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Black phosphorus (bP) is a promising material for mid-infrared (mid-IR) optoelectronic applications, exhibiting high performance light emission and detection. Alloying bP with arsenic extends its operation toward longer wavelengths from 3.7 μm (bP) to 5 μm (bP3As7), which is of great practical interest. Quantitative optical characterizations are performed to establish black phosphorus-arsenic (bPAs) alloys optoelectronic quality. Anisotropic optical constants (refractive index, extinction coefficient, and absorption coefficient) of bPAs alloys from near-infrared to mid-IR (0.2-0.9 eV) are extracted with reflection measurements, which helps optical device design. Quantitative photoluminescence (PL) of bPAs alloys with different As concentrations are measured from room temperature to 77 K. PL quantum yield measurements reveal a 2 orders of magnitude decrease in radiative efficiency with increasing As concentration. An optical cavity is designed for bP3As7, which allows for up to an order of magnitude enhancement in the quantum yield due to the Purcell effect. Our comprehensive optical characterization provides the foundation for high performance mid-IR optical device design using bPAs alloys.

Low‐cost, room‐temperature operating light emitters in the 3–8 µm mid‐wave infrared wavelengths are desired for a broad range of applications such as chemical spectroscopy and infrared scene projection. The photoluminescence properties of PbSnSe ternary alloys grown by molecular beam epitaxy on (001) GaAs are studied in this work. Despite a large threading dislocation density, much greater than 109 cm−2, room temperature photoluminescence out to a peak wavelength of 8.4 µm is observed. In situ surface passivation with amorphous GeSe and an optimized thick PbSe buffer layer are shown to be necessary for enabling bright photoluminescence in PbSnSe heterostructures, stemming from Sn‐related defect complexes generated from ambient exposure and lying near the GaAs interface. Luminescence is found to be strongly blueshifted by unrelaxed tensile strains associated with thermal expansion and lattice constant mismatch, which makes emission wavelengths farther in the infrared more difficult to achieve. Carrier recombination analysis indicates room for further improvement as luminescence efficiencies are still limited by extrinsic mechanisms rather than intrinsic Auger recombination. Overall, this work demonstrates the exciting potential of PbSnSe for spontaneous emission in the mid‐wave infrared at room temperature, alongside more established material platforms like InAsSb. [ABSTRACT FROM AUTHOR]

A compact and responsive thermoelectric photodetector is introduced for the mid‐infrared. By resonantly coupling mid‐infrared light to a Sb2Te3‐Bi2Te3 thermoelectric junction, a thermocouple is formed that is directly heated by narrow‐band mid‐infrared radiation. Near‐perfect absorption is achieved at this hot junction through the resonantly enhanced coupling of light to free‐electrons in the Bi2Te3 and Sb2Te3 materials. The fabricated devices operate at 3.6 µm and demonstrate a responsivity of 10.2 V W−1, a specific detectivity of 4.6  × 106 cm Hz1/2 W−1, and a bandwidth in the order of 1 kHz. The optimal detection wavelength can be spectrally tuned by changing the resonant cavity dimensions. This work shows a path toward miniaturized mid‐infrared detectors and spectrometers with high sensitivity, responsivity, and bandwidth. Importantly, the device presented here is ideal for industrial production, which it is hoped will provide wider access to mid‐infrared technologies for chemical sensing, medicine, and security. [ABSTRACT FROM AUTHOR]

Lead chalcogenides are compelling materials for nanophotonics and optoelectronics due to their high refractive indices, extreme thermo‐optic coefficients, and high transparency in the mid‐infrared (MIR). In this study, PbTe hoppercubes (HC, face‐open box cubes) are synthesized and explored for their MIR resonant characteristics. Single‐particle microspectroscopy uncovered deep‐subwavelength light localization, with a spectral response dominated by both fundamental and multiple high‐order Mie‐resonant modes. Nanoimaging mapping using scattering‐type scanning near‐field optical microscopy (s‐SNOM) reveals that the scattering at the center of the HC is reduced by more than five times compared to the edges. 2D‐Hyperspectral scans conducted using a low‐power broadband MIR source and nanometer spatial resolutions provided information on the local amplitude and phase‐resolved near‐fields, including amplitude and phase mapping of higher order modes with measured Q‐factors of close to 100. Employing s‐SNOM to characterize complex resonant nanophotonic structures holds implications for quantum sensing, IR photodetection, non‐linear generation, and ultra‐compact high‐Q metaphotonics. [ABSTRACT FROM AUTHOR]

