Far-infrared Spectral Region Research Articles

INVESTIGATION OF THE INFLUENCE OF PULSED RADIATION GENERATED BY FUNCTIONAL CERAMICS BASED ON THE PRINCIPLE OF PTE ON THE CHARACTERISTICS OF THE CR2O3-SIO2-FE2O3-CAO-AL2O3-MGO-CUO SYSTEM

This work investigates methods for producing ceramic materials based on the Cr2O3-SiO2-Fe2O3-CaO-Al2O3-MgO-CuO system capable of generating modulated pulsed radiation in the far-infrared spectral region. The possibility of synthesizing such ceramics, in addition to helio-technology, using thermomechanical processing and mechanoactivation of the initial carbonates is considered. A comprehensive analysis of the structure and properties of the obtained materials using X-ray structural, electron microscopic analysis, and other methods has been carried out. It has been established that activation by pulsed infrared radiation generated by the principle of pulsed tunneling effect (PTE) leads to changes in the microstructure of the samples, accompanied by the formation of metastable phases at the interfaces and the generation of radiation.

HgBr2: an easily growing wide-spectrum birefringent crystal.

Birefringent materials are of great significance to the development of modern optical technology; however, research on halide birefringent crystals with a wide transparent range remains limited. In this work, mercuric bromide (HgBr2) has been investigated for the first time as a promising birefringent material with a wide transparent window spanning from ultraviolet (UV) to far-infrared (far-IR) spectral regions (0.34-22.9 μm). HgBr2 has an exceptionally large birefringence (Δn, 0.235 @ 546 nm), which is 19.6 times that of commercial MgF2. The ordered linear motif [Br-Hg-Br] with high polarizability anisotropy within the molecule is the inherent source of excellent birefringence, making it an efficient building block for birefringent materials. In addition, HgBr2 can be easily grown under mild conditions and remain stable in air for prolonged periods. Studying the birefringent properties of HgBr2 crystals would provide new ideas for future exploration of wide-spectrum birefringent materials.

GBB-Nadir and KLIMA: Two Full Physics Codes for the Computation of the Infrared Spectrum of the Planetary Radiation Escaping to Space

In 2019 the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission was selected to be the 9th Earth Explorer mission of the European Space Agency (ESA). In the preparatory phase of the mission there was the need for accurate and versatile codes to compute the spectrally resolved Earth radiation escaping to space ( outgoing long-wave radiation, OLR), targets for the FORUM measurements.Moreover, for the study of planetary atmospheres, several instruments measuring the planetary radiation escaping to space have been deployed (i.e., the planetary Fourier spectrometer on Mars express or composite infrared spectrometer on Cassini). For both the analysis of the measurements of these instruments and the design of new instruments, reliable radiative transfer codes need to be available. In this paper, we describe two full physics codes, Geofit broadband-Nadir (GBB-Nadir) and Kyoto protocol-informed management of adaptation (KLIMA), both able to compute the OLR spectrum, while GBB-Nadir is only a forward model, and therefore computes the spectra only, KLIMA implements the computation of spectral radiance derivatives with respect to atmospheric parameters and therefore it is suitable to be used in retrieval codes. The GBB-Nadir code can be interfaced with radiative transfer solvers that include representations of multiple scatterings, making it suitable to compute the radiances in all-sky conditions. KLIMA has been extensively validated comparing its radiances to ones generated by the widely used line-by-line radiative transfer model (LBLRTM) code. In this paper, we describe the latest version of both codes and their comparison. We compared the optical depth computed by GBB-Nadir and KLIMA for given values of pressure, temperature and gas columns for most gases active in the far-infrared and thermal-infrared spectral regions. We show that the optical depths computed by the two codes are in very good agreement. We compared the simulated spectra in clear sky conditions for three different atmospheres (equatorial, mid-latitude and polar) at resolutions of the FORUM instrument. The differences found are well below the expected noise of the FORUM instrument. The KLIMA code has already been used to simulate the observations of the Mars atmosphere, while the limb version of the GBB code has been used to simulate the radiances measured in the limb geometry of planetary atmospheres (Titan and Jupiter). Therefore, we may safely affirm that both codes can be used to simulate the nadir measurements of planetary atmospheres.

