Changes In Optical Absorption Research Articles

Effects of Hydrostatic Pressure on Optical Absorption and Refractive Index Changes of Exciton in Semimagnetic CdMnTe Quantum Wells

Effects of Hydrostatic Pressure on Optical Absorption and Refractive Index Changes of Exciton in Semimagnetic CdMnTe Quantum Wells

Electric field induced optical absorption and refractive index changes in a diluted magnetic quantum well

Binding energy of a confined exciton is investigated in a Cd1−xoutMnxoutTe/Cd1−xinMnxinTe/Cd1−xoutMnxoutTe diluted magnetic quantum well in the influence of electric field. Calculations are performed for various Mn incorporation in Cd1−xMnxTe material within a single band effective mass approximation using variational method. Spin polaronic shifts are estimated using mean field theory for different Mn concentration and the well sizes. A theoretical study of diluted magnetic semiconductors treating local sp–d exchange interaction J between the itinerant carriers and the Mn electrons is treated within a realistic band structure. The optical absorption and the refractive index changes as a function of normalized photon energy in the presence of electric field strength and the Mn ion content are analysed. Our results show that the occurred red shift of the absorption resonant peak due to the electric field gives the information about the variation of two energy levels in the quantum well. The optical absorption coefficients and the refractive index changes strongly depend on the incident optical intensity, the electric field strength and Mn content.

Fast optical signal in visual cortex: Improving detection by General Linear Convolution Model

In this study we applied the General Linear Convolution Model to fast optical signals (FOS). We modeled the Impulse Response Function (IRF) as a rectangular function lasting 30ms, with variable time delay with respect to the stimulus onset. Simulated data confirmed the feasibility of this approach and its capability of detecting simulated activations in case of very unfavorable Signal to Noise Ratio (SNR), providing better results than the grand average method. The model was tested in a cohort of 10 healthy volunteers who underwent to hemi-field visual stimulation. Experimental data quantified the IRF time delay at 80-100ms after the stimulus onset, in agreement with classical visual evoked potential literature and previous optical imaging studies based on grand average approach and a larger number of trails. FOS confirmed the expected contralateral activation in the occipital region. Correlational analysis between hemodynamic intensity signal, phase and intensity FOS supports diffusive rather than optical absorption changes associated with neuronal activity in the activated cortical volume. Our study provides a feasible method for detecting fast cortical activations by means of FOS.

Exciton-related nonlinear optical absorption and refractive index change in GaAs–Ga1−xAlxAs double quantum wells

In this work the variations of the exciton-related optical absorption and the change of the refractive index in a GaAs–(Ga,Al)As double quantum well as functions of the geometric parameters of the heterostructure are investigated. The variational method is applied within the framework of the parabolic band and effective mass approximations, in order to obtain the 1s-like exciton energy spectrum. The outcome for the related optical coefficients shows a quenched and redshifted light absorption as a result of the increment in the inner barrier and right-hand well widths, with the possibility of an enhancement of the excitonic contribution to the relative change in the refractive index.

Effects of hydrostatic pressure on the nonlinear optical properties of a donor impurity in a GaAs quantum ring

The effects of hydrostatic pressure, size quantization and impurity position on the binding energies of a hydrogenic-like donor impurity in a two-dimensional GaAs quantum ring and together with the linear and nonlinear intraband optical absorption and the relative refractive index changes are studied using the variational method and effective-mass approximation. The binding energies of 1s and 2s states are examined as functions of the structure (inner and outer radii), impurity position and hydrostatic pressure. We have also investigated the dependencies of the linear, nonlinear, and total optical absorption coefficients and relative index changes as a function of incident photon energy for different geometric configurations, hydrostatic pressure and impurity position. It is found that the variation of distinct sizes of the structure and impurity position leads to either a redshift and/or a blueshift of the resonant peaks of the intraband optical absorption and relative refractive index change spectrum. In addition we have found that the application of a hydrostatic pressure leads to a blueshift.

A Pronounced Dispersion Effect of Crystalline Silicon Nanoparticles on the Performance and Stability of Polymer:Fullerene Solar Cells

We investigated the dispersion effect of crystalline silicon nanoparticles (SiNP) on the performance and stability of organic solar cells with the bulk heterojunction (BHJ) films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM). To improve the dispersion of SiNP in the BHJ films, we attached octanoic acid (OA) to the SiNP surface via esterification reaction and characterized it with Raman spectroscopy and high-resolution transmission electron microscopy. The OA-attached SiNP (SiNP-OA) showed improved dispersion in chlorobenzene without change of optical absorption, ionization potential and crystal nanostructure of SiNP. The device performance was significantly deteriorated upon high loading of SiNP (10 wt %), whereas relatively good performance was maintained without large degradation in the case of SiNP-OA. Compared to the control device (P3HT:PC(61)BM), the device performance was improved by adding 2 wt % SiNP-OA, but it was degraded by adding 2 wt % SiNP. In particular, the device stability (lifetime under short time exposure to 1 sun condition) was improved by adding 2 wt % SiNP-OA even though it became significantly decreased by adding 2 wt % SiNP. This result suggests that the dispersion of nanoparticles greatly affects the device performance and stability (lifetime).

Electron-related nonlinearities in GaAs–Ga1−xAlxAs double quantum wells under the effects of intense laser field and applied electric field

The combined effects of intense laser radiation and applied electric fields on the intersubband-related linear and nonlinear optical properties in GaAs-based quantum wells are discussed. It is shown that for asymmetric double quantum well, the increasing laser field intensity causes progressive redshifts in the peak positions of the second and third harmonic coefficients. However, the resonant peaks of the nonlinear optical rectification can suffer a blueshift or a redshift, depending on the laser strengths. The same feature appears in the case of the resonant peaks corresponding to the total coefficients of optical absorption and relative change in the refractive index.

Photoacoustic tomography can detect cerebral hemodynamic alterations in a neonatal rodent model of hypoxia-ischemia.

Hypoxic-Ischemic Encephalopathy (HIE) is one of the most recognized causes of neurological deficits in children. Cerebral blood flow (CBF) reductions, as seen with HIE, resulting in neuronal injury have not been evaluated in real-time. Photoacoustic Tomography (PAT) is a form of optical imaging which can detect cerebral hemodynamic alterations in a noninvasive, non-ionizing fashion via changes in hemoglobin optical absorption. Further, this technology has the potential to capture cerebral blood volume (CBV) fluctuations and perhaps CBF changes in real-time. We hypothesized that PAT can detect a reduction in cerebral hemoglobin optical absorption, and therefore CBF, in a neonatal model of hypoxia-ischemia. To investigate, P7 rats underwent right carotid artery ligation and exposure to 8 percent oxygen for 60 minutes while imaged with PAT every 20 minutes. Cerebral hemodynamic alterations, as measured by mean optical absorption (MOA), were calculated as a change from baseline. Global and regional MOA was analyzed using a linear mixed model. Global MOA was reduced within the right hemisphere as compared to the left during hypoxia. Regional differences in MOA were detected between the left and right sides for the middle and posterior cortical regions. Injury was confirmed using immunohistochemistry. We conclude that a reduction in global and regional MOA, and hence CBF, could be identified by PAT in a neonatal rat model of HIE. This is the first study described in the literature utilizing a neonatal rat model of HIE to demonstrate in vivo alterations in cerebral hemodynamics in a non-invasive and near real-time fashion.

Approximation error method can reduce artifacts due to scalp blood flow in optical brain activation imaging

Diffuse optical tomography can image the hemodynamic response to an activation in the human brain by measuring changes in optical absorption of near-infrared light. Since optodes placed on the scalp are used, the measurements are very sensitive to changes in optical attenuation in the scalp, making optical brain activation imaging susceptible to artifacts due to effects of systemic circulation and local circulation of the scalp. We propose to use the Bayesian approximation error approach to reduce these artifacts. The feasibility of the approach is evaluated using simulated brain activations. When a localized cortical activation occurs simultaneously with changes in the scalp blood flow, these changes can mask the cortical activity causing spurious artifacts. We show that the proposed approach is able to recover from these artifacts even when the nominal tissue properties are not well known.

Optical ammonia gas sensor based on nanostructure dye-doped polypyrrole

Design and fabrication of an optical gas sensor based on ammonia vapor sensing system are described. A dye-doped nanostructure polypyrrole (PPy) film, as sensitive layer, has been synthesized by chemical processing. The dye-doped PPy gas sensor was prepared by the doping of eriochrome cyanine R (ECR) in polypyrrole (PPy–ECR) and used for selective optical ammonia gas sensor. The sensing properties of the optical PPy–ECR and morphology of the polymer film was studied by UV–vis spectroscopy and scanning electron microscopy (SEM) respectively. The change in optical absorption of thin polymer layers in the presence of ammonia gas is used as a sensor signal. Sensing tests performed under exposition to calibrated amounts of ammonia reveal that these simple and inexpensive sensors are able to detect ammonia gas at room temperature in a fast reliable way, with a sensitivity linearly related to concentration in the range between 15 μg L−1 and 260 μg L−1. Optical PPy–ECR gas sensor presents low detection limit (5 μg L−1) and good reproducibility in construction (RSDs < 5%). The sensor is highly selective to ammonia gas over diverse amine gases, water and organic solvents. The method was applied for determination of ammonia in environmental real samples with satisfactory recovery.

Morphology and optical absorption change of Ag/SiO2 core-shell nanoparticles under thermal annealing

Ag/SiO2 core shell nanoparticles can be potentially useful in solar cells with the expectation to enhance light absorption. Thermal stability of such nanoparticles would be an important issue and has not been investigated much in the past. This research studied both morphological and optical absorption changes when heat-treating Ag/SiO2 core shell nanoparticles at various temperatures. At lower temperatures, Ag diffuses out through the porous shell, and remains inside at higher temperatures when the shell becomes solid. The results are instructive to further apply such nanoparticles into solar cells.

Improved Hardness of Single-Crystal ${\rm Nd}^{3+}:{\rm YAG}$ to Ionizing Radiation via Co-Doping with ${\rm Cr}^{3+}$

The proliferation of optical elements in systems that are subject to harsh ionizing radiation environments prompts the need for commercially available radiation-hard materials, such as laser materials. In the present study, changes in the material optical absorption are compared for a suite of single-crystal YAG, Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> :YAG, and Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ,Nd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> :YAG laser materials in response to high-dose-rate gamma radiation events. It is found that the addition of Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> yields radiation hardened material.

The Influence of Gap Length on Cooperativity and Rate of Association in DNA-Modified Gold Nanoparticle Aggregates

Polyvalent gold nanoparticle–DNA conjugates hybridize with complementary linker DNA strands to form aggregates that exhibit sharp dissociation curves indicative of cooperative behavior. Introducing single-stranded gaps consisting of thymidines (T1–T20) into the linker strand resulted in a decrease in the number of duplexes that dissociate cooperatively. Upon adding one base insertion (T1) the cooperative number drops from 6.3(2) to 2.8(2) duplexes. The cooperative number then increases slightly for the T3 gap and thereafter decreases for T8 and T10, with a slight increase again for the T20 gap. As the presence of a shared condensed cation cloud has been implicated in neighboring duplex cooperativity, we measured the salt-dependent behavior of Tn gap-linked unmodified duplexes and the number of ions released per duplex dissociation. Interestingly, the number of cations released for the duplexes with a longer gap sequence is significantly larger than the number released for a T1 gap-linked duplex or a nicked duplex (T0). Overall there is a correlation between the change in condensed cation density and the dissociation entropy for the unmodified Tn gap-linked duplexes, and the cooperative unit for the Tn gap-linked GNP–DNA aggregates. Using dynamic light scattering and changes in optical absorbance, we also found that aggregation of GNP–DNA is more rapid when hybridization occurs at a nicked versus gap site, which was previously observed but attributed to slower hybridization as a result of the longer linker strand. By comparing the aggregation rate of a prehybridized GNP–DNA:T10-linker complex with a completely complementary GNP–DNA and a GNP–DNA that led to a T10 gap, we were able to establish that the presence of the gap, not DNA length or accessibility, caused the decrease in aggregation rate. Our results support that flexibility in aggregates decreases the rate of aggregation as well as the extent of cooperativity, which has important implications in genomic DNA detection.

Linear and nonlinear intersubband optical absorption and refractive index change in asymmetrical semi-exponential quantum wells

Linear and nonlinear intersubband optical absorption and refractive index change in asymmetrical semi-exponential quantum wells are theoretically investigated within the framework of the compact–density–matrix approach and iterative method. The wave functions are obtained by using the effective mass approximation. The energy levels are obtained by numerical method. It is found that the optical absorption coefficients and refractive index changes are strongly affected not only by σ and U0, but also by the incident optical intensity.

Hydrostatic pressure on optical absorption and refractive index changes of a shallow hydrogenic impurity in a GaAs/GaAlAs quantum wire

The effect of hydrostatic pressure on the binding energy of a hydrogenic impurity in a GaAs/GaAlAs quantum wire is discussed. Calculations have been performed using Bessel functions as an orthonormal basis within a single band effective mass approximation. Pressure induced photoionization cross section of the hydrogenic impurity is investigated. The total optical absorption and the refractive index changes as a function of normalized photon energy between the ground and the first excited state in the presence of pressure are analysed. The optical absorption coefficients and the refractive index changes strongly depend on the incident optical intensity and the pressure.

Optical fibre-coupled cryogenic radiometer with carbon nanotube absorber

A cryogenic radiometer was constructed for direct-substitution optical-fibre power measurements. The cavity is intended to operate at the 3 K temperature stage of a dilution refrigerator or 4.2 K stage of a liquid cryostat. The optical fibre is removable for characterization. The cavity features micromachined silicon centring rings to thermally isolate the optical fibre as well as an absorber made from micromachined silicon on which vertically aligned carbon nanotubes were grown. Measurements of electrical substitution, optical absorption and temperature change indicate that the radiometer is capable of measuring a power level of 10 nW with approximate responsivity of 155 nW K−1 and 1/e time constant of 13 min. An inequivalence between optical and electrical power of approximately 10% was found, but the difference was largely attributable to unaccounted losses in the optical fibre.

Synthesis and optical properties of 1,1-binaphthyl–thiophene alternating copolymers with main chain chirality

A series of axially chiral 1,1-binaphthyl–thiophene copolymers were synthesized by Migita–Kosugi–Stille polycondensation and electrochemical polymerization. The polymers exhibit circular dichroism not only in the ultraviolet region but also in the visible region, both in solution and film states. Optical and electrochemical properties of the polymers are in good agreement with theoretical results obtained by density functional theory calculations. Although effective conjugation of the polymers was cleaved at the orthogonally oriented two naphthalene rings in the 1,1-binaphthyl unit, the electrochemically synthesized polymer film shows good electrochemical redox behavior with repeatable changes in optical absorption and circular dichroism. The changes in circular dichroism may reflect a change in the dihedral angle of the binaphthyl unit caused by reorientation of conjugated main chains between oxidized and neutral states.

Production of novel microporous porphyrin materials with superior sensing capabilities

New robust microporous cobalt 5,10,15,20-meso-tetraphenyl porphyrin (CoTPP) thin films for sensing applications have been produced by a novel plasma-based deposition technique named Glow Discharge Induced Sublimation (GDS). CoTPP films have been also produced by conventional vacuum evaporation (VE) and spin coating (SPIN) methods for comparison. The chemical properties of the films were assessed by FT-IR and ESI-MS analyses demonstrating the integrity and the purity of the GDS films. The physical properties of the samples were thoroughly analysed: thickness measurements coupled with surface density analyses showed the impressive free volume of the GDS samples (30 times higher than that of VE ones), SEM images show the extremely rough morphology of GDS samples, and physisorption measurements indicate both the extremely high specific surface area (184 m2 g−1) and the microporosity of the GDS porphyrin films. The sensing capabilities of the samples were investigated by exposing them to low concentrations of NO2 and by monitoring their optical absorption changes. These measurements clearly demonstrate that the GDS process leads to superior sensing materials as a result of the distinctive attainable molecular architectures. GDS-grown CoTPP sensors exhibit very high intense and sensitive responses as well as faster responses and much lower detection limits (<1 ppm) than conventionally deposited ones. Finally, the repeatability and reproducibility of sorption–desorption processes demonstrate the robustness of these assemblies. The GDS method can be extended to several other organic receptors, opening the way for the production of new improved sensing materials.

PH-Responsive Optical Modulation of Chemically Reduced Graphene through Noncovalent Interaction with Poly(acrylic acid)

Abstract pH-responsive chemically reduced graphene/poly(acrylic acid) (PAA) assembly is simply formulated through noncovalent interaction. Graphene/PAA assembly shows optical absorbance change depending on pH of the solution. Structural rearrangement of graphene/PAA assembly might contribute this optical modulation at different pH through either “hydrophobic interaction dominating” mode in strongly acidic medium or “charge repulsion dominating” mode in strongly basic medium.

Nonlinear optical properties of a three-electron quantum dot with account of the Rashba spin–orbit interaction

In this study, a detailed investigation of the nonlinear optical properties such as optical absorption and refractive index change associated with intersubband transitions in a three-electron quantum dot in two dimensions in the presence of the Rashba spin–orbit interaction has been carried out. We present the exact wave functions and energy levels of the system. Numerical results on typical GaAs/AlGaAs materials show that the decrease of the quantum dot radius blueshifts and amplifies the absorption coefficients as well as the refractive index changes, as expected. Additionally, an increase of the optical intensity and relaxation time considerably changes the absorption coefficients and the refractive index changes.