Hollow-core fiber gas lasers [Invited

Near infrared spectroscopic imaging assessment of cartilage composition: Validation with mid infrared imaging spectroscopy

The mid-infrared (MIR) region is one of the most thriving spectral regions as it contains the vibrational resonances of several molecules of interest, as well as the absorption bands for hot bodies. In this work, we propose a novel dielectric waveguide that confines the light in a nanoscale air gap. This dielectric waveguide is a suitable candidate for on-chip sensing. Detailed dispersion analysis of this 3D waveguide is also provided. The effect of the refractive index change in the gap is studied and shows very high sensitivity and causes significant changes in the modal parameters. We also show that these waveguide modes exhibit plasmonic-like characteristics at the MIR region with controllable plasma frequency, without the inclusion of any metals. This waveguide is also utilized in various on-chip applications with nanoscale confinement at the MIR region.

Invasiveness of actual glucose measuring methods for diabetic patients has impulsed studies of non-invasive techniques, within which infrared (IR) spectroscopy has become one of the most accepted approaches. However, there is a disagreement between studies that identify measuring characteristics in near infrared (NIR) or mid infrared (MIR) regions. In this work a quantitative analysis in both regions is presented in order to identify important peaks and parts that could contain information related to glucose concentration. 12 solutions of glucose diluted in ultrapure water were prepared, in order to cover the clinical range of glucose concentration in human blood (40mg/dL -400mg/dL). Transmittance spectrums in NIR and MIR regions were collected, and different wavelengths and regions that could be related with glucose concentration were identified. 22 calibration models were built with algorithms like partial least squares regression (PLS) and principal component regression (PCR). The models were compared by using the standard error of calibration (SEC), the standard error of prediction (SEP) and a contrast test of significance. Important information was identified in the MIR region between 990 cm− 1 and 1130 cm− 1 with a peak in 1042 cm− 1, and in the NIR region between 7700 cm− 1 and 4000 cm− 1. These results will be used in further analysis with more complex matrixes as plasma, whole blood and some tissue simulators, in order to identify interferences in identified regions.

Spectroscopy of molecules in the mid-infrared (MIR) region has important applications in various fields, such as astronomical observation, environmental detection, and fundamental physics. However, compared to that in the near-infrared, precision spectroscopy in the MIR is often limited by the light source and has not shown full potential in sensitivity. Here we report a cavity ring-down spectroscopy system using a tunable narrow-linewidth optical parametric oscillator, which fulfills the requirement of high sensitivity and high precision in the MIR region. The Lamb-dip spectrum of the N2O molecule at 2.7 μm was measured as a demonstration of spectroscopy in the MIR with kilohertz accuracy.

The generation of tunable narrow-band light in the mid-infrared region is of considerable interest in pollution monitoring, remote sensing and trace gas detection. Nonlinear difference frequency mixing (DFM) is one of several techniques which allow access to the important 3-20 μm region, where many molecules have fundamental absorptions. Recent advances in single longitudinal mode high-power diode-laser technology, coupled with new developments based on periodic poling of non-linear crystals and improved growth techniques for birefringently phasematched crystals (such as GaSe), has made DFM a promising approach for a compact broadly tunable, narrowband infrared source. A judicious selection of pump laser wavelengths and non-linear crystals will allow most of the mid-infrared spectral region to be accessible by DFM. Our approach involves DFM of two high power single longitudinal mode diode lasers in the non-linear crystal, GaSe. We combine the broad tuning-range of the diode lasers with the extended critical phasematching range of GaSe to produce a widely tunable system with which we have implemented computer controlled diode frequency tuning, automatic crystal angle positioning, and automatic alignment of the pump beams into the crystal.

Integrated graphene-based semimetal/metallic-like MSe2 (M = Pt, Re) heterostructures for mid-infrared ultrafast photonics

In this study, a low Schottky-barrier photodetector with a plasmonic assist using a two-dimensional (2D) nanohole array was demonstrated, which receives mid-infrared (MIR) light at room temperature. In the structural design, it was confirmed that the 2D nanohole-array photodetector has high absorbance in the MIR region using rigorous coupled-wave analysis. The result showed that the nanoholes formed in p-type silicon (p-Si), platinum silicide (PtSi), to form Schottky barriers, and gold (Au), for photocurrent extraction, had high absorbance in the MIR region along with the Fabry-Perot resonance mode toward the depth of the nanohole. The 2D nanohole array, with Au/PtSi/p-Si layers, has high absorbance for illuminating MIR light near 3.46 µm from the backside. The current-voltage characteristics indicated a low Schottky barrier of 0.32 eV, confirming the photoresponsive potential in the MIR photodetection. The photocurrent response to the modulation signal was obtained at room temperature. In addition, signal processing through transimpedance and lock-in amplifiers enabled us to obtain characteristics with high linearity for light intensities in milliwatts. Light acquisition for 2.5-3.8-µm-long MIR wavelength became possible, and applications in gas sensing, including vibrational absorption bands of alkane groups, are expected.

