Spectral Region Research Articles

A spectral–structural characterization of European temperate, hemiboreal, and boreal forests

Abstract. Radiative transfer models of vegetation play a crucial role in the development of remote sensing methods by providing a theoretical framework to explain how electromagnetic radiation interacts with vegetation in different spectral regions. A limiting factor in model development has been the lack of sufficiently detailed ground reference data on both the structural and spectral characteristics of forests needed for testing and validating the models. In this data description paper, we present a dataset on the structural and spectral properties of 58 stands in temperate, hemiboreal, and boreal European forests. It is specifically designed for the development and validation of radiative transfer models for forests but can also be utilized in other remote sensing studies. It comprises detailed data on forest structure based on forest inventory measurements, terrestrial and airborne laser scanning, and digital hemispherical photography. Furthermore, the data include spectral properties of the same forests at multiple scales: reflectance spectra of tree leaves and needles (based on laboratory measurements), the forest floor (based on in situ measurements), and entire stands (based on airborne measurements), as well as transmittance spectra of tree leaves and needles and entire tree canopies (based on laboratory and in situ measurements, respectively). We anticipate that these data will have wide use in testing and validating radiative transfer models for forests and in the development of remote sensing methods for vegetation. The data can be accessed at Hovi et al. (2024a, https://doi.org/10.23729/9a8d90cd-73e2-438d-9230-94e10e61adc9) (for laboratory and field data) and Hovi et al. (2024b, https://doi.org/10.23729/c6da63dd-f527-4ec9-8401-57c14f77d19f) (for airborne data).

WISE J152614.95-111326.4, an Unusual Variable Star

ABSTRACT New time series photometry of WISE J152614.95-111326.4, an eclipsing binary candidate, has been obtained. Full cycles of variation were covered in five filters, ranging from B to z. Archival time series photometry is also available from several sources. The phased light curve shape changes from a double wave form in the red, to a single wave at shorter wavelengths. Analysis of the spectral energy distribution and SALT spectra shows the presence of a cool (∼ 7250−7900 K) white dwarf and an M6 star. The light curves can be explained by a hot spot on the opposing hemisphere of the white dwarf. The star may be in a pre-cataclysmic variable phase with a very low rate of mass flow from the red dwarf to the white dwarf, such that no flickering is evident. Evidence in favour of this hypothesis is that the period of the system (2.25 h) is in the cataclysmic variable period gap. It is speculated that a weak magnetic field associated with the white dwarf funnels accreted material onto a magnetic pole. Amplitudes of the W1 and W2 WISE light curves are anomalously large. The possibility is discussed that variability in this spectral region is primarily driven by electron cyclotron radiation.

Ammonium hydrosulfide (NH_4SH) as a potentially significant sulfur sink in interstellar ices

