Photoluminescence of Cesium-Doped Sodium Iodide Films Irradiated by UV LED.

Alkali metal halides have long been used as scintillators for applications as sensors and detectors. Usually, a small amount of impurities are added to these inorganic materials to improve their luminescence efficiencies. We investigate the structures and luminescent properties of un-doped sodium iodide (NaI) and cesium-doped NaI (NaI:Cs) films deposited by thermal vacuum evaporation. Instead of using the toxic element thallium (Tl), we introduced cesium dopant into NaI. This is the first study for the NaI:Cs film excited by UV LED's ultraviolet C (273 nm, 4.54 eV). The luminescence spectra show two main peaks at 3.05 and 4.32/3.955 eV (for fused silica/B270 substrate), originating from the intrinsic defects and/or activator excited states and the intrinsic self-trapped excitons (STEs), respectively. In general, both Cs-doping and post-annealing processes enhance the luminescence performance of NaI films.

Editorial for the Special Issue "Applications of Optical Thin Films".

Optical thin films have been vital to enhancing optical performance for many years [...].

Highly Sensitive, Robust, and Recyclable TiO2/AgNP Substrate for SERS Detection.

Label-free biosensors provide an important platform for detecting chemical and biological substances without needing extra labeling agents. Unlike surface-based techniques such as surface plasmon resonance (SPR), interference, and ellipsometry, surface-enhanced Raman spectroscopy (SERS) possesses the advantage of monitoring analytes both on surfaces and in solutions. Increasing the SERS enhancement is crucial to preparing high-quality substrates without quickly losing their stability, sensitivity, and repeatability. However, fabrication methods based on wet chemistry, nanoimprint lithography, spark discharge, and laser ablation have drawbacks of waste of time, complicated processes, or nonreproducibility in surface topography. This study reports the preparation of recyclable TiO2/Ag nanoparticle (AgNP) substrates by using simple arc ion plating and direct-current (dc) magnetron sputtering technologies. The deposited anatase-phased TiO2 ensured the photocatalytic degradation of analytes. By measuring the Raman spectra of rhodamine 6G (R6G) in titrated concentrations, a limit of detection (LOD) of 10-8 M and a SERS enhancement factor (EF) of 1.01 × 109 were attained. Self-cleaning was performed via UV irradiation, and recyclability was achieved after at least five cycles of detection and degradation. The proposed TiO2/AgNP substrates have the potential to serve as eco-friendly SERS enhancers for label-free detection of various chemical and biological substances.

High Sensitivity SERS Substrate of a Few Nanometers Single-Layer Silver Thickness Fabricated by DC Magnetron Sputtering Technology.

Surface-enhanced Raman spectroscopy (SERS) is commonly used for super-selective analysis through nanostructured silver layers in the environment, food quality, biomedicine, and materials science. To fabricate a high-sensitivity but a more accessible device of SERS, DC magnetron sputtering technology was used to realize high sensitivity, low cost, a stable deposition rate, and rapid mass production. This study investigated various thicknesses of a silver film ranging from 3.0 to 12.1 nm by field emission scanning electron microscope, X-ray diffraction, and X-ray photoelectron spectroscopy. In the rhodamine 6G (R6G) testing irradiated by a He-Ne laser beam, the analytical enhancement factor (AEF) of 9.35 × 108, the limit of detection (LOD) of 10-8 M, and the relative standard deviation (RSD) of 1.61% were better than the other SERS substrates fabricated by the same DC sputtering process because the results showed that the 6 nm thickness silver layer had the highest sensitivity, stability, and lifetime. The paraquat and acetylcholine analytes were further investigated and high sensitivity was also achievable. The proposed SERS samples were evaluated and stored in a low humidity environment for up to forty weeks, and no spectrum attenuation could be detected. Soon, the proposed technology to fabricate high sensitivity, repeatability, and robust SERS substrate will be an optimized process technology in multiple applications.

Highly Reflective Silver-Enhanced Coating with High Adhesion and Sulfurization Resistance for Telescopes.

Highly reflective metal coatings are essential for manufacturing reflecting telescope mirrors to achieve the highest reflectivity with broad spectral bandwidth. Among metallic materials, enhanced silver-based coatings can provide higher reflectivity in the 400-500 nm spectral range to better performance from visible to near IR. Moreover, over-coating a dielectric protective layer on the mirror's front side attains additional hardness and oxidation stability. In this paper, we study a combination of thermal and electron beam evaporation as a technology to form protected enhanced high reflective Ag coatings. A newly designed multiplayer film can pass ASTM 5B adhesive performance testing and give sulfurization inhibition. The average specular reflectivity for the enhancement coating is about 98% in wavelengths across the spectral range from 400-1000 nm. This innovation has been demonstrated on a Newtonian type telescope, with storage in an ambiance humidity H = 60-85%, and temperature T = 10-35 °C, for more than six months without degradation in coating performance.

Fabrication of a Bare Optical Fiber-Based Biosensor.

