Absolute Absorption Research Articles

Nearly perfect absorption of light in monolayer molybdenum disulfide supported by multilayer structures.

A novel multilayer photonic structure is proposed to achieve the strong enhancement of light absorption in monolayer molybdenum disulfide (MoS2). Both numerical and analytical results illustrate that the absolute absorption of light in this atomically thin layer can approach as high as 96% at the visible wavelengths due to the excitation of Tamm plasmon mode. It is also found that the operating wavelength and height of sharp absorption peak are particularly dependent on the layer thicknesses and period number of dielectric grating, MoS2 position in the spacer, and incident angle of light, which contribute to the tunability and selectivity of light-MoS2 interaction. These results would provide a new pathway for the improvement of MoS2 photoluminescence and photodetection.

Ultrathin tunable terahertz absorber based on MEMS-driven metamaterial

The realization of high-performance tunable absorbers for terahertz frequencies is crucial for advancing applications such as single-pixel imaging and spectroscopy. Based on the strong position sensitivity of metamaterials’ electromagnetic response, we combine meta-atoms that support strongly localized modes with suspended flat membranes that can be driven electrostatically. This design maximizes the tunability range for small mechanical displacements of the membranes. We employ a micro-electro-mechanical system technology and successfully fabricate the devices. Our prototype devices are among the best-performing tunable THz absorbers demonstrated to date, with an ultrathin device thickness (~1/50 of the working wavelength), absorption varying between 60% and 80% in the initial state when the membranes remain suspended, and fast switching speed (~27 μs). The absorption is tuned by an applied voltage, with the most marked results achieved when the structure reaches the snap-down state. In this case, the resonance shifts by >200% of the linewidth (14% of the initial resonance frequency), and the absolute absorption modulation measured at the initial resonance can reach 65%. The demonstrated approach can be further optimized and extended to benefit numerous applications in THz technology.

Development of a multichannel hyperspectral imaging probe for property and quality assessment of horticultural products

This paper reports on the development and calibration of a new multipurpose, multichannel hyperspectral imaging probe for property and quality assessment of horticultural and food products. The new multichannel probe consists of a 910μm diameter fiber as a point light source and 30 light receiving fibers of three sizes (i.e., 50μm, 105μm and 200μm) arranged in a symmetrical pattern, for simultaneous acquisition of 30 spatially-resolved reflectance spectra of horticultural and food samples with either flat or curved surface over the spectral region of 550–1650nm. Three types of calibration for the multichannel probe were carried out, i.e., linearity calibrations for each fiber of the hyperspectral imaging system to ensure consistent linear responses of individual fibers, spectral response calibrations of individual fibers for each fiber size group and between the three groups of different size fibers, and optical property calibrations using four reference liquid samples. The calibration results showed that all 30 fibers had high linearity with exposure time with the coefficient of determination being greater than 0.990 for wavelengths greater than 700nm; however, linear responses, as measured by the slope and intercept, for the individual fibers differed greatly and also varied with wavelength. After the calibrations, the probe was able to measure the scattering coefficient of the reference liquid samples with the relative errors between 3%–21%, whereas higher measurement errors were obtained for the absorption coefficient due to low absolute absorption values for the samples. Furthermore, the probe was demonstrated for measuring the absorption and scattering coefficients of tomato fruit at four maturity stages for the wavelengths of 550–1350nm.

Measurement of line strengths in the à 2A’ ← [formula omitted]2A” transition of HO2 and DO2

