Simple Optical Model Research Articles

Selective observation of transverse optical phonons of Au modes to evaluate free charge carrier parameters in β -Ga2O3 substrate and homoepitaxial film

Polarized infrared reflectance spectra from β-Ga2O3 (001) unintentionally doped (undoped) and Sn-doped substrates were investigated. Spectra from an undoped homoepitaxial film grown on the Sn-doped substrate were also investigated. By setting the electric field vector of the incident light E parallel to the crystallographic b-axis in the s-polarized configuration, the spectra for pure transverse optical phonons of Au modes were well reproduced by the Drude–Lorentz model. Subsequently, the free-carrier concentrations and carrier mobilities were determined to be in reasonable agreement with those determined by Hall-effect measurements, and at the same time, the film thickness was determined for the homoepitaxial layer. The results ensure the validity of the simplified optical model analyses for any arbitrary surface orientations where the b-axis is parallel to the surface only if no birefringence effects are present by choosing as E//b in s-polarized configuration.

Back-side illuminated optical stack optimized with a high refractive index micro-lens array for CMOS image sensors

We simulate a simplified optical stack model with Lumerical FDTD over wide ranges of optical parameters: wavelength, 550 nm to 1.4 μm, and pixel array pitch, 1 to 10 μm. By increasing the lens and the planarization layer refractive indices from the usual 1.5 to 2, we study the improved lens focalization abilities, in terms of spot width, power, and depth below the micro-lens. We show an NIR wavelength range benefit from slightly increasing the optical stack index to 1.7 while visible wavelengths have less interest. Indeed, increasing the lens and planarization layer index enables thinner back-side illuminated optical stacks for higher quantum efficiencies and angular response improvement.

Structural vibration measurement with multiple synchronous cameras

This paper presents a simulation and experimental study on the measurement with multiple, i.e. more than 2, synchronous cameras of the flexural deflection shape of a beam excited by a transverse force at the fundamental resonance frequency, which gives a good estimate of the first flexural natural mode shape. The first part of the paper presents the simplified optical model employed to simulate how the accuracy of the measurements varies with respect to: a) the distance of the cameras from the structure; b) the opening angle between pairs of cameras, which is analysed with respect to the azimuthal and elevation spherical coordinates, c) the resolution of cameras and d) the number of cameras. The second part of the paper provides experimental results taken on a beam rig and camera setup assembled using off-the-shelf devices. The study shows that the accuracy of the measurements significantly rises as the setup is equipped with an increasingly larger number of cameras, which should be arranged close to the vibrating structure with large opening angles between pairs of cameras characterized by large azimuthal and small elevation spherical coordinates. For instance, the average error of the measured flexural deflection shape falls from 15% to 3% when a setup with cameras arranged along a circumference oriented parallel the beam surface, with a small offset from the beam, passes from 2 cameras to 12 cameras.

Kinematics and dynamics of the stratocumulus average diurnal dissipation process at Atacama Desert

Stratocumulus-type clouds constitute a stable system over the Southeast Pacific Ocean. Their phenomenology is well known particularly over the ocean, but a quantitative analysis of their diurnal cycle over the Pacific continental coastal desert strip of South America remained unaddressed. In this work, the stratocumulus average morning dissipation process at two sites in the Atacama Desert is documented through simple empirical mathematical equations. Based on the change rate analysis of the visible cloud optical depth (COD) inferred from solar irradiance measurements, the kinematics and the dynamics of the stratocumulus dissipation process is established, showing that it develops at an average constant “acceleration of dissipation” (d2COD/dt2) of about −3.3 h−2, equivalent to a cloud geometrical vertical thickness change at an average constant “acceleration of thinning” of about −40 m/h2. Towards the end of the dissipation process when cloudiness vanishes, the average “velocity of dissipation” (dCOD/dt) and “velocity of thinning” round −16 h−1 and −175 m/h, respectively. A simple plane-parallel optical and geometrical model explains the average behavior of the phenomenon in the region on a wholly radiative base, with an absorbed fraction of solar irradiance acting as exclusive driver of the average dissipation process. About 13 kJ/m2 of net irradiation is needed to change the COD of the stratocumulus deck by one unit. The progressive morning cloud dissipation from the land on the east towards the sea on the west is evidenced. The analysis of the cloud effective transmittance and a full set of checks, including test of robustness, support these results and their interpretation.

