Serial Bideposition Research Articles

Polarization-universal bandgaps realized with columnar thin films

Equichiral sculptured thin films (ECSTFs) with unit cells comprising a sequence of four identical columnar thin films were fabricated using asymmetric serial bideposition to exhibit the polarization-universal Bragg phenomenon. Oblique-angle optical transmission measurements of the ECSTFs showed bands of total transmittance values under 20%, regardless of the polarization state of the incident plane wave. These polarization-universal bandgaps can be tuned by adjusting the angle of incidence.

Multilayer thin-film flake dispersion gel for surface-enhanced Raman spectroscopy

Ag nanorod arrays/dielectrics/mirror-structured multilayer thin-film are well known, highly sensitive surface-enhanced Raman scattering (SERS) substrates that enhance the Raman scattering cross-section by the interference of light. However, extracting biomarkers directly from human skin using these solid substrates is difficult. To overcome this problem, we propose a multilayer thin-film flake dispersion gel by centrifugal mixing of the multilayer thin-film and hydroxyethyl cellulose (HEC) gel. The multilayer thin-film was prepared by serial bideposition using the dynamic oblique angle deposition technique. The mixing process was optimized to obtain flakes of ~10 {\mu}m so that the optical properties of the multilayer film can be preserved, and there is no risk of adverse effects on humans. The SERS features of the flakes dispersion gel were tested using 4, 4'-bipyridine (BPY). The BPY molecules diffused through the highly porous gel within a few seconds, generating significant SERS signals. The multilayer film flakes dispersion gel showed a SERS signal about 20 times better than the gel-dispersed Ag nanorod arrays without a multilayer film structure. These SERS active flakes dispersion gel can be used directly on the skin surface to collect body fluids from sweat, for biomarker sensing.

Plasmonic Nanoslit Arrays Fabricated by Serial Bideposition: Optical and Surface-Enhanced Raman Scattering Study.

Recently, studies have been carried out to combine surface-enhanced Raman spectroscopy substrates that are based on either localized surface plasmon or surface plasmon polariton structures. By combining these two systems, the individual drawbacks of each can be overcome. However, the manufacturing methods involved so far are sophisticated, labor-intensive, expensive, and technically demanding. We propose a facile method for the fabrication of a flexible plasmonic nanoslit surface-enhanced Raman scattering (SERS) sensor. We utilized the pattern on periodic optical disks as an inexpensive substitute for printing the periodic pattern on polydimethylsiloxane with soft imprint lithography. The Ag nanoslits were fabricated by serial bideposition using the dynamic oblique angle deposition technique. The nanoslit structures were physically and optically characterized, and the experimental results were compared to the results of the numerical simulation: Monte Carlo and finite-difference time-domain simulation. The AgNS samples showed excellent SERS performance with an enhancement factor of ∼105 and a limit of detection of 5 × 10-7 g/mL for a Rhodamine 6G solution. Their biosensing capability was demonstrated by the sensing of bilirubin.

Dual‐band circular‐polarization filter for obliquely incident light

ABSTRACTChiral sculptured thin films of zinc selenide with different helical periods were deposited on both sides of a glass slide using the serial bideposition technique in order to fabricate a dual‐band circular‐polarization filter polarizer that exhibits two distinct circular Bragg regimes for a wide range (about 45°) of the angle of incidence. This device can be angle tuned within a ±8% spectral range. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2381–2384, 2016

Low-stress phase plates produced by serial bideposition of TiO2thin films

The stresses in thin films are one of the main problems in the development of small dimensions or thin optical components. In order to produce low-tension anisotropic coatings, the characterization of serial bideposition is essential. Structural and optical performance have been tested for anisotropic TiO2 thin films deposited by serial bideposition technique. Three-dimensional surface interferometric measurements were performed for samples deposited at various angles. Also experimental observations of anisotropic tensions in annealed two-dimensional sculptured thin films of TiO2 material are presented. The deposition angle of 80 deg is considered to be the most applicable for developing optical components due to the befitting optical performance and the lowest tensions before and after the annealing.

Optical properties and microstructures of inorganic wave plates prepared by serial bideposition

We investigated the optical characteristics and microstructures of wave plates com- posed of Ta2O5ð100 − x Þþ TiO2ðxÞ and prepared by the so-called serial bideposition technique. While a single-layer film prepared by conventional oblique deposition technique has a tilted columnar structure (i.e., tilted optical axis), a serial bideposition film has a narrow, long col- umnar structure; this ensures that the optical axis of the film is along the quasinormal to the substrate, thus reducing haze. The influence of using additives with Ta2O5 was investigated as well. It was found that additive TiO2 improves optical transmittance at shorter wavelengths. For verifying the advantage of this type of wave plates, quarter wave plates with optimized TiO2 content were fabricated and their optical performance and reliability were evaluated against those of organic-type wave plates. The results show that the inorganic wave plate prepared by serial bideposition is advantageous for applications where high-transmittance and high-temperature durability are essential. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.JNP.8.083991)

