mid-IR Region Research Articles

Dispersion engineered mid-IR supercontinuum generation using multi-core silicon-rich nitride photonic integrated waveguide

The multi-core silicon-rich nitride photonic integrated waveguide is presented here to numerically model the spanning of supercontinuum generation in the mid-IR region beyond the 4 μm wavelength range by engineering the dispersion profile such that maximum phase-matching can be achieved. Tunable dispersive wave generation has been observed in a long mid-IR region up to 5.2 μm wavelength by the variation of gap spacing between the cores. This unique geometry enables a higher phase matching from 1.2 μm to 2.6 μm wavelength range resulting in the flat dispersion profile in the near-IR region by coupling the fundamental mode in narrow waveguide core to the higher-order mode in wide waveguide core that gives rise to anomalous GVD which plays an essential role in compensating chromatic dispersion in the mid-IR region.

Fatty Acid Determination in Human Milk Using Attenuated Total Reflection Infrared Spectroscopy and Solvent-Free Lipid Separation.

This study introduces the first mid-infrared (IR)-based method for determining the fatty acid composition of human milk. A representative milk lipid fraction was obtained by applying a rapid and solvent-free two-step centrifugation method. Attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy was applied to record absorbance spectra of pure milk fat. The obtained spectra were compared to whole human milk transmission spectra, revealing the significantly higher degree of fatty acid-related spectral features in ATR FT-IR spectra. Partial least squares (PLS)-based multivariate regression equations were established by relating ATR FT-IR spectra to fatty acid reference concentrations, obtained with gas chromatography-mass spectrometry (GC-MS). Good predictions were achieved for the most important fatty acid sum parameters: saturated fatty acids (SAT, R2CV = 0.94), monounsaturated fatty acids (MONO, R2CV = 0.85), polyunsaturated fatty acids (PUFA, R2CV = 0.87), unsaturated fatty acids (UNSAT, R2CV = 0.91), short-chain fatty acids (SCFA, R2CV = 0.79), medium-chain fatty acids (MCFA, R2CV = 0.97), and long-chain fatty acids (LCFA, R2CV = 0.88). The PLS selectivity ratio (SR) was calculated in order to optimize and verify each individual calibration model. All mid-IR regions with high SR could be assigned to absorbances from fatty acids, indicating high validity of the obtained models.

Numerical optimization of 1D superconductor photonic crystals pressure sensor for low temperatures applications

In this research, reflectance spectra for a one-dimensional (1D) photonic crystal (PC) consists of a high-temperature superconductor Hg-1223 and a semiconductor GaAs have been calculated using the well-known transfer matrix method (TMM) and the two-fluid model. The proposed PC was introduced as an optical pressure sensor at the mid-IR region. The pressure effect is studied on two types of PCs; a defective PC [Air (GaAs/Hg 1223)5 (GaAs) (GaAs/Hg 1223)5 substrate] and periodic PC [Air (GaAs/Hg-1223)10 substrate]. Herein, we have found that the pressure sensitivity reached a high value of 1.7467 nm/GPa at an applied pressure of 6 GPa. However, a small blue shift is noticed for the periodic design. Therefore, the number of periods, thickness, and incident angle were taken after many optimization steps to get the highest-pressure sensitivity for the defective PC. Based on the optimization process, the results reveled higher performance for the other sensitivity parameters such sensor resolution, signal to noise ratio, figure of merit, and quality factor (QF). Finally, we have studied the effect of the operating temperature on the defect mode, which shows a small redshift with increasing the operating temperature. We hope this research may be utilized for applications of tunable filters and hydrostatic pressure sensors.

Reversibly controlled infrared transparency of conjugated polymer films incorporated into a compact electrochemical cell structure.

We demonstrate a reversible control of the mid-infrared (mid-IR) transparency of conjugated polymer films in a compact electrochemical cell structure with planar IR-transparent electrodes. The mid-IR absorbance of up to more than 1.0 is observed when a 1.0 µm thick polythiophene layer is electrochemically doped with an ionic liquid under variable bias voltage. Furthermore, the doping and dedoping processes can be repeated with reproductive absorbance spectra, in contrast to the conventional one-way chemical doping. Our device structure works for actively variable attenuation filters in the mid-IR region and can also be extended potentially to the terahertz region.