Extending the photoacoustic microscopy (PAM) into the mid-infrared (MIR) molecular fingerprint region constitutes a promising route toward label-free imaging of biological molecular structures. Realizing this objective requires a high-energy nanosecond MIR laser source. However, existing MIR laser technologies are limited to either low pulse energy or free-space structure that is sensitive to environmental conditions. Fiber lasers are promising technologies for PAM for their potential to offer both high pulse energy and robust performance, which however have not yet been used for PAM because it is still at the infant research stage. We aim to employ the emerging gas-filled anti-resonant hollow-core fiber (ARHCF) laser technology for MIR-PAM for the purpose of imaging myelin-rich regions in a mouse brain. This laser source is developed with a high-pulse-energy nanosecond laser at , targeting the main absorption band of myelin sheaths, the primary chemical component of axons in the central nervous system. The laser mechanism relies on two-order gas-induced vibrational stimulated Raman scattering for non-linear wavelength conversion, starting from a 1060-nm pump laser to through the two-stage gas-filled ARHCFs. The developed fiber Raman laser was used for the first time for MIR-PAM of mouse brain regions containing structures rich in myelin. The high peak power of and robust performance of the generated MIR Raman pulse addressed the challenge faced by the commonly used MIR lasers. We pioneered the potential use of high-energy and nanosecond gas-filled ARHCF laser source to MIR-PAM, with a first attempt to report this kind of fiber laser source for PAM of lipid-rich myelin regions in a mouse brain. We also open up possibilities for expanding into a versatile multiwavelength laser source covering multiple biomarkers and being employed to image other materials such as plastics.

Body: Surface plasmon polariton (SPP), which result from the strong coupling of bound electromagnetic (EM) waves and collective charge oscillations, enable subwavelength manipulation of light and matter interaction. Conventional noble metals-based SPP excited in the visible and near-Infrared spectra ranges suffer from large energy losses and cannot be dynamically tuned. Thanks to its 2D nature, graphene has emerged as a promising alternative plasmonic material with well- confined SPPs in the mid-infrared (MIR) and terahertz (THz) ranges and remarkable tunability. On the other hand, polar dielectric materials support low optical losses and sub-wavelength mode in the MIR and THz regions via surface phonon polariton (SPhP)mode' stimulation within the Reststrahlen band, a narrow spectral range between transverse optical (TO) and longitudinal optical (LO) frequencies. Due to the excellent polaritonic responses in both graphene and silicon carbide in the MIR and THz regions, devices combining the two materials are inferred to greatly advance photonics and THz technologies. We have recently theoretically demonstrated the strong confinement and large propagation figures of merit (FOM) for hybrid surface plasmon and surface phonon polariton in the epitaxial graphene on silicon carbide. The ability to confine such MIR light into small volume through excitation of SPP and SPhP has many implications for optoelectronic technologies such as the MIR photodetection[2], sensing[3], and solar cells[4]. Epitaxial graphene(EG) grown on silicon carbide(SiC) on silicon(Si) has been investigated as a suitable platform for graphene growth on semiconductors to meet the prospective technologies [5]. Despite this platform being very advantageous, experimental and theoretical works demonstrating the potentials properties/applications of EG on SiC on Si are still missing. Here we combine electromagnetic simulations based on finite elements method (FEM) with attenuated total reflectance Fourier transformed infrared (ATR-FTIR) spectroscopy to indicate improved MIR absorption, E field enhancement, and tunability for graphene-coated SiC nanowires (NWs). As anticipated, low absorption was confirmed from both simulations and measurements on bare SiC NWs, further supported by the calculated weak electric field due to the thin size of SiC NWs being incapable of absorbing sufficient light. On the other hand, enhanced absorption for the SiC NWs was measured by ATR-FTIR after a conformal graphitization of NWs[6]. Our previous study of this material system revealed a low-density oxide layer formed between graphene and SiC NWs[6]. This oxide layer between EG and SiC NW has been implemented in the FEM model, leading to field enhancement of incident EM field and strongly confined in the thin oxide. This substantial absorption and field enhancement originates from the coupling between the incident MIR photon and the plasmon/ phonon in graphene and SiC, respectively, with the oxide layer acting as a coupling medium. Furthermore, we demonstrate the modes' tunability whereby tuning the graphene's Fermi energy, the modes' resonance frequencies can be shifted to ~ 100 cm-1. Lastly, we realize substantial field enhancement, which is achieved by carefully tuning the graphene's Fermi energy and adjusting the spectra position of mode 1(mode on the left of LO frequency) closer to TO frequency. These results hold promise for various photonic applications such as perfect absorber to improve solar cell, MIR photodetectors, and other tunable photonics. [1]P. Rufangura, et al., J. Phys. Mater., vol. 3, no. 3, p. 032005, 2020. [2]T. Low and P. Avouris, ACS nano, vol. 8, no. 2, pp. 1086-1101, 2014. [3]A. G. Brolo, Nat. Photon., vol. 6, no. 11, p. 709, 2012. [4]H. A. Atwater and A. Polman, Nat. Mater., vol. 9, no. 3, p. 205, 2010. [5] N. Mishra et al., Phys. Status Solidi (a), vol. 213, no. 9, pp. 2277-2289, 2016. [6]N. Mishra et al., J. Appl. Phys., vol. 126, no. 6, p. 065304, 2019.