Emissivity retrievals with FORUM's end-to-end simulator: challenges and recommendations

Abstract. Spectral emissivity is a key property of the Earth's surface, of which only very few measurements exist so far in the far-infrared (FIR) spectral region, even though recent work has shown that the FIR is important for accurate modelling of the global climate. The European Space Agency's 9th Earth Explorer, FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) will provide the first global spectrally resolved measurements of the Earth's top-of-the-atmosphere (TOA) spectrum in the FIR. In clear-sky conditions with low water vapour content, these measurements will provide a unique opportunity to retrieve spectrally resolved FIR surface emissivity. In preparation for the FORUM mission with an expected launch in 2027, this study takes the first steps towards the development of an operational emissivity retrieval for FORUM by investigating the sensitivity of the emissivity product of a full spectrum optimal estimation retrieval method to different physical and operational parameters. The tool used for the sensitivity tests is the FORUM mission's end-to-end simulator. These tests show that the spectral emissivity of most surface types can be retrieved for dry scenes in the 350–600 cm−1 region, with an absolute uncertainty ranging from 0.005 to 0.01. In addition, the quality of the retrieval is quantified with respect to the precipitable water vapour content of the scene, and the uncertainty caused by the correlation of emissivity with surface temperature is investigated. Based on these investigations, a road map is recommended for the development of the operational emissivity product.

Assessment of Sampling Sufficiency for Low-Cost Satellite Missions: Application to PREFIRE

AbstractThe Polar Radiant Energy in the Far Infrared Experiment (PREFIRE) mission will, for the first time, systematically document the far-infrared (15–54 µm) spectral region from space. The environmental sampling characteristics of the PREFIRE CubeSats, defined in terms of surface temperature (Tsfc) and column water vapor (CWV) are evaluated for a range of possible orbit scenarios for both clear-sky and all-sky conditions over a variety of surface types (land, ocean, sea ice, snow, glacier ice) at both poles. Using NASA Aqua’s Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Sounding Unit (AMSU) retrievals to define the climatological ranges of Tsfc and CWV, the fraction of environmental regimes observed by distinct PREFIRE configurations are evaluated. The sampling rates within any single year for two-orbit CubeSat launches spanning both polar regions are ~75% for clear-sky and ~85% for all-sky compared to the AIRS/AMSU climatology. Decreasing mission duration from 12 to 3 months decreases sampling much more (10%–20%) than decreasing the swath width from 15 to 8 footprints (6%–9%). For a single CubeSat launch, a 98° orbital inclination provides slightly better sampling than either 93° or 103°. For a two-orbit CubeSat launch, a combination of 93° + 98° is somewhat preferable to 103° + 98°. Finally, a 50% data loss rate simulated by dropping out every other orbit leads to only a modest 7%–8% reduction in sampling from full data coverage. This statistical analysis demonstrates that low-cost platforms could offer similar coverage as present-day flagship missions for sampling wide-ranging Tsfc and CWV states over polar regions.

Ring a ring o’ pulses

Quantum cascade lasers are bright and compact semiconductor lasers that emit light in the mid- to far-infrared spectral region. The use of a closed ring cavity has now set them on the path towards ultrafast pulses.

Far infrared spectroscopy and other spectral and thermal properties of [Li@C60]PF6

The endohedral fullerene [Li@C60]+PF6− was characterized by differential scanning calorimetry (DSC), by electronic absorption spectroscopy in the spectral range from 195 to 800 nm and by FT-IR and far-infrared spectroscopy. [Li@C60]+PF6− shows a sharp melting point with the onset at 86 °C and peak at 90 °C and melting enthalpy of 9.5 kJ/mol. The electronic absorption spectrum of [Li@C60]+PF6− in the UV shows both a hypsochromic shift and a hyperchromic effect with respect to the corresponding electronic transitions of C60. In the mid-infrared spectrum [Li@C60]+ cation displays the typical four band pattern of C60, with small shifts. The far-infrared spectroscopy appears to be the most effective diagnostic tool to distinguish between C60 and endohedral fullerenes like [Li@C60]+. In fact, while C60 does not show any far-infrared vibration, [Li@C60]+PF6− shows four far-infrared bands. Also in an astrochemical perspective the search of endohedral fullerenes must rely mainly on the far-infrared spectral region.

