Wavelength-dependent Absorption Research Articles

Photoreflectance/scattering measurements of spider silks informed by standard optics.

The silks of certain orb weaving spiders are emerging as high-quality optical materials. This motivates study of the optical properties of such silk and particularly the comparative optical properties of the silks of different species. Any differences in optical properties may impart biological advantage for a spider species and make the silks interesting for biomimetic prospecting as optical materials. A prior study of the reflectance of spider silks from 18 species reported results for three species of modern orb weaving spiders (Nephila clavipes, Argiope argentata and Micrathena Schreibersi) as having reduced reflectance in the UV range. (Modern in the context used here means more recently derived.) The reduced UV reflectance was interpreted as an adaptive advantage in making the silks less visible to insects. Herein, a standard, experimental technique for measuring the reflectance spectrum of diffuse surfaces, using commercially available equipment, has been applied to samples of the silks of four modern species of orb weaving spiders: Phonognatha graeffei, Eriophora transmarina, Nephila plumipes and Argiope keyserlingi. This is a different technique than used in the previous study. Three of the four silks measured have a reduced signal in the UV. By taking the form of the silks as optical elements into account, it is shown that this is attributable to a combination of wavelength-dependent absorption and scattering by the silks rather than differences in reflectance for the different silks. Phonognatha graeffei dragline silk emerges as a very interesting spider silk with a flat ‘reflectance'/scattering spectrum which may indicate it is a low UV absorbing dielectric micro-fibre. Overall the measurement emerges as having the potential to compare the large numbers of silks from different species to prospect for those which have desirable optical properties.

Near-infrared absorption enhancement for perovskite solar cells via the rear grating design

Optical management and design is one of the most effective methods to unlock the full potential of photocurrent density and power conversion efficiency for perovskite solar cells (PSCs). However, the common-used optical designs are still limited by the near-infrared response. Here, we proposed an effective optical structure in PSCs by texturing the rear-side perovskite layer with a rectangular grating to promote the optical response in the near-infrared region. Through the comprehensive numerical simulations, the new design under the optimized structural configuration shows an improved photocurrent density, i.e., from 22.75 (flat structure) to 23.72 mA/cm2. We confirm that the optical absorption enhancement occurs at the near-infrared region by the wavelength-dependent absorption and reflection spectra. Besides, the light-trapping mechanism was clarified by the comprehensive analysis of the electric field distributions. Moreover, PSCs having the rear-grating design demonstrate the superior optical performance compared to that of flat design in the varied incident angles. Our simulation results displayed in this study provide an easy scheme to promote the optical absorption in near-infrared region.

A study on ultrafast laser micromachining and optical properties of amorphous polyether(ether)ketone (PEEK) films

Polyether(ether)ketone (PEEK) has been widely used in the electronics and biomedical industries due to excellent electrical, chemical and thermal stability. The ablation and optical properties of amorphous PEEK have been investigated using ultrahigh intensity fs laser pulses (180 fs/ 1 kHz) for precision manufacture. The single pulse ablation threshold was found to be 2.01±0.05 J cm-2 and 0.23±0.02 J cm-2 at 775 nm and 387 nm respectively. The significant difference has been attributed to the change in wavelength dependent nonlinear absorption order from multi-photon at 775 nm (via virtual states) to sequential 2-photon at 387 nm with photon energy within the material band edge. The 2-photon absorption coefficient at 387 nm was measured to be β ∼ 38 cm GW-1 at I ∼3 GW cm-2 leading to a strong light-matter coupling. Fs ablation quality is much superior at 387 nm with almost no melting and debris, allowing the observation of laser induced periodic surface structures (LIPSS) with pitch Λ < 0.4 µm. Large area LIPSS have been machined on 200 µm thick amorphous PEEK film with the direction of LIPSS parallel to the direction of linear polarisation.