The first atmospheric window of 3–5 μm in the mid-infrared (MIR) spectral range pertains to crucial application fields, with particular scientific and technological importance. However, conventional narrow-bandgap semiconductors operating at this band, represented by mercury cadmium telluride and indium antimonide, suffer from limited specific detectivity at room temperature and hindered optoelectronic integration. In this study, a plasmonic hot electron-empowered MIR photodetector based on Al-doped ZnO (AZO)/bi-layer graphene heterostructure is demonstrated. Free electrons oscillate coherently in AZO disk arrays, resulting in strong localized surface plasmon resonance (LSPR) in the MIR region. The photoelectric conversion efficiency at 3–5 μm is significantly improved due to plasmon-induced hot-electron extraction and LSPR-enhanced light absorption. The specific detectivity reaches about 1.4 × 1011 Jones and responsivity is up to 4712.3 A/W at wavelength of 3 μm at room temperature. The device’s specific detectivity is among the highest performance of commercial state-of-the-art photodetectors and superior to most of the other 2D materials based photodetectors in the MIR region. These results demonstrate that a plasmonic heavily doped metal oxides/2D material heterostructure is a suitable architecture for constructing highly sensitive room-temperature MIR photodetectors.

Novel chalcogenide glass-based fiber opens up the mid-infrared (MIR) range for real-time monitoring and control in medical diagnostics and chemical processing. Fibers with long wavelength cut-off are of interest here. Sulfide, selenide and telluride based chalcogenide glass are candidates, but there are differences in their glass forming region, thermal stability and in the short and long wavelength cut-off positions. In general sulfide and selenide glasses have greater glass stability, but shorter long-wavelength cut-off edge, compared to telluride glasses; selenide-telluride glasses are a good compromise. Low optical loss selenide-telluride based long wavelength fibers could play a substantial role in improving medical diagnostic systems, chemical sensing, and processing, and in security and agriculture. For biological tissue, the molecular finger print lies between ~3-15 μm wavelengths in the MIR region. Using MIR spectral mapping, information about diseased tissue may be obtained with improved accuracy and in vivo using bright broadband MIR super-continuum generation (SCG) fiber sources and low optical loss fiber for routing. The Ge-As-Se-Te chalcogenide glass system is a potential candidate for both MIR SCG and passive-routing fiber, with good thermal stability, wide intrinsic transparency from ~1.5 to 20 μm and low phonon energy. This paper investigates Ge-As-Se-Te glass system pairs for developing high numerical aperture (NA) small-core, step-index optical fiber for MIR SCG and low NA passive step-index optical fiber for an in vivo fiber probe. Control of fiber geometry of small-core optical fiber and methods of producing the glass material are also included in this paper.

Mid-infrared (MIR) represents a crucial spectral region for applications in spectroscopy, sensing, imaging, security and industry screening, owing to the strong characteristic vibrational transitions of many important molecules. However, the current MIR compatible materials are fragile, hazardous, and costly, which hampers the performance of MIR devices. Here, we developed a versatile transmittance-based Kramers-Kronig method and obtained the optical properties of graphene oxide in the MIR region, unveiling its application potentials as a novel MIR compatible material. As an example, we demonstrated free-standing graphene oxide MIR polarizers with large extinction ratio (∼20 dB) and controllable working wavelength up to 25 μm, by using the low-cost and flexible direct laser writing technique. Our transmittance-based KK method offers a versatile approach to obtain the optical properties of novel atomic-scale low-dimensional materials in the less developed MIR region and opens up opportunities in high performing functional MIR devices.