Detailed infrared study of amorphous to crystalline propionitrile ices relevant to observed spectra of Titan's stratospheric ice clouds

We have conducted a comprehensive study of propionitrile (C2H5CN) ice from the amorphous to crystalline phase in order to provide detailed information on this specific cyanide, which may potentially contribute to the chemical composition of the Haystack ice cloud observed in Titan's stratosphere by the Cassini Composite InfraRed Spectrometer (CIRS). Infrared transmission spectra of thin films of pure propionitrile ices deposited at low temperature (30–160 K) were collected from 50 cm−1 to 11,700 cm−1 (200–0.85 μm). The far-infrared spectral region was specifically targeted to compare with CIRS far-infrared limb spectra. The temperature and time evolution of C2H5CN ice was thoroughly investigated to better understand discrepancies reported in previously published laboratory studies on the crystalline phase of C2H5CN. Specifically, we observe peculiar temperature and time-driven ice phase transitions, revealed by significant spectral variations in the ice, which stabilizes once a complete crystalline phase is achieved. From these results, the crystalline phase of propionitrile ice was identified at deposition temperatures greater than or equal to 135 K and <140 K. Our findings corroborate previous studies that ruled out pure propionitrile ice as the sole chemical identity of Titan's observed Haystack emission feature. In order to understand and identify the Haystack cloud, we have initiated co-deposition experiments that incorporate mixtures of Titan-relevant organics, many of which have corresponding vapors that are abundantly present in Titan's stratosphere. In this paper, we present the result of one example of a co-deposited ternary ice mixture containing 16% hydrogen cyanide (HCN), 23% C2H5CN, and 61% benzene (C6H6). Although this co-condensed ice mixture is the best fit thus far obtained to match the broad width of the Haystack, it is still not the appropriate chemical candidate. However, it reveals an intriguing result: the strong lattice mode of pure C2H5CN ice is drastically altered by the surrounding molecules as a result of mixing in a co-condensed phase. The laboratory results reported here on propionitrile ice may help to further constrain the chemical identification of Titan's stratospheric Haystack ice cloud, as well as improve on the current state of knowledge of Titan's stratospheric ice cloud chemistry.

Highly localized H2O librational motion as a far-infrared spectroscopic probe for microsolvation of organic molecules.

The most prominent spectroscopic observable for the hydrogen bonding between individual molecules in liquid water is the broad absorption band detected in the spectral region between 300 and 900 cm-1. The present work demonstrates how the associated large-amplitude out-of-plane OH librational motion of H2O molecules also directly reflects the microsolvation of organic compounds. This highly localized OH librational motion of the first solvating H2O molecule causes a significant change of dipole moment and gives rise to a strong characteristic band in the far-infrared spectral region, which is correlated quantitatively with the complexation energy. The out-of-plane OH librational band origins ranging from 324.5 to 658.9 cm-1 have been assigned experimentally for a series of four binary hydrogen-bonded H2O complexes embedded in solid neon involving S-, O- and N-containing compounds with increasing hydrogen bond acceptor capability. The hydrogen bond energies for altogether eight binary H2O complexes relative to the experimental value of 13.2 ± 0.12 kJ mol-1 for the prototypical (H2O)2 system [Rocher-Casterline et al., J. Chem. Phys., 2011, 134, 211101] are revealed directly by these far-infrared spectroscopic observables. The far-infrared spectral signatures are able to capture even minor differences in the hydrogen bond acceptor capability of O atoms with slightly different alkyl substituents in the order H-O-C(CH3)3 > CH3-O-CH3 > H-O-CH(CH3)2 > H-O-CH2CH3.

