High Spectral Resolution Measurements Research Articles

Design and inversion study of a 1.6 µm high resolution non-modulated laser heterodyne radiometer (NM-LHR)

Laser heterodyne detection boasts exceptional advantages such as high spectral resolution and high signal-to-noise ratio (SNR). It excels at capturing spectral line broadening information of upper atmospheric molecules, which presents substantial research value in the realms of greenhouse gas profile measurement and the assessment of laser propagation effects in the atmosphere. This paper delves into the investigation of the processing method for heterodyne signals, adopting a non-modulated signal processing method to construct a near-infrared non-modulated laser heterodyne radiometer. This innovative design significantly enhanced the response speed and SNR. The radiometer achieved a spectral resolution of 0.006 cm-1 and an SNR of 300. This facilitated the acquisition of vertical profile distribution and column concentration of CH4 by measuring the absorption spectrum. Comparative tests revealed compelling advantages of the non-modulated device. The modulated device collected data 6 times in 6 minutes, yielding an SNR of 58. In contrast, the non-modulated device demonstrated superior efficiency by collecting data 6000 times in 2 minutes, resulting in a remarkable SNR of 103. In the process of inversion, the influence of the solar spectrum was coupled to improve the accuracy of inversion results. The inversion results of the CH4 column concentration from the laser heterodyne radiometer were compared with those from the Fourier transform spectrometer (EM27/SUN), with average concentrations of 1.946 ppmv and 1.930 ppmv, and exhibited an overall deviation of approximately 0.8%. The non-modulated laser heterodyne radiometer provides a new reference for the rapid, accurate and high spectral resolution measurements of greenhouse gas concentration.

Study of chemiluminescence of methane–air flame stabilized on a flat porous burner

In this work, the spatial distribution and spectral characteristics of the chemiluminescence of chemically excited species, OH∗ and CH∗, are experimentally and numerically studied by using a stationary premixed methane–air flame stabilized on the surface of a flat porous burner for various equivalence ratio and normal pressure. Numerical simulations are carried out using detailed reaction mechanisms, and the experimental study includes high-resolution spatial and spectral optical measurements. Despite the data reported in the literature, it is found that (i) the rotational degrees of freedom of OH∗ and CH∗ are not in thermal equilibrium with the surrounding gas and therefore cannot be used to measure flame temperature; (ii) there is no direct correlation between the heat release rate and the distribution of OH∗ and CH∗; (iii) the detailed reaction mechanisms not only quantitatively, and also qualitatively differ in description of the OH∗ and CH∗ concentrations. Since the chemically excited species are well localized in a direction normal to the flame surface, they are demonstrated to be a very accurate markers of flame location. The shape of the combustion front can be reconstructed and resolved up to the accuracy of tens of microns, which is very important for estimation of blow-off critical parameters and measurement of the laminar burning velocity.Novelty and significance statementCurrently, there is a growing interest in the development of sensors for combustion control systems, including active control and suppression of instabilities, in combustion chambers of various devices and engines based on chemiluminescence of excited reaction species. The possibility of non-invasive determination of parameters such as flame temperature, stoichiometry, heat release rate location, etc. using this technique is discussed. We have found that most of these parameters cannot be estimated either due to fundamental limitations or insufficient knowledge of the reaction kinetics involved in the production of these species. Nevertheless, since OH* and CH* are well localized in the direction normal to the flame surface, they can be used as very accurate markers of flame shape and position, allowing us to reconstruct the flame surface to within tens of microns resolution, which is very important for estimating blow-off critical parameters and measuring laminar burning velocity.

CMOS-compatible reconstructive spectrometers with self-referencing integrated Fabry–Perot resonators

Miniaturized reconstructive spectrometers play a pivotal role in on-chip and portable devices, offering high-resolution spectral measurement through precalibrated spectral responses and AI-driven reconstruction. However, two key challenges persist for practical applications: artificial intervention in algorithm parameters and compatibility with complementary metal-oxide-semiconductor (CMOS) manufacturing. We present a cutting-edge miniaturized reconstructive spectrometer that incorporates a self-adaptive algorithm referenced with Fabry-Perot resonators, delivering precise spectral tests across the visible range. The spectrometers are fabricated with CMOS technology at the wafer scale, achieving a resolution of ~2.5 nm, an average wavelength deviation of ~0.27 nm, and a resolution-to-bandwidth ratio of ~0.46%. Our approach provides a path toward versatile and robust reconstructive miniaturized spectrometers and facilitates their commercialization.

