Synthesis Methods and Optical Sensing Applications of Plasmonic Metal Nanoparticles Made from Rhodium, Platinum, Gold, or Silver.

The purpose of this paper is to provide an in-depth review of plasmonic metal nanoparticles made from rhodium, platinum, gold, or silver. We describe fundamental concepts, synthesis methods, and optical sensing applications of these nanoparticles. Plasmonic metal nanoparticles have received a lot of interest due to various applications, such as optical sensors, single-molecule detection, single-cell detection, pathogen detection, environmental contaminant monitoring, cancer diagnostics, biomedicine, and food and health safety monitoring. They provide a promising platform for highly sensitive detection of various analytes. Due to strongly localized optical fields in the hot-spot region near metal nanoparticles, they have the potential for plasmon-enhanced optical sensing applications, including metal-enhanced fluorescence (MEF), surface-enhanced Raman scattering (SERS), and biomedical imaging. We explain the plasmonic enhancement through electromagnetic theory and confirm it with finite-difference time-domain numerical simulations. Moreover, we examine how the localized surface plasmon resonance effects of gold and silver nanoparticles have been utilized for the detection and biosensing of various analytes. Specifically, we discuss the syntheses and applications of rhodium and platinum nanoparticles for the UV plasmonics such as UV-MEF and UV-SERS. Finally, we provide an overview of chemical, physical, and green methods for synthesizing these nanoparticles. We hope that this paper will promote further interest in the optical sensing applications of plasmonic metal nanoparticles in the UV and visible ranges.

Manipulation of resonance orders and absorbing materials for structural colors in transmission with improved color purity.

We present an improved color purity of additive transmissive structural color filters by controlling a resonance order and by inserting a highly absorbing material. The proposed structure consists of a single metal sandwiched by two transparent dielectric media serving as a cavity to minimize the ohmic loss in the metal mirrors, which is distinctly different from a conventional Fabry-Perot (FP) cavity that is in general designed to have two metal mirrors. Low reflections at an air-dielectric interface cause a quality-factor of a resonance to be reduced, causing a degraded color purity, which can be improved by employing a 1st order resonance that exhibits a narrower bandwidth than a fundamental FP resonant mode (0th order). For a red color with the improved purity, introducing an ultrathin absorbing layer in the middle of a top cavity enables the 1st resonance to be trivially influenced while selectively suppressing a 2nd order resonance appearing at the shorter wavelength region. Moreover, angle-insensitive performances up to 60° are attained by utilizing a cavity material with high index of refraction. Besides, the fabrication of the structural coloring devices involves a few deposition steps, thus rendering the approach suitable for applications over the large area. The described concept could be applied to diverse applications, such as colored solar panels, sensors, imaging devices, and decorations.

Efficient electron-mediated electrochemical biosensor of gold wire for the rapid detection of C-reactive protein: A predictive strategy for heart failure.

C-reactive protein (CRP) is considered a promising biomarker for the rapid and high-throughput real-time monitoring of cardiovascular disease and inflammation in unprocessed clinical samples. Implementation of this monitoring would enable various transformative biomedical applications. We have fabricated a highly specific sensor chip to detect CRP with a detection limit of 2.25 fg/mL. The protein was immobilized on top of a gold (Au) wire/polycarbonate (PC) substrate using 1-ethyl-3-(3-dimethylamino-propyl) carbodiimide hydrochloride/N-hydroxy succinimide-activated 3-mercaptoproponic acid (MPA) as a self-assembled monolayer agent and bovine serum albumin (BSA) as a blocking agent. In contrast to the bare PC substrate, the CRP/BSA/anti-CRP/MPA/Au substrate exhibited a considerably high electrochemical signal toward CRP. The influence of the experimental parameters on CRP detection was assessed via various analysis methods, and these parameters were then optimized. The linear dynamic range of the CRP was 5-220 fg/mL for voltammetric and impedance analysis. Morever, the strategy exhibited high selectivity against various potential interfering species and was capable of directly probing trace amounts of the target CRP in human serum with excellent selectivity. The analytical assay based on the CRP/BSA/anti-CRP/MPA/Au substrate could be exploited as a potentially useful tool for detecting CRP in clinical samples.

Nanogap Engineering for Enhanced Transmission of Wire Grid Polarizers in Mid-Wavelength Infrared Region.

