Changes In Absorption Coefficient Research Articles

Effect of Phytoplankton Community Composition and Size Structure on Light Absorption Properties

In the fields of phytoplankton ecology, water optics, and water color remote sensing, phytoplankton absorption properties represent the light absorption capacity of phytoplankton, which affects photosynthesis efficiency and carbon fixation. Here, the biomass, community composition, and the absorption properties of phytoplankton were measured alongside other bio-optical parameters in Lake Tianmu are examined using data collected between January and November 2013 (except February). Based on the relationships between phytoplankton biomass, community composition, and absorption, the effects of abundance, biomass, and equivalent sphere diameter on phytoplankton absorption and specific absorption were revealed. The highest biomass and abundance of phytoplankton were recorded in the autumn and the lowest in the winter. Cryptomonas, Synedra, and Cyclotella were the dominant genera throughout the year. The dominant genera structure type was Bacillariophyta-Cryptophyta in the winter and spring, Bacillariophyta-Chlorophyta-Pyrroptata in the summer, and Cryptophyta-Bacillariophyta-Chlorophyta in the autumn. Phytoplankton diameter was ranked in the order summer>autumn>winter>spring, with mean values of 64.83 μm in summer and 29.54 μm in spring. Phytoplankton absorption coefficients of were ranked in the order autumn > spring > winter > summer, with mean values at 440 nm and 675 nm of (0.66±0.18) m-1 and (0.33±0.10) m-1 in autumn and (0.17±0.02) m-1 and (0.08±0.01) m-1 in summer, respectively. The specific absorption coefficients of the phytoplankton were ranked in the order spring > winter > autumn > summer, with mean values at 440 nm and 675 nm of (0.07±0.02) m2·mg-1 and (0.04±0.01) m2·mg-1 in spring and (0.03±0.004) m2·mg-1 and (0.01±0.002) m2·mg-1 in summer, respectively. Significant linear correlations were found between phytoplankton biomass, abundance, and absorption coefficients. Variations of Bacillariophyta and Cyanophyta biomass caused by temperature explained the seasonal variation in absorption coefficients. The specific absorption coefficient decreased with an increase in equivalent sphere diameter, and variations in phytoplankton community composition explained seasonal changes in the specific absorption coefficient.

Temperature induced changes in the optical properties of skin in vivo

Knowledge of temperature-induced changes of skin optical properties is required for accurate dosimetry of photothermal treatments. We determined and compared in vivo optical properties of mouse ear skin at different temperatures. The diffuse reflectance, total and diffuse transmittance were measured in the spectral range from 400 to 1650 nm using an integrating sphere spectrometer at the temperatures of 25 °C, 36 °C and 60 °C. Target temperatures were attained and maintained using an automated heater equipped with a sensor for feed-back and control. Temperature and temperature induced morphological changes of skin were monitored using an infrared thermal camera and reflectance confocal microscopy, respectively. An inverse Monte Carlo technique was utilized to determine absorption, scattering, and anisotropy factors from the measured quantities. Our results indicate significant differences between the optical properties of skin at different temperatures. Absorption and scattering coefficients increased, whereas anisotropy factors decreased with increasing temperature. Changes in absorption coefficients indicate deoxygenation of hemoglobin, and a blue shift of water absorption bands. Confocal imaging confirmed that our observations can be explained by temperature induced protein denaturation and blood coagulation. Monitoring spectral responses of treated tissue may become a valuable tool for accurate dosimetry of light treatments.

Microfluidic tuning of linear and nonlinear absorption in graphene oxide liquid crystals.

Manipulation of the nonlinear optical response of materials plays a significant role in photonics applications; however, it may be irreversible, untunable, and uncontrollable, which makes it difficult. In this Letter, we present a mechanical-hydrodynamical approach through a microchannel to tune the nonlinear absorption response of graphene oxide liquid crystals. In this material, the optical properties depend on the flake orientation. This feature has helped us to study empirically the dependency of the nonlinear absorption coefficients to external hydrodynamical force by employing the Z-scan technique. The experimental results show that increasing the flow rate in the microchannel enhances both linear and nonlinear absorption coefficients and, as a result, reduces the laser beam transmission through the sample. It has been observed that the percentage change in the nonlinear absorption coefficient of the sample is significant due to the flow rate.