Opto-Electro-Thermal Modeling of Thin-Film Hydrogenated Amorphous Silicon (a-Si:H) Photovoltaic Cell

Abstract A one-dimensional opto-electro-thermal simulator for an a-Si:H-based thin-film solar cell is developed. The simulator includes optical, electrical, and thermal modeling for a complete modeling of energy conversion in a-Si:based solar cells. Particularly, the thermal impact on the performance of the cell has been studied. Cell performance is worse when thermal simulation is included compared to simulations where the temperature is kept constant. This implication suggests that cell performance is typically overestimated in simulations where the thermal effect is ignored. A simplified optical model was used, reducing the computational resources required in the study. The electrical model proposed in this study extends the classical drift-diffusion model to include the effects of temperature. The proposed thermal model considers the energy conservation in a non-thermal equilibrium condition between electron, optical phonons, and acoustic phonons. While simplifications have been used to reduce the complexity of the simulations, the program captures the essential behavior of the cell with reasonable accuracy. Hence, the developed program is useful for a-Si:H-based thin-film solar cell design and optimization.

Hollow Plasma Acceleration Driven by a Relativistic Reflected Hollow Laser.

In order to address the present difficulty in experimentally generating the relativistic Laguerre-Gaussian laser, primarily due to damage caused to optical modulators, a high-reflectivity phase mirror is applied in the femtosecond petawatt laser system to generate a relativistic hollow laser at the highest intensity of 6.3×10^{19} W/cm^{2} for the first time. A simple optical model is used to verify that the vortex laser may be generated in this new scheme; using such a relativistic vortex laser, the hollow plasma drill and acceleration are achieved experimentally and proven by particle-in-cell simulations. With the development of the petawatt laser, this scheme opens up possibilities for the convenient production of the relativistic hollow laser at high repetition and possible hollow plasma acceleration, which is important for a wide range of applications such as the generation of radiation sources with orbital angular momentum, fast ignition for inertial confinement fusion, and jet research in the astrophysical environment.

Effect of phytoplankton community size structure on remote-sensing reflectance and chlorophyll a products

Abstract Remotely-sensed ocean colour is the main tool for estimating chlorophyll a (Chl-a) concentration and primary productivity on the global scale. In order to investigate the source of errors in remotely-sensed Chl-a concentration we obtained in situ bio-optical properties, in situ reflectances, satellite-derived reflectances and the Chl-a concentration satellite products of the Ocean and Land Colour Imager (OLCI) Instrument on board Sentinel-3 A in waters off eastern Australia. The mesoscale eddies of these oligotrophic waters provide contrasting phytoplankton communities that allowed us to focus on the effect of phytoplankton size as a source of errors. In these waters, cold-core cyclonic eddies (CE) are dominated by large phytoplankton cells, while small cells dominate warm-core anticyclonic eddies (ACE). The chlorophyll-specific absorption and backscattering from contrasting sites show significant difference due to the differing package effect of phytoplankton size distributions. After normalising the absorption and backscattering spectra to Chl-a associated with just small phytoplankton, the spectra of optical properties become much more similar, showing that small-sized phytoplankton dominate IOPs even when large cells contain the greater fraction of Chl-a concentration of the phytoplankton community. Measured in situ reflectances agreed with reflectances calculated using a simple optical model based on measured IOPs. Furthermore, the in situ measured reflectances agreed well with the OLCI reflectance (mean normalised bias (MNB) of 7% for wavelengths

Intra-abdominal hypertension assessment based on Near Infra-Red reflectometry: New model and low-cost device

This work describes a simple optical model of the abdominal wall for non-invasive assessment of intra-abdominal pressure during laparoscopy, and the optimized design of a low-cost device for clinical measurements. A pilot run performed on a cadaver of a Sus scrofa domesticus (domestic pig) is case-studied for validation of a proposed rule-of-thumb design. Intra-abdominal hypertension was induced by inflating the abdomen. An inverse correlation between intra-abdominal pressure and Near Infra-Red (NIR) reflectometry measurements verified the model. Previous work claimed that changes in NIR reflectometry, during intra-abdominal hypertension, depended on the oxygenation of the abdominal wall tissues. However, in our work on a cadaver, oxygenation is not relevant; therefore, we conclude that the changes in NIR reflectometry are connected to changes in the thickness and density of the abdominal wall due to hypertension, confirming the proposed rule-of-thumb design for a new device.

