Increase In Optical Path Length Research Articles

Electronic Excitation Characteristics and Spectral Behavior of Phosphate Anions.

Absorption spectra are fundamental bases for the qualitative and quantitative analysis of the target chemical, and the development of an analytical model can be improved by studying its characteristics and rules. In the present study, the electronic excitation characteristics of phosphate anions (H2PO4-, HPO42-, and PO43-) were analyzed based on the charge-transfer spectrum. In addition, the absorption spectra of phosphate anions at the energy level of PBE0/6-311+G (d,p) were recorded based on the time-dependent density functional theory (TD-DFT) method. Different (HPO42-)n·(H2O)10-n molecular clusters were theoretically constructed, and the combined TD-DFT method and independent gradient model revealed that red shift of the maximum absorption wavelength (λmax) with the increase of phosphate anion concentration (0-10 mM) may be caused by the decrease of hydrogen bond interaction. In addition, the prominent dispersion in the short-wave region mainly resulted in the red shift of λmax with the increase in optical path length (1-100 mm). Moreover, with the increase in spectral bandwidth (0.4-3.0 nm), λmax slightly blue-shifted because of the increase in energy through the slit, and repeatability of the corresponding absorbance measurement at λmax gradually improved. As the spectral bandwidth increased, light monochromaticity became poor, resulting in the decrease of the linearly fitted correlation coefficient of the concentration-absorbance curve. Finally, the multivariate analysis of variance results showed that the optical path length was the most significant factor that influenced the absorption spectral characteristics of phosphate anions. This study provides a basis for the qualitative and quantitative analysis of phosphate anions by using absorption spectra and also renders a theoretical reference for absorption spectroscopy of other chemicals.

Comparison of random upright pyramids and inverted pyramid photonic crystals in thin crystalline silicon solar cells: An optical and morphological study

The potential of enabling a wide range of applications with lightweight, flexible, thin silicon photovoltaic (PV) devices has led to research interest in various light-trapping technologies for thin silicon absorbers. Herein, using both experiments and photonic modelling, we present a detailed comparison between random pyramid texturing and inverted pyramid photonic crystal patterning in thin silicon vis-à-vis light trapping. In particular, we investigate the potential of uniform single-dimension periodic structures in contrast to conventional multi-dimensional random structures for high broadband absorption efficiency in ultra-thin silicon. We find that inverted pyramid photonic crystals cause a large increase in optical path length especially for long wavelength photons due to strong wave interference producing exceptionally high absorption and so does random pyramids though with slightly lower absorption than that of photonic crystals. On the other hand, the random pyramids, fabricated via a one-step etching process with feature sizes ranging from sub-micron to 4 µm, have higher absorption at short wavelengths due to their multi-dimensional size distribution. Overall, we find that random textured pyramids appropriately fabricated can yield a comparable ideal photocurrent density to that by photonic crystals for 10 µm thick silicon.

Effect of Plasmonic Ag Nanoparticles on the Performance of Inverted Perovskite Solar Cells

Recently, perovskite solar cells (PSCs) have attracted phenomenal research interest due to their potential as the next‐generation photovoltaics. Despite rapid development in this field, further increasing their power conversion efficiency (PCE) remains a critical issue for the commercialization of PSCs. Herein, the application of Ag nanoparticle (NP) layers via vapor‐phase deposition onto perovskite active layers is investigated. The formation of unique, crescent‐shaped Ag NPs is confirmed by scanning electron microscopy (SEM), which shows that the NPs self‐assemble along the grain boundaries of perovskite, leading to their unique shape. The PCE for devices incorporating an optimized size of Ag NPs of 79 ± 6 nm increase from 11.63% to 13.46% with an improvement factor of 15.74%. The increase in PCE is can be attributed to an increase in short‐circuit current density (Jsc) that is assigned to an increase in optical path length and absorption. NPs exhibit the ability to increase the optical path length of photons in the device due to the near‐field and far‐field enhancement (plasmonic scattering) and consequently may act to improve the photon‐to‐electron conversion efficiency (ΔIPCE) and PCE of PSCs. Moreover, ultraviolet photoelectron spectroscopy reveals a decrease in hole injection barrier (ϕh), which also contributes to enhanced performance.