Mid-Infrared Nonlinear Frequency Conversion Using Monolithic Barium Titanate on Silicon-on-Insulator

Efficient nonlinear frequency conversion in the mid-infrared (mid-IR) region was demonstrated by a barium titanate (BTO) on a silicon-on-insulator (SOI) substrate. The BTO thin film was epitaxially grown by pulsed-laser deposition (PLD). The tensorial second order optical nonlinearity and the ferroelectric domain property of the BTO were characterized by the polarimetric mid-IR second harmonic generation (SHG) measurements. Azimuthal-dependent polarized SHG was modeled at different combinations of the nonlinear coefficient <formula><tex>$d_{ij}$</tex></formula> and the domain fraction factor <formula><tex>$\Delta D^{Y}/\Delta D^{X}$</tex></formula>. Strong SHG intensity was experimentally observed over a broad spectrum at &#x03BB; = 3.1 &#x03BC;m - 3.7 &#x03BC;m. A characteristic linear dependence between the SHG signal and the square of the pumping laser intensity was observed. The monolithic integration between the epitaxial BTO and the Si provides a compact platform for efficient on-chip light generation and quantum photonic technologies.

Impact of strain engineering and Sn content on GeSn heterostructured nanomaterials for nanoelectronics and photonic devices.

Heterostructures based on the GeSn nanocompound have high impact on integrated photonics devices. The promising feature of GeSn nanostructures is its direct bandgap transition that is a result of Sn incorporation in the Ge networks, forming a strained structure. Herein, we demonstrate a deep survey of the strain-controlling mechanisms in GeSn nanomaterials with different methodologies. Using either layer configurations, Sn incorporation, or by external stressors, the emission of different photonic and nanoelectronic applications is controlled. We find that strain engineering modulates the bandgap of GeSn active media to control the region of emission for light emitting diodes, lasing applications, and spectral response for photodetection applications within the mid-IR region of the spectrum and enhances the performance of MOSFETs. This gives GeSn nanocompounds the chance to contribute greatly to IoT physical devices and compete with unstable perovskite materials since GeSn materials can achieve a stable and more reliable performance.

Development of a Compact and Robust Mid-Infrared Spectrometer by Using a Silicon/Air Hyperspectral Filter

The molecular absorption spectrum in the mid-infrared (mid-IR) region is typically measured by Fourier-transform infrared spectroscopy (FTIR). Such spectrometers require complex and delicately aligned interferometers, which increases their size and cost. Here, we present an alternative compact mid-IR spectrometer, which uses a variable mid-IR filter with a gradual change of cavity length between two silicon/air dielectric mirrors. When combined with a modern uncooled thermal imaging camera, hyperspectral filtering can provide a powerful solution for the miniaturization of mid-IR spectrometers. By using the hyperspectral filter, light from a broadband light source can be dispersed and assigned to individual pixels of a microbolometer array of the thermal imaging camera. This technology offers an inexpensive and compact mid-IR spectrometer design with no moving parts and rapid acquisition time.

Complex Spectroscopic Study for Fusarium Genus Fungi Infection Diagnostics of “Zalp” Cultivar Oat

At present, one of the critical problems in agriculture is the identification of cereals, including oats, infected by Fusarium spp. genus fungi. Timely diagnostics can prevent the further disease spread and help to identify the already stored infected grains. In this regard, the aim of this work is to develop the spectroscopic approaches that determine the infected grains. As an object of the investigation the “Zalp” cultivar oat, both healthy and infected grains of the 2020 harvest were chosen. The spectroscopic diagnostics included FTIR in the mid-IR region, Raman, and luminescence methods. Combination of chemometric tools with FTIR and Raman spectroscopy allowed obtaining approaches based on identified characteristic spectral features which may be used as infection markers. These approaches make it possible to detect the infection on the grain husk. The carotenoid type fungi pigment was identified within the resonance conditions of Raman scattering excitation. The luminescence study of infected oat husk revealed the presence of characteristic chlorophyll α peak which is absent in healthy grain husk.