Sulfur is depleted with respect to its cosmic standard abundance in dense star-forming regions. It has been suggested that this depletion is caused by the freeze-out of sulfur on interstellar dust grains, but the observed abundances and upper limits of sulfur-bearing ices remain too low to account for all of the missing sulfur. Toward the same environments, a strong absorption feature at sim 6.85 mu m is observed, but its long-standing assignment to the NH$_ $ cation remains tentative. We aim to spectroscopically investigate the plausibility of NH$_ $SH salt serving as a sulfur reservoir and a carrier of the 6.85 mu m band in interstellar ices by characterizing its IR signatures and apparent band strengths in water-rich laboratory ice mixtures. We then use this laboratory data to constrain NH$_ $SH abundances in observations of interstellar ices. Laboratory transmission IR spectra of NH$_ $:H$_ $S ice mixtures both with and without H$_ $O were collected. The apparent band strengths of the NH$_ $ asymmetric bending ($ $) mode and the SH$^ $ stretching mode in H$_ $O-containing mixtures were calculated with Beer's law plots. The IR features of the laboratory salts were compared to those observed toward a sample of four protostars (two low-mass, two high-mass) and two cold dense clouds without star formation. Apparent band strengths ranging from 3.2(pm 0.3)-3.6(pm 0.4)times 10$^ $ cm molec$^ $ and 3.1(pm 0.4)-3.7(pm 0.5)times 10$^ $ cm molec$^ $ are calculated for the NH$_ $ mode at sim 6.8 mu m/1470 cm$^ $ and the SH$^ $ stretching mode at sim 3.9 mu m/2560 cm$^ $, respectively, in NH$_ $SH:H$_ $O mixtures. The peak position of the NH$_ $ mode redshifts with increasing temperature but also with increasing salt concentration with respect to matrix species H$_ $O and NH$_ $. The observed 6.85 mu m feature is fit well with the laboratory NH$_ $SH:H$_ $O ice spectra. NH$_ $ column densities obtained from the 6.85 mu m band range from 8-23<!PCT!> with respect to H$_ $O toward the sample of protostars and dense clouds. These column densities are consistent with the optical depths observed at 3.9 mu m (the SH$^ $ stretching mode spectral region). A weak and broad feature observed at sim 5.3 mu m/1890 cm$^ $ is tentatively assigned to the combination mode of the NH$_ $ mode and the SH$^ $ libration. The combined upper limits of four other counter-anion candidates, OCN$^ $, CN$^ $, HCOO$^ $, and Cl$^ $, are determined to be lesssim 15-20<!PCT!> of the total NH$_ $ column densities toward three of the protostars. The redshift of the 6.85 mu m feature correlates with higher abundances of NH$_ $ with respect to H$_ $O in both the laboratory data presented here and observational data of dense clouds and protostars. The apparent band strength of the SH$^ $ feature is likely too low for the feature to be detectable in the spectrally busy 3.9 mu m region, but the 5.3 mu m NH$_ $ + SH$^ $ R combination mode may be an alternative means of detection. Its tentative assignment adds to mounting evidence supporting the presence of NH$_ $ salts in ices and is the first tentative observation of the SH$^ $ anion toward interstellar ices. If the majority (gtrsim 80-85<!PCT!>) of the NH$_ $ cations quantified toward the investigated sources in this work are bound to SH$^ $ anions, then NH$_ $SH salts could account for up to 17-18<!PCT!> of their sulfur budgets.

Semi-supervised spectral classification of DESI white dwarfs by dimensionality reduction

Abstract As a new generation of large-sky spectroscopic surveys comes online, the enormous data volume poses unprecedented challenges in classifying spectra. Modern unsupervised techniques have the power to group spectra based on their dominant features, circumventing the complete reliance on training data suffered by supervised methods. We outline the use of dimensionality reduction to generate a 2D map of the structure of an intermediate-resolution spectroscopic dataset. This technique efficiently separates white dwarfs of different spectral classes in the Dark Energy Spectroscopic Instrument’s Early Data Release (DESI EDR), identifying spectral features that had been missed even by visual classification. By focusing the method on particular spectral regions, we identify white dwarfs with helium features at 90percnt recall, and cataclysmic variables at 100percnt recall, illustrating rapid selection of low-contamination samples from spectroscopic surveys. We also demonstrate the use of dimensionality reduction in a supervised manner, outlining a procedure to classify any white dwarf spectrum in comparison with those in the DESI EDR. With upcoming surveys promising tens of millions of spectra, our work highlights the potential for semi-supervised techniques as an efficient means of classification and dataset visualisation.

Ultrafast Polarization‐Maintaining Fiber Lasers: Design, Fabrication, Performance, and Applications

AbstractUltrafast polarization‐maintaining fiber lasers (UPMFLs), with superior optical performance and high immunity to environmental disturbances, are highly preferable in a variety of industrial and scientific applications such as high‐precision micromachining and biomedical imaging. Especially, the utilization of PM fibers endows the laser intrinsic stability, thereby enabling the construction of robust and low‐noise optical frequency comb systems. To meet more demanding application challenges, continuous efforts have been invested in the design and fabrication of UPMFLs, aiming to reach unprecedented levels of various pulse parameters, that is, to achieve shorter pulse duration, higher or lower repetition rate, and higher pulse energy. This review presents a detailed overview of different passive mode‐locking techniques for pulsed operation and the most significant achievements in UPMFLs. Representative advances at 1.0, 1.55, and 2.0 µm spectral regions are presented and summarized. The state‐of‐the‐art lasing performance is application‐oriented, and conversely, optical improvements in all‐PM pulsed lasers promote emerging applications, which are also discussed and analyzed. How to overcome the bottlenecks of UPMFLs in terms of pulse duration, repetition rate, emission wavelength, and pulse energy to make them powerful tools for physical, medical, and biological applications remains challenging in the future.