CA-GAN: Class-Condition Attention GAN for Underwater Image Enhancement

Underwater images suffer from serious color distortion and detail loss because of the wavelength-dependent light absorption and scattering, which seriously influences the subsequent underwater object detection and recognition. The latest methods for underwater image enhancement are based on deep models, which focus on finding a mapping function from the underwater image subspace to a ground-truth image subspace. They neglect the diversity of underwater conditions which leads to different background colors of underwater images. In this paper, we propose a Class-condition Attention Generative Adversarial Network (CA-GAN) to enhance an underwater image. We build an underwater image dataset which contains ten categories generated by the simulator with different water attenuation coefficient and depth. Relying on the underwater image classes, CA-GAN creates a many-to-one mapping function for an underwater image. Moreover, in order to generate the realistic image, attention mechanism is utilized. In the channel attention block, the feature maps in the front-end layers and the back-end layers are fused along channels, and in the spatial attention block, feature maps are pixel-wise fused. Extensive experiments are conducted on synthetic and real underwater images. The experimental results demonstrate that CA-GAN can effectively recover color and detail of various scenes of underwater images and is superior to the state-of-the-art methods.

Energy Budgets for Terrestrial Extrasolar Planets

Abstract The pathways through which incoming energy is distributed between the surface and atmosphere have been analyzed for the Earth. However, the effect of the spectral energy distribution of a host star on the energy budget of an orbiting planet may be significant given the wavelength-dependent absorption properties of atmospheric CO2, water vapor, surface ice, and snow. We have quantified the flow of energy on aqua planets orbiting M-, G-, and F-dwarf stars, using a 3D Global Climate Model with a static ocean. The atmosphere and surface of an M-dwarf planet receiving an instellation equal to 88% of the modern solar constant at the top of the atmosphere absorb 12% more incoming stellar radiation than those of a G-dwarf planet receiving 100% of the modern solar constant, and 17% more radiation than an F-dwarf planet receiving 108% of the modern solar constant, resulting in climates similar to that of modern-day Earth on all three planets, assuming a 24 hr rotation period and fixed CO2. At 100% instellation, a synchronously rotating M-dwarf planet exhibits smaller flux absorption in the atmosphere and on the surface of the dayside, and a dayside mean surface temperature that is 37 K colder than its rapidly rotating counterpart. Energy budget diagrams are included to illustrate the variations in global energy budgets as a function of host star spectral class, and can contribute to habitability assessments of planets as they are discovered.

Wavelength-dependent nonlinear absorption and ultrafast dynamics process of WS2

The nonlinear absorption and ultrafast dynamic process of WS2 nanosheets were investigated by using a broadband (ranging from 450 to 700 nm) nanosecond Z-scan technique. Z-scan measurements showed that WS2 nanosheets exhibit saturable absorption (SA), and the magnitude of SA is wavelength-dependent. Besides, the ultrafast dynamics process of WS2 was also investigated with femtosecond transient absorption spectrum. It was found that there is a double-exponential energy relaxation in the process. The investigation shows that WS2 nanosheets can be used for ultrashort pulse generation and a wide spectral range optical absorber.

Underwater scene prior inspired deep underwater image and video enhancement

In underwater scenes, wavelength-dependent light absorption and scattering degrade the visibility of images and videos. The degraded underwater images and videos affect the accuracy of pattern recognition, visual understanding, and key feature extraction in underwater scenes. In this paper, we propose an underwater image enhancement convolutional neural network (CNN) model based on underwater scene prior, called UWCNN. Instead of estimating the parameters of underwater imaging model, the proposed UWCNN model directly reconstructs the clear latent underwater image, which benefits from the underwater scene prior which can be used to synthesize underwater image training data. Besides, based on the light-weight network structure and effective training data, our UWCNN model can be easily extended to underwater videos for frame-by-frame enhancement. Specifically, combining an underwater imaging physical model with optical properties of underwater scenes, we first synthesize underwater image degradation datasets which cover a diverse set of water types and degradation levels. Then, a light-weight CNN model is designed for enhancing each underwater scene type, which is trained by the corresponding training data. At last, this UWCNN model is directly extended to underwater video enhancement. Experiments on real-world and synthetic underwater images and videos demonstrate that our method generalizes well to different underwater scenes.