Theoretical modeling of inter-sublevel optical properties of a ZnSe/ZnSxSe1-x and ZnSe/ZnSxSe1-x/ZnS core/shell and core/shell/shell quantum dot

In this work, we have modelling a ZnSe/ZnSxSe1-x and ZnSe/ZnSxSe1-x/ZnS core/shell and core/shell /shell quantum dot (QDs) respectively in the intent to study the effect of the second shell ZnS on the electronic and optical properties. To solve the three dimensional Schrödinger equation, we have used a theoretical method which is combined the coordinate transformation and the finite difference method (FDM). We have evaluated the influence of ZnS shell material, first on the energy eigenvalues and on the transition lifetime, then on the linear, non-linear and total absorption coefficient and refractive index changes. The first linear and third order nonlinear susceptibilities are also investigated.

Ultrafast dynamics of hot charge carriers in an oxide semiconductor probed by femtosecond spectroscopic ellipsometry

Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorption partially blocked and screened by the photo-excited electron occupation of the conduction band and hole occupation of the valence band (absorption bleaching). Exciton absorption turns spectrally narrower upon pumping and sustains the Mott transition, indicating Mahan excitons. Simultaneously, intra-valence-band transitions occur at sub-picosecond time scales after holes scatter to the edge of the Brillouin zone. Our results pave new ways for the understanding of non-equilibrium charge-carrier dynamics in materials by reliably distinguishing between changes in absorption coefficient and refractive index, thereby separating competing processes. This information will help to overcome the limitations of materials for high-power optical devices that owe their properties from dynamics in the ultrafast regime.

Thermo-Optical Property Degradation of ITO-Coated Aluminized Polyimide Thin Films Under VUV and Low-Energy Proton Radiation

We studied thermo-optical property degradation of indium tin oxide (ITO)-coated aluminized polyimide thin films under exposure to vacuum ultraviolet radiation and low-energy (3 and 5 keV) protons during ground tests using the Complex Irradiation Facility at the DLR site in Bremen. Changes in solar absorption and thermal emission coefficients caused by the irradiation were analyzed. We report a significant increase in solar absorptance of the samples irradiated by protons. We also attempted to identify any defects on the surface of the samples. The study was motivated by a unique opportunity that is provided by the Complex Irradiation Facility to study the degradation effects induced by exposure to protons with an energy below 10 keV and short-wavelength light below 115 nm.

Nonlinear optical absorption coefficient and refractive index change in symmetric semiconductor nanostructures: Comparison between different methods

Abstract We present a systematic study of the optical absorption coefficient (OAC) and refractive index change (RIC) for a symmetric semiconductor nanostructure. We derived the OAC and RIC following two approaches, one without approximations, besides the usual ones (rotating-wave approximation and application of a two-level model), and the other using an approximation keeping only the contribution of the linear and third-order nonlinear terms. In the derivation we consider properly the contribution of the population decay time and the dephasing time, which may be different to each other, in general, in semiconductor nanostructures, extending previous works. We also apply the derived formulae for the calculation of the OAC and RIC in a specific semiconductor quantum well with position-dependent mass and compared the results obtained from the derived formulae. We find that while for low light intensities the different formulae for OAC and RIC give similar results for higher light intensities only the formulae obtained without approximations can lead to correct physical results. We also present a critical intensity below which the different formulae for OAC and RIC give practically the same results. This critical intensity can be used for obtaining correct behavior for the OAC and RIC in any symmetric semiconductor nanostructure.