Mode division multiplexing reconstructive spectrometer with an all-fiber photonics lantern

This study presents a high-accuracy, all-fiber mode division multiplexing (MDM) reconstructive spectrometer (RS). The MDM was achieved by utilizing a custom-designed 3 × 1 mode-selective photonics lantern to launch distinct spatial modes into the multimode fiber (MMF). This facilitated the information transmission by increasing light scattering processes, thereby encoding the optical spectra more comprehensively into speckle patterns. Spectral resolution of 2 pm and the recovery of 2000 spectral channels were accomplished. Compared to methods employing single-mode excitation and two-mode excitation, the three-mode excitation method reduced the recovered error by 88% and 50% respectively. A resolution enhancement approach based on alternating mode modulation was proposed, reaching the MMF limit for the 3 dB bandwidth of the spectral correlation function. The proof-of-concept study can be further extended to encompass diverse programmable mode excitations. It is not only succinct and highly efficient but also well-suited for a variety of high-accuracy, high-resolution spectral measurement scenarios.Graphical

Interference correction for polarization spectral intensity modulation (PSIM) with spatial heterodyne spectroscopy (SHS)

Spectropolarimetry, capable of measuring both the polarization parameters and spectral information of light, is widely utilized in astrophysical research and atmospheric remote sensing. Spatial Static Polarization Modulation Interferometric Spectroscopy merges polarization spectral intensity modulation (PSIM) with spatial heterodyne spectroscopy to achieve high spectral resolution and static synchronous measurement of continuous band polarization spectral information. This article is based on spatial static polarization modulation interferometric spectroscopy principles and describes correction techniques for non-uniform response and phase errors observed in interferograms. A quantitative relationship was established through simulation between the non-uniform response, phase errors in the interferograms, and the demodulation accuracy of polarization parameters. An experimental setup based on these principles was constructed to validate the effectiveness of the correction. The experimental results demonstrated that by using the reported data processing workflow the error in the degree of polarization was reduced from 0.1354 to 0.0095.

High-resolution spectral atmospheric attenuation measurement for solar power plants

One of the main challenges in solar tower renewable technologies is measurement of solar radiation attenuation at the plants at surface level. This paper describes an improved version of the optical spectrum analysis method for measuring solar radiation attenuation in real time at solar tower plants, as presented in a previous work. The new hardware design and calibration process are explained in detail. This new system has a resolution of 0.25 nm in the VIS range and a precision of 0.5% in real time, improving over the state of the art for the measurement of spectral atmospheric attenuation. These specifications allow the assessment of the contribution of different attenuation factors at surface level, such as absorbance peaks (Na, H, H2O) or scattering and the search for correlation with different weather conditions. They also enable experimental validation of atmospheric extinction simulation methods and their parameters. This work also includes the results of a 6 month-long campaign of solar weighted atmospheric extinction measurements at Atlantica's operating central receiver solar plant PS10 at Sanlúcar la Mayor (Seville, Spain). Fluctuations in the daily averaged transmittance greater than 10% are observed, which shows the importance of monitoring this parameter during the operation of solar plants.

Induced luminescence in water and PMMA - spectral analysis for irradiations with proton beams within the clinical energy range

It was recently discovered that water and PMMA emit a weak luminescence signal when irradiated with protons within the clinically used energy range. This could offer a fast approach for range measurements in water. However, a complete explanation or investigation on the origin of the signal has not been published. In this work, a setup for the high-resolution spectral measurement of the weak luminescence signal in water and PMMA was designed. The measurement environment in the vicinity of a proton accelerator represented a major challenge for the sensitive optical measurements due to the presence of ionizing scattered radiation. A high-sensitive spectrometer in combination with a custom-made fiber was used to build a foundation for further analysis of the luminescence signal by providing accurate spectral information. For water, a broad distribution in the range from 240 to 900 nm with a maximum at 480 nm was obtained. A comparison of the spectra with previously published work indicates that the signal originates from excited states produced during the radiolysis of water. In comparison, differences between the water and the PMMA spectrum were observed. When examining the signal in PMMA, spectral differences were found compared to the measurements in water. The signal in PMMA was approximately 10 times stronger, had a narrower distribution and was shifted to lower wavelengths. Nevertheless, for the investigated proton energies, no spectral energy dependence was detected. In addition to the results for water and PMMA, a further luminescence signal was measured when the silica fiber used was directly irradiated with primary protons. All spectra, obtained in this work, describe the signal of proton-induced luminescence in water and PMMA with a high resolution of 3.4 nm and thus form a basis for further research, which could be a powerful tool in proton range verification.