High-power tunable mid-infrared (MIR) and far-infrared (FIR) lasers in a range of 3-20 μm, especially in the atmospheric windows of 3-5 μm and 8-12 μm are essential for the applications, such as in remote sensing, minimally invasive surgery, telecommunication, national security, etc. At present, the technology of MIR and FIR laser have become a research hotspot. As the core component of all-solid-state laser frequency conversion system, nonlinear optical (NLO) crystals for coherent MIR and FIR laser are urgently needed by continuously optimizing and developing. However, compared with several outstanding near infrared, visible, and ultraviolet NLO crystals, such as <i>β</i>-BaB<sub>2</sub>O<sub>4</sub>, LiB<sub>3</sub>O<sub>5</sub>, LiNbO<sub>3</sub>, KTiOPO<sub>4</sub>, and KBe<sub>2</sub>BO<sub>3</sub>F<sub>2</sub>, the generation of currently available NLO crystals for 3-20 μm laser is still underdeveloped. Traditional NLO oxide crystals are limited to output wavelengths ≤ 4 μm due to the multi-phonon absorption. In the past decades, the chalcopyrite-type AgGaS<sub>2</sub>, AgGaSe<sub>2</sub> and ZnGeP<sub>2</sub> have become three main commercial crystals in the MIR region due to their high second-harmonic generation coefficients and wide IR transparency ranges. Up to now, ZnGeP<sub>2</sub> is still the state-of-the-art crystal for high energy and high average power output in a range of 3-8 μm. Unfortunately, there are still some intrinsic drawbacks that hinder their applications, such as in poor thermal conductivity and low laser damage threshold for AgGaS<sub>2</sub>, non-phase-matching at 1.06 μm pumping for AgGaSe<sub>2</sub>, and harmful two-photon absorption at 1.06 μm for ZnGeP<sub>2</sub>. In addition, ZnGeP<sub>2</sub> has significant multi-phonon absorption in an 8-12 μm band, which restricts its applications in long wavelength MIR. With the development of research, several novel birefringent crystals, as well as all-epitaxial processing of orientation-patterned semiconductors GaAs (OP-GaAs) and GaP (OP-GaP), have been explored together with attractive properties, such as large NLO effect, wide transparency ranges, and high resistance to laser damage.<br/>In this paper, from the angle of the compositions of NLO crystal materials, several kinds of phosphide crystals (ZnGeP<sub>2</sub> CdSiP<sub>2</sub>) and chalcogenide crystals (CdSe, GaSe, LiInS<sub>2</sub> series, and BaGa<sub>4</sub>S<sub>7</sub> series) are summarized. In addition, the latest achievements of the orientation-patterned materials such as OP-GaAs and OP-GaP are also reviewed systematically. In summary, we review the above-mentioned attractive properties of these materials such as in the unique capabilities, the crystal growth, and the output power in the MIR and FIR region.

Applying NIR and MIR spectroscopy for C and soil property prediction in northern cold-region ecosystems. Which approach works better?

Infrared spectroscopy in the visible to near‐infrared (vis–NIR) and mid‐infrared (MIR) regions is a well‐established approach for the prediction of soil properties. Different data fusion and training approaches exist, and the optimal procedures are yet undefined and may depend on the heterogeneity present in the set and on the considered scale. The objectives were to test the usefulness of partial least squares regressions (PLSRs) for soil organic carbon (SOC), total carbon (C t ), total nitrogen (N t ) and pH using vis–NIR and MIR spectroscopy for an independent validation after standard calibration (use of a general PLSR model) or using memory‐based learning (MBL) with and without spiking for a national spectral database. Data fusion approaches were simple concatenation of spectra, outer product analysis (OPA) and model averaging. In total, 481 soils from an Austrian forest soil archive were measured in the vis–NIR and MIR regions, and regressions were calculated. Fivefold calibration‐validation approaches were carried out with a region‐related split of spectra to implement independent validations with n ranging from 47 to 99 soils in different folds. MIR predictions were generally superior over vis–NIR predictions. For all properties, optimal predictions were obtained with data fusion, with OPA and spectra concatenation outperforming model averaging. The greatest robustness of performance was found for OPA and MBL with spiking with R 2 ≥ 0.77 (N), 0.85 (SOC), 0.86 (pH) and 0.88 (C t ) in the validations of all folds. Overall, the results indicate that the combination of OPA for vis–NIR and MIR spectra with MBL and spiking has a high potential to accurately estimate properties when using large‐scale soil spectral libraries as reference data. However, the reduction of cost‐effectiveness using two spectrometers needs to be weighed against the potential increase in accuracy compared to a single MIR spectroscopy approach.