Coloration of tungsten oxide electrochromic thin films at high bias potentials and low intercalation levels

The optical absorption of amorphous tungsten oxide thin films was studied at low intercalation levels. The electronic density of states was obtained by an electrochemical method and bias potential regions were assigned to the conduction band and gap states according to the experimentally estimated conduction band edge. Differences between the coloration in the conduction band and gap states were observed when comparing the experimental results to a theoretical site-saturation model that considers electronic transitions between localized tungsten sites. The model could reproduce the optical response due to conduction band states. However, it underestimated the rate of change of the optical absorption coefficient with intercalation level for gap states. This discrepancy is discussed in the context of small-polaron optical absorption.

Changing optical properties of ZnO powder upon separate and sequential irradiation by electrons and protons with an energy of 100 keV

The authors examined synergistic effects during sequential irradiation of zinc oxide powders by electrons and protons with an energy of 100 keV. Two modes were employed: 1) electrons followed by protons; 2) protons followed by electrons. It was found that the additive coefficient, when determined by the change in the integral absorption coefficient of solar radiation and calculated via diffuse reflectance spectra within 0.3–2.1 μm, depends on the irradiation mode and on the type of radiation-induced defects (i.e., intrinsic point defects or free electrons).

Design of Hybrid Plasmonic Multi-Quantum-Well Electro-Reflective Modulators Towards <100 fJ/bit Photonic Links

Realization of on-board and inter-chip optical interconnects requires a photonic data link with power consumption well below their electrical counterparts (i.e., 50 Gb/s requires 2–4 pJ/bit/channel. External reverse-biased modulators could drastically reduce this power consumption. Here we design ultralow power GaAs/AlGaAs multi quantum well electro-reflective modulators operating at 1 V for facile integration with polymer “optical bridges”, utilizing coupled quantum confined Stark effect between adjacent quantum wells and optical coupling to hybrid surface plasmon-slab modes for significantly enhanced extinction ratio and spectral bandwidth. Distinctive from conventional electro-optical or electro-absorption modulators, this new design synergistically leverages ultra-large changes in both refractive index (|Δn|∼0.05) and absorption coefficient (Δα∼104 cm−1), achieving 35-50 dB extinction ratio at 1 V reverse bias with a low insertion loss of 1–3 dB, an incident angle tolerance of ∼5°, and a spectral bandwidth of 7–10 nm. The modulator power consumption is ∼1.9 fJ/bit without the need of thermal tuning, and the RC-limited bandwidth well exceeds 100 GHz. This new modulator enables high bandwidth and ultralow power optical interconnect networks at >100 Gb/s/channel and <100 fJ/bit/channel compatible with ever-scaling CMOS technologies.

Spectroscopic Ellipsometry for Simultaneous Thickness and Defect Concentration Changes in Oxide Thin Films