Theoretical analysis of a multi-grating-based cross-dispersed spatial heterodyne spectrometer.

This paper presents a multi-grating-based cross-dispersed spatial heterodyne spectrometer (MGCDSHS). The principle of generation of two-dimensional interferograms for two cases, where the light beam is diffracted by one sub-grating or two sub-gratings, is given and equations for the interferogram parameters in these two cases are derived. An instrument design with numerical simulations is presented that demonstrates the spectrometer's ability to simultaneously record separate interferograms corresponding to different spectral features with high resolution over a broad spectral range. The design solves the mutual interference problem caused by overlapping of the interferograms, and also provides the high spectral resolution and broad spectral measurement range that cannot be achieved using conventional SHSs. Additionally, by introducing cylindrical lens groups, the MGCDSHS solves the throughput loss and light intensity reduction problems caused by direct use of multi-gratings. The MGCDSHS is compact, highly stable, and high-throughput. These advantages make the MGCDSHS suitable for high-sensitivity, high-resolution, and broadband spectral measurements.

CDAP: A portable CCD-based daytime airglow photometer for investigations of ionosphere-thermosphere phenomena

A photometric technique to obtain daytime optical airglow emissions is described. Airglow emissions occur naturally due to photochemical and chemiluminescent processes taking place in the upper atmosphere. Airglow emissions that occur in the daytime are referred to as dayglow. It is a challenge to detect these dayglow emissions as the solar scattered background against which they occur is extremely bright. Thus, it is required that high spectral resolution measurements are carried out to delineate the difference between the dayglow signal and the solar scattered background brightness. In the method being described here, a low-resolution Fabry-Perot etalon is used as a high spectral resolution filter to obtain the dayglow signal. The spectral resolution achieved by this instrument is 0.026 nm at 630.0 nm, which is sufficient to separate the signal and neighbouring spectral regions. The present technique builds up on the success of employing such methods in the past and brings in innovations that make it reliable, rugged, and suitable for unattended field operations. The details of the technique and the new elements brought in are described in this paper.

High-Resolution and Broadband Spectral Measurement of CO2 Based on Virtually Imaged Phased Array Spectrometer

High-Resolution and Broadband Spectral Measurement of CO2 Based on Virtually Imaged Phased Array Spectrometer

Ultra-high-Q resonances in terahertz all-silicon metasurfaces based on bound states in the continuum

High-Q metasurfaces have important applications in high-sensitivity sensing, low-threshold lasers, and nonlinear optics due to the strong local electromagnetic field enhancements. Although ultra-high-Q resonances of bound states in the continuum (BIC) metasurfaces have been rapidly developed in the optical regime, it is still a challenging task in the terahertz band for long years because of absorption loss of dielectric materials, design, and fabrication of nanostructures, and the need for high-signal-to-noise ratio and high-resolution spectral measurements. Here, a polarization-insensitive quasi-BIC resonance with a high-Q factor of 1049 in a terahertz all-silicon metasurface is experimentally achieved, exceeding the current highest record by 3 times of magnitude. And by using this ultra-high-Q metasurface, a terahertz intensity modulation with very low optical pump power is demonstrated. The proposed all-silicon metasurface can pave the way for the research and development of high-Q terahertz metasurfaces.

Echelle Grating Spectroscopic Technology for High-Resolution and Broadband Spectral Measurement

Echelle grating provides high spectral resolving power and diffraction efficiency in a broadband wavelength range by the Littrow mode. The spectrometer with the cross-dispersed echelle scheme has seen remarkable growth in recent decades. Rather than the conventional approach with common blazed grating, the cross-dispersed echelle scheme achieves the two-dimensional spatial distribution of the spectrum by one exposure without scanning in the broadband spectral range. It is the fastest and most sensitive spectroscopic technology as of now, and it has been extensively applied in commercial and astronomical spectrometers. In this review, we first highlight the characteristics of the echelle and then present the optical layout, detection approach, and method of calibration. Finally, we discuss the state-of-the-art implementations and applications of commercial and astronomical instruments.