The development of new methods of identification of active pharmaceutical ingredients (API) is a subject of paramount importance for research centers, the pharmaceutical industry, and law enforcement agencies. Here, a system for identifying and classifying pharmaceutical tablets containing acetaminophen (AAP) by brand has been developed. In total, 15 tablets of 11 brands for a total of 165 samples were analyzed. Mid-infrared vibrational spectroscopy with multivariate analysis was employed. Quantum cascade lasers (QCLs) were used as mid-infrared sources. IR spectra in the spectral range 980–1600 cm−1 were recorded. Five different classification methods were used. First, a spectral search through correlation indices. Second, machine learning algorithms such as principal component analysis (PCA), support vector classification (SVC), decision tree classifier (DTC), and artificial neural network (ANN) were employed to classify tablets by brands. SNV and first derivative were used as preprocessing to improve the spectral information. Precision, recall, specificity, F1-score, and accuracy were used as criteria to evaluate the best SVC, DEE, and ANN classification models obtained. The IR spectra of the tablets show characteristic vibrational signals of AAP and other APIs present. Spectral classification by spectral search and PCA showed limitations in differentiating between brands, particularly for tablets containing AAP as the only API. Machine learning models, specifically SVC, achieved high accuracy in classifying AAP tablets according to their brand, even for brands containing only AAP. [ABSTRACT FROM AUTHOR]

A low‐cost single frequency laser, emitting in the mid‐infrared spectral region and dissipating minimal electrical power, is a key ingredient for the next generation of portable gas sensors for high‐volume applications involving chemical sensing of important greenhouse and pollutant gases. Herein, a Quantum Cascade Surface Emitting Laser (QCSEL) is proposed, which is implemented as a short linear cavity with high reflectivity coated end‐mirrors to suppress any edge emission and employs a buried semiconductor diffraction grating to extract the light from the surface. By wafer‐level testing, the cavity length scaling is investigated, mirror reflectivities larger than 0.9 are extracted, and a pulsed threshold power dissipation of 237 mW for an emission wavelength near 7.5 µm is achieved. Finally, single‐mode emission with a side‐mode suppression ratio larger than 33 dB is demonstrated for a 248 µm short cavity, which is mounted with the epitaxial layer up and operated in continuous wave at 20 ∘C$^{\circ}{\rm C}$. [ABSTRACT FROM AUTHOR]

Wide-field mid-infrared (MIR) hyperspectral imaging offers a promising approach for studying heterogeneous chemical systems due to its ability to independently characterize the molecular properties of different regions of a sample. However, applications of wide-field MIR microscopy are limited to spatial resolutions no better than ∼1 μm. While methods exist to overcome the classical diffraction limit of ∼λ/2, chromatic aberration from transmissive imaging reduces the achievable resolution. Here we describe the design and implementation of a simple MIR achromatic lens combination that we believe will aid in the development of resolution-enhanced wide-field MIR hyperspectral optical and chemical absorption imaging. We also examine the use of this doublet lens to image through polystyrene microspheres, an emerging and simple means for enhancing spatial resolution. [ABSTRACT FROM AUTHOR]

Nanophotonic structures have shown promising routes to controlling and enhancing nonlinear optical processes at the nanoscale. However, most nonlinear nanostructures require a handling substrate, reducing their application scope. Due to the underwhelming heat dissipation, it has been a challenge to evaluate the nonlinear optical properties of free-standing nanostructures. Here, we overcome this challenge by performing shot-controlled fifth harmonic generation (FHG) measurements on a SiC meta-membrane – a free-standing transmission metasurface with pronounced optical resonances in the mid-infrared (λres ≈ 4,000 nm). Back focal plane imaging of the FHG diffraction orders and rigorous finite-difference time-domain simulations reveal at least two orders of magnitude enhancement of the FHG from the meta-membrane, compared to the unstructured SiC film of the same thickness. Single-shot measurements of the meta-membrane with varying resonance positions reveal an unusual spectral behavior that we explain with Kerr-driven intensity-dependent resonance dynamics. This work paves the way for novel substrate-less nanophotonic architectures. [ABSTRACT FROM AUTHOR]