Limb Correction of Geostationary Infrared Imagery in Clear and Cloudy Regions to Improve Interpretation of RGB Composites for Real-Time Applications

AbstractRed–green–blue (RGB) composites are increasingly used by operational forecasters to interpret vast amounts of satellite imagery by reducing several bands into a single, easily understood product which identifies important atmospheric features with unique colors. Limb effects, a result of an increase in optical pathlength of the absorbing atmosphere between the satellite and Earth as viewing zenith angle increases, adversely affects RGB composite interpretation by causing anomalous reductions in brightness temperature, thus changing the color interpretation of the RGB composites. In a previous paper, Elmer et al. demonstrated a limb correction technique that effectively removes limb effects from polar-orbiting infrared channels in both clear and cloudy regions using latitudinally and seasonally varying correction coefficients. This study applies the Elmer et al. limb correction to Air Mass RGB composites derived from geostationary sensors and compares the limb-corrected geostationary imagery to limb-corrected polar-orbiter imagery and satellite-derived atmospheric profiles. A statistical comparison in overlapping regions shows that the limb correction reduces the absolute mean brightness temperature difference from 4–12 K to 0–2 K for all infrared bands, demonstrating that the Elmer et al. limb correction algorithm is a robust method of removing limb effects from infrared imagery derived from both geostationary and polar-orbiting sensors. The limb-corrected RGB composites derived from geostationary sensors present several advantages, including the improved depiction of atmospheric features and enabling wider use of imagery from overlapping geostationary coverage regions where viewing zenith angles are large for both geostationary sensors.

Investigation of effective near-infrared light-trapping structure with submicron diameter for crystalline silicon thin film solar cells

To obtain high efficiency in crystalline silicon thin-film solar cells, it is necessary to develop a novel light-trapping structure that not only has high light absorptance and can be fabricated with a small etching margin but also has a small surface area for a small effect of surface recombination. In this study, optical properties of silicon nanowires (SiNWs) with submicron diameters were investigated by the finite-difference time-domain (FDTD) method in the infrared wavelength region, in which the absorption coefficient of crystalline Si is quite low and the increase in optical path length is very important. To verify the simulation results, SiNWs with submicron diameters were successfully fabricated by the metal-assisted chemical etching method using two types of etching masks. The optical properties and minority carrier lifetime of the obtained structures were measured, and the results suggest that SiNWs with submicron diameters such as ∼700 nm are effective for both light trapping and the suppression of surface recombination.

Thin hydrogel films based on lectin-saccharide biospecific interaction for label-free optical glucose sensing

Concanavalin A (Con A) has been widely used as glucose recognition element in glucose sensors, however, these sensors typically detect glucose by fluorescence, and suffer from laborious labeling processes. Here a new label-free optical Con A-based glucose sensor was designed. The sensor uses Con A-containing thin hydrogel films as sensing material and Fabry-Perot cavity simultaneously, which were fabricated by layer-by-layer assembly of Con A and dextran, using lectin-saccharide biospecific interaction as driving force. These films display Fabry-Perot fringes on their reflection spectra, from which optical path length of the films can be calculated. The films swell upon addition of glucose, which causes a shift of Fabry-Perot fringes on the reflection spectra of the film, from which the glucose concentration can be reported. The glucose-induced swelling is reversible, and the increase in optical path length increases linearly with glucose concentration over a wide range of glucose concentration. The sensor works well at physiological temperature, pH and ionic strength. Other sugars, which may present in blood, do not interfere with glucose detection, because of their very low concentrations in the blood. Particularly, unlike other hydrogel-based sensors, this sensor responds quite fast, because the hydrogel films are very thin. The new sensor may have potential for real time, continuous glucose monitoring.