Direct measurement of refractive index and dispersion of optical glass by dual-prism configuration with imaging spectrograph

We present a simple and precise method based on a dual-prism configuration combined with spectrograph for wavelength-dependent refractive index and absorption measurements in optical glass. This technique measures the angle deviation due to the refractive index difference between two prisms. The refractive indices of dense flint N-SF11 glass from 400 to 650 nm were measured, and the limitations of the system were evaluated. From our results, the three-term Sellmeier coefficients of the glass sample were calculated showing excellent agreement with standard values.

Intrinsic polarization of elongated porous dust grains

ABSTRACT Observations of the Atacama Large Millimeter Array (ALMA) revealed recently polarized radiation of several protoplanetary discs in the (sub)millimetre wavelength range. Besides self-scattering of large particles, thermal emission by elongated grains is a potential source for the detected polarization signal. We calculate the wavelength dependent absorption and intrinsic polarization of spheroidally shaped, micrometre, and submillimetre sized dust grains using the discrete dipole approximation. In particular, we analyse the impact of dust grain porosity that appears to be present in discs when small grains coagulate to form larger aggregates. For the first time, our results show that (a) the intrinsic polarization decreases for increasing grain porosity and (b) the polarization orientation flips by 90 deg for certain ratios of wavelength to grain size. We present a new method to constrain grain porosity and the grain size in protoplanetary discs using multiwavelength polarization observations in the far-infrared to millimetre wavelengths. Finally, we find that moderate grain porosities ($\mathcal {P}\lesssim 0.7$) potentially explain the observed polarization fraction in the system HD 142527 while highly porous grains ($\mathcal {P}\gt 0.7$) fail unless the grain’s axis ratio is extraordinarily large.

Wavelength-dependent nonlinear absorption and ultrafast dynamics process of Au triangular nanoprisms.

The nonlinear absorption and ultrafast dynamics process of Au triangular nanoprisms were investigated by using broadband (ranging from 550 to 700 nm) nanosecond Z-scan measurements and femtosecond time-resolved transient absorption spectrum, respectively. We found that Au triangular nanoprisms exhibit saturation absorption (SA) at low excitation intensities. With the increase of incident intensity, a switch from SA to reverse saturation absorption (RSA) occurs. Photo-dynamics process was found to be a double-exponential energy relaxation with a fast and a slow decay component. Interestingly, when probe wavelength is away from the plasma resonance peak, the decay of relaxation also shows the modulation due to the vibration mode of the coherent excitation.

Absorption properties of black carbon aerosols over environmentally distinct locations in south-western India: Temporal, spectral characterization and source apportionment

The near-simultaneous measurements of wavelength dependent light absorption properties of black carbon (BC) aerosols employing the seven-channel, model AE-33 Aethalometers are carried out from the campuses of Ahmednagar College, Ahmednagar (AC-Ahmednagar) and Indian Institute of Tropical Meteorology, Pune (IITM-Pashan) from December 2015 to December 2016. Analysis brings out statistically significant higher values of BC absorption coefficient [σabs,880 nm (BC), Mm−1] (hence BC mass concentration) at site AC-Ahmednagar (annual mean = 99.95 ± 79.8 Mm−1) as compared to those at site IITM-Pashan (annual mean = 47.19 ± 59.9 Mm−1). The seasonal mean values of σabs,880 nm (BC) illustrate conspicuous intra-seasonal variability at both the locations. This variability seems to be influenced by the seasonal nature and strengths of BC emission sources, local meteorology, dynamics in atmospheric boundary layer (ABL) and modulation in advection pathways. The absorption Ångström exponent (AAE) analysis reveals that there exists a predominant contribution to the total BC by fossil fuel emission enriched mixed carbonaceous aerosols as well as freshly emitted fossil fuel burning BC aerosols at site AC-Ahmednagar while at site IITM-Pashan the major contribution to the total BC appears to come from fossil fuel/biomass emission enriched mixed carbonaceous aerosols. The source apportionment of BC mass depicts that the fractional contributions of BC_FF to BC mass dominate throughout the year at both observing locations. Seasonal variability of BC_BB fraction of BC at both locations is found to be complimentary to the seasonal variability of BC_FF fraction of BC at respective sites.