Wire-grid polarizers (WGPs) have been widely used in various fields, such as polarimetry, imaging, display, spectroscopy, and optical isolation. However, conventional WGPs used in diverse mid-wavelength infrared (MWIR) applications show high reflection losses, which intrinsically arise from high refractive indices of their IR-transmitting substrates, such as silicon (Si) and germanium (Ge). This study demonstrated the enhanced transmittance of a transverse magnetic (TM) wave that surpassed ~80% over the entire MWIR range from 3000 to 5000 nm in a narrow air gap of a WGP, where aluminum (Al) was selectively deposited on a nanopatterned Si substrate using an oblique angle deposition method. Moreover, a higher TM wave transmittance was achieved by reducing the air gaps of the WGPs in the nanopatterns, which were distinctly different from the traditional WGPs comprising metal wires patterned directly on a flat substrate. A finite-difference time-domain simulation was performed to investigate optical properties of the proposed WGPs, which showed that the electric field in the air nanogap was remarkably enhanced. The characteristic performances were further investigated using a combination of an effective medium approximation and an admittance diagram, revealing that the broadband transmission enhancement could be attributed to a combined effect of a strong electric field and a better admittance matching. The approach and results described in this paper hold promise for the design and the fabrication of high-quality WGPs, as well as their numerous applications.

General Strategy for Broadband Coherent Perfect Absorption and Multi-wavelength All-optical Switching Based on Epsilon-Near-Zero Multilayer Films.

We propose a general, easy-to-implement scheme for broadband coherent perfect absorption (CPA) using epsilon-near-zero (ENZ) multilayer films. Specifically, we employ indium tin oxide (ITO) as a tunable ENZ material, and theoretically investigate CPA in the near-infrared region. We first derive general CPA conditions using the scattering matrix and the admittance matching methods. Then, by combining these two methods, we extract analytic expressions for all relevant parameters for CPA. Based on this theoretical framework, we proceed to study ENZ CPA in a single layer ITO film and apply it to all-optical switching. Finally, using an ITO multilayer of different ENZ wavelengths, we implement broadband ENZ CPA structures and investigate multi-wavelength all-optical switching in the technologically important telecommunication window. In our design, the admittance matching diagram was employed to graphically extract not only the structural parameters (the film thicknesses and incident angles), but also the input beam parameters (the irradiance ratio and phase difference between two input beams). We find that the multi-wavelength all-optical switching in our broadband ENZ CPA system can be fully controlled by the phase difference between two input beams. The simple but general design principles and analyses in this work can be widely used in various thin-film devices.

Broadband Epsilon-Near-Zero Perfect Absorption in the Near-Infrared.

Perfect absorption (PA) of incident light is important for both fundamental light-matter interaction studies and practical device applications. PA studies so far have mainly used resonant nanostructures that require delicate structural patterning. Here, we realize tunable and broadband PA in the near-infrared region using relatively simple thin film coatings. We adjust the growth condition of an ITO film and control its epsilon-near-zero (ENZ) wavelength. We show that this results in highly tunable PA in the telecommunication window. Then, using an ITO multilayer of different ENZ wavelengths, we demonstrate broadband PA that covers a wide range of near-infrared wavelengths. The use of ENZ coatings makes PA adjustable during the film growth and does not require any structural patterning afterward. It also facilitates the chip-scale integration of perfect absorbers with other device components. Broadband PA relaxes the single wavelength condition in previous PA studies, and thus it is suitable for many practical device applications, including sensors, photodetectors, and energy harvesting devices.

Binary mask designs with single- and double-layer absorber stacks for extreme ultraviolet lithography and actinic inspection.

In this study, we propose binary mask (BIM) designs with single- and double-layer absorber stacks with high optical contrast at a wavelength of 13.5 nm for use in extreme ultraviolet lithography (EUVL) and actinic defect inspection. The optimum thickness of the absorber stack was estimated using a method based on the transfer matrix. In the double-layer designs, [Ag/SnTe] has a minimum thickness of ∼32 nm with almost 100% optical contrast compared to the TaN layer. In addition, a SnTe absorber layer was deposited using radio frequency magnetron sputtering. The optical constant of the SnTe layer at 13.5 nm wavelength was determined using the density of the layer, which was obtained from x-ray reflectivity measurements. The reflectance of the SnTe single-layer absorber stack was measured in the EUV region and compared with the simulated reflectance by using the calculated optical constants. The results show that the new BIM designs for EUVL and actinic inspection can be helpful in reducing the geometric shadow effect compared to the TaN absorber layer with a thickness of ∼70 nm.

Optical properties of Ag hemisphere-like nanoparticles.