Limb Correction of MODIS and VIIRS Infrared Channels for the Improved Interpretation of RGB Composites

AbstractRed–green–blue (RGB) composite imagery combines information from several spectral channels into one image to aid in the operational analysis of atmospheric processes. However, infrared channels are adversely affected by the limb effect, the result of an increase in optical pathlength of the absorbing atmosphere between the satellite and the earth as viewing zenith angle increases. This study develops a technique to quickly correct for limb effects in both clear and cloudy regions using latitudinally and seasonally varying limb correction coefficients for real-time applications. These limb correction coefficients account for the increase in optical pathlength in order to produce limb-corrected RGB composites. The improved functionality of limb-corrected RGB composites is demonstrated by multiple case studies of Air Mass and Dust RGB composites using Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) and Suomi–National Polar-Orbiting Partnership (SNPP) Visible Infrared Imaging Radiometer Suite (VIIRS) imagery. However, the limb correction can be applied to any polar-orbiting sensor infrared channels, provided the proper limb correction coefficients are calculated. Corrected RGB composites provide multiple advantages over uncorrected RGB composites, including increased confidence in the interpretation of RGB features, improved situational awareness for operational forecasters, and the ability to use RGB composites from multiple sensors jointly to increase the temporal frequency of observations.

THE EFFECT OF GREEN LASER ON HYDROGEN PRODUCTION OF TIO2/CDS ELECTRODE

Enhancing the response of TiO2 semiconductor nanocomposites to visible light is topically important for hydrogen production. TiO2/CdSnanocomposite films (NCFs) are grown on p-type [100] silicon substrate using electron beam deposition (EBD). The UV–Vis reflectance spectra and PEC measurement reveal the enhanced absorption and induced photo-currents due to CdS coating on TiO2 nanofilm. The improved visible light absorption is attributed to the increase in optical path length and subsequent decrease in back-scattered radiation near TiO2 – Si interface. The excellent features of our results suggest that NCF electrodes may be nominated for potential applications in water splitting, renewable energy and environmental fields.

Detection of pH-induced aggregation of “smart” gold nanoparticles with photothermal optical coherence tomography

We report the feasibility of a novel contrast agent, namely "smart" gold nanoparticles (AuNPs), in the detection of cancer cells with photothermal optical coherence tomography (PT-OCT). "Smart" AuNPs form aggregation in low pH condition, which is typical for cancer cells, and this aggregation results in a shift of their absorption spectrum. A PT-OCT system was developed to detect this pH-induced aggregation by combining an OCT light source and a laser with 660 nm in wavelength for photothermal excitation. Optical detection of pH-induced aggregation was tested with solution samples at two different pH conditions. An increase in optical path length (OPL) variation was measured at mild acidic condition, while there was not much change at neutral condition. Detection of cancer cells was tested with cultured cell samples. HeLa and fibroblast cells, as cancer and normal cells respectively, were incubated with "smart" gold nanoparticles and measured with PT-OCT. An elevated OPL variation signal was detected with the HeLa cells while not much of a signal was detected with the fibroblast cells. With the novel optical property of "smart" AuNPs and high sensitivity of PT-OCT, this technique is promising for cancer cell detection.