Nonuniversal Transverse Electron Mean Free Path through Few-layer Graphene.

In contrast to the in-plane transport electron mean-free path in graphene, the transverse mean-free path has received little attention and is often assumed to follow the "universal" mean-free path (MFP) curve broadly adopted in surface and interface science. Here we directly measure transverse electron scattering through graphene from 0 to 25eV above the vacuum level both in reflection using low energy electron microscopy and in transmission using electronvolt transmission electron microscopy. From these data, we obtain quantitative MFPs for both elastic and inelastic scattering. Even at the lowest energies, the total MFP is just a few graphene layers and the elastic MFP oscillates with graphene layer number, both refuting the universal curve. A full theoretical calculation taking the graphene band structure into consideration agrees well with experiment, while the key experimental results are reproduced even by a simple optical toy model.

Numerical study on the energy performance of building zones with transparent water storage envelopes

Researches on water wall passive solar technologies, especially the numerical study on the energy performance of building zones with Transparent Water Storage Envelopes (TWSEs), are reported in this paper. TWSE is a climatic adaptive building envelope consisting of visually transparent modular water containers, exterior shading devices, water supply and return pipes. It is an upgraded water wall that can serve both as an energy efficient facade and an auxiliary water cistern in a building. Currently the energy simulation involving transparent water walls cannot be explicitly done in all energy simulation programs because transparent envelopes are always predefined as the surfaces without thermal mass due to the embedded algorithms complied with ISO15099 standard. A numerical approach based on the integrated energy and computational fluid dynamics (IE&CFD) simulation was developed for solving the energy simulation problem related with TWSEs. A simplified optical model of TWSE was proposed and validated through light transmission testing. Meanwhile, cooling and heating loads of a zone with TWSEs and conventional glazing in summer and winter months were studied. The thermal performance of a TWSE relative to the conventional glazing was also investigated via the comparative thermal box testing. Based on the simulation and testing results, it reveals that TWSEs can exceed most high performance coated glazing with regard to solar radiation control, and they are also exceptional energy efficient transparent envelopes that can outperform the ASHRAE standard window and conventional glazing in terms of cooling and heating load reduction as long as being configured and operated properly according to their physical characteristics and outdoor climatic condition. Furthermore, the innovative technical paradigm of TWSEs, along with the numerical approach developed for their energy simulation, demonstrates a wide range of versatility to be implemented in research and practice for ultra low energy buildings.

Optical changes of molecular contamination thin-film outgassed from epoxy-based resin during deposition and desorption process

Molecular contaminants outgassed from organic materials used for the spacecraft degrade the performance of optical surfaces of spacecraft. The influence of contaminants outgassed from epoxy resin on the spectral transmittance of the quartz substrate was investigated with an in-situ measurement system. The system can deposit the contaminants on temperature-controlled quartz substrates and the transmittance spectra were measured immediately after deposition in vacuum ambient. We obtained the optical constants of the contaminant using transmittance spectrum and simple optical models for optical calculations. The optical constants were described with a harmonic oscillator model and an effective medium approximation model. This paper reports the in-situ measurement results of transmittance spectra of the epoxy-resin-induced contaminants in deposition and desorption process. The thin contamination layer decreased the transmittance in the ultraviolet region. However, the contamination was entirely desorbed at −20 °C and transmittance recovered to the initial value. In addition, the results of optical calculations using the obtained optical constants were compared to the measurement results.

Hazy Al₂O₃-FTO Nanocomposites: A Comparative Study with FTO-Based Nanocomposites Integrating ZnO and S:TiO₂ Nanostructures.

In this study, we report the use of Al2O3 nanoparticles in combination with fluorine doped tin oxide (F:SnO2, aka FTO) thin films to form hazy Al2O3-FTO nanocomposites. In comparison to previously reported FTO-based nanocomposites integrating ZnO and sulfur doped TiO2 (S:TiO2) nanoparticles (i.e., ZnO-FTO and S:TiO2-FTO nanocomposites), the newly developed Al2O3-FTO nanocomposites show medium haze factor HT of about 30%, while they exhibit the least loss in total transmittance Ttot. In addition, Al2O3-FTO nanocomposites present a low fraction of large-sized nanoparticle agglomerates with equivalent radius req > 1 μm; effectively 90% of the nanoparticle agglomerates show req < 750 nm. The smaller feature size in Al2O3-FTO nanocomposites, as compared to ZnO-FTO and S:TiO2-FTO nanocomposites, makes them more suitable for applications that are sensitive to roughness and large-sized features. With the help of a simple optical model developed in this work, we have simulated the optical scattering by a single nanoparticle agglomerate characterized by bottom radius r0, top radius r1, and height h. It is found that r0 is the main factor affecting the HT(λ), which indicates that the haze factor of Al2O3-FTO and related FTO nanocomposites is mainly determined by the total surface coverage of all the nanoparticle agglomerates present.