Suppression of circular Bragg phenomenon in chiral sculptured thin films produced with simultaneous rocking and rotation of substrate during serial bideposition

Chiral sculptured thin films (STFs) produced by substrate rotation during physical vapor deposition exhibit the circular Bragg phenomenon, whereby normally incident left- and right-circularly polarized plane waves are discriminated in a spectral regime called the circular Bragg regime. Theory had predicted that substrate rocking, in synchrony with substrate rotation, during deposition could suppress the propensity to exhibit the circular Bragg phenomenon. Therefore chiral STFs of a dielectric material were fabricated with/without substrate rocking, and their transmittance spectrums for incident linearly and circularly polarized plane waves were measured. With sufficient rocking amplitude, the discrimination between incident left- and right-circularly polarized light nearly vanished, whereas a Bragg phenomenon for all normally incident plane waves was observed. Thus, chiral STF technology can be used to produce both ordinary and circular-polarization Bragg filters.

Enhanced birefingence of MgF2 thin film at 193 nm by serial bideposition

Magnesium fluoride films were coated by serial bideposition with a varying number of sub-deposited layers. The deposition angle 70° remained constant throughout the process by a single electron-beam source. One, 4, 20, and 100 sub-deposition layers were prepared. The microstructure, residual stress, absorption of water, and linear birefringence of serial bideposition of anisotropic thin film at 193 nm were investigated and discussed. Microstructures of the films were observed by scanning electron microscopy. The residual stress was measured by phase-shifting interferometer. The water absorption bands of the thin films were measured by Fourier transform infrared (FTIR) spectroscopy, and refractive indices in two different directions (S- and P-polarized) were determined from a spectrometer equipped with a polarizer. The linear birefringence would increase and the stress would decrease when the sub-deposited layers were increased. When the number of layers was 100, the difference between the refractive indices for two different polarizations was able to reach 0.082.

Analysis of the optical properties and structure of serial bi-deposited TiO<sub>2</sub> chiral sculptured thin films using Mueller matrix ellipsometry

A titanium dioxide (TiO2) chiral sculptured thin film (STF) fabricated using a serial bi-deposition (SBD) method based on electron beam evaporation has been studied using spectroscopic Mueller matrix ellipsometry (MME). Complete Mueller matrices for the SBD TiO2 chiral STF have been measured using a dual-rotating compensator spectroscopic ellipsometer over the spectral range from 250 to 825 nm in transmission mode, both at normal incidence (i = 0°) and over a range of oblique angles (5° i 60°). A multilayer structurally-graded optical model has been applied to deduce spectra in the three principal indices of refraction that characterize the locally biaxial structure, using as input the complex amplitude transmission ratios deduced from the Mueller matrix measured at normal incidence. A Bragg resonance feature has been observed, and this feature blue-shifts with increasing angle of incidence. Predictions of the transmittance for circularly polarized light normally incident upon the SBD TiO2 chiral STF can be obtained simply by multiplying the unnormalized Mueller matrix by the appropriate Stokes vector, and the results are in excellent agreement with direct measurements.

Prospects for nanowire sculptured-thin-film devices

In this paper, we introduce a modified serial bideposition (SBD) method, asymmetric SBD, for depositing sculptured thin films (STFs) which may allow added control of the surface area, void accessibility, and optical properties of STFs. Results on using TiO2 STFs with spectral-hole filters as vapor/liquid sensors are presented. In addition, solar cell, microcavity, biological sensing, and bioabsorption devices utilizing STFs are discussed.

Fabrication of structurally chiral Sc2O3 films and observation of Bragg resonance at deep UV wavelengths

The authors report serial bideposition of structurally chiral films for deep ultraviolet (DUV) wavelengths. Scanning electron micrographs confirm that the Sc2O3 films have a twisted columnar nanostructure similar to a double-start screw. Observation of circular Bragg resonances in reflection and transmission at 230nm indicates a dielectric pitch of 65nm, an average refractive index of 1.75, and local in-plane linear birefringence of 0.05. Possible applications of DUV chiral media include novel thin film polarizing devices for use in the DUV spectral region, where conventional designs are currently severely constrained by material properties.

Birefringence enhancement in annealed TiO2 thin films

Postdeposition thermal annealing is used to enhance the form birefringence of nanostructured TiO2 thin films grown by electron-beam evaporation using the serial bideposition technique. Thin films were grown on fused silica substrates using oblique deposition angles between 60° and 75° and repetitive 180° substrate rotations to produce birefringent thin films that are structurally anisotropic. Postdeposition annealing in air, between 200 and 900°C, was used to increase the form birefringence of the films by changing the TiO2 phase from the as-deposited amorphous state to a polycrystalline state that exhibits a greater inherent density and larger bulk refractive index. The optical properties, microstructure, and crystallinity were characterized by Mueller matrix ellipsometry, scanning electron microscopy, atomic force microscopy, and x-ray diffraction. It was found that the in-plane birefringence increased significantly upon thermal annealing, in some cases yielding birefringence values that doubled in magnitude, from 0.11 to 0.22 at a wavelength of 550nm for films annealed at 400°C.