Utilizing VSWIR spectroscopy for macronutrient and micronutrient profiling in winter wheat

This study explores the use of leaf-level visible-to-shortwave infrared (VSWIR) reflectance observations and partial least squares regression (PLSR) to predict foliar concentrations of macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur), micronutrients (boron, copper, iron, manganese, zinc, molybdenum, aluminum, and sodium), and moisture content in winter wheat. A total of 360 fresh wheat leaf samples were collected from a wheat breeding population over two growing seasons. These leaf samples were used to collect VSWIR reflectance observations across a spectral range spanning 350 to 2,500 nm. These samples were then processed for nutrient composition to allow for the examination of the ability of reflectance to accurately model diverse chemical components in wheat foliage. Models for each nutrient were developed using a rigorous cross-validation methodology in conjunction with three distinct component selection methods to explore the trade-offs between model complexity and performance in the final models. We examined absolute minimum predicted residual error sum of squares (PRESS), backward iteration over PRESS, and Van der Voet’s randomized t-test as component selection methods. In addition to contrasting component selection methods for each leaf trait, the importance of spectral regions through variable importance in projection scores was also examined. In general, the backward iteration method provided strong model performance while reducing model complexity relative to the other selection methods, yielding R2 [relative percent difference (RPD), root mean squared error (RMSE)] values in the validation dataset of 0.84 (2.45, 6.91), 0.75 (1.97, 18.67), 0.78 (2.13, 16.49), 0.66 (1.71, 17.13), 0.68 (1.75, 14.51), 0.66 (1.72, 12.29), and 0.84 (2.46, 2.20) for nitrogen, calcium, magnesium, sulfur, iron, zinc, and moisture content on a wet basis, respectively. These model results demonstrate that VSWIR reflectance in combination with modern statistical modeling techniques provides a powerful high throughput method for the quantification of a wide range of foliar nutrient contents in wheat crops. This work has the potential to advance rapid, precise, and nondestructive field assessments of nutrient contents and deficiencies for precision agricultural management and to advance breeding program assessments.

Femtosecond-laser-induced suprawavelength two-dimensional ZnO microstructures for multi-wavelength photodetector devices.

Herein, we report on two-dimensional (2D) suprawavelength crystalline ZnO microstructures induced by a single ultraviolet (UV) femtosecond laser beam (400 nm, 35 fs, 666 Hz) with significant absorption enhancement. The achieved absorption values of 90-99% and 75-80% in the UV and visible spectral regions, respectively, were approximately 1.16 and 12 times higher than those of the blank ZnO crystal. Furthermore, large-area 2D ZnO microstructures were fabricated to be used as photodetectors (PDs). The experimental results demonstrated that, compared with the blank ZnO, these 2D ZnO microstructures effectively enhanced the PD performance by nearly four times at 375 nm. More importantly, the ZnO microstructure exhibited great response value, ∼7.12 A/W at 532 nm as well as acceptable response at 660 and 808 nm, whereas the blank ZnO crystal showed almost no response. Raman analyses demonstrated that no change occurred after the femtosecond laser induced the microstructure on ZnO. Thus, the enhancement in photoelectric performance can be attributed to the strong absorption of the ZnO microstructure.

Waste Material Classification Based on a Wavelength-Sensitive Ge-on-Si Photodetector

Waste material classification is critical for efficient recycling and waste management. This study proposes a novel, low-cost material classification system based on a single, voltage-tunable Ge-on-Si photodetector operating across the visible and short-wave infrared (SWIR) spectral regions. Thanks to its tunability, the sensor is able to extract spectral information, and the system effectively distinguishes between seven different materials, including plastics, aluminum, glass, and paper. The system operates with a broadband illuminator, and material identification is obtained through the processing of the photocurrent signal at different bias voltages with classification algorithms. Here, we demonstrate the basic system functionality and near real-time classification of different waste materials.