In situ monitoring of atomic layer epitaxy via optical ellipsometry

We report on the use of time-resolved optical ellipsometry to monitor the deposition of single atomic layers with subatomic sensitivity. Ruddlesden–Popper thin films of SrO(SrTiO3)n=4 were grown by means of metalorganic aerosol deposition in the atomic layer epitaxy mode on SrTiO3(1 0 0), LSAT(1 0 0) and DyScO3(1 1 0) substrates. The measured time dependences of ellipsometric angles, Δ(t) and Ψ(t), were described by using a simple optical model, considering the sequence of atomic layers SrO and TiO2 with corresponding bulk refractive indices. As a result, valuable online information on the atomic layer epitaxy process was obtained. Ex situ characterization techniques, i.e. transmission electron microscopy, x-ray diffraction and x-ray reflectometry verify the crystal structure and confirm the predictions of optical ellipsometry.

Engineering the Emission of Broadband 2D Perovskites by Polymer Distributed Bragg Reflectors

Thanks to their broadband emission and solution processability, 2D hybrid perovskite materials are promising for the realization of large area and flexible lighting devices. The deposition of 2D perovskites, however, requires wide range solvents that are incompatible with commodity polymers used for structural support and light management. Here we demonstrate coupling of broad-emitting 2,2-(ethylenedioxy)bis(ethylammonium)PbCl4 perovskite with solution processed polymer distributed Bragg reflectors on both rigid fused silica and flexible polymer substrates. The optical functions of the chemically engineered perovskite were determined by ellipsometric measurements and used to design dielectric multilayer structures with photonic bandgap tunable over the entire visible range. The resulting photonic structures control directionality and spectral enhancement or suppression of the perovskite photoluminescence, in agreement with simple analytical optical models. These results pave the way to the development of ...

Designing an efficient graphene quantum dot-filled luminescent down shifting layer to improve the stability and efficiency of perovskite solar cells by simple optical modeling.

Degradation of perovskite material under UV light is a problem hampering the practical application of perovskite solar cells (PSCs) despite attaining high efficiency. This paper studies the application of a luminescent down shifting (LDS) layer containing graphene quantum dots (GQDs) on top of a PSC as an efficient strategy to improve the stability and light harvesting efficiency of PSCs under UV light. With absorption and emission bands in the UV and visible regions respectively, and simple synthesis of GQDs with a high luminescence quantum efficiency (QE), GQDs are a suitable candidate as a down shifting material in the LDS layer. Here, a simple optical model is used to investigate the effect of parameters such as the concentration of GQDs, LDS layer thickness, absorption/emission bands of GQDs and the luminescence quantum efficiency on the performance of the LDS layer. The calculated results show that application of a GQD-filled LDS layer, with 94% QE and negligible RO and PA, on a PSC causes a remarkable enhancement in the incident photon to current conversion efficiency (IPCE) and thereby the short circuit current density (JSC) in the 300–400 nm spectral range of more than 400%. This strategy is also very effective in improving the stability of the PSC by suppressing the UV light from entering the device.