We investigated the optical properties of Ag hemisphere-like nanoparticles. The nanoparticles were prepared by using magnetron sputtering and post-thermal annealing. The scanning-electron microscopy images reveal that hemisphere-like nanoparticles were successfully produced. The transmission spectra exhibit the surface plasmon resonance effect. From the spatial intensity distributions obtained by solving Maxwell equations using the three-dimensional, finite-difference time-domain method, we found that there is strong confinement of the optical field near the nanoparticle. This optical confinement makes it easy to exhibit the plasmon resonance effect.

Optical performance of extreme ultraviolet lithography mask with an indium tin oxide absorber.

In this study, we propose a new extreme ultraviolet (EUV) binary mask with an indium tin oxide (ITO) absorber. The optical constant of ITO film at 13.5 nm wavelength in the EUV regime was determined by means of X-ray reflectivity measurements and the chemical composition was determined using Rutherford backscattering spectrometry. The reflectance of a binary mask with an ITO absorber layer at various thicknesses was also measured to investigate the optical performance in the EUV regime. It was found that the extinction coefficient of ITO film is higher than that of a typical absorber layer, TaN, and that the reflectance of the ITO absorber in the binary mask at a wavelength of 13.5 nm is reduced to 0.62% at a thickness of 45 nm. Therefore, it is expected that the ITO film can be employed as a thin absorber of a binary mask to reduce the geometrical shadow effect in extreme ultraviolet lithography.

Effects of seed layers on structural, morphological, and optical properties of ZnO nanorods.

We studied the effects of seed layers on the structural and optical properties of ZnO nanorods. ZnO and Ag-doped ZnO (ZnO:Ag) seed layers were deposited on glass substrates by magnetron co-sputtering. ZnO nanorods were grown on these seed layers by the chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. SEM micrographs clearly reveal that ZnO nanorods were successfully grown on both kinds of seed layers. The XRD patterns indicate that crystallization of ZnO nanorods is along the c-axis. Meanwhile, the packing density and the vertical alignment of the ZnO nanorods on the ZnO seed layer are better than those of the ZnO nanorods on ZnO:Ag. The enhanced growth of nanorods is thought to be due to the fact that the ZnO layer exhibits a higher crystalline quality than the ZnO:Ag layer. According to the low-temperature photoluminescence spectra, the ZnO nanorods on the ZnO seed layer show a narrow strong ultraviolet emission band centered at 369 nm, while those on ZnO:Ag exhibit multiple bands. These results are thought to be related with the crystallinity of ZnO nanorods, the morphologies of ZnO nanorods, and the reflectivities of seed layers. More detailed studies for clarification of the seed layer effect on the growth of ZnO nanorods are desirable.

Linear polarization-discriminatory state inverter fabricated by oblique angle deposition.

In this paper, we report a linear polarization-discriminatory state inverter made of three-layer sculpture thin film fabricated by oblique angle deposition technique. The first and third layers are quarter-wave plates of zigzag structure and the middle of them is a circular Bragg reflector of left-handed helical structure. It is found that the normal incidence of P-polarized light on this polarization-discriminatory state inverter becomes the S-polarized light at output, while the incident S-polarized light of wavelength lying in the Bragg regime is reflected. The microstructure of the linear polarization-discriminatory state inverter is also investigated by using a scanning electron microscope.

Wideband circular polarization reflector fabricated by glancing angle deposition.

We demonstrate a wideband circular polarization reflector fabricated as cascades of helical films with different pitch thickness by using glancing angle deposition (GLAD) technique. The full-width-at-half-maximum bandwidth of this reflector is measured from the reflectance spectra and is found about 200 nm indicating the feasibility of wideband reflector. A helical TiO(2) film with three sections, each of different pitch thickness, is also studied. It shows three Bragg peaks at different wavelengths. To select appropriate material for this circular reflector, the optical properties of 5-turns TiO(2), ZrO(2), and Ta2O(5) helical films and the porosity effect on the TiO(2) helical film are investigated.

Optical design of hybrid-type attenuated phase-shift masks for extreme-ultraviolet lithography by use of a Fabry-Perot interference filter.

We have designed what we believe to be new hybrid-type attenuated phase-shift masks for extreme-ultraviolet optical lithography by use of a Fabry-Perot interference filter. The designs for the attenuated phase-shift masks show a smaller step height for less geometric shadow effects than additive- and subtractive-type attenuated phase-shift masks, a contrast higher than 94% for both deep-ultraviolet and extreme-ultraviolet wavelength regimes, and a 180 degrees phase-shift in the extreme-ultraviolet wavelength regime.

Effects of annealing on the optical, structural, and chemical properties of TiO2 and MgF2 thin films prepared by plasma ion-assisted deposition.