Optical properties of Meloxicam in the far-infrared spectral region

One of the most commonly used nonsteroidal anti-inflammatory active pharmaceutical ingredient called Meloxicam has been characterized spectroscopically both by Terahertz (THz) time domain spectroscopy (THz-TDS) and by Fourier Transform Infrared (FTIR) spectroscopy in far-IR regions of electromagnetic spectrum; 0.2 THz–20 THz. While many relatively sharp features are observed in the far-IR range between 2 THz–20 THz as expected for being an organic substance, very distinct and relatively strong absorption bands are also observed at 1.00, 1.66, 2.07 and 2.57 THz in the THz range. These well separated, defined, and fairly strong spectral features can be used for discrimination and quantification of Meloxicam in drug analysis. Frequency dependent refractive index of the drug was determined in a range of 0.2 THz–2.7 THz, where an almost constant index was observed with an average index of 1.75. Powder XRD, and solid-state Density Functional Theory (SS-DFT) calculations were utilized to determine the crystalline form of the Meloxicam sample in its enolic crystalline form. Single molecule DFT calculations were also performed in all four possible structures of Meloxicam. In addition, the capability of THz waves transmission through common packaging materials is demonstrated for possibility of future on-site analysis. The results suggest that drug analysis will be possible to perform not only at every stage of manufacturing without destruction but also directly at the shelf of a market after development of portable THz technologies.

Fiber-type difference frequency generation infrared optical frequency comb based on the femtosecond pulses generated by a mode-locked fiber laser

Optical frequency comb (OFC) is a new type of high-quality laser source. The visible and near-infrared OFCs have become mature, and it has been widely used in optical frequency metrology, time/frequency transfer, precision laser spectroscopy and other fields. Since the mid and far-infrared spectral regions contain a large number of baseband absorption lines for molecules and the absorption intensities are several orders of magnitude higher than those in the visible and near-infrared spectral region, one has made great efforts to develop the mid and far-infrared OFCs in recent years. Although a variety of approaches to achieving infrared OFCs directly have been proposed, the method of difference frequency generation (DFG) infrared OFC based on the optical rectification technique is still more efficient. DFG infrared OFCs with widely tuning ability have been demonstrated based on fiber lasers so far. However, how to obtain the broadband spectrum for a DFG infrared OFC with widely tuning ability still needs to be solved. In this paper we report a fiber-type DFG infrared OFC by using the femtosecond pulses from a mode-locked erbium-doped fiber laser as the fundamental light. Based on the self-developed mode-locked fiber laser oscillator with repetition rate locked, the two-color fundamental pulse trains with the central wavelengths of 1.5 and 2.0 m are respectively achieved after the chirped pulse fiber amplification and all-fiber supercontinuum (SC) generation techniques have been utilized. With a time-domain synchronous detection system based on the intensity autocorrelation principle, the accurate synchronization with the fundamental two-color pulses is obtained by optimizing the OFS compensated fiber length and adjusting a tunable optical delay line. Finally, by using the optical rectification technique, a fiber-type DFG infrared OFC is successfully generated with the help of a suitable designed GaSe nonlinear crystal. Our experimental results also show that the spectral location of the DFG infrared OFC can be tuned by controlling the spectral shape of the SC combined with the adjustment of the phase-matching for the nonlinear crystal. The measured tuning range of the DFG infrared OFC is from 6 to 10 m, and the maximum spectral width is 1.3 m. This fiber-type DFG infrared OFC may play an important role in the molecular spectroscopy, the atmospheric environmental monitoring, and other fields.

Properties of hydrogen bonds in the protic ionic liquid ethylammonium nitrate

Comparative molecular dynamics simulations of a hexamer cluster of the protic ionic liquid ethylammonium nitrate are performed using density functional theory and density functional-based tight-binding (DFTB) methods. The focus is on assessing the performance of the DFTB approach to describe the dynamics and infrared spectroscopic signatures of hydrogen bonding between the ions. Average geometries and geometric correlations are found to be rather similar. The same holds true for the far-infrared spectral region. Differences are more pronounced for the NH- and CH-stretching bands, where DFTB predicts a broader intensity distribution. DFTB completely fails to describe the fingerprint range shaped by nitrate anion vibrations. Finally, charge fluctuations within the H bonds are characterized yielding moderate dependencies on geometry. On the basis of these results, DFTB is recommended for the simulation of H bond properties of this type of ionic liquid.