Biaxial thin-film coated-plate polarizing beam splitters

We present a design for a biaxial thin-film coated-plate polarizing beam splitter that transmits the p-polarized component of a beam of light without change of direction and reflects the s-polarized component. The beam splitter has a periodic structure and is planned for fabrication by serial bideposition in mutually orthogonal planes. Recent experimental data for form-birefringent silicon is used to establish the feasibility of the design for a beam splitter to be used at 1310 nm and at an angle of 45 degrees in air.

Blue-shifting of circular Bragg phenomenon by annealing of chiral sculptured thin films

The wavelength regime of the circular Bragg phenomenon, exhibited by chiral sculptured thin films fabricated using the serial bideposition technique, blue shifts as a result of post-deposition annealing. This blue shift can be attributed to the net effect of three material changes that occur during annealing: a small reduction in helical pitch, an increase in the relative permittivity of the column material changing from amorphous to crystalline, and a density reduction due to columnar thinning from the same amorphous-to-crystalline transition.

Growth and characterisation of birefringent films on textured silicon substrates

The effects of textured substrates on the serial bi-deposition (SBD) technique of thin film growth have been investigated. Obliquely deposited coatings exhibit optical anisotropy as a consequence of the internal nanostructure developed during self-shadowed growth, which should be enhanced by growth onto textured substrates. We examine the growth and some subsequent optical properties of SBD coatings deposited obliquely onto lithographically-defined fine line and dot arrays with periods as small as 100 nm. Reflection retardance optical measurements show clear differences over a range of wavelengths for the structures in the patterned areas.

Enhanced birefringence in vacuum evaporated silicon thin films

We report an experimental study of enhanced optical birefringence in silicon thin films on glass substrates. Form anisotropy is introduced as an atomic-scale morphological structure through dynamic control of growth geometry. The resulting birefringence is large compared with naturally anisotropic crystals and is comparable to two-dimensional photonic crystals. The films are fabricated with serial bideposition onto a substrate held at a fixed tilt angle relative to the impinging vapor. Films were analyzed by spectroscopic ellipsometry and scanning electron microscopy, the latter clearly revealing form anisotropy in a morphology of bunched columns perpendicular to the deposition plane with dimensions of hundreds of nanometers and smaller. The observed linear birefringence varies with wavelength and tilt angle, with a maximum of 0.4 at a 630-nm wavelength and 0.25 at 1500 nm.

Inorganic positive uniaxial films fabricated by serial bideposition

The physical vapor deposition process of serial bideposition is adapted to the fabrication of uniaxial optical coatings. During the coating process the vapor impinges at an angle of incidence of about 70 on to the substrate, and a stepwise axial rotation with 90 increments causes a columnar structure to grow normal to the substrate. Symmetry considerations that follow from the choice of 90 for the stepwise increment ensure that the film is achiral and has negligible in-plane linear birefringence. Optical characterization techniques confirm that films of tantalum oxide, titanium oxide and zirconium oxide are positive uniaxial with ne -no in the range 0.10 to 0.14.

All-silicon polarizing filters for near-infrared wavelengths

We discuss the design and fabrication of optical filters that are transparent to a preferred linear polarization, but are partially or substantially reflective to the orthogonal linear polarization. The filters are fabricated by depositing thin films of form-birefringent silicon onto silicon wafers using the serial bideposition method. Characteristic curves that are presented for the in-plane principal refractive indices np and ns of the porous films as functions of vapor impingement angle facilitate critical aspects of the filter design. These include matching of a p principal index and an s principal index for films deposited at different angles, and realization of p and s principal indices that satisfy the antireflection condition for the silicon substrate in air.

Bideposited thin-film retardation plates for use at deep UV wavelengths

The physical vapour deposition process of serial bideposition is used to fabricate inorganic retardation plates of LaF 3 , NdF 3 , Sc 2 O 3 and SmF 3 . For a deposition angle of 70° the form-birefringent materials exhibit linear birefringence in the range 0.09–0.13 at wavelength 248 nm. The study shows that NdF 3 and Sc 2 O 3 are unsuitable for use at 193 nm, due to excessive absorptive loss, and that LaF 3 is the preferred material.

On calibration of a nominal structure–property relationship model for chiral sculptured thin films by axial transmittance measurements

A chiral sculptured thin film is fabricated from patinal titanium oxide using the serial bideposition technique. Axial transmittance spectrums are measured over a spectral region encompassing the Bragg regime for axial excitation. The same spectrums are calculated using a nominal structure–property relationship model and the parameter space of the model is explored for best fits of the calculated and measured transmittances. Ambiguity arising on calibrating the model against axial transmittance measurements is shown to be resolvable using non-axial transmittance measurements.