In-Plane Anisotropic Raman Response and Electrical Conductivity with Robust Electron-Photon and Electron-Phonon Interactions of Air Stable MoO2 Nanosheets.

The monoclinic m-MoO2 shows strong in-plane anisotropy that is essential to design anisotropic nanodevices. However, there is not yet detailed study on the anisotropy of m-MoO2 nanostructures. Here, the in-plane anisotropic Raman spectra and electrical conductivity of single-crystal m-MoO2 nanosheets were investigated systematically for the first time. Angle-resolved polarized Raman spectroscopy (ARPRS) shows distinct dependence on the excitation laser wavelength and sample thickness, demonstrating the robust wavelength-dependent optical absorption, electron-photon/electron-phonon interactions, and anisotropic phonon polarization in m-MoO2 nanosheets. Such results agree well with the semiclassical Placzek model. Moreover, the angle-dependent electrical conductivity of starburst-like m-MoO2 nanosheet devices shows a strong anisotropy with a conductivity ratio (σmax/σmin) of up to 10.1, which is the largest value in the previously reported 2D materials. This work underscores the importance of understanding the in-plane anisotropic light-matter interactions for the application of m-MoO2 nanosheets in anisotropic plasmonics and artificial synapse.

Black carbon: source apportionment and its implications on CCN activity over a rural region in Western Ghats, India.

This study presents the characteristics of black carbon aerosol (BC) over a high-altitude site, Mahabaleshwar during the monsoon season. The mass concentration of BC exhibits a morning peak and a daytime build-up with a mean mass concentration of 303 ± 142ngm-3. The simultaneous measurements of aerosol particle number concentration (PNC), cloud condensation nuclei concentration (CCN), and non-refractory particulate matter less than 1μm size (NR-PM1) were also made by using a Wide-Range Aerosol Spectrometer (WRAS), CCN counter and Aerosol Chemical Speciation Monitor (ACSM) respectively. The source apportionment using wavelength-dependent light absorption model reveals the dominance by wood burning sources during morning hours and traffic sources during remaining hours of the day. The diurnal variation of PNC follows the variability of BC mass concentration. However, CCN concentrations were high during the morning hours coinciding with the increased fractional contribution of organics. The k-means clustering coupled with fuzzy algorithm highlights the effect of different sources on aerosol size distribution. On the basis of size distribution curve, the 3 clusters were attributed to wood burning (mean diameter range: 50-100nm), traffic (30-50nm), and background aerosols (65-95nm). The combined analysis of k-means clustering, fractional contribution of organics, and kappa variation suggests that higher CCN concentration during morning is mainly attributed to probable emission of the water-soluble organic/inorganic compounds from wood burning.

An Improved Absorption Ångström Exponent (AAE)-Based Method for Evaluating the Contribution of Light Absorption from Brown Carbon with a High-Time Resolution