Infrared optical properties of SiO2 films on silicon substrate under different temperatures

SiO2 is a very important low refractive index film material that can operate in the UV to mid-IR regions. The SiO2 films used were deposited on Si substrates by different deposition techniques. We designed and manufactured a high-temperature infrared spectrum measuring equipment connected with a Fourier transform infrared (FTIR) spectrometer; the measuring temperature could range from room-temperature to 600°C. The FTIR transmission spectra of SiO2 films under different working temperatures were measured in the wavenumber region from 4000 to 400 cm − 1. From the measured spectra, it can be seen that IBS-SiO2 and IAD-SiO2 films are more stable and operating temperature can reach 400°C. EB-SiO2 film is worse and operating temperature is only 300°C. Complex dielectric functions of SiO2 films under different temperature conditions were calculated from FTIR transmittance spectra, and the best fitted method was obtained for calculating optical constants of dielectric materials in the high temperature condition. The results show that the structure of the Si-O-Si network in IBS-SiO2 films was the most stable structure. The experimental results were of great significance to practical application.

Metal-insulator phase transition in the δ−(BEDT−TTF)4 [2,6-anthracene- bis(sulfonate)] ·4H2O studied by infrared spectroscopy

Temperature dependence of polarized infrared (IR) reflectance spectra of two-dimensional weakly dimerized organic conductor $\ensuremath{\delta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{4}$[2,6-anthracene-bis(sulfonate)]$\ifmmode\cdot\else\textperiodcentered\fi{}4{\mathrm{H}}_{2}\mathrm{O}$ with $\frac{1}{4}$-filled band has been measured to investigate the metal-insulator phase transition at ${T}_{\mathrm{MI}}=85\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ linked to charge ordering (CO). Vibrational bands related to $\mathrm{C}=\mathrm{C}$ stretching vibrations of the BEDT-TTF donor molecule give evidence of the CO and charge fluctuations both in the metallic and insulating phases above and below 85 K, respectively. Considerable modifications of the vibrational features due to electron-molecular vibration coupling are also observed. The phase transition is associated with the reorganization of hydrogen $\mathrm{OH}\ensuremath{\cdots}\mathrm{O}$ bonds in anion layers between the sulfonate moieties of the anion and the trapped water molecules, which is transmitted to conducting BEDT-TTF layers through weaker C-$\mathrm{H}\ensuremath{\cdots}\mathrm{O}$ bonds. The analysis of electronic bands confirms the presence of strong electronic correlations in the material. Two electronic absorptions are observed in the mid-IR region, attributed to transitions between Hubbard bands and transitions inside BEDT-TTF dimers. A low-energy electronic absorption in the far-IR region is attributed to collective excitations of the short-range charge order. Below 85 K, the opening of a charge gap of \ensuremath{\sim}24 meV is observed both parallel and perpendicular to the BEDT-TTF stacking direction. The spectroscopic data indicate that the CO phase transition is a consequence of both Coulomb interactions between charge carriers and donor-acceptor interactions.

Near-Infrared Quantum Cascade Lasers Designed with van der Waals Materials

Quantum cascade lasers (QCLs) constitute a leading source of coherent radiation in the mid-IR region. However, their performance outside this region remains unsatisfactory. Indeed, there are currently no QCLs in the near-IR region. Here, we propose that a superlattice of atomically thin layers held together by van der Waals forces may operate near room temperature as a compact and powerful near-IR QCL emitting at a wavelength of $1.66\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$. It can compress over 100 stages within $0.5\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$. The electric field required for operation is about $3\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.2em}{0ex}}{\mathrm{V~cm}}^{\ensuremath{-}1}$, while the lasing threshold current density is about $22.4\phantom{\rule{0.2em}{0ex}}{\mathrm{kA~cm}}^{\ensuremath{-}2}$ depending on parameters. Rate-equation analysis reveals that the peak power per unit volume can reach over $0.1\phantom{\rule{0.2em}{0ex}}\mathrm{mW}\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}3}$ in cw operation. Unlike existing QCLs, our device is $p$-type working with holes.