Sub-terahertz silicon-based on-chip absorption spectroscopy using thin-film model for biological applications

Spectroscopy in the sub-terahertz (sub-THz) range of frequencies has been utilized to study the picosecond dynamics and interaction of biomolecules. However, widely used free-space THz spectrometers are typically limited in their functionality due to low signal-to-noise ratio and complex setup. On-chip spectrometers can revolutionize THz spectroscopy allowing integration, compactness, and low-cost fabrication. In this paper, a low-loss silicon-based platform is proposed for on-chip sub-THz spectroscopy. Through functionalization of silicon chip and immobilization of bio-particles, we demonstrate the ability to characterize low-loss nano-scale biomolecules across the G-band (0.14–0.22 THz). We also introduce an electromagnetic thin-film model to account for the loading effect of the immobilized biomolecules, i.e. dehydrated streptavidin and immunoglobulin antibody, as two key molecules in the biosensing discipline. The proposed platform was fabricated using a single mask micro-fabrication process, and then measured by a vector network analyzer (VNA), which offers high dynamic range and high spectral resolution measurements. The proposed planar platform is general and paves the way towards low-loss, cost-effective and integrated sub-THz biosensors for the detection and characterization of biomolecules.

Extremely high spectral resolution measurements of the 450 µm atmospheric window at Chajnantor with APEX,

Ground-based telescopes observing at millimeter (mm) and submillimeter (submm) wavelengths have to deal with a line-rich and highly variable atmospheric spectrum, both in space and time. Models of this spectrum play an important role in planning observations that are appropriate for the weather conditions and also calibrating those observations. Through magnetic dipolar (M1) rotational transitions and electric dipolar (E1) transitions O2 and H2O, respectively, dominate the atmospheric opacity in this part of the electromagnetic spectrum. Although O2 lines, and more generally the so-called dry opacity, are relatively constant, the absorption related to H2O can change by several orders of magnitude leading from a totally opaque atmosphere near sea level with high H2O columns to frequency windows with good transmission from high and dry mountain sites. Other minor atmospheric gases, such as O3 and N2O among others, are present in the atmospheric spectrum which also includes nonresonant collision-induced absorption due to several mechanisms. The aim of our research is to improve the characterization of the mm/submm atmospheric spectrum using very stable heterodyne receivers with excellent sideband separation and extremely high (kHz) spectral resolutions at the 5000 m altitude Chajnantor site in northern Chile. This last aspect (spectral resolution) is the main improvement (by more than three orders of magnitude) in the presented data with respect to our previous work conducted ~20 yr ago from Mauna Kea in Hawai’i. These new measurements have enabled us to identify slight modifications needed in the Atmospheric Transmission at Microwaves (ATM) model to better take into account minor constituent vertical profiles, include a few missing lines, and adjust some high-energy O3 line frequencies. After these updates, the ATM model is highly consistent with all data sets presented in this work (within ~2% at 1 GHz resolution).

FT-ICR Mass Spectrometry Imaging at Extreme Mass Resolving Power Using a Dynamically Harmonized ICR Cell with 1ω or 2ω Detection.

MALDI mass spectrometry imaging (MALDI MSI) is a powerful analytical method for achieving 2D localization of compounds from thin sections of typically but not exclusively biological samples. The dynamically harmonized ICR cell (ParaCell) was recently introduced to achieve extreme spectral resolution capable of providing the isotopic fine structure of ions detected in complex samples. The latest improvement in the ICR technology also includes 2ω detection, which significantly reduces the transient time while preserving the nominal mass resolving power of the ICR cell. High-resolution MS images acquired on FT-ICR instruments equipped with 7T and 9.4T superconducting magnets and the dynamically harmonized ICR cell operating at suboptimal parameters suffered severely from the pixel-to-pixel shifting of m/z peaks due to space-charge effects. The resulting profile average mass spectra have depreciated mass measurement accuracy and mass resolving power under the instrument specifications that affect the confidence level of the identified ions. Here, we propose an analytical workflow based on the monitoring of the total ion current to restrain the pixel-to-pixel m/z shift. Adjustment of the laser parameters is proposed to maintain high spectral resolution and mass accuracy measurement within the instrument specifications during MSI analyses. The optimized method has been successfully employed in replicates to perform high-quality MALDI MS images at resolving power (FWHM) above 1,000,000 in the lipid mass range across the whole image for superconducting magnets of 7T and 9.4T using 1 and 2ω detection. Our data also compare favorably with MALDI MSI experiments performed on higher-magnetic-field superconducting magnets, including the 21T MALDI FT-ICR prototype instrument of the NHMFL group at Tallahassee, Florida.