The absorption spectrum of the à 2A’ ← X˜2A” 000–000 band of HO2 and DO2 radicals has been measured in the range 6941–7077 cm−1 with a resolution of around 0.005 cm−1. HO2 and DO2 radicals were generated from the reaction of Cl-atoms with CH3OH and CD3OD, respectively, whereby Cl-atoms were generated by pulsed photolysis of (COCl)2 at 248 nm. One time-resolved absorption curve was measured by cw-Cavity Ring Down Spectroscopy (cw-CRDS) for each wavelength. The relative absorption coefficient for each wavelength was then obtained from the ring down time τ, extrapolated to t = 0 s with respect to the photolysis pulse, and the ring down time τ0 before the photolysis pulse. Absolute absorption coefficients were quantified for several selected lines relative to a well characterized absorption line of HO2 at 6638.21 cm−1 by a second cw-CRDS absorption path. The maximum absorption cross section at 50 Torr Helium was for HO2 the transition N Ka Kc J = 11 0 11 11.5 ← 11 1 11 11.5 with σ7000.28 cm−1 = 2.12 × 10−19 cm2 and for DO2 the doublet transition N Ka Kc J = 18 1 18 18.5 ← 18 0 18 18.5 and 18 1 18 17.5 ← 18 0 18 17.5 with σ7019.83 cm−1 = 2.97 × 10−19 cm2. The full spectrum has been very well reproduced by employing spectroscopic data from earlier works.

Measurement of the absolute absorbance based on wavelength modulation spectroscopy

Ratios of the 2nd and 1st harmonics at the line center are very sensitive to absorbance, but not to line profiles when the modulation index is set to 0.94. Based on this characteristic, we proposed a method which uses the 1st and 2nd harmonics to measure the absolute absorbance, and then obtain gas pressure and concentration. Some transitions of CO2 and H2O molecules near 6980 cm−1 are selected to verify this method. The satisfactory agreement between measurement results and theoretical values validates the proposed method.

Excitonic metal oxide heterojunction (NiO/ZnO) solar cells for all-transparent module integration

Could transparent solar cells work as the invisible power generator? Is it possible, in order to satisfy the on-site energy production, to install the transparent solar cell into the window of buildings and vehicles without recognizing the existence of energy harvesting entities?Here we propose and demonstrate the wide energy bandgap materials for visible light transmittance and UV photon absorption for power generation. All-transparent solar cell was achieved by the heterojunction of metal oxide layers. By using the solid-state sputtering method, transparent heterojunction (p-type NiO/n-type ZnO) was realized. A unit cell gave the record-high conversion efficiency of 6% with the enormous current density (2.7mA/cm2) and open circuit voltage of 532mV. The remarkable transparent solar power is mainly attributed to the absolute UV absorption to induce the substantial excitonic effect for ZnO/NiO heterojunction.For integration, the transparent solar cell units were fabricated on a glass substrate to demonstrate the module of unit solar cells. Using the 3 × 3 unit cell solar module, a significant output voltage (> 2V) was achieved to confirm the excellent connection manipulation of transparent solar cell units. By putting the transparent solar cells on buildings and vehicles, the electric power is spontaneously supplied from the Sun power but human eyes have no visible scarification. Transparent solar cells would provide a route for on-site energy generation.

Self-Referenced Method for Estimating Refractive Index and Absolute Absorption of Loose Semiconductor Powders

The absolute absorption coefficient, α(E), is a critical design parameter for devices using semiconductors for light harvesting associated with renewable energy production, both for classic technologies such as photovoltaics and for emerging technologies such as direct solar fuel production. While α(E) is well-known for many classic simple semiconductors used in photovoltaic applications, the absolute values of α(E) are typically unknown for the complex semiconductors being explored for solar fuel production due to the absence of single crystals or crystalline epitaxial films that are needed for conventional methods of determining α(E). In this work, a simple self-referenced method for estimating both the refractive indices, n(E), and absolute absorption coefficients, α(E), for loose powder samples using diffuse reflectance data is demonstrated. In this method, the sample refractive index can be deduced by refining n to maximize the agreement between the relative absorption spectrum calculated from bidire...