Standoff Detection of Plastics

Rapid identification of different plastics for recycling is very important in overcoming the plastic waste challenge our society is facing today. An ideal plastic detector should be able to discern different plastics at the molecular level in a standoff fashion where the detector does not come in contact with the target plastic. We have demonstrated a standoff technique for the rapid identification of plastics based on their mid-IR absorption spectra. Since mid-IR region is free of overtones, this region is known as molecular finger print regime This technique is based on a variation of photoacoustic spectroscopy (PAS), where the light from a tunable infrared source, a quantum cascade laser (QCL), is scattered/reflected off the target plastic object. The returning scattered light is collected and made to create acoustic waves on a microfabricated cantilever sensor. Our experimental results show that the cantilever response, resonance frequency, amplitude, and bending, changes with infrared absorption characteristics of the target. A plot of cantilever response as a function of illuminating wavelength mimic the infrared absorption peaks of the plastics material producing a mechanical spectrum. The generated spectrum obtained for different plastics is then compared with that conventional spectrum for polymer identification.

Two-color, intracavity pump-probe, cavity ringdown spectroscopy.

We report a proof-of-principle demonstration of intracavity pump-probe, cavity ringdown (CRD) detection in a three-mirror, traveling-wave cavity. With cavity-enhanced pump power and probe absorption path length, the technique is a generally applicable, high-sensitivity, high-selectivity detection method. In our experiments, the pump radiation is switched off during every other probe ringdown, which allows uncorrelated measurements of analyte and background cavity decay rates. The net, two-color signal from the difference between the pump-on and pump-off decay rates is immune to empty-CRD drifts and spectral overlaps from non-target molecular transitions. The immunity to the ringdown drifts allows longer signal-averaging and, thus, higher detection sensitivity. The ability to compensate for the background absorption enhances the detection selectivity in spectrally congested regions. Our technique is well-suited for trace-detection in the mid-IR region, where pump-probe schemes based on strong rovibrational transitions can be applied. In this work, two-color CRD detection is implemented on a ladder-type, three-level system based on the N2O, ν3 = 1 ← 0, P(19) (pump) and ν3 = 2 ← 1, R(18) (probe), rovibrational transitions. By frequency-locking two-quantum cascade lasers to the p-polarization (pump, Finesse = 5280) and s-polarization (probe, Finesse = 67700) cavity modes, we achieve high intracavity pump power (36W) and high probe ringdown rates (>2 kHz). The observed two-color spectra are simulated by a density-matrix, three-level system model that is solved under the constraints of the cavity resonance conditions. In addition to its background compensation capability, experimental flexibility in the selection of pump-probe schemes and signal insensitivity to intracavity laser power are further features that enhance the utility of our technique for mid-IR trace-detection.

Cavity ring-down spectroscopy of l-type doubling in 15N-β-site N2O isotopomer near 7.8 µm

The spectroscopic features of l-type doublets of the site-specific 15N-isotopomers of nitrous oxide (N2O) are interesting to study as it would reveal the effect of l-type doubling in different branches of the transition band and on the spectroscopic parameters. Here, we have explored various spectroscopic features that are influenced by the l-type doubling of the fine structure lines of the β-site isotopomer of N2O, i.e. 15N14N16O (or 15Nβ) in the (1110)←(0110) transition for l = 1 (i.e. Π state) vibrational state in the mid-IR region. High-resolution spectroscopic measurements of rotationally-resolved l-type splitting involving the parity doublet e and f sub-states of 15N-β isotopomer corresponding to the P-and R-branches of the (1110)←(0110) vibrational band near 7.8 μm were carried out by cavity ring-down spectroscopy (CRDS) coupled with a continuous-wave external-cavity quantum cascade laser (cw-EC-QCL). Several new spectroscopic parameters such as l-type doubling constants, Coriolis constants, band centres, rotational constants and centrifugal distortion constants were determined for both the parity doublet e and f components of 15Nβ isotopomer. We also investigated the effects of temperature and pressure on the l-type doublets along with the dependency of pressure broadening coefficients on the rotational quantum number J. The new experimental data will improve our understanding of the molecular properties of the site-specific 15N-isotopomers of N2O and intramolecular distribution of 15N-isotopes within the N2O isotopomers.