Convenient models of the atmosphere: optics and solar radiation

Simple optical models of clear and cloudy atmosphere are proposed. Four versions of atmospheric aerosols content are considered: a complete lack of aerosols in the atmosphere, low background concentration (500 cm−3), high concentrations (2000 cm−3) and very high content of particles (5000 cm−3). In a cloud scenario, the model of external mixture is assumed. The values of optical thickness and single scattering albedo for 13 wavelengths are calculated in the short wavelength range of 0.28–0.90 µm, with regard to the molecular absorption bands, that is simulated with triangle function. A comparison of the proposed optical parameters with results of various measurements and retrieval (lidar measurement, sampling, processing radiation measurements) is presented. For a cloudy atmosphere models of single-layer and two-layer atmosphere are proposed. It is found that cloud optical parameters with assuming the “external mixture” agrees with retrieved values from airborne observations. The results of calculating hemispherical fluxes of the reflected and transmitted solar radiation and the radiative divergence are obtained with the Delta-Eddington approach. The calculation is done for surface albedo values of 0, 0.5, 0.9 and for spectral values of the sandy surface. Four values of solar zenith angle: 0°, 30°, 40° and 60° are taken. The obtained values are compared with data of radiative airborne observations. Estimating the local instantaneous radiative forcing of atmospheric aerosols and clouds for considered models is presented together with the heating rate.

Multi-transverse mode operation of alkali vapor lasers: modeling and comparison with experiments

In high-power diode pumped alkali lasers with stable resonators the radius of the pump beam is usually larger than that of the fundamental laser mode and thus several high order transverse modes of the resonator can participate in the lasing. A simple optical model of multi-transverse mode operation of alkali vapor lasers is reported. The model is based on calculations of the pump and laser beam intensities in the gain medium, where the laser beam intensity is a linear combination of the azimuthally-symmetric Laguerre-Gaussian modes. It was applied to Ti:Sapphire and diode pumped cesium vapor lasers. The model predicts that for low pump power only the fundamental lasing mode oscillates. However, for higher pump powers several transverse modes participate in oscillation. The number and intensities of the oscillating modes as a function of the pump beam power and radius were found. The model predicts linear dependence of the laser power on the pump power, the values of the former being in agreement with the experimental results obtained for diode pumped cesium laser [Electron. Lett.44, 582 (2008)]. The mode-matching efficiency for the multi-transverse mode lasing is ~0.8 - 0.85 which means that in this case almost complete overlap of the laser and pump beams takes place. The laser beam quality factorM2increases with increasing pump power from 1 at the threshold power to 5-6 at maximum values of the pump power resulting in lower beam quality at high powers.

Non-contact video-based estimation of heart rate variability spectrogram from hemoglobin composition

Non-contact HR measurement is becoming an active research area. Recently, remote photoplethysmography (rPPG) measurement based on simple skin optics model has been proposed and shown to be effective. In this paper, we propose an accurate remote observation of the heart rate (HR) and heart rate variability (HRV) based on hemoglobin component estimation which is based on a detailed skin optics model. We perform experiments to measure subjects at rest and under cognitive stress with the proposed method putting a polarized filter in front of the camera to evaluate the principle of the framework. From the results of the experiments, the proposed method shows a high correlation with the electrocardiograph (ECG) which is assumed as the ground truth. We also evaluate the proposed method without putting any polarized filter and confirm the usefulness for the remote observation of HRV which requires accurate detection of HR.

A 2-D transient numerical heat transfer model of the solar absorber plate to improve PV/T solar collector systems

This paper presents a genuine 2-D transient numerical model to evaluate the temperature field and the dependent electrical performances of different solar absorber plates. It can readily be adapted for solar thermal absorber plates, standard PV plates or thermally enhanced PV plates that can be used in PV/T solar collectors. The model can also be combined with the appropriate heat exchanger equations to simulate a complete solar collector. A simple optical model is combined with the well-known five parameters PV module electrical model and the 2-D heat conduction equation to compute the temperature field of the PV plate and the electrical power generated for every time step of the simulation. The resulting differential equation is solved through finite volume discretization using an implicit time discretization scheme. The model relies on widely available weather parameters such as total horizontal solar radiation, ambient temperature, wind speed, etc. Validation of the electrical model is first carried out in real outdoor weather conditions comparing simulated and measured electrical current. During this process, measured temperature of the plate and incident solar radiation are used as inputs. Then, the global multi-physic model is validated using a temperature measurement at a selected point and the measured electrical performances. In the multi-physic model, measurements of the incident solar radiation, air temperature, wind speed and relative humidity are used as inputs. The validation results for the temperature showed a maximum discrepancy of the numerical predictions with respect to the measurements of less than 2 [°C] and a discrepancy below 7 [%] for the electric power production both over a complete day of simulation.

Errors in the detection and discrimination of vehicles: a simple optical model to explain driver and pedestrian misjudgements

Errors in the detection and discrimination of vehicles: a simple optical model to explain driver and pedestrian misjudgements