Resonance enhanced multiphoton ionization − time of flight (REMPI-TOF) detection of Br ( 2 P j ) atoms in the photodissociation of 4-bromo-2,3,5,6-tetrafluoropyridine at 234 nm: Effect of low-lying πσ* states

Abstract The photodissociation dynamics of 4-bromo-2,3,5,6-tetrafluoropyridine (BTFP) have been studied in a supersonic molecular beam around 234 nm, which prepares the molecule in its ππ* state. The dynamics of the C Br bond dissociation have been investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS) by detecting the nascent spin orbit states of the primary bromine atoms, namely, Br (2P3/2) and Br*(2P1/2). State specific polarization dependent TOF profiles were obtained, from which the translational energy distributions and recoil anisotropy parameters, βi, were extracted using forward convolution method. A strong polarization dependence of TOF profiles suggests anisotropic distributions of the Br and Br* fragments. Two components, namely, the fast and the slow, are observed in the translational energy distribution of Br and Br* atoms, formed from different potential energy surfaces. The average translational energies released into the Br and Br* channels for the fast component are 17.3 ± 2.0 and 11.1 ± 2.0 kcal/mol, respectively. Similarly, for the slow component, the average translational energies imparted into the Br and Br* channels are 2.6 ± 1.0 and 1.5 ± 1.0 kcal/mol, respectively. The relative quantum yields of Br and Br* are 0.87 ± 0.15 and 0.13 ± 0.06. The anisotropy parameters for Br and Br* are characterized by a similar value of 0.60 ± 0.05. The energy partitioning into the translational modes is interpreted with the help of various models. The experimental studies revealing the nature of translational energies distribution for Br and Br* and its average value along with the theoretical calculations employing Time-Dependent Density Functional Theory (TD-DFT) and Multi Configuration Quasi Degenerate second order Perturbation Theory (MCQDPT2) suggest that the initially prepared ππ* state crosses over to a nearby σ* repulsive state along the C Br bond, mainly, πσ* state from where the dissociation takes place. The results also indicate the process of fast internal conversion to ground state from the initially prepared ππ* state, which eventually forms slow Br atoms after C-Br bond dissociation process on the ground state. During the course of experiments, the absorption spectrum was also obtained with its absolute absorption cross section. Non-adiabatic curve crossing plays an important role in the C Br bond dissociation of BTFP.

GPU-based acceleration and mesh optimization of finite-element-method-based quantitative photoacoustic tomography: a step towards clinical applications.

Finite element method (FEM)-based time-domain quantitative photoacoustic tomography (TD-qPAT) is a powerful approach, as it provides highly accurate quantitative imaging capability by recovering absolute tissue absorption coefficients for functional imaging. However, this approach is extremely computationally demanding, and requires days for the reconstruction of one set of images, making it impractical to be used in clinical applications, where a large amount of data needs to be processed in a limited time scale. To address this challenge, here we present a graphic processing unit (GPU)-based parallelization method to accelerate the image reconstruction using FEM-based TD-qPAT. In addition, to further optimize FEM-based TD-qPAT reconstruction, an adaptive meshing technique, along with mesh density optimization, is adopted. Phantom experimental data are used in our study to evaluate the GPU-based TD-qPAT algorithm, as well as the adaptive meshing technique. The results show that our new approach can considerably reduce the computation time by at least 136-fold over the current central processing unit (CPU)-based algorithm. The quality of image reconstruction is also improved significantly when adaptive meshing and mesh density optimization are applied.

Bile Salt Stabilized Vesicles (Bilosomes): A Novel Nano-Pharmaceutical Design for Oral Delivery of Proteins and Peptides.

With the advent of novel vesicular drug delivery systems especially bilosomes, for large molecular weight proteins and peptides, their oral administration seems a viable approach. These nano-vesicles have shown promising results for the effective delivery of insulin and other therapeutics, perhaps due to their structural composition. The present review has elaborated the biopharmaceutical challenges for the oral delivery of therapeutic proteins and peptides as well as presented a novel approach to deliver the essential macromolecules through oral route as bilosomes. The extensive search has been presented related to the formulation, evaluation and in vivo performance of bilosomes. Some of the crucial findings related to bilosomes have corroborated them superior to other colloidal carriers. The successful drug delivery through bilosomes requires significant justifications related to their interaction with the biological membranes. The other aspects such as absolute absorption, safety and toxicity of bilosome drug delivery should also be equally considered.