Quantitative mineralogical analysis of clay-containing materials using ATR-FT-IR spectroscopy with PLS method.

A universal method for quantitative analysis of clay components, in terms of mineral composition, using ATR-FT-IR spectroscopy (in the mid-IR and far-IR regions) combined with partial least squares (PLS) regression technique (ATR-FT-IR-PLS) is reported. For the PLS method development, altogether 222 samples covering natural clay sources and various archaeological/cultural heritage artefacts were used as calibration and validation standards. This is the largest calibration set used for creating an ATR-FT-IR-PLS method for clay minerals. The quantitative compositions of these standards containing combinations of altogether 29 minerals for the PLS method were determined using XRD analysis. The developed ATR-FT-IR with PLS method is quick and easy to use, and enables analysis of very small sample amounts (down to a few mg). This is very important when working with samples from archaeological and cultural heritage objects. The developed ATR-FT-IR-PLS method enables quantifying the contents of 7 main classes of minerals in different clays with a root mean square error of prediction (RMSEP) ranging from 0.9 to 5.1 g/100g. This means that in some cases, depending also on the content of the mineral in the sample, the accuracy is at a semiquantitative level. This quantitative method was successfully applied to 11 cultural heritage case-study samples.

Infrared thermospectroscopic imaging of heat and mass transfers in laminar microfluidic reactive flows

In this work, a novel image-based method is presented to characterize the heat and mass transfer rates in a Hele-Shaw microfluidic reactor. A Fourier transform infrared (FTIR) spectrometer is used in transmission mode in combination with an infrared (IR) camera to simultaneously measure the molar concentration and the thermal fields in the microfluidic chip within few seconds. A classical exothermic NaOH + HCl → NaCl + H2O chemical reaction is used to produce a multiphase flow and a heat source in the reactor. The molar concentration fields of all the species are measured using the IR spectrum in the mid-IR region, and the heat fields are obtained simultaneously from the proper emission. The quantitative aspect of the method is illustrated by comparing the molar concentration profiles to a reactor model, based on the advection-diffusion-reaction equations. The good agreement between the model and experimental data validates the method, and the expected strong diffusion-limited reaction regime in laminar microfluidic reactor is achieved. Thus, the results of this work provide a new and efficient thermospectroscopic imaging method to perform rapid, contactless and in operando heat and mass transfer characterizations in laminar microfluidic reactive flows.

Investigation of Mid-Infrared Broadband Second-Harmonic Generation in Non-Oxide Nonlinear Optic Crystals

The mid-infrared (mid-IR) continuum generation based on broadband second harmonic generation (SHG) (or difference frequency generation) is of great interest in a wide range of applications such as free space communications, environmental monitoring, thermal imaging, high-sensitivity metrology, gas sensing, and molecular fingerprint spectroscopy. The second-order nonlinear optic (NLO) crystals have been spotlighted as a material platform for converting the wavelengths of existing lasers into the mid-IR spectral region or for realizing tunable lasers. In particular, the spectral coverage could be extended to ~19 µm with non-oxide NLO crystals. In this paper, we theoretically and numerically investigated the broadband SHG properties of non-oxide mid-IR crystals in three categories: chalcopyrite semiconductors, defect chalcopyrite, and orthorhombic ternary chalcogenides. The technique is based on group velocity matching between interacting waves in addition to birefringent phase matching. We will describe broadband SHG characteristics in terms of beam propagation directions, spectral positions of resonance, effective nonlinearities, spatial walk-offs between interacting beams, and spectral bandwidths. The results will show that the spectral bandwidths of the fundamental wave allowed for broadband SHG to reach several hundreds of nm. The corresponding SH spectral range spans from 1758.58 to 4737.18 nm in the non-oxide crystals considered in this study. Such broadband SHG using short pulse trains can potentially be applied to frequency up-conversion imaging in the mid-IR region, in information transmission, and in nonlinear optical signal processing.