Full Stokes-vector Inversion of the Solar Mg ii h and k Lines

The polarization of the Mg ii h and k resonance lines is the result of the joint action of scattering processes and the magnetic field–induced Hanle, Zeeman, and magneto-optical effects, thus holding significant potential for the diagnostic of the magnetic field in the solar chromosphere. The Chromospheric LAyer Spectro-Polarimeter sounding-rocket experiment, carried out in 2019, successfully measured at each position along the 196″ spectrograph slit the wavelength variation of the four Stokes parameters in the spectral region of this doublet around 280 nm, both in an active-region plage and in a quiet region close to the limb. We consider some of these CLASP2 Stokes profiles and apply to them the recently developed HanleRT Tenerife Inversion Code, which assumes a one-dimensional model atmosphere for each spatial pixel under consideration (i.e., it neglects the effects of horizontal radiative transfer). We find that the nonmagnetic causes of symmetry breaking, due to the horizontal inhomogeneities and the gradients of the horizontal components of the macroscopic velocity in the solar atmosphere, have a significant impact on the linear polarization profiles. By introducing such nonmagnetic causes of symmetry breaking as parameters in our inversion code, we can successfully fit the Stokes profiles and provide an estimation of the magnetic field vector. For example, in the quiet region pixels, where no circular polarization signal is detected, we find that the magnetic field strength in the upper chromosphere varies between 1 and 20 G.

Unprecedented Silver-Based Hybrid Pyroelectric Enabling Wide Spectral Photo-Pyroelectricity for Birefringence Photomodulation.

Birefringent crystal, which has the capacity to manipulate polarization light, holds an indispensable position in optics and optoelectronics, while it remains challenging to fulfill the modulation of birefringence. Here, we present wide spectral photo-pyroelectric effect in a silver-based hybrid pyroelectric, (N-CHM)Ag2I3 (N-CHM=N-cyclohexylmethylamine), serving as a feasible strategy to regulate birefringence through light stimuli. As the first silver-based hybrid pyroelectric, (N-CHM)Ag2I3 exhibits strong room-temperature photo-pyroelectricity with a large polarization of ~3.23 μC/cm2 and high voltage responsivity of ~0.96 m2/C across the UV-NIR spectral region. Strikingly, the photomodulation of its in-plane birefringence is established through pyroelectric effect, giving a saturation value of ~1.68×10-2 that is among the highest level achieved to date. This study on the birefringence photomodulation of lead-free hybrid pyroelectric is anticipated to boost future development of new smart optical and optoelectronic devices.

Chemometric approaches for discriminating manufacturers of Korean handmade paper using infrared spectroscopy

The objective of this study was to identify the manufacturer of Hanji, Korean handmade paper widely used in conservation science. To achieve this, machine learning models utilizing attenuated total reflectance–infrared spectroscopy (ATR–IR) were developed to assess the robustness and effectiveness of the computed models. Principal component analysis (PCA), partial least squares–discriminant analysis (PLS–DA), decision tree (DT), and k-NN models were constructed using IR spectral data, with the spectral region between 1800 and 1500 cm⁻1 identified as the critical input variable through Variable Importance in Projection (VIP) scores. The transformation of the obtained spectra into second derivative spectra proved beneficial in this key spectral region, leading to significant improvements in model performance. Additionally, the application of DBSCAN for outlier detection was effective in refining the dataset, further enhancing the performance of the models. Specifically, the k-NN model, when applied to the selected variables and preprocessed with the second derivative transformation, achieved an F1 score of 0.92. These findings underscore the importance of focusing on the 1800–1500 cm⁻1 spectral range and applying outlier detection techniques, such as DBSCAN, to enhance the robustness and accuracy of the Hanji classification models by eliminating the influence of atypical data points.

Coconut Testa Flour Sub-Fractions: Correlation Between FTIR Spectral Data and α-Glucosidase Inhibitory Activities