A bare optical fiber-based biosensor is proposed for measuring the refractive index of different liquids and the binding kinetics of biomolecules to the sensor surface. This optical fiber sensor is based on the Kretschmann's configuration to attain total internal reflection (TIR) for surface plasmon resonance (SPR) excitation. One end of the bare optical fiber is coated with a gold film. By guiding the light source from the other end into the optical fiber, the light is reflected from the gold-deposited end and the surface evanescent wave is excited in the gold film-transparent material interface. Methanol and ethanol solutions with different refractive indices are used for measuring the corresponding changes in the peak values of the spectra and calculating the corresponding sensitivities. These values are experimentally determined to be in the order of 10-4~10-5 refractive index unit (RIU). Binding of proteins onto the sensor surface is also monitored in real time to obtain the binding kinetics. We believe that, in the future, this optical fiber sensor can serve as a useful biosensor for in situ measurement of allergens, antibody-antigen interactions, and even circulating tumor cells in the blood.

Integration of Curved D-Type Optical Fiber Sensor with Microfluidic Chip.

A curved D-type optical fiber sensor (OFS) combined with a microfluidic chip is proposed. This OFS, based on surface plasmon resonance (SPR) of the Kretchmann's configuration, is applied as a biosensor to measure the concentrations of different bio-liquids such as ethanol, methanol, and glucose solutions. The SPR phenomenon is attained by using the optical fiber to guide the light source to reach the side-polished, gold-coated region. Integrating this OFS with a polymethylmethacrylate (PMMA)-based microfluidic chip, the SPR spectra for liquids with different refractive indices are recorded. Experimentally, the sensitivity of the current biosensor was calculated to be in the order of 10-5 RIU. This microfluidic chip-integrated OFS could be valuable for monitoring subtle changes in biological samples such as blood sugar, allergen, and biomolecular interactions.

Analysis of the thickness uniformity improved by using wire masks for coating optical bandpass filters.

Layer uniformity was improved by using a wire mask in the fabrication of a coarse wavelength division multiplexer (CWDM) filter. Theoretical simulations determined the optimal diameter of the wire to be placed just below the substrate, which was rotated at 800 rpm during E-beam evaporation. This simulation also demonstrated the correction of the thickness distribution by the etching effect of ion-assisted deposition. In the corresponding experiment, a distribution uniformity of 0.083% in the radial range from 35 to 65 mm was achieved by the coating of the CWDM filter.

Spectroscopic ellipsometry studies on various zinc oxide films deposited by ion beam sputtering at room temperature.

Various zinc oxide films were deposited by ion-beam sputter deposition (IBSD) under different oxygen partial pressures (P(O2)) at room temperature. The as-deposited ZnO films fabricated at P(O2)>1.0×10(-4) Torr had poly-crystalline structures to absorb water on the surface at ambient condition. Simultaneously, the film surfaces were covered and smoothed by the surface layers formed with the water, hydroxyl (OH(-)) groups, and ZnO materials investigated by X-ray photoelectron spectroscopy (XPS). When the compositions of the surface layers were used in a multilayer fitting model of spectroscopic ellipsometry, the actual optical refractive index of the ZnO film deposited at P(O2)=1.2×10(-4) Torr was found to be about 1.9618 at λ=550 nm.

X-ray photoelectron spectroscopy study of thin TiO2 films cosputtered with Al.

In this study, titanium dioxide (TiO(2)) films were fabricated by cosputtering of a titanium (Ti) target and an aluminum (Al) slice in a smaller area by an ion-beam sputtering deposition method. The sputtered films were postannealed at 450 degrees C. The x-ray photoelectron spectroscopy spectra were categorized by their oxygen bonding variations, which include high-binding-energy oxygen, (HBO), bridging oxygen, low-binding-energy oxygen, and shifts of the binding energies (BEs) of oxygen (O) and Ti signals. The enhancement of HBO and higher BE shifts of the O 1s spectra as a function of cosputtered Al in the film imply the formation of an Al-O-Ti linkage. Corresponding changes in the Ti 2p spectra further confirm the modification of properties of the cosputtered film that results from the variation of the chemical bonding environment. An observed correlation between the chemical structure and optical absorption of the Al cosputtered films can be used to modify the optical properties of the film.

Coating uniformity improvement for a dense-wavelength-division-multiplexing filter by use of the etching effect.

The uniformity of optical narrow-bandpass filters for dense wavelength division multiplexing (DWDM) has been improved by control of the coating parameters of electron guns and the ion source. The optical film was deposited by the electron gun and was etched by the ion source during the ion-assisted deposition process. The uniformity of the coating of a 100 GHz DWDM filter is better than +/- 0.003% over a circular area of 50 mm in diameter when such an etching process is used.

Internal stress and optical properties of Nb2O5 thin films deposited by ion-beam sputtering.

The influence on the internal stress and optical properties of Nb2O5 thin films with ion-beam energy was investigated. Nb2O5 thin films were deposited on unheated glass substrates by means of ion-beam sputtering with different ion-beam voltage, Vb. The refractive index, extinction coefficient, and surface roughness were found to depend on the ion-beam energy. The stresses in thin films were measured by the phase-shifting interferometry technique. The film stress was also found to be related to Vb, and a high compressive stress of -0.467 GPa was measured at Vb = 850 V. The Nb2O5-SiO2 multilayer coatings had smaller average compressive stress as compared with single-layer Nb2O5 film.