ABSTRACT While brown carbon (BrC) might play a substantially important role in radiative forcing, an estimation of its light absorption contribution with high-time resolution is still challenging. In this study, a multi-wavelength (370–950 nm) Aethalometer was applied to obtain the wavelength dependent light absorption coefficient (σabs) of aerosols both before and after being heated to 250°C. An improved absorption Angstrom exponent (AAE)-based method was developed to evaluate the contribution of BrC to light absorption at a wavelength of 370 nm (σabs,BrC/σabs,370nm). The σabs,BC at 370 nm was determined from the field measured AAE values for the wavelengths from 880 to 950 nm with a one-hour resolution. The simultaneous measurements of heated aerosols help confirm the negligible influence of BrC on the σabs values across the range of 880–950 nm. Meanwhile, σabs,BrC/σabs,370nm was also estimated with previously reported methods by assuming that the AAE was equal to 1 (Method I) as well as a new approach based on the light absorption enhancement (Method II). While the estimated σabs,BrC/σabs,370nm based on our developed method and Method I is highly correlated (r2 = 0.78), the difference could be as large as > 20% on average. The obtained mean σabs,BrC/σabs,370nm was negative with Method II, indicating the net production of BrC when the aerosols were heated. The difference between the values for σabs,BrC/σabs,370nm obtained by our developed method and by Method II was ~40% on average and much higher (> 50%) during the noon hour, when secondary organic aerosols and sulfate were abundant. We propose that it is more suitable to use an AAE around 0.7 for “pure” BC to evaluate the contribution of BrC to light absorption in the PRD region. The developed method thus helps improve our understanding of the light absorption and climate forcing of BrC.

Near-Infrared Hyperspectral Absorption/Scattering Imaging Method Using Multiple Ground Plates for Evaluating Polymer Composites.

This paper proposes a nondestructive method of evaluating polymer composites using near-infrared (NIR) diffuse reflection spectroscopy with multiple ground plates. Wavelength-dependent absorption and reduced scattering coefficients were acquired to evaluate the chemical structure and the concentration of the substances from absorption and to determine the size and the dispersity of filler in the polymer domain from scattering. NIR spectra of the sample were measured on multiple ground plates, namely, "ground-plate-dependent" diffuse reflection spectra. The effects of the external reflection on the ground-plate-dependent diffuse reflection spectra were subsequently removed. The internal reflection coefficient was calculated based on the difference between the diffuse reflectances of the neat resin and ground plates without prior information on the incident angle of light and the refractive index of sample. The external reflection coefficient was evaluated by the gap of diffuse reflectances between the sample and a white ground plate. After the corrections of reflections, the spectra were fitted by a physical model of light propagation based on the two-flux theory to acquire the absorption and the reduced scattering coefficients. The calculated absorption coefficients indicated a good linear relationship with particle concentration. The calculated reduced scattering coefficients agreed with the theoretical values by Mie scattering theory. It was demonstrated that the proposed method achieved the simultaneous evaluation of particulate-filler concentrations and sizes in polymer composites.

Enhanced broadband optical limiting and switching of nonlinear absorption in functionalized solar exfoliated reduced graphene oxide–Ag-Fe2O3 nanocomposites

Wavelength dependent nonlinear absorption (NLA) studies of Ag-Fe2O3–reduced graphene oxide (rGO) hybrids were performed using high-repetition rate (80 MHz), femtosecond (150 fs), and near-infrared (NIR) (700-900 nm) laser pulses. Tunable nonlinear absorption properties and broadband optical limiting performance of the pure and Ag-Fe2O3–(15, 25, and 40 wt. %) rGO nanocomposites were achieved by varying the parameters such as composition, defect states, and morphology. Detailed Z-scan experiments revealed that the NLA coefficient increases with an increase in the excitation wavelength. A switching behavior in the NLA mechanism for Ag-Fe2O3 and Ag-Fe2O3–(15 wt. %)rGO from saturable absorption (700 nm) to reverse saturable absorption (800 nm and 900 nm) was witnessed, while rGO and Ag-Fe2O3–(25 wt. % and 40 wt. %)rGO exhibited reverse saturable absorption, which is ascribed to be the 2-photon absorption (2PA) process. The origin of 2PA involves the electronic states of irregular conjugate carbon bonds based on sp2 domains of rGO for 700 nm and 800 nm excitations and surface plasmon resonance of Ag metals for 900 nm excitations. Ag-Fe2O3–(25 wt. %)rGO hybrid possesses a stronger NLA coefficient and a lower onset optical limiting threshold at all the wavelengths studied and, therefore, can be considered as a favorable candidate for broadband ultrafast optical limiters toward protection from NIR ultrashort pulse laser damages.