Preparation and photoelectrochemical properties of TiO2 hollow spheres embedded TiO2/CdS photoanodes for quantum-dot-sensitized solar cells

A novel process was developed by introducing TiO2 hollow spheres (THS) into TiO2 nanoparticles film as light scattering centers to improve the light harvesting of CdS sensitized TiO2 (TiO2/CdS) photoanodes for quantum-dot-sensitized solar cells (QDSCs). The THS in TiO2 nanoparticles film can largely enhance the light scattering due to the multi-reflection among or within the THS, leading to the increase of optical path length in TiO2 film. Furthermore, the THS in TiO2 particles favors the penetration of CdS nanocrystals in the porous TiO2 film. As a result, the photocurrent is significantly improved with THS embedding in TiO2/CdS photoanodes. The experiment results suggest that an optimal content of THS is highly desired, too high content of THS in the photoanode will degrade the light to electricity conversion efficiency due to the strong back-scattering in the interfacial area of TiO2 film and the conducting glass, penetration of excess and large CdS nanocrystals, and poor charge transfer between TiO2 particles. The photoelectrochemical measurement shows that the highest photocurrent of 2.4mA/cm2 was obtained with 10% THS in the TiO2/CdS photoanode, which is nearly four times higher than that of the TiO2/CdS photoanode without THS (0.5mA/cm2).

Remarkable improvement of visible light photocatalysis with PANI modified core–shell mesoporous TiO2 microspheres

PANI modified core–shell mesoporous TiO2 (PANI/M-TiO2) with efficient photocatalytic capability under visible light irradiation was fabricated by hydrothermal method and chemisorption approach. The nitrogen adsorption–desorption characterization indicated that the specific surface area of mesoporous M-TiO2 was 2.8 times as great as that of P25, which resulted in the increased uptake of PANI molecule on the surface of M-TiO2. Environmental scanning electron microscopy and transmission electron microscopy images demonstrated that the PANI/M-TiO2 possessed a unique core–shell structure, which allowed multiple reflection or scattering of light in the photocatalyst and led to the increase of optical path length subsequently. Both the increased uptake of PANI molecules and optical path length in photocatalyst contributed to enhance the visible light absorption. The UV–vis diffuse reflectance spectra confirmed that the optical absorption for PANI/M-TiO2 was more intensive than that for PANI modified TiO2 nanoparticle (PANI/NP-TiO2) in the visible light region. The intensive visible light absorption and effective charge separation owing to the heterojunction built between TiO2 and PANI lead to remarkable improvement of visible light photocatalysis. The pseudo-first-order kinetic constant of photocatalytic degradation of rhodamine B and 4-chlorophenol under visible light irradiation with 6% PANI/M-TiO2 was 5.04 and 2.03 times as great as that with PANI/NP-TiO2 respectively, showing the advantage of the unique core–shell mesoporous structure in the PANI/M-TiO2 for efficient photocatalysis.

Pattern scaling effect on the radiative properties of wafer’s surface

This paper presents a parametric study of the radiative properties of patterned wafers with a polysilicon gate array on the Si substrate, considering the effect of wavelength and polarization. The finite-difference time-domain (FDTD) method was employed to examine the pattern scaling effect on the spectral absorptance via numerical solutions of the Maxwell’s equations. The effective medium theory (EMT) was also used to help explain the absorptance predictions. While the gate sizes are very small compared to wavelength, the results show rather unusual phenomena. The absorptance calculated by EMT is in agreement with FDTD in the cases with small gate and period sizes. With the increase of period and decrease of the ratio of the gate width to the grating period, both EMT and FDTD results for the TM (transverse magnetic) mode approach to pure silicon since the grating effect diminishes. However, the error of EMT results for the TE mode in some cases becomes considerable due to the limitation of EMT, which requires the period to be significantly smaller than the wavelength. Besides, the TE (transverse electric) absorptance curve separates from that of plain Si when the wavelength equals the grating period, this is because the gate can interact with its neighbering region by diffraction and the diffraction effects are weak, when the wavelength is small. The peak absorptance can be attributed to the thin film effect. It shows that a slight increase in the gate height can drastically increase the absorptance and the increased gate height shifts the peak absorptance to longer wavelength due to the increase of optical path length in the effective thin film. This work is of great importance for optimization of advanced annealing techniques in semiconductor manufacturing.