Fourier-transform infrared spectroscopy (FTIR) serves as a rapid analytical technique to characterize food specimens chemically. The purpose of this study was to investigate the potential of FTIR combined with multivariate statistics to detect Alpha-glucosidase (Alpha-glu) inhibitory activities of a non-cereal flour-like coconut testa flour (CTF). CTF of five distinct local cultivars was sequentially extracted with hexane, ethyl acetate (EtOAc), and methanol (MeOH) to assay the Alpha-glu inhibitory activity. FTIR spectra of CTF extracts were obtained within the range of 4000–500 cm−1 and the prominent spectral peaks obtained for both hexane and EtOAc extracts were roughly similar but some additional peaks were observed in EtOAc extracts representing phenolic constituents. The major absorbance peaks found in MeOH extracts were primarily indicative of the occurrence of the hydroxyl group associated with carbohydrates and phenolic compounds. The multivariate predictive models developed using partial least squares (PLS) and orthogonal partial least squares (OPLS) regression analyses indicated a strong correlation between Alpha-glu inhibitory activity and spectral data. Models developed for the spectral regions 3700–2800 cm−1 and 1800–500 cm−1 exerted the highest regression coefficients with the lowest root mean square errors. In OPLS regression analysis, the model obtained with third-derivative spectral data was identified as the best, exhibiting the highest regression coefficients and the lowest root mean square errors. Both PLS and OPLS regression analyses indicated a potential correlation of Alpha-glu inhibitory activity with FTIR spectral regions. Notably, OPLS models offered enhanced interpretability of the model parameters. This study suggests that the application of multivariate regression analysis of FTIR spectral data on coconut-based products could help to detect Alpha-glu inhibitory activities.

Plasmonic Silver Nanoparticles Facilitate Electron Emission from Diamond upon Sun‐like Excitation

The development of a stable, non‐toxic material that emits electrons following absorption of visible light may have a major impact on the solar photocatalysis of difficult reactions such as CO$_2$ and N$_2$ reduction, as well as for targeted chemical transformations in general. Diamond is a good candidate, however it is a wide bandgap material requiring deep UV photons (λ < 227 nm) to promote electrons from the valence band into the conduction band. Embedding silver nanoparticles under the diamond surface allows the photoconductivity of the diamond in the spectral region of the surface plasmon resonance to be increased, while also leading to an enhancement of visible light photoemission. Considering the low intensity of the light sources used in this work and the spectral properties of the enhanced photoconductivity and photoemission a mechanism based on plasmonically enhanced photoconductivity which in turn allows surface states emptied by photoemission to be recharged thus leading to enhanced photoemission in the visible range is proposed.

Identification of Potential Biomarkers and Spectral Fingerprinting for Detection of Foodborne Pathogens in Raw Chicken Meat Matrix Using GCMS and FTIR

Foodborne illnesses pose a serious threat to public health, with increasing global incidence rates driven by factors such as rising meat consumption. Rapid detection of foodborne pathogens in meat is critical for preventing outbreaks. This study investigates the potential of gas chromatography-mass spectrometry (GC-MS) and Fourier-transform infrared spectroscopy (FTIR) for identifying biomarkers and spectral fingerprints indicative of foodborne pathogens in raw chicken meat. Raw broiler chicken meat samples were surface-sterilized and inoculated with foodborne pathogens. The samples were challenge inoculated with the specific pathogen and the physical quality parameters like pH, color, texture, drip loss, and water activity were assessed. GC-MS analysis identified 113 metabolites, including potential biomarkers like ureidopropionic acid, 5-sulfosalicylic acid, 11,14-eicosadienoic acid, methyl ester for E. coli O157:H7; 11-bromoundecanoic acid, neocurdione, glafenin, eicosanoic acid for Salmonella; azepan-1-yl-acetic acid, methyl ester, tramadol, cytarabine, dipipanone for Staphylococcus and cyclopentaneundecanoic acid, phosphonofluoridic acid, î-n-formyl-l-lysine for Pseudomonas. Pathway analysis revealed the involvement of fatty acid metabolism and amino acid degradation pathways. FTIR spectral data showed significant variances between control and spiked samples, particularly in the fatty acid spectral region. The identified metabolites and spectral patterns could serve as biomarkers for developing rapid pathogen detection methods, contributing to enhanced food safety protocols.

Spin switching in Sm0.7Er0.3FeO3 triggered by terahertz magnetic-field pulses.

