has led to the incorporation of ferroelectric thin films on single-crystal substrates. These structures have appealing electro-optic modulation characteristics, interesting strain-dependent bandgaps and refractive index, as well as promising possibilities for solar harvesting. Here, we review the work on epitaxial ferroelectric (FE) films for optical applications. We first show that FE thin film materials are attractive for integrated electro-optic modulators and then show that epitaxial strain can be used to enhance the FE and optical functionality of films. Next, we describe some of the photovoltaic functionality of FE thin film materials' systems and conclude the review by highlighting some thin-film devices that exploit the aforementioned optical effects.

Periodic Dielectric Metasurfaces with High‐Efficiency, Multiwavelength Functionalities

AbstractMetasurfaces are thin‐film optical devices for tailoring the phase fronts of light. The extension of metasurfaces to multiple wavelengths has remained a major challenge, and existing design techniques do not yield devices with high efficiency. This study reports a new design method, based on inverse freeform optimization, that enables high‐efficiency, multiwavelength metasurfaces. Using an iterative optimization solver, this study incorporates multiple wavelength responses into wavelength‐scale design domains in a straightforward and automated manner. In principle, this method can readily scale to a very large number of wavelengths. As a proof of concept, this study designs and characterizes periodic transmissive metasurfaces, made from silicon, that deflect N different incident near‐infrared wavelengths to N unique diffraction orders. The theoretical and experimental efficiencies of these devices scale as 1/N0.5, which is significantly better than current state‐of‐the‐art devices. The implementation of large‐angle, broadband blazed grating devices is also demonstrated. This study envisions that this inverse design method can generalize to high‐performance, multiwavelength, aperiodic devices, and that it serves as a potential route to broadband metasurfaces.

Six Degrees of Separation: Connecting Research with Users and Cost Analysis

Shreya H. Dave graduated with her PhD in Mechanical Engineering from MIT. Her research focused on the design and manufacture of graphene oxide membranes for water desalination, including fundamental characterization methods of graphene oxide, membrane synthesis, and economic analysis of the role of membranes in cost constraints of desalination plants. She also holds Bachelor's and Master's degrees from MIT in Mechanical Engineering and Technology & Policy. Brent D. Keller completed his PhD in Materials Science & Engineering working with Professor Jeffrey C. Grossman on novel materials development for electronic devices. His work includes the first ever investigation of natural carbon materials such as coal and asphaltenes for thin film electronic and optical devices. He also led an industry and academic collaboration to improve and better understand atomic-layer-deposition-based approaches to two-dimensional semiconductor synthesis. He is a graduate of the University of Minnesota Twin Cities and holds degrees in Chemistry and Chemical Engineering. Karen Golmer is currently the Innovator in Residence at the MIT Deshpande Center. She is an innovative industry leader who facilitates collaboration to advance new technologies, identify technical challenges, and find solutions to mitigate business risks. Karen is a chemist with an MBA and 35 years of corporate experience from lab professional to global program director with companies including GE, Ecolab, Calgon, Diversey, and Kodak. Jeffrey C. Grossman is a Professor in the Department of Materials Science and Engineering at the Massachusetts Institute of Technology. He received his PhD in theoretical physics from the University of Illinois, performed postdoctoral work at U.C. Berkeley, and was a Lawrence Fellow at the Lawrence Livermore National Laboratory. Dr. Grossman's group uses a combination of modeling and experiment to gain fundamental understanding, develop new insights based on this understanding, then use these insights to develop new materials with improved properties for energy conversion, energy storage, and membrane-based separation technologies.

Preparation of random stamps for thermal nanoimprint

Tailoring of surfaces or interfaces by patterning is a well-known approach used for the improvement of optical and electrical properties of thin film optical devices, and nanoimprint is a strong candidate to realise this at low cost over large areas. For such purposes we provide stamps with random texture. Contamination masking is used instead of lithography to further reduce the costs. Reactive ion etching in SF6/O2 at high oxygen content is effective to provide pyramidal structures of about 300-500nm in height. Loading affects the etch result and has to be considered when establishing a process suitable for large area processing. The geometry of the pyramids is characterised by SEM and AFM measurements, and also by thermal imprint to show the shape of the grooves in between the pyramids. Optical characterisation in a single-axis goniometer-type set-up documents a substantial broadening of the angular distribution of the intensity of the reflected light compared to a flat surface, well suited for the purpose envisaged.