Cavity-Based Medium Resolution Spectroscopy (CBMRS) in the THz: A Bridge Between High- and Low-Resolution Techniques for Sensor and Spectroscopy Applications

System technologies and sensor applications in the THz largely bifurcate between high-resolution ( 1 GHz) methods. There is an extensive high-resolution literature and many approaches to low-resolution spectroscopy have been proposed and demonstrated. In this paper, we discuss and apply an approach, cavity-based medium resolution spectroscopy (CBMRS) that is applicable continuously across this intermediate resolution gap. CBMRS systems use high brightness and spectrally pure continuous wave electronic sources to determine absolute absorption coefficients by the measurement of 100–1000 mode widths of a high-finesse Fabry–Perot cavity. This eliminates the challenge of distinguishing broad spectral features from a complicated baseline that results from variations in source power, transmission efficiency, and detector response. CBMRS has many fewer data points than observed with high-resolution systems interrogating near Doppler-limited spectra. Thus, CBMRS sacrifices the “absolute” specificity of high resolution Doppler-limited systems, but still can retain excellent specificity. Indeed, this smaller sampling does not set the specificity limit, but rather it is set by whether or not the analyte has a specific THz signature. Results from pressure broadened (including atmospheric pressure) applications and molecules with dense and/or quasi-continuous spectra will be presented. Comparisons with related observations in the literature are discussed.

Cavity ring-down Faraday rotation spectroscopy for oxygen detection

A combination of the path length enhancement provided by cavity ring-down spectroscopy together with the selectivity and noise suppression capabilities of Faraday rotation spectroscopy is utilized for highly sensitive detection of oxygen at ~762.3 nm. The system achieves a noise-equivalent rotation angle of 1.3 × 10−9 rad/√Hz, and a trace O2 detection limit of 160 ppb for 100 s of averaging. The technique relies on measurements of the losses in two orthogonal polarization directions simultaneously, whereby an absolute assessment of the magnetically induced polarization rotation can be retrieved, analogous to the absolute absorption measurement provided by stand-alone cavity ring-down spectroscopy. The differential nature of the technique described here eliminates the need for off-resonance decay measurements and thereby allows for efficient shot-to-shot fluctuation suppression. This is especially advantageous when operating the system under measurement conditions that severely affect the non-absorber related losses, such as particulate matter contamination typically present in combustion or open-path applications.

(Invited) Recent Advances in SWCNT Analytical Spectroscopy

Since the 2002 assignment of absorption and emission transitions to specific (n,m) structures, optical spectroscopy has offered the potential for simple, detailed structure-resolved characterization of SWCNT samples. However, this goal has not been fully realized because of incomplete spectroscopic and photophysical knowledge and a lack of specialized instrumentation. We describe here recent progress in overcoming these problems to enable quantitative sample analysis through combined fluorescence and absorption spectroscopy. One important step was determining absolute E11 absorption cross sections (molar absorptivities) for a number of common (n,m) species. This was achieved by using a direct counting method to calibrate the absolute concentrations of reference samples, and then by extending the list of measured cross sections through variance spectroscopy determinations of (n,m)-resolved particle concentrations in mixed samples. To properly interpret fluorimetric data, we have measured, interpreted, and modeled emission spectra and excitation spectra for a range of semiconducting (n,m) species. Combinations of single particle and ensemble spectroscopies were used for these studies. The models were extrapolated to other (n,m) species to predict E11 line widths and shapes plus the positions, amplitudes, and widths of spectral sidebands. When combined with data on structure-dependent fluorimetric efficiencies, the results allow fluorescence spectra of nanotube mixtures to be simulated as a sum of contributions from multiple (n,m) species of determined relative concentrations. To convert those concentrations from relative into absolute values, the absorption spectrum of a mixed sample is simulated as a background function plus components determined from the fluorescence emission and excitation profiles for the (n,m) species that were found to be present. Then the separate E11 (n,m) absorbance components are divided by their structure-specific absorption cross sections to find absolute species concentrations. The overall process allows quantitation of semiconducting (n,m) concentrations from simple and quick spectral measurements.