Anisotropy, phonon modes, and lattice anharmonicity from dielectric function tensor analysis of monoclinic cadmium tungstate

We determine the frequency dependence of four independent CdWO$_4$ Cartesian dielectric function tensor elements by generalized spectroscopic ellipsometry within mid-infrared and far-infrared spectral regions. Single crystal surfaces cut under different angles from a bulk crystal, (010) and (001), are investigated. From the spectral dependencies of the dielectric function tensor and its inverse we determine all long wavelength active transverse and longitudinal optic phonon modes with $A_u$ and $B_u$ symmetry as well as their eigenvectors within the monoclinic lattice. We thereby demonstrate that such information can be obtained completely without physical model line shape analysis in materials with monoclinic symmetry. We then augment the effect of lattice anharmonicity onto our recently described dielectric function tensor model approach for materials with monoclinic and triclinic crystal symmetries [Phys. Rev. B, 125209 (2016)], and we obtain excellent match between all measured and modeled dielectric function tensor elements. All phonon mode frequency and broadening parameters are determined in our model approach. We also perform density functional theory phonon mode calculations, and we compare our results obtained from theory, from direct dielectric function tensor analysis, and from model lineshape analysis, and we find excellent agreement between all approaches. We also discuss and present static and above reststrahlen spectral range dielectric constants. Our data for CdWO$_4$ are in excellent agreement with a recently proposed generalization of the Lyddane-Sachs-Teller relation for materials with low crystal symmetry [Phys. Rev. Lett. 117, 215502 (2016)].

Surface-Enhanced Infrared Absorption by Optical Phonons in Nanocrystal Monolayers on Au Nanoantenna Arrays

We report on a study of surface-enhanced infrared absorption (SEIRA) by optical phonons in monolayers (MLs) of CdSe, CdS, and PbS nanocrystals (NCs) deposited on arrays of linear nanoantennas the optimized structural parameters of which allow coupling between the localized surface plasmon resonance (LSPR) and diffraction modes in the far-infrared spectral region. The Langmuir–Blodgett technique was used for homogeneous deposition of the NCs. The structural parameters of the arrays and the NC MLs were determined by scanning electron microscopy. According to the three-dimensional electrodynamic simulations of the electromagnetic field distribution around the antennas, the maximal SEIRA enhancement is realized for an array period of about 15 μm when the energy of a diffraction mode coincides with that of the LSPR mode. SEIRA experimental results are in perfect quantitative agreement with the simulation. The maximal SEIRA enhancement is observed for the nanoantenna length and transverse periodicity predicted ...

Anomalous diffraction in super-wavelength plasmonic metasurfaces

We demonstrate that super-wavelength sandwich plasmonic metasurfaces, with the top metal layer consisting of a periodic arrangement of differently sized rectangle patches, can support anomalous diffraction – one first-order diffraction is boosted greatly in a broad bandwidth while another first-order and zero-order diffractions are suppressed. More importantly, by adding and subtracting the patches in a unit cell and adjusting the geometric parameters, high efficiency and broadband first-order diffraction can be easily tuned from visible to near-, mid- and even far-infrared spectral regions. The metasurfaces can be applied as high-performance light dispersive components in beam splitters, thin-film solar cells and spectrometers.

Phosphate structure and lithium environments in lithium phosphorus oxynitride amorphous thin films

Lithium ion-conducting glasses attract wide interest for electrochemical applications like efficient energy storage devices. This work presents a structural study on involved bonding units, based on X-ray photoelectron spectroscopy and infrared spectroscopy, of lithium phosphorus oxide and oxynitride amorphous thin films prepared by RF magnetron sputtering. A thorough consideration of the mid- and far-infrared spectral regions demonstrated structural changes at the phosphate units and the lithium ion environments triggered by nitrogen incorporation and post-deposition thermal treatment. It was found that films prepared by sputtering in pure nitrogen atmosphere have about 75 % of their nitrogen atoms in sites doubly coordinated with phosphorus (P–N=P), and the rest in triply coordinated sites. It was shown also that nitrogen incorporation favors the stability of lithium ions, while annealing enhances ionic conductivity of the oxynitride films.