Effects of thermal annealing at 400 degrees C on the optical, structural, and chemical properties of TiO2 single-layer, MgF2 single-layer, and TiO2/MgF2 narrow-bandpass filters deposited by conventional electron-beam evaporation (CE) and plasma ion-assisted deposition (PIAD) were investigated. In the case of TiO2 films, the results show that the annealing of both CE and PIAD TiO2 films increases the refractive index slightly and the extinction coefficient and surface roughness greatly. Annealing decreases the thickness of CE TiO2 films drastically, whereas it does not vary that of PIAD TiO2 films. For PIAD MgF2 films, annealing increases the refractive index and decreases the extinction coefficient drastically. An x-ray photoelectron spectroscopy analysis suggests that an increase in the refractive index and a decrease in the extinction coefficient for PIAD MgF2 films after annealing may be related to the enhanced concentration of MgO in the annealed PIAD MgF2 films and the changes in the chemical bonding states of Mg 2p, F 1s, and O is. It is found that (TiO2/MgF2) multilayer filters, consisting of PIAD TiO2 and CE MgF2 films, are as deposited without microcracks and are also thermally stable after annealing.

Optical monitoring of nonquarterwave layers of dielectric multilayer filters using optical admittance.

The optical monitoring method is described to compensate for the thickness error of nonquarterwave layers of dielectric multilayer filters, using optical admittance during deposition. Stability is confirmed by computer simulation of random thickness error generation in layers. In addition, a band split filter consisting of 61 nonquarterwave and nonperiodic layers is deposited in the proposed method, resulting in high spectral performance, as the design requires.

Derivation of the center-wavelength shift of narrow-bandpass filters under temperature change.

We derive a simple equation to predict the center-wavelength shift of a Fabry-Perot type narrow-bandpass filter by using the conventional characteristic matrix method and the elastic strain model as the temperature varies. We determine the thermal expansion coefficient of substrate from the zero-shift condition of the center wavelength of the filter. The calculated shifts are in a good agreement with the experimental ones, in which the narrow-bandpass filters are prepared by plasma ion-assisted deposition on four substrates with different thermal expansion coefficients.

Deposition of a conductive near-infrared cutoff filter by radio-frequency magnetron sputtering.

We have designed a conductive near-infrared (NIR) cutoff filter for display application, i.e., a modified low-emissivity filter based on the three periods of the basic design of [TiO2[Ti]Ag] TiO2] upon a glass substrate and investigated the optical, structural, chemical, and electrical properties of the conductive NIR cutoff filter prepared by a radio frequency magnetron sputtering system. The results show that the average transmittance is 61.1% in the visible, that the transmittance in the NIR is less than 6.6%, and that the sheet resistance and emissivity are 0.9 ohms/square (where square stands for a square film) and 0.012, respectively, suggesting that the conductive NIR cutoff filter can be employed as a shield against the hazard of electromagnetic waves as well as to cut off the NIR.

Design of omnidirectional high reflectors with quarter-wave dielectric stacks for optical telecommunication bands.

A design for omnidirectional high reflectors with quarter-wave dielectric stacks in the optical telecommunication band that uses conventional optical thin-film design theory is described. The omnidirectional bandwidth is derived in units of wavelength and investigated as a function of its high- and low-refractive-index values in the near infrared. The results show that the high refractive index should be larger than 2.26 for an omnidirectional high reflector and that the low refractive index for maximum onmidirectional bandwidth should be approximately 1.5. It is shown that one can design broad-bandwidth omnidirectional high reflectors for S, C, and L bands for optical telecommunication simply by connecting the band edges of omnidirectional high reflectors.

Control of resonant wavelength from organic light-emitting materials by use of a Fabry-Perot microcavity structure.

We report the fabrication of Fabry-Perot microcavity structures with the organic light-emitting material tris-(8-hydroxyquinoline) aluminum (Alq3) and derive their optical properties by measuring their photoluminescence (PL) and absorption. Silver and a TiO2-SiO2 multilayer were used as metal and dielectric reflectors, respectively, in a Fabry-Perot microcavity structure. Three types of microcavity were prepared: type A consisted of [air[Ag[Alq3]Ag]glass]; type B, of [air[dielectric[Alq3]dielectric]glass]; and type C, of [air[Ag[Alq2]dielectric]glass]. A bare Alq3 film of [air[Alq3]glass] had its PL peak near 514 nm, and its full width at half-maximum (FWHM) was 80 nm. The broad FWHM of a bare Alq3 film was reduced to 15-27.5, 7-10.5, and 16-16.6 nm for microcavity types A, B, and C, respectively. Also, we could control the PL peak of the microcavity structure by changing the spacer thickness, the amount of phase change on reflection, and the angle of incidence.