Microstructural and optical investigations of Ce-doped barium titanate thin films by FTIR and spectroscopic ellipsometry

(BaTiO3)1− x (CeO2) x (with x = 0, 1, 3, 5.7 and 16 at.%) thin films were prepared by electron beam evaporation method. According to X-ray diffraction, nanosized and amorphous Ce-doped barium titanate thin films were obtained. The crystallinity diminished as Ce content increased. The increase in the Ce slightly shifts the absorption peaks at 433–450 and 416–424 cm−1 to higher wavenumbers. A two-layer model was used to describe the experimental ellipsometric data. The Bruggeman effective medium approximation (BEMA) was used to describe the surface roughness layer and the Cauchy dispersion relation was used to describe the main Ce-doped barium titanate layer. The refractive index and the extinction coefficient were found to increase with increasing Ce content; however, the optical band gap decreased with increasing Ce content. The accurate determination of the optical constants of Ce-doped barium titanate thin films may favor them in the applications of optical thin film devices.

Nanostructured optical thin films fabricated by oblique angle deposition

Oblique angle deposition (OAD) is a sophisticated technique to fabricate engineered nanostructured thin films for next generation optical nanodevices. In this technique, oblique angle deposition and substrate rotation are employed to control the columnar and helical nanostructures of thin films. The films deposited by this technique show the optical anisotropy, the porosity, or the chirality, depending on the controlled morphologies at the nano-scale. In this review paper, the nanostructured optical thin film devices, such as a circular polarization handedness inverter, a linear polarization-discriminatory inverter and the selective coatings on nanopatterns, are fabricated by electron beam evaporation using the OAD technique, and their optical and structural properties as nanooptical devices are described.

Composition dependence of optical constants in ferroelectric Ba(Ti 1 − x,Ni x)O 3 thin films by optical transmittance technique

Ba(Ti 1 − x ,Ni x )O 3 ferroelectric thin films with perovskite structure are prepared on fused quartz substrates by a sol–gel process. Optical transmittance measurements indicate that Ni-doping has an obvious effect on the energy band structure of BaTiO 3. It has been found that the refractive index n, extinction coefficient k, and band gap energy E g of the films are functions of the film composition. The E g of Ba(Ti 1 − x ,Ni x )O 3 decreases approximately linearly as the Ni content increases, which is attributed to the decline of conduction band energy level with increasing the Ni content. On the other hand, n and k both increase linearly with increasing the Ni content because of the increase of packing density. These results indicate that thin films might have potential applications in BaTiO 3-based thin-film optical devices.

Developing Langmuir–Blodgett strategies towards practical devices

Several industrially important aspects of functionalized surfaces are discussed including applications to artificial membranes, patterning materials, thin film optical devices, and displays. Special attention is focused on the design of nanostructured surfaces by update of Langmuir–Blodgett method. The potentialities of the method of horizontal precipitation of monolayer films for the formation of artificial membranes are demonstrated, and the formation of “raft” domains resulting in the self-organization of lipids in a monolayer is reported. Also, many approaches to flexible polymeric support modification using specially developed LB equipment are described, including examples dealing with the effective ways of capsulation of drugs, formation of hybrid structures, and surface patterning, from the standpoint of medical or plastic electronic applications. © 2009 Elsevier B.V. All rights reserved.