Far-Infrared Radiative Properties of Water Vapor and Clouds in Antarctica

Abstract Water vapor and clouds are among the most important greenhouse components whose radiative features cover all the broad spectral range of the thermal emission of the atmosphere. Typically more than 40% of the total thermal emission of Earth occurs in the far-infrared (FIR) spectral region from 100 to 667 cm−1 (wavelengths from 100 to 15 µm). Nevertheless, this spectral region has not ever been fully covered down to 100 cm−1 by space missions, and only a few ground-based experiments exist because of the difficulty of performing measurements from high altitude and very dry locations where the atmosphere is sufficiently transparent to observe the FIR emission features. To cover this lack of observations, the Italian experiment “Radiative Properties of Water Vapor and Clouds in Antarctica” has collected a 2-yr dataset of spectral measurements of the radiance emitted by the atmosphere and by clouds, such as cirrus and polar stratospheric clouds, from 100 to 1,400 cm−1 (100–7 µm of wavelength), including the underexplored FIR region, along with polarization-sensitive lidar observations, daily radiosondes, and other ancillary information to characterize the atmosphere above the site. Measurements have been performed almost continuously with a duty cycle of 6 out of 9 h, from the Italian–French base of Concordia at Dome C over the Antarctic Plateau at 3,230 m MSL, in all-sky conditions since 2012. Because of the uniqueness of the observations, this dataset will be extremely valuable for evaluating the accuracy of atmospheric absorption models (both gas and clouds) in the underexplored FIR and to detect possible daily, seasonal, and annual climate signatures.

Molecular Plasmonics.

Graphene supports surface plasmons that have been observed to be both electrically and geometrically tunable in the mid- to far-infrared spectral regions. In particular, it has been demonstrated that graphene plasmons can be tuned across a wide spectral range spanning from the mid-infrared to the terahertz. The identification of a general class of plasmonic excitations in systems containing only a few dozen atoms permits us to extend this versatility into the visible and ultraviolet. As appealing as this extension might be for active nanoscale manipulation of visible light, its realization constitutes a formidable technical challenge. We experimentally demonstrate the existence of molecular plasmon resonances in the visible for ionized polycyclic aromatic hydrocarbons (PAHs), which we reversibly switch by adding, then removing, a single electron from the molecule. The charged PAHs display intense absorption in the visible regime with electrical and geometrical tunability analogous to the plasmonic resonances of much larger nanographene systems. Finally, we also use the switchable molecular plasmon in anthracene to demonstrate a proof-of-concept low-voltage electrochromic device.

Ge-Te-Se and Ge-Te-Se-S alloys as new materials for acousto-optic devices of the near-, mid-, and far-infrared spectral regions

The optical, acoustic, and acousto-optic properties of new Ge-Se-Te-S and Ge-Se-Te alloys are studied. As a result of parameter optimization, efficient acousto-optic materials transparent in a wide infrared range (λ = 1.5–18 μm) are fabricated.

UPDATED ANALYSIS OF A “DARK” GALAXY AND ITS BLUE COMPANION IN THE VIRGO CLOUD H I 1225 + 01

HI 1225+01 is an intergalactic gas cloud located on the outskirts of Virgo cluster. Its main components are two large clumps of comparable HI masses (M_HI ~ 10^9 Msun) separated by about 100 kpc. One of the clumps hosts a blue low-surface-brightness galaxy J1227+0136, while the other has no identified stellar emission and is sometimes referred to as a promising candidate of a "dark galaxy", an optically invisible massive intergalactic system. We present a deep optical image covering the whole HI 1225+01 structure for the first time, as well as a collection of archival data from ultraviolet to far-infrared (IR) spectral region of the brightest knot "R1" in J1227+0136. We find that R1 has a young stellar population of age 10-100 Myr and mass ~ 10^6 Msun, near-IR excess brightness which may point to the presence of hot dust with color temperature ~ 600 K, and relatively faint mid- to far-IR fluxes corresponding to the dust mass of up to ~ 100 Msun. Overall, it seems to share the general properties with low-metallicity blue compact dwarf galaxies. On the other hand, no optical counterpart to the other clump is found in our deepest-ever image. Now the limiting surface brightness reaches down to R_AB > 28 mag/arcsec2 for any emission extended over 10" (comparable to R1), which is more than one hundred times fainter than the brightest part of the companion galaxy J1227+0136.