Ultra-broadband metamaterial absorbers (UBMAs) that are compatible with CMOS technology for use in the mid-infrared and long-wave infrared regions are crucial for a variety of applications, including radiative cooling, thermal photovoltaic, and thermal imaging. In this regard, we propose, in this work, a design of an UBMA based on the heavily doped silicon (D-Si) and silicon carbide (SiC). The 3D finite-difference time-domain method is used, mainly, to numerically calculate the optical characteristics of the proposed UBMA. The absorber, which is made up of a periodic array of symmetrical multilayered square rings of D-Si and SiC, achieves high absorption with an average absorption of 95% over a wavelength range of 2.5–22 µm. This broad range of wavelength absorption is attained, encompassing the mid-, long-wave, and partial far-infrared regions. In addition to the materials' inherent absorption, the stimulation of magnetic polaritons, surface plasmon polaritons, localized surface plasmon resonance, and cavity resonance are responsible for the nearly perfect broadband absorption. Under normal incidence, the proposed UBMA is polarization-independent due to the symmetrical design of the absorber. Furthermore, the impact of the incidence angle on the absorption of transverse electric and transverse magnetic waves is examined. [ABSTRACT FROM AUTHOR]

Manipulating the propagation of mid-infrared (mid-IR) light is crucial for optical imaging, biosensing, photocatalysis, and guiding photonic circuits. Artificially engineered metamaterials were introduced to comprehensively control optical waves. However, fabrication challenges and optical losses have impeded the progress. Fortunately, two-dimensional van der Waals (vdW) materials are alternatives because of their inherent optical properties, such as hyperbolic behavior, high confinement, low loss, and atomic-scale thickness. In this research, we conducted theoretical and numerical investigations on the α-phase molybdenum trioxide, a biaxial vdW material, with patterned graphene to assess the potential of the tunable focusing of mid-IR light. Our proposed method directly alters the path of excited light to focus mid-IR light by negative refraction. Further, the patterned graphene in our design offers enhanced focusing characteristics, featuring a significantly reduced waist diameter with 1/92 of the free-space wavelength, an enhanced beam quality without pronounced field ripples, and a fivefold increase in field intensity. Moreover, our approach significantly preserves the waist diameter of the focused beam while facilitating directional steering. Thus, the focused beam can propagate in a canalized manner toward the desired direction. These advancements lay the foundation for promising applications in planar photonics. [ABSTRACT FROM AUTHOR]

In this paper, we present a single crystal, KTA (potassium titanyl-arsenate, KTiOAsO4) based picosecond optical parametric amplifier pumped by an in-house built 1030 nm Yb:YAG thin-disk laser, capable of tunability from 1.46 to 3.5 µ m, operating at 90 kHz, with high average power in the signal and idler beams. The highest output power of 8.9 W was reached for the 1750 nm signal beam with 19% conversion efficiency and the respective 2500 nm idler beam power was 6.2 W with 13% efficiency. The highest combined signal and idler mid-infrared power was 17 W at the 2060 nm wavelength degeneracy point. [ABSTRACT FROM AUTHOR]

Fiber combiner is an effective device to enhance the power level or broaden the spectral wavelengths through a single monolithic output aperture, by fusing multiple fibers together. This is particularly important in mid‐infrared (MIR) 2–5 µm range, in which the output power of a single laser is limited. Due to the high MIR transmission, chalcogenide glass is an ideal host for constructing MIR fiber combiner. In this study, we report the design, fabrication, and characterization of a homemade 3 × 1 chalcogenide glass fiber combiner. The fiber combiner includes a 3 × 1 As–S input fiber bundle and an output fiber. The input fiber bundle was made by tapering three As–S multimode fibers with a core diameter of 200 µm and a cladding diameter of 250 µm. The taper reduction ratio R was 2, and the length of the taper transition region was ∼2 cm. The output fiber with core diameter of 360 µm and cladding diameter of 570 µm was then spliced with the tapered end of the fiber bundle. The total length of the combiner was ∼53 cm. The measurement of power combination at 4.56 µm indicated that the transmission efficiency of three ports reached 80%, whereas the power combining efficiency of the combiner was measured to be ∼48%. In addition, efficient wavelength combining over one octave has been demonstrated, by launching three different lasers at 2.1, 3.39, and 4.56 µm through the 3 × 1 combiner. It proves that chalcogenide fiber combiner is promising for the applications of high‐level power combination and broadband wavelength combining in MIR 2–5 µm range. [ABSTRACT FROM AUTHOR]