Hg3P2S8: A New Promising Infrared Nonlinear Optical Material with a Large Second-Harmonic Generation and a High Laser-Induced Damage Threshold

The development of new infrared (IR) nonlinear optical (NLO) materials with strong second-harmonic generation (SHG) responses and high laser-induced damage thresholds (LIDTs) is urgent but challenging. Herein, the defective diamond-like chalcogenide Hg3P2S8 (HPS) was rationally designed and fabricated by a flexible and rigid tetrahedral motif combined strategy. HPS exhibits a phase-matching SHG response of similar to 3.6 x AGS (the largest one in the reported ternary sulfides), high LIDT (similar to 3 x AGS), wide IR transparent region (0.43-16.3 mu m), moderate birefringence (0.043 at 1064 nm), and good physicochemical stability and crystal growth habits. Theoretical analyses confirm that the large SHG effect originates from the synergistic effects between distorted HgS4, PS4, and vacancy-induced nonbonding electrons. The results indicate that HPS is a promising NLO candidate for high-power, high-efficiency laser output in the mid-IR region, which provides an insight into the exploration of new IR NLO materials.

KCs2[Pb2Br5(HCOO)2]: A Polar 3D Lead‐Bromide Framework Exhibiting Strong Second‐Harmonic Generation Response

The discovery of new nonlinear optical (NLO) crystals with excellent properties is in urgently demand because of their ability to generate coherent light. Herein, we report an unique NLO lead bromide formate, KCs2 [Pb2 Br5 (HCOO)2 ], which has been synthesized by a mix-solvothermal method. KCs2 [Pb2 Br5 (HCOO)2 ] exhibits strong phase-matching second-harmonic generation (SHG) response (6.5×KDP), large birefringence (0.16@ 1064 nm), and a wide transparent window in most visible light and mid-IR region. Interestingly, KCs2 [Pb2 Br5 (HCOO)2 ] features a polar 3D lead-bromide framework in which adjacent Pb-Br layers containing coplanar Pb6 Br6 rings are not only parallel to each other, but also orient in the same direction. These oriented arrangements are responsible for the strong SHG response and large birefringence that are elucidated by both local dipole moment and theoretical calculations. This research provides a new strategy to explore subsequent NLO crystals.

A3Te(Zn2Ge)Ge2O14 (A = Sr, Ba, and Pb): New Langasite Mid-infrared Nonlinear Optical Materials by Rational Chemical Substitution

Mid-infrared (mid-IR) nonlinear optical (NLO) crystals with excellent comprehensive performances are urgently demanded in the laser technology field. Herein, a series of new langasite members, A3Te(Zn2Ge)Ge2O14 (A = Sr, Ba, and Pb), were designed and synthesized through a rational chemical substitution strategy with classic A3BC3D2O14 langasites as ideal prototypes. These crystals exhibited excellent mid-IR NLO properties including a large second harmonic generation (SHG) response (2.7, 3.2, 13.0 × KDP under 1064 nm and 0.11, 0.12, 0.54 × AGS under 2090 nm), a large band gap (4.03, 3.98, and 3.43 eV), and a wide transparency region from the ultraviolet (UV) to mid-IR region. Compared to those langasites with light D-site cations, the titled crystals highlighted the reduction of vibration frequency by the introduction of heavier Ge4+ cations, which helps extend the transparency range in the mid-IR region. Additionally, theoretical calculation results revealed the significant enhancement of the SHG effect by Pb2+ cations on the A-site. This work confirmed the feasibility of our chemical co-substitution strategy and enriched the langasites.