Measurements of strain and stoichiometry changes needed to determine coefficients of chemical expansion (CCEs) can be very time-intensive for bulk materials due to the long equilibration times needed at the multiple instruments and conditions. Additionally, bulk polycrystalline materials give macroscopic responses that include both lattice and microstructural contributions, and the responses are often therefore averaged over the randomized orientation of grains. Thin films provide an avenue to rapidly assess steady-state properties of such chemically expanding materials after short dwells due to their low volume and high surface area. Additionally, the controlled growth of thin films can be used to fabricate polycrystalline samples or single crystals with tailored crystallographic orientation, grain dimensions, and induced strain for controlled experiments. Although techniques exist for in situ analysis of thickness changes or defect concentration changes in thin films, there are few (if any) techniques that can measure both simultaneously. Further, the limited existing techniques to probe defect concentrations of thin films, e.g., in-plane electrical conductivity, cross-plane chemical capacitance, or in situ X-ray diffraction, require additional knowledge such as the dominant carrier mobility, the interface capacitance, or the coefficient of chemical expansion respectively (which is what we wish to determine).Spectroscopic ellipsometry has been widely used to analyze film thicknesses and optical properties, but we show that the optical properties obtained from the measurement can also be used to quantify defect concentrations, specifically oxygen non-stoichiometry. This work builds on our prior in situ optical absorption studies of oxygen concentrations and dynamics in thin films [1-4]. When we induce an oxygen stoichiometry change in a film, ellipsometry is, in principle, able to provide a measurement of the corresponding thickness change and oxygen stoichiometry change by a single technique.To evaluate the effectiveness of this technique initially, a SrTi0.65Fe0.35O3 thin film has been scanned after anneals in O2 and Ar. The modelled expansion and optical constants, derived from the data, were compared with previous work, and thicknesses were verified by X-ray reflectometry (XRR). For materials whose extinction coefficient corresponding to oxygen concentration is already known, the oxygen stoichiometry change can be estimated from the measured absorption coefficient change. Additionally, a variation of the Clausius-Mossotti equation is proposed to estimate the defect concentration in materials that lack prior optical extinction coefficient data corresponding to the defect of interest. These preliminary results suggest that spectroscopic ellipsometry may be used for the rapid acquisition of in situ data to quantify thin film CCEs in various environments.[1] Perry, N. H., Kim, J. J., & Tuller, H. L. (2018). Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr (Ti, Fe) O3-x thin film case study. Science and Technology of advanced MaTerialS, 19(1), 130-141.[2] Perry, N. H., Pergolesi, D., Bishop, S. R., & Tuller, H. L. (2015). Defect chemistry and surface oxygen exchange kinetics of La-doped Sr (Ti, Fe) O3− α in oxygen-rich atmospheres. Solid State Ionics, 273, 18-24.[3] Chen, T., Harrington, G. F., Sasaki, K., & Perry, N. H. (2017). Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach. Journal of Materials Chemistry A, 5(44), 23006-23019.[4] Perry, N. H., Kim, N., Ertekin, E., & Tuller, H. L. (2019). Origins and Control of Optical Absorption in a Nondilute Oxide Solid Solution: Sr (Ti, Fe) O3–x Perovskite Case Study. Chemistry of Materials, 31(3), 1030-1041.

Optical second harmonic generation in LiB3O5 modulated by intense femtosecond X-ray pulses.

Many of the scientific applications for X-ray free-electron lasers seek to exploit the ultrashort pulse durations of intense X-rays to obtain femtosecond time resolution of various processes in a "pump-probe" scheme. One of the limiting factors for such experiments is the timing jitter between the X-rays and ultrashort pulses from more conventional lasers operating at near-optical wavelengths. In this work, we investigate the potential of using X-ray-induced changes in the optical second harmonic generation efficiency of a nonlinear crystal to retrieve single-shot arrival times of X-ray pulses with respect to optical laser pulses. Our experimental results and simulations show changes to the efficiency of the second harmonic generation of 12%, approximately three times larger than the measured changes in the transmission of the 800 nm center-wavelength fundamental pulse. Further experiments showing even larger changes in the transmission of 400 nm center-wavelength pulses show that the mechanism of the second harmonic generation efficiency modulation is mainly the result of X-ray-induced changes in the linear absorption coefficients near 400 nm. We demonstrate and characterize a cross-correlation tool based on this effect in reference to a previously demonstrated method of X-ray/optical cross-correlation.

Isosbestic Point in Optical Mapping; Theoretical and Experimental Determination With Di-4-ANBDQPQ Transmembrane Voltage Sensitive Dye.

Optical mapping methods utilize fluorescence dyes to measure dynamic response of cardiac tissue such as changes in transmembrane potential (Vm). For the commonly used Vm sensitive dyes, a dye absorption and emission spectra shift as Vm changes. Signals relevant to Vm are calculated as a relative fluorescence change with respect to the fluorescence baseline. The amplitude of the change depends on the long-pass (LP) filter cut-on wavelength, placed on the sensor side, and the excitation wavelength. An excitation wavelength near the absorption peak, termed the isosbestic point, results in minimal absorption coefficient change as absorption spectra shifts. Consequentially the fluorescence intensity virtually does not change, when fluorescence across the entire emission spectra is measured, irrelevant of Vm changes. In this study we experimentally determined the isosbestic point for a near infrared dye Di-4-ANBDQPQ. We then present a theoretical study examining the dye linear or non-linear response as the fractional fluorescence change of Vm change, due to emission spectra shift and amplitude change, over a range of excitation wavelengths and LP filters. Linear "optical" response is important to quantify certain aspects of cardiac dynamics such as the action potential (AP) shape and duration, especially when studying drug effects and dynamical substrates for arrhythmia development.