Estimating the water content of hydrated minerals using reflectance spectroscopy: II. Effects of particle size

Visible-near infrared reflectance spectra for five particle size fractions of a Hawaiian palagonite (HWMK101) and a nontronite (ferruginous smectite, Clay Minerals Society source clay SWa-1) were measured under ambient, purged, and heated conditions to characterize the effects of surface and volume scattering on the relationship between absolute H 2O content and the strength of the 3 μm absorption feature. Both materials were ground and dry sieved to particle sizes of <25, 25–45, 45–75, 75–125, and 125–250 μm. Particles of the bulk palagonite have an approximate bimodal distribution consisting of small, amorphous particles < 5 μm in diameter mixed with crystalline and glass particles < 1 mm in diameter, whereas the nontronite particles are polycrystalline aggregates. We find that band parameters value relating the strength of the 3 μm hydration feature to water content increase with particle size for a given water content, regardless of whether reflectance or single scattering albedo spectra are used. Spectra generally increase in reflectance as particle size decreases, a result of the relative increase in volume to surface scattering. Spectra of large particles are commonly saturated in the 3 μm region due to an increase in optical path length, making an accurate estimate of water content indeterminate until the samples dehydrate to the volume-scattering regime. We find that the presence of fines in several of the size fractions of palagonite cause their spectra to be representative of the finest fraction rather than the mean particle size. The nontronite spectra appear to be representative of an effective particle size within the range of the sieved size fractions. Many planetary surfaces are expected to have a large number of small particles which can dominate their spectral signature. Our results for particles < 45 μm provide a reasonable model for estimating the H 2O content of hydrated asteroids and regions of Mars.

Effect of white pigments on UV curing of inks

The dynamic viscosity of ultraviolet (UV) inks during curing was measured by the use of an oscillating plate rheometer and the cure rate was determined rheologically. The effect of white pigments on the cure rate is discussed in relation to absorption and scattering of UV radiation by the pigments. In white inks, the scattering, which depends primarily on the difference in refractive index between the pigment and the vehicle, has an important role in controlling the cure rate. A pigment with a refractive index very close to that of the vehicle has little adverse effect on the cure rate. Thin films of ink containing a pigment with a high refractive index are rapidly cured because of the increase in optical path length by scattering. However, the cure rate is greatly reduced when the thickness is increased due to the rapid decay of UV radiation with depth in the film. The absorption of UV radiation by the photoinitiator in a film which absorbs and scatters radiation is calculated, from which the cure rate is predicted. The theoretical prediction shows good agreement with the experimental results.

Path length effect in direct scanning gel chromatography

The apparent absorbancies of phenol red, myoglobin, and blue dextran were measured in the presence of scattering matrices including solid glass beads (200 μm) and controlled-pore glyceryl glass (70- or 1000-Apores). The increase in optical path length was greatest with the most highly scattering material (1000-A-controlled-pore glyceryl glass). When tested in a direct scanning gel chromatograph the 1000-A-pore glass showed a much more marked dependence on wavelength of incident light than did the 70-A-pore material. This is explicable on the basis of light-scattering theory for small particles. It is proposed that the path length effect is a constant for a particular configuration of scanner and matrix, independent of the absorbing solute present.

Prediction of a Forbidden Zone in Light Interference Fringes of Glass Microbubbles

Light interference microscopy has been used to characterize 50 to 100 μm o.d. glass microbubbles for use as laser fusion targets. In support of this programme, we have written a parallel-light, ray-trace code that follows the refracted paths of rays passing through the microbubbles. The code ultimately calculates the increase in optical path length as one moves from rays passing through the centre of the microbubble to those at the edge. The code also predicts four types of ray paths, of which only two are strong enough to produce measurable light in the interference microscope. Most interesting is the code's prediction of a forbidden zone in which no interference fringes appear. Attempts to verify this effect with laser target microbubbles gave inconclusive results because of improper dimensions and lack of sphericity in the microbubbles. Experiments with high-refractive-index, thick-walled glass cylinders might confirm the code's predictions.