Spectroscopic and electrical detection of intermediate phases and chemical bonding self-organizations in (i) dielectric films for semiconductor devices, and (ii) chalcogenide alloys for optical memory devices

This paper presents a discussion of intermediate phases in thin film materials that have been incorporated into liquid crystal displays, LCDs, and optical memory thin film devices. The formation of intermediate phases in the a-Si 3N 4:H (a-Si:N:H) alloys used for gate dielectrics in thin film transistors, TFTs, of LCDs, and the a-Ge–Sb–Te (GST) alloys used for read-write optical writing and storage in optical memory discs are qualitatively different than those first addressed by the Boolchand group in Ge–Se bulk glass alloys. In the a-Si:N:H and a-GST thin films, the chemical self-organizations that suppress percolation of strain, involve chemically-ordered bonding arrangements that break bond bending constraints at the four-fold coordinated Si and Ge atoms in a-Si:N:H and a-GST, respectively. In the GST alloys, this results in over-coordinated and under-coordinated atomic constituents, or valence alternation pairs, VAPs, of charged defects. Finally, other technologically important systems in which broken constraints, and/or VAP defects are important in intermediate phase formation include group IVB (Ti, Zr and Hf) Si oxynitride alloys, and hydrogenated amorphous Si (a-Si:H).

Studies on single layer CeO 2 and SiO 2 films deposited by rotating crucible electron beam evaporation

High index CeO 2 films and low index SiO 2 films are widely used in multi layer optical thin film devices, such as high reflectors, interference filters, anti reflection coatings, etc. Optical properties, such as refractive index, transmittance in the UV, Visible and IR depends upon the deposition conditions, especially the substrate temperature. Single layer films of CeO 2 and SiO 2 have been deposited using a novel rotating crucible electron beam evaporation technique. The refractive indices and UV, VIS and IR transmittance of the films have been measured by varying substrate temperature in the range ambient to 400 °C. The structure of CeO 2 films has also been studied. The refractive index of CeO 2 films at 550 nm increased from 2.00 to 2.41 as the substrate temperature was increased from ambient to 400 °C. The extinction coefficient of these films was negligibly small even at elevated substrate temperatures. CeO 2 films deposited even at ambient temperature were crystalline. The refractive index of SiO 2 films increased marginally from 1.46 to 1.48. UV, Visible and IR transmission characteristics of SiO 2 films indicate that the films are stoichiometric. The density of CeO 2 and SiO 2 films has also been estimated with substrate temperature.

Low-temperature dependence of midinfrared optical constants of lead–germanium–telluride thin film

The design and manufacture of diode lasers for gas analysis or multilayer thin-film optical devices used at low-temperature require the refractive index and the temperature coefficient of IV–VI compound over a significant temperature range. In this article, the refractive index and the absorption coefficient of Pb0.94Ge0.06Te thin film have been determined from transmission spectra measured at temperature between 80 and 300 K in the spectral range of 2.5–8.5 μm by fitting based on a Lorentz-oscillator model. It is found that the maximum refractive index occurs at 150 K, which corresponds to the structural phase transition from rocksalt to rhombohedrally distorted structure and reflects an increase of lattice polarizability. The value of the index of refraction is 5.350–6.000 in the spectral range of 4.0–8.5 μm for all measured temperatures, which reveals that Pb1−xGexTe is a highly refractive infrared material. The temperature coefficient of refractive index, dn/dT, is found to be −0.006–0.002 K−1 in the spectral range of 3.0–8.5 μm for all measured temperature. An empirical formula that fits the temperature coefficient in the spectral range of 4.0-8.5 μm is presented. The dependence of the transmission and absorption spectra on decreasing temperature can be explained by the modification of the energy-band structure due to rhombohedral distortions. The conclusion can be drawn that anomalies corresponding to the ferroelectric phase transition occur in both refractive index and absorption coefficient of Pb1−xGexTe alloy.

Optical loss mechanism in yttria thin film waveguides

It is well known that the imperfections in thin film optical devices would cause absorption, radiation, and scattering loss of the guiding beam, and such losses are mostly unavoidable. To reduce the propagation loss, the conditions for film deposition should therefore be optimized. Yttria (Y 2O 3) thin film was chosen for the study. Yttria thin film waveguide was deposited on silicon and thermally oxidized silicon wafers by reactive sputtering in a rare gas–oxygen discharge. The deposition process was controlled by the substrate temperature, total gas pressure, oxygen flow rate, and rare gases (Ar, Ne). The yttria thin films were characterized for the optical losses based upon the film structures. Relationships between deposition parameters and the propagation losses were discussed.