Automated room temperature optical absorbance CO sensor based on In-doped ZnO nanorod

Metal oxide based optical absorbance gas sensor (MOAGS) exhibits underlying potential to be the most promising energy-saving device with long-term stability and excellent sensing performance. In this work, a self-customized automated MOAGS setup was employed to study the carbon monoxide (CO) gas sensing performance between the hydrothermal synthesized ZnO nanorod (ZNR) and Indium-doped ZnO nanorod (IZNR) operating at room temperature of (25±1)°C. Specifically, it was found that the In3+ was evenly doped into ZnO lattice which in turn increased the defect density and generated an impurity state within the energy band structure of ZnO. The results showed that 0.04 mol% IZNR exhibited an optimum sensing performance with absolute optical absorbance change (OAC) of 0.067 a.u. under the 10ppm of CO gas testing environment with fastest respond and recovery time. The dependency of sensing performances to the optical band gap energy alteration and the defect state was evaluated. This work provides a simple and feasible route to develop the room temperate operating MOAGS for hazardous gas detection.

Hydrogen dimer features in low temperature collision-induced spectra

The absorption of radiation in pure hydrogen (H2) gas around the S(0) and S(1) rotational transitions is computed at 20 K and compared with laboratory data. All transitions involving free state are included in the calculations of the absolute absorption. These calculations are done with an isotropic approximation for the H2–H2 pair potential. Agreement with the experiment is observed around the S(0) transition, while the computational approach appears to be slightly worse around the S(1) transition. The positions for bound-to-bound transitions are computed including the full anisotropic pair potential. The anisotropy seems to be crucial to achieve agreement with the measured bound-to-bound transition frequencies. However, those transitions contribute little to the total absorption. The present computed absolute absorptions will provide improved input for radiative transfer models of planetary atmospheres.

Integrating Sphere Microscopy for Direct Absorption Measurements of Single Nanostructures.

Nanoscale materials are promising for optoelectronic devices because their physical dimensions are on the order of the wavelength of light. This leads to a variety of complex optical phenomena that, for instance, enhance absorption and emission. However, quantifying the performance of these nanoscale devices frequently requires measuring absolute absorption at the nanoscale, and remarkably, there is no general method capable of doing so directly. Here, we present such a method based on an integrating sphere but modified to achieve submicron spatial resolution. We explore the limits of this technique by using it to measure spatial and spectral absorptance profiles on a wide variety of nanoscale systems, including different combinations of weakly and strongly absorbing and scattering nanomaterials (Si and GaAs nanowires, Au nanoparticles). This measurement technique provides quantitative information about local optical properties that are crucial for improving any optoelectronic device with nanoscale dimensions or nanoscale surface texturing.

Molar Absorptivity Measurements in Absorbing Solvents: Impact on Solvent Absorptivity Values.

Molar absorptivity is a fundamental molecular property that quantifies absorption strength as a function of wavelength. Absolute measurements of molar absorptivity demand accounting for all mechanisms of light attenuation, including reflective losses at interfaces associated with the sample. Ideally, such measurements are performed in nonabsorbing solvents and reflective losses can be determined in a straightforward manner from Fresnel equations or effectively accounted for by path length difference methods. At near-infrared wavelengths, however, many solvents, including water, are absorbing which complicates the quantification of reflective losses. Here, generalized equations are developed for calculating absolute molar absorptivities of neat liquids wherein the dependency of reflective loss on absorption properties of the liquid are considered explicitly. The resulting equations are used to characterize sensitivity of absolute molar absorptivity measurements for solvents to the absorption strength of the solvent as well as the path length of the measurement. Methods are derived from these equations to properly account for reflective losses in general and the effectiveness of these methods is demonstrated for absolute molar absorptivity measurements for water over the combination region (5000-4000 cm-1) of the near-infrared spectrum. Results indicate that ignoring solvent absorption effects can incorporate wide ranging systematic errors depending upon experimental conditions. As an example, systematic errors range from 0 to 10% for common conditions used in the measurement of absolute molar absorptivity of water over the combination region of the near-infrared spectrum.