On the use of polychromatic cameras for high spatial resolution spectral dose measurements

Objective. Despite the demonstrated benefits of hyperspectral formalism for stem effect corrections in the context of fiber dose measurements, this approach has not been yet translated into volumetric measurements where cameras are typically used for their distinguishing spatial resolution. This work investigates demosaicing algorithms for polychromatic cameras based spectral imaging. Approach. The scintillation and Cherenkov signals produced in a radioluminescent phantom are imaged by a polychromatic camera and isolated using the spectral formalism. To do so, five demosaicing algorithms are investigated from calibration to measurements: a clustering method and four interpolation algorithms. The resulting accuracy of scintillation and Cherenkov images is evaluated with measurements of the differences (mean ± standard deviation) between the obtained and expected signals from profiles drawn across a scintillation spot. Signal-to-noise ratio and signal-to-background ratio are further measured and compared in the resulting scintillation images. Finally, the resulting differences on the scintillation signal from a 0.2 × 0.2 cm2 region-of-interest (ROI) were reported. Main results. Clustering, OpenCV, bilinear, Malvar and Menon demosaicing algorithms respectively yielded differences of 3 ± 5%, 1 ± 3%, 1 ± 3%, 1 ± 2% and 2 ± 4% in the resulting scintillation images. For the Cherenkov images, all algorithms provided differences below 1%. All methods enabled measurements over the detectability (SBR > 2) and sensitivity (SNR > 5) thresholds with the bilinear algorithm providing the best SNR value. Clustering, OpenCV, bilinear, Malvar and Menon demosaicing algorithms respectively provided differences on the ROI analysis of 7 ± 5%, 3 ± 2%, 3 ± 2%, 4 ± 2%, 7 ± 3%. Significance. Radioluminescent signals can accurately be isolated using a single polychromatic camera. Moreover, demosaicing using a bilinear kernel provided the best results and enabled Cherenkov signal subtraction while preserving the full spatial resolution of the camera.

Optical and magnetic properties of fluoride-doped CeO2 obtained by an oxidative insertion method

• CeO 2 : F sample is synthesized using molten flux promoted oxidative insertion method. • PXRD and Raman spectral measurements confirmed the fluorite structure of CeO 2 :F. • Besides Ce and O, EDX analysis also revealed the presence of 18 at. % of fluoride. • PL and magnetic studies revealed a lower concentration of O vacancies in CeO 2 :F. • The F-doped sample had a higher concentration of Ce 3+ in CeO 2 :F. In the present study, we generated a fluoride-doped CeO 2 sample through the molten hydroxide flux promoted oxidative insertion approach using CeF 3 as the precursor. We analyzed the results and compared them with pure CeO 2 prepared by the same approach starting from Ce(NO 3 ) 3 . High-resolution powder X-ray diffraction, FTIR, and Raman spectral measurements confirmed both samples’ fluorite structure. The CeO 2 resulted from the oxidative flux treatment of CeF 3 had 18 at.% of fluoride ion-doped in it. The optical band gap narrowed marginally from 2.98 to 2.88 eV upon fluoride doping in CeO 2 . The blue emission intensity decreased with fluoride doping, indicating a reduction in oxygen vacancies compared to undoped CeO 2 . F-doped CeO 2 showed ferromagnetic behavior with magnetization, H c, and M r values higher than the undoped sample. A higher concentration of Ce 3+ and a lower concentration of oxygen vacancies were the consequences of fluoride doping in CeO 2 by this synthetic method.