Lithium niobate (LN) crystal with anti‐reflective subwavelength structures (ASSs) has great applications in high‐power tunable optical parametric oscillator (OPO) lasers. However, it is still a great challenge to 3D nanofabrication of LN ASSs. Herein, a wet‐etching‐assisted laser polarization domain inversion (WE‐LPDI) technology is proposed to fabricate periodic cone arrays with a period from 200 nm to 4 µm on LN. Based on the optimized structural parameters, large‐area LN ASSs with a period of 1.3 µm and a height of 1.7 µm were fabricated on LN, which exhibits an average transmittance of 3–5 µm increasing from 78% to 85% and a highest transmittance of 88% at 5 µm. It has been demonstrated that the LN ASSs show high stability under high‐temperature and high‐power laser irradiation, which shows potential applications for high‐power mid‐IR lasers. The results indicate that the WE‐LPDI technology provides a novel way for 3D nanofabrication of LN. [ABSTRACT FROM AUTHOR]

Highly responsive, low noise, and inexpensive photodetectors that operate in the mid‐infrared (MIR) wavelength regime are in high demand for applications ranging from fundamental science to large scale industries. However, simultaneously achieving all this in one device architecture is very challenging. In this work, mercury telluride (HgTe) colloidal quantum dot (cQD) based photodetectors are systematically improved by the introduction of new metamaterial designs. The new designs are found by utilizing simulations. Thereby the structures are optimized to increase the responsivity and simultaneously decrease the noise spectral current density. This is achieved by focusing on improving the photogenerated charge carrier collection efficiency while reducing the active material volume without altering the near unity absorption. A standard metamaterial perfect absorber architecture based on disc resonators is used as a starting point for the optimization process. By optimizing the carrier extraction through contact engineering, resulting in a narrow slot metamaterial, an overall ≈13‐fold responsivity and ≈345‐fold detectivity increase is achieved. The final metamaterial design reaches a responsivity of 16.2 A W−1 and detectivity of 6×108 Jones at a wavelength of 2710 nm. The analysis therefore provides a route to improve the responsivity and noise characteristics of mid‐infrared photodetectors based on cost‐efficient colloidal quantum dots. [ABSTRACT FROM AUTHOR]

We demonstrate substrate-emitting resonant cavity interband cascade light emitting diodes (RCICLEDs) based on a single distributed Bragg reflector (DBR). These devices operate in continuous wave mode at room temperature. Compared to standard ICLEDs without a cavity, we achieved an 89% reduction in the emission spectrum width, as indicated by the Full Width Half Maximum (FWHM) of 70 nm. Furthermore, we observed far-field narrowing and improved thermal stability. A single DBR configuration allows the cavity length to be adjusted by adding refractive index-matched material to the top of the epitaxial structure after epitaxial growth. This modification effectively shifts the cavity response towards longer wavelengths. We fabricated emitters comprising two cavities of different lengths, resulting in the emission of two distinct spectral lines that can be independently controlled. This dual-color capability enables one of the emission lines to serve as a built-in reference channel, making these LEDs highly suitable for cost-effective gas-sensing applications. [ABSTRACT FROM AUTHOR]

Multi-band circular dichroism (CD) response and tunability on the chiral metasurface are crucial for this device's applications in sensing and detection. This work proposes a dual-band CD Au-CaF2-Au dimer elliptical metasurface absorber, where chiroptical sensing is realized by breaking the geometric symmetry between two ellipses. The proposed metasurface can achieve high CD values of 0.8 and −0.74 for the dual-band within the 3–5 μm region, and the CD values can be manipulated by independently adjusting the geometric parameters of the metasurface. Furthermore, a slotted nanocircuit is introduced onto the metasurface