Driving spin systems to states far from equilibrium is indispensable in investigations of functional nonlinearities of antiferromagnets for spintronics. So far, it has been shown that electric-field pulses in the spectral region from the visible to the terahertz range can be used to induce ultrafast switching between different spin states. Here we demonstrate that a multicycle terahertz magnetic-field pulse can be used to induce non-thermal spin switching in antiferromagnets. When a strong pulse is applied to Sm0.7Er0.3FeO3, the magnetic order parameter is first driven away from the barrier between the two potential minima of this antiferromagnet and then, in the subsequent inertial motion towards the opposite direction, it crosses the barrier. Our analysis reveals that the initial motion is driven by a dynamical modification of the magnetic potential, and this modification is enhanced through coupling between the two magnon modes.

An enhanced method for reconstruction of full SIF spectrum for near-ground measurements

Recently the applications of remotely sensed Solar-Induced chlorophyll Fluorescence (SIF) in the study of photosynthesis, stress conditions, and gross primary production have increased significantly. The full SIF spectrum spans over a spectral region of 650 ∼ 850 nm with two characteristic peaks around 685 nm and 740 nm. Over recent decades, many retrieval algorithms have been developed to estimate SIF at Top-Of-Canopy (TOC) using in-situ measurements of solar irradiance and canopy radiance spectra. Although the majority of retrieval methods retrieve SIF at a narrow spectral window, there exists a potential for retrieval of SIF in the full emission spectrum. Moreover, solar irradiance and canopy radiance spectra should ideally be measured at the same time but are usually measured sequentially with a certain time lag, raising potential errors in SIF retrieval. In this study, an enhanced retrieval algorithm of the full SIF spectrum at TOC is proposed. The proposed algorithm attempts to minimize the errors owing to time mismatch in measurements of solar irradiance and canopy radiance spectra. As an improvement to the previous algorithm (advanced Fluorescence Spectrum Reconstruction, aFSR), this proposed algorithm (aFSR-SVE) models the SIF-free contribution with principal components using the singular value decomposition technique. The optimal parameter settings in the forward model were determined for the experimental data collected by spectrometers used in the study. Firstly, the proposed algorithm was used to reconstruct full SIF spectrum for simulated data. The results were compared with known reference SIF values. After achieving satisfying results from simulated data, the proposed algorithm was compared with retrievals from established algorithms using experimental data. The results show improved SIF retrieval accuracy, without the need to simultaneously measure solar irradiance and canopy radiance spectra. The retrieval values comply with the results of previous algorithms in terms of spectral shape, diurnal trend, and temporal variations. The induced errors in SIF retrievals due to non-simultaneous measurements of solar irradiance and canopy radiance spectra were also investigated and the proposed algorithm was found to be less prone to such errors. Hence, the proposed algorithm is an improvement in reconstructing the full SIF spectrum with near-ground measurements. With the help of the proposed algorithm, field measurements using sequential systems and automated measurements of multiple targets can be performed effectively as it relaxes the requirement of concurrent measurement of solar irradiance and canopy radiance spectra. For future work, the applicability of this method can be investigated under more variable illumination conditions, like high cirrus clouds, passing clouds or persistent thin clouds.

Sub-picosecond biphasic ultrafast all-optical switching in ultraviolet band

Abstract Ultrafast all-optical control has been a subject of wide-spread attention as a method of manipulating optical fields using light excitation on extremely short time scales. As a fundamental form of ultrafast all-optical control, all-optical switching has achieved sub-picosecond switch speeds in the visible, infrared, and terahertz spectral regions. However, due to the lack of suitable materials, ultrafast all-optical control in the ultraviolet range remains in its early stages. We demonstrate sub-picosecond all-optical switching in the ultraviolet wavelength by designing a Si3N4-ITO Fabry–Pérot resonance aligns with the edge of the interband transition region of ITO. The response time of 500 fs achieved at a pump fluence as low as 0.17 mJ/cm2. Notably, unlike conventional binary switches (0, 1), this biphasic all-optical switch enables the modulation of optical intensity with positive, zero, and negative ΔR/R (0, 1, −1) at the same wavelength, all achieved with a switching speed of 680 fs at a pump fluence of 0.45 mJ/cm2. This work establishing a new pathway for all-optical control in the ultraviolet spectrum, the biphasic switch provides an extra degree of freedom for all-optical modulation.