Empirical Constraints on Core-collapse Supernova Yields Using Very Metal-poor Damped Lyα Absorbers

Abstract We place empirical constraints on the yields from zero- and low-metallicity core-collapse supernovae (CCSNe) using abundances measured in very metal-poor (VMP; [Fe/H] ≤ −2) damped Lyα absorbers (DLAs). For some abundance ratios ([N,Al,S/Fe]), VMP DLAs constrain the metal yields of the first SNe more reliably than VMP stars. We compile a large sample of high-S/N VMP DLAs from over 30 yr of literature, most with high-resolution spectral measurements. We infer the initial-mass-function-averaged CCSNe yield from the median values from the DLA abundance ratios of C, N, O, Al, Si, S, and Fe (over Fe and O). We assume that the DLAs are metal-poor enough that they represent galaxies in their earliest stages of evolution, when CCSNe are the only nucleosynthetic sources of the metals we analyze. We compare five sets of zero- and low-metallicity theoretical yields to the empirical yields derived in this work. We find that the five models agree with the DLA yields for ratios containing Si and S. Only one model (Heger & Woosley 2010, hereafter HW10) reproduced the DLA values for N, and one other model (Limongi & Chieffi 2018, hereafter LC18) reproduced [N/O]. We found little change in the theoretical yields with the adoption of an SN explosion landscape (where certain progenitor masses collapse into black holes, contributing no yields) onto HW10, but fixing explosion energy to progenitor mass results in wide disagreements between the predictions and DLA abundances. We investigate the adoption of a simple, observationally motivated initial distribution of rotational velocities for LC18 and find a slight improvement.

Cytotoxic Phenylpropanoid Derivatives and Alkaloids from the Flowers of Pancratium maritimum L.

Regarding our growing interest in identifying biologically active leads from Amaryllidaceous plants, the flowers of Pancratium maritimum L. (Amaryllidaceae) were investigated. Purification of the cytotoxic fractions of the alcoholic extract of the flowers gave a new glycoside, 3-[4-(β-D-glucopyranosyloxy)phenyl]-2-(Z)-propenoic acid methyl ester (1), together with the previously reported compounds 3-methoxy-4-(β-D-glucopyranosyloxy)benzoic acid methyl ester (2), 3-(4-methoxyphenyl)propan-1-ol-1-O-β-D-glucopyranoside (3), (E)-3-(4-hydroxyphenyl)acrylic acid methyl ester (4), caffeic acid (5), dihydrocaffeic acid methyl ester (6), and pancratistatin (7). Interestingly, compounds 1 and 2 are phenolic-O-glycosides, while the glucose moiety in 3 is attached to the propanol side chain. This is the first report about the existence of 1–6 in the genus Pancratium. Further, glycosides 1–3 from the Amaryllidaceae family are reported on here for the first time. The structures of 1–7 were determined by analyses of their 1D (1H and 13C) and 2D (COSY, HMQC, HMBC) NMR spectra, and by high-resolution mass spectral measurements. Pancratistatin displayed potent and selective growth inhibitory effects against MDA-MB-231, HeLa, and HCT 116 cells with an IC50 value down to 0.058 µM, while it possessed lower selectivity towards the normal human dermal fibroblasts with IC50 of 6.6 µM.

Development of electrical substitution Fourier transform spectrometry for absolute optical power measurements.

We have demonstrated the first continuous-scan electrical substitution Fourier transform spectrometer (ES-FTS), which serves initially as an apparatus for absolute spectral responsivity calibrations of detectors over the wavelength range from 1.5 µm to 11 µm. We present data on the realization of a spectral detector-comparator system with high accuracy, high dynamic range, high spectral resolution and fast measurement in the infrared region, which is tied directly to an absolute power scale through electrical substitution. The ES-FTS apparatus employs a commercial Fourier transform spectrometer and a custom electrical substitution bolometer detector to enable spectrally-resolved absolute optical power measurements. A generalization of electrical substitution techniques enables determination of the voltage waveform that must be applied to the bolometer's electrical heater to cancel the optical signal from a Michelson interferometer in order to quantify the time-dependent optical power incident on the bolometer. The noise floor of the electrical substitution bolometer is on the order of 10 pW/Hz½ and its response is expected to be linear from the noise floor to 1 mW. A direct comparison between a pyroelectric standard detector and the ES-FTS has been performed, and experimental results reported here show great potential for this technique.