Highly Efficient Copper Sulfide-Based Near-Infrared Photothermal Agents: Exploring the Limits of Macroscopic Heat Conversion.

Among the foreseeable therapeutic approaches at the cellular level, nanoplatform-driven photothermal therapy is a thriving tool for the selective eradication of malignant tissues with minimal side effects to healthy ones. Hence, chemically versatile, near-infrared absorbing plasmonic nanoparticles are distinctly appealing and most sought after as efficient photothermal agents. In this work, a straightforward method to synthesize monodisperse PEGylated copper sulfide nanoparticles of pure covellite (CuS) phase, featuring strong localized surface plasmonic resonance absorption in the near-infrared and flexible surface chemistry, imparted by monomethyl ether polyethylene glycol molecules, is developed and optimized. These nanoparticles show a remarkable photothermal heat conversion efficiency (HCE) of 71.4%, which is among the highest for CuS systems and rivals that of plasmonic noble metal nanostructures. Moreover, through critical evaluation and mathematical modeling of the material's properties and measurement methodology, it is assessed that the calculated HCE values drastically depend on experimental conditions such as wavelength-dependent solvent absorption properties, sol concentration, and optical path. These findings are of paramount relevance to the photothermal community, since they call for a standardization of the procedure for the evaluation of the HCE of proposed photothermal agents, in order to make the reported values universally and reliably comparable.

Wavelength-Dependent Nonlinear Optical Absorption and Broadband Optical Limiting in Au-Fe2O3-rGO Nanocomposites

Broadband optical limiting and wavelength-dependent nonlinear absorption studies of Au-Fe2O3-(15, 25, 40 wt %)rGO (reduced graphene oxide) nanocomposites were investigated using the Z-scan technique and excited with femtosecond (700–900 nm, 80 MHz, 150 fs) laser pulses. A reduction in the graphene perfect (La) domain and ID/IG ratio observed in the Raman data and the appearance of a Au 4f and Fe 2p singlet along with a shift in the C 1s state toward higher energy in the XPS data, peak broadening in the XRD data, and different mass residues left out with respect to content of rGO in the TG-DTA data strongly indicated the incorporation of Au-Fe2O3 nanostructures on solar exfoliated reduced graphene oxide. Selected aread electron diffraction data ascertained the presence of Au and Fe2O3 as a composite, and transmission electron microscopy (TEM) data revealed that the nano-octahedra Au-Fe2O3 turned themselves into nanospindles during incorporation of rGO. Ground state absorption spectra illustrated a pure and...

Black and brown carbon over central Amazonia: long-term aerosol measurements at the ATTO site

Abstract. The Amazon rainforest is a sensitive ecosystem experiencing the combined pressures of progressing deforestation and climate change. Its atmospheric conditions oscillate between biogenic and biomass burning (BB) dominated states. The Amazon further represents one of the few remaining continental places where the atmosphere approaches pristine conditions during occasional wet season episodes. The Amazon Tall Tower Observatory (ATTO) has been established in central Amazonia to investigate the complex interactions between the rainforest ecosystem and the atmosphere. Physical and chemical aerosol properties have been analyzed continuously since 2012. This paper provides an in-depth analysis of the aerosol's optical properties at ATTO based on data from 2012 to 2017. The following key results have been obtained. The aerosol scattering and absorption coefficients at 637 nm, σsp,637 and σap,637, show a pronounced seasonality with lowest values in the clean wet season (mean ± SD: σsp,637=7.5±9.3 M m−1; σap,637=0.68±0.91 M m−1) and highest values in the BB-polluted dry season (σsp,637=33±25 M m−1; σap,637=4.0±2.2 M m−1). The single scattering albedo at 637 nm, ω0, is lowest during the dry season (ω0=0.87±0.03) and highest during the wet season (ω0=0.93±0.04). The retrieved BC mass absorption cross sections, αabs, are substantially higher than values widely used in the literature (i.e., 6.6 m2 g−1 at 637 nm wavelength), likely related to thick organic or inorganic coatings on the BC cores. Wet season values of αabs=11.4±1.2 m2 g−1 (637 nm) and dry season values of αabs=12.3±1.3 m2 g−1 (637 nm) were obtained. The BB aerosol during the dry season is a mixture of rather fresh smoke from local fires, somewhat aged smoke from regional fires, and strongly aged smoke from African fires. The African influence appears to be substantial, with its maximum from August to October. The interplay of African vs. South American BB emissions determines the aerosol optical properties (e.g., the fractions of black vs. brown carbon, BC vs. BrC). By analyzing the diel cycles, it was found that particles from elevated aerosol-rich layers are mixed down to the canopy level in the early morning and particle number concentrations decrease towards the end of the day. Brown carbon absorption at 370 nm, σap,BrC,370, was found to decrease earlier in the day, likely due to photo-oxidative processes. BC-to-CO enhancement ratios, ERBC, reflect the variability of burnt fuels, combustion phases, and atmospheric removal processes. A wide range of ERBC between 4 and 15 ng m−3 ppb−1 was observed with higher values during the dry season, corresponding to the lowest ω0 levels (0.86–0.93). The influence of the 2009/2010 and 2015/2016 El Niño periods and the associated increased fire activity on aerosol optical properties was analyzed by means of 9-year σsp and σap time series (combination of ATTO and ZF2 data). Significant El Niño-related enhancements were observed: in the dry season, σsp,637 increased from 24±18 to 48±33 M m−1 and σap, 637 from 3.8±2.8 to 5.3±2.5 M m−1. The absorption Ångström exponent, åabs, representing the aerosol absorption wavelength dependence, was mostly &lt;1.0 with episodic increases upon smoke advection. A parameterization of åabs as a function of the BC-to-OA mass ratio for Amazonian aerosol ambient measurements is presented. The brown carbon (BrC) contribution to σap at 370 nm was obtained by calculating the theoretical BC åabs, resulting in BrC contributions of 17 %–29 % (25th and 75th percentiles) to σap 370 for the entire measurement period. The BrC contribution increased to 27 %–47 % during fire events under El Niño-related drought conditions from September to November 2015. The results presented here may serve as a basis to understand Amazonian atmospheric aerosols in terms of their interactions with solar radiation and the physical and chemical-aging processes that they undergo during transport. Additionally, the analyzed aerosol properties during the last two El Niño periods in 2009/2010 and 2015/2016 offer insights that could help to assess the climate change-related potential for forest-dieback feedbacks under warmer and drier conditions.

Absorption by water increases fluorescence image contrast of biological tissue in the shortwave infrared

Recent technology developments have expanded the wavelength window for biological fluorescence imaging into the shortwave infrared. We show here a mechanistic understanding of how drastic changes in fluorescence imaging contrast can arise from slight changes of imaging wavelength in the shortwave infrared. We demonstrate, in 3D tissue phantoms and in vivo in mice, that light absorption by water within biological tissue increases image contrast due to attenuation of background and highly scattered light. Wavelengths of strong tissue absorption have conventionally been avoided in fluorescence imaging to maximize photon penetration depth and photon collection, yet we demonstrate that imaging at the peak absorbance of water (near 1,450 nm) results in the highest image contrast in the shortwave infrared. Furthermore, we show, through microscopy of highly labeled ex vivo biological tissue, that the contrast improvement from water absorption enables resolution of deeper structures, resulting in a higher imaging penetration depth. We then illustrate these findings in a theoretical model. Our results suggest that the wavelength-dependent absorptivity of water is the dominant optical property contributing to image contrast, and is therefore crucial for determining the optimal imaging window in the infrared.