The role of losses in determining hyperbolic material figures of merit

Uniaxial materials have achieved new prominence in photonics because they can have hyperbolic spectral regions with metallic (ε<0) and dielectric (ε>0) permittivities along different crystal axes. In the lossless case, this results in an open hyperboloid dispersion relation, allowing materials to support highly confined modes with extremely large wavevectors. However, even small losses change the character of the hyperbolic dispersion from open hyperboloids to closed surfaces with finite maximum k, significantly limiting the extent to which highly-confined modes can be achieved. Here, we derive a simple analytic formula for the dispersion relation in the presence of loss and show that for some typical materials the maximum wavevector in hyperbolic materials is roughly ten times the free-space. The scaling of the maximum wavevector is derived, and it is shown that there is a universal scaling relation between the propagation length and the wavelength, which implies that the shortest wavelengths in any hyperbolic material are strongly attenuated.

Hypoxia-Active Iridium(III) Bis-terpyridine Complexes Bearing Oligothienyl Substituents: Synthesis, Photophysics, and Phototoxicity toward Cancer Cells.

In an effort to develop hypoxia-active iridium(III) complexes with long visible-light absorption, we synthesized and characterized five bis(terpyridine) Ir(III) complexes bearing oligothienyl substituents on one of the terpyridine ligands, i.e., nT-Ir (n = 0-4). The UV-vis absorption, emission, and transient absorption spectroscopy were employed to characterize the singlet and triplet excited states of these complexes and to explore the effects of varied number of thienyl units on the photophysical parameters of the complexes. In vitro photodynamic therapeutic activities of these complexes were assessed with respect to three melanoma cell lines (SKMEL28, A375, and B16F10) and two breast cancer cell lines (MDA-MB-231 and MCF-7) under normoxia (∼18.5% oxygen tension) and hypoxia (1% oxygen tension) upon broadband visible (400-700 nm), blue (453 nm), green (523 nm), and red (633 nm) light activation. It was revealed that the increased number of thienyl units bathochromically shifted the low-energy absorption bands to the green/orange spectral regions and the emission bands to the near-infrared (NIR) regions. The lowest triplet excited-state lifetimes and the singlet oxygen generation efficiency also increased from 0T to 2T substitution but decreased in 3T and 4T substitution. All complexes exhibited low dark cytotoxicity toward all cell lines, but 2T-Ir-4T-Ir manifested high photocytotoxicity for all cell lines upon visible, blue, and green light activation under normoxia, with 2T-Ir showing the strongest photocytotoxicity toward SKMEL28, MDA-MB-231, and MCF-7 cells, and 4T-Ir being the most photocytotoxic one for B16F10 and A375 cells. Singlet oxygen, superoxide anion radicals, and peroxynitrite anions were found to likely be involved in the photocytotoxicity exhibited by the complexes. 4T-Ir also showed strong photocytotoxicity upon red-light excitation toward all cell lines under normoxia and retained its photocytotoxicity under hypoxia toward all cell lines upon visible, blue, and green light excitation. The hypoxic activity of 4T-Ir along with its green to orange light absorption, NIR emission, and low dark cytotoxicity suggest its potential as a photosensitizer for photodynamic therapy applications.

Optimal Design and Analysis of Wide-Band Near-Infrared Hybrid Dielectric Gratings with High Transmission Efficiency

Since surface relief transmission gratings have very strict requirements on operators and use environment, according to the semiconductor laser external cavity spectral beam combining system, this paper proposes a design scheme for a semiconductor laser array spectral beam combining system based on the grating-external cavity. The finite element approach was used to create a wideband, high-efficiency fill-in multilayer dielectric transmission grating structure for a high-power spectrum beam combining system. The incidence angle, ridge height, duty cycle, and sidewall inclination angle of the transmission grating were tuned and evaluated, and a link between the transmission grating’s diffraction efficiency and grating characteristics was discovered. The calculated design of the high-power fused silica transmission grating has a negative first-order peak diffraction efficiency of 99.5% in the 800 nm range. In the spectral region of 765–872 nm, the transmission grating’s diffraction effectiveness exceeds 92%. The filled ultra-high diffraction efficiency multilayer dielectric transmission grating design addresses the issue of resistance to high-power lasers under complicated operating settings. It is intended to maintain a high diffraction efficiency even after several cleaning cycles, and it is an ideal component for high-power spectrum beam combining systems.