Interlayer distance effects on absorption coefficient and refraction index change in p-type double-δ-doped GaAs quantum wells**Project supported by PISA 2016–1 UJAT and PRODEP Folio UJAT-245 of México.

In the framework of the Thomas–Fermi (TF) approach, a model for the p-type double-δ-doped (DDD) system in GaAs is presented. This model, unlike other works in the literature, takes into account that the Poisson equation associated with the system is nonlinear. The electronic structure is calculated for heavy and light holes. The changes in the electronic structure result of the distance d between the doped layers are studied. In particular, the relative absorption coefficient as well as the relative refractive index change is calculated as a function of the incident photon energy for heavy holes. The effect of the interlayer distance exhibits, in the absorption coefficient, a red shift of the peak position and a decrease in amplitude when the distance increases. In addition, the relative refractive index change node has a red shift as well as the interlayer distance increases. The calculations show that the effect of the separation between layers has a greater influence on the linear terms. These results are very important for theoretical calculations and engineering of optical and electronic devices based in δ-doped GaAs.

Electronic structure and optical anisotropy in Sr1−x BaxFBiS2 (x = 0, 0.25, 0.5, 0.75, 1) based solar cell materials

We conducted theoretical simulations for exploring the overall physical properties of the pristine SrFBiS2 as well as its derivative alloys (Sr1−x BaxFBiS2 (x = 0, 0.25, 0.5, 0.75, 1)) resulting from BiS2 layers via the compositional effect of substituting strontium with barium. In this respect, we computed the electronic and optical characteristics of Sr1−xBaxFBiS2 systems within the concentration limits of x varying from 0 to 1. Based on full-potential linear augmented plane wave scheme, we determined the structural properties of the Sr1−xBaxFBiS2 alloys, and the lattice parameter was found to increase with the augmentation of Ba concentration, which in turn agrees well with the available experimental works. The electronic band structures and density of states of Sr1−xBaxFBiS2 (0 ≤ x ≤ 1) alloys were also examined. According to our calculations, the parent compound SrFBiS2 has a semiconducting behavior and its energy gap is about 0.8 eV. The electronic band structures illustrated that the substitution of the divalent Ba+2 element instead of Sr+2 produces a semiconducting behavior for all Sr1−xBaxFBiS2 (0 ≤ x ≤ 1) alloys. When substituting Ba+2 with Sr+2, the acquired optical features, such as the dielectric function, refractive index, reflectivity, absorption coefficient, energy loss functions, and optical conductivity for Sr1−xBaxFBiS2 (0 ≤ x ≤ 1) alloys also exhibited a semiconducting nature. It is easily noticed that the maximum absorption coefficient changes with the variation of Ba concentration. Furthermore, a superior birefringence occurs in the concerned Sr1−xBaxFBiS2 alloys when the composition variation undergoes from x = 0 to 1. Intriguingly, the optical spectra for both parent compounds SrFBiS2 and BaFBiS2 and their ultimate Sr1−xBaxFBiS2 alloys displayed a high anisotropy. The viable significant role of the spin-orbit coupling in deciding the electronic structures and optical characteristics is due to the presence of heavy bismuth atoms. All our results determined for Sr1−xBaxFBiS2 systems are compared with the existing theoretical and experimental reports.

Femtosecond laser direct writing of diffraction grating and its refractive index change in chalcogenide As2Se3 film.

Chalcogenide glasses possess novel optical property in infrared range, which make them ideal candidates for photonic devices by laser direct writing. Precisely control of the refractive index manipulation in the material is the key to achieve high quality optical devices. In this work, diffraction gratings in chalcogenide As2Se3 thin films were fabricated with femtosecond laser direct writing and its refractive index change was carefully studied. Grating diffraction efficiency was measured from visible to near-infrared light by using multiple single-wavelength lasers and a supercontinuum laser. Clear diffraction patterns and high diffraction efficiency indicated the good optical quality of the prepared gratings. Results show that the grating with period of 5 µm inscribed under pulse energy of 30 nJ demonstrated a 1st-order diffraction efficiency of 30% at 808 nm testing wavelength, due to the changes of refractive index and absorption. The relationship of the change of refractive index and absorption coefficient of film under different laser irradiation intensity is carefully studied. The maximum refractive index change was estimated to be 0.087 at 808 nm.

Consideration of Absorption Coefficient Changes in Numerical Simulations of Laser Forming

Material processing with laser beams is well-known in nowadays production. Compared to CO2 lasers, modern solid state lasers are, amongst others, popular because of higher energy efficiency and higher absorption when metals like steel and aluminum are irradiated. However, the absorption of metals is not only dependent on the chemical composition of the work piece metal and the laser beam wavelength. Previous investigations determined the oxidation of the surface as an influence on laser beam absorption changes due to multiple irradiation. In this study, a method is presented for considering the absorption coefficient changes caused by surface oxidation in numerical simulations. Reproductions of single trajectories were assigned with appropriate absorption coefficients calculated from a function generated by reference tryouts. With the described approach, benefits are gained for numerical simulations of laser beam forming (like bending) and other processes with an iterative heat input.

The effect of quantum ring size on shifting the absorption coefficient from infrared region to ultraviolet region

In this paper, the effects of magnetic flux and the width of the quantum ring on plasmon frequency are studied in a narrow quantum ring threaded by magnetic flux. Plasmon frequency is obtained by random phase approximation in the zero wave vector limits. Calculation of plasmon frequency as a function of magnetic flux shows Aharonov–Bohm oscillations. The periodicity and the amplitude of Aharonov–Bohm oscillations depend on magnetic flux and the width of the ring. In other words, the plasmon frequency of the quantum ring is sensitive to the variation of the magnetic flux and when the width is small, the modulation is more sensitive since the period and amplitude are smaller. The absorption coefficient versus ring radius changes from infrared region to ultraviolet region, which it can be of relevant application in solar cells.

Fabrication of graphene-magnetite multi-granule nanocluster composites for microwave absorption application

Ferrite multi-granule nanoclusters are fabricated for microwave absorption materials in different sized particles and granules by modified polyol process. Various sizes of ferrite nanoclusters are placed on graphene-based composites and the behavior of their microwave absorbing properties is studied. The absorbing properties are measured using the free-space method with two horn antennas for X-band range (8.2 GHz–12.4 GHz). Relative permittivity and permeability values are calculated in measured frequency domain. The absorption coefficient changes by forming ferrite-graphene composites are presented as well.

Impurity related optical properties in tuned quantum dot/ring systems

ABSTRACTIn this paper, we explore the linear, third-order nonlinear, and total optical absorption coefficient (OAC) and refractive index change coefficient (RICC) of a GaAs doped quantum dot/quantum ring (QD/QR) with parabolic-inverse squared potential in conjunction with modified Gaussian confinement and taking into account the presence of on-centre shallow donor and or acceptor impurity. Calculations are done via the compact density matrix formalism and the iterative method. The two-dimensional parabolic QD/QR is subjected to uniform magnetic field oriented perpendicularly to the plane of the structure. The energy levels and wave function are derived within the framework of effective-mass and parabolic band approximation. The results exhibit that the OACs and RICC are clearly affected by different parameters of the applied confinement, strength of magnetic field, and the presence of impurity. The variation of confinement potential, nature of impurity, dot radius, cyclotron frequency of the parabolic confinement potential, and geometric parameter of the on-centre repulsive potential lead to either a red-shift or a blue-shift of the resonant peaks of the OACs and of the maximum and minimum of the RICC together with significant variations of the magnitudes of these resonant structures.