Components of antioxidant systems in the cells of aerotolerant sulfate-reducing bacteria of the genus Desulfovibrio (strains A2 and TomC) isolated from metal mining waste

Two strains of sulfate-reducing bacteria of the genus Desulfovibrio (A2 and TomC) isolated from metal mining waste were able to grow on agar Postgate C nutrient medium under microaerobic conditions. Since their growth in liquid nutrient medium was just slightly affected by 1% O2 (initial concentration in the gas phase) and 0.05–0.1 mM H2O2, these strains were relatively oxygen-tolerant. Only the presence of oxidants in high concentrations (5–10% О2 or 0.3–1.0 mM H2O2) resulted in practically complete inhibition of their growth. Strain A2 was more resistant to oxidative stresses than strain TomC. Activities of the key enzymes of antioxidant defense—superoxide dismutase (SOD), catalase, and peroxidase—were revealed in the cell-free extracts of strain A2 grown under strict anaerobic conditions. While strain TomC was found to possess no peroxidase activity, its catalase activity was much higher than that of strain A2 (36 and 2 U/mg protein, respectively). SOD activity of both strains was almost the same (5 U/mg protein). Sublethal H2O2 doses (concentration of 0.05–0.15 mM and exposure for 45–240 min) resulted in a drastic increase of catalase activity, especially in strain A2. Sublethal О2 doses (1–2% in the gas phase) had no significant effect on activities of the antioxidant enzymes of both strains. The cytochrome composition determined from the absolute absorption spectra of the whole cells of strains TomC and A2 revealed the presence of the c heme (438 and 831 pmol/mg protein) and the d heme (336 and 303 pmol/mg protein, respectively). The presence of the d heme indicated the presence of the bd heme–heme quinol oxidase, which together with the c heme may provide for the functioning of the electron transport segment of the antioxidant defensive system, which is responsible for aerotolerance of sulfate-reducing bacteria.

(n,m)-Specific Absorption Cross Sections of Single-Walled Carbon Nanotubes Measured by Variance Spectroscopy.

A new method based on variance spectroscopy has enabled the determination of absolute absorption cross sections for the first electronic transition of 12 (n,m) structural species of semiconducting single-walled carbon nanotubes (SWCNTs). Spectrally resolved measurements of fluorescence variance in dilute bulk samples provided particle number concentrations of specific SWCNT species. These values were converted to carbon concentrations and correlated with resonant components in the absorbance spectrum to deduce (n,m)-specific absorption cross sections (absorptivities) for nanotubes ranging in diameter from 0.69 to 1.03 nm. The measured cross sections per atom tend to vary inversely with nanotube diameter and are slightly greater for structures of mod 1 type than for mod 2. Directly measured and extrapolated values are now available to support quantitative analysis of SWCNT samples through absorption spectroscopy.

Ab initio calculations for the first-positive bands of N2

The absolute absorption spectral line intensities of the 0-0, 1-1, 2-2, 3-3, and 4-4 bands of N2(A3Σu+−B3Пg) are computed for the first time with the accurate potential energy curves (PECs) and transition dipole moment curves (TDMCs) using the ab initio calculation. During the calculation process, the precise multireference configuration interaction (MRCI) approach with the augmented correlation consistent polarized valence quadruple-zeta set (aug-cc-pVQZ) and active space 3110311 are used. The calculated PECs of the two states (A3Σu+, B3Пg) agree well with Rydberg-Klein-Rees (RKR) potential, and the calculated TDMCs are more reasonable than the results of other studies. Our calculated Einstein A coefficients are also well consistent with the recent experiment results. The intensities of five bands are obtained from the summation of their respective absolute line intensities, for instance, the 0-0 band intensities are 3.25×10−16cm−1/(moleculecm−2) at 300K and 1.65×10−16cm−1/(moleculecm−2) at 3000K. The theoretical results are credible. They are also beneficial for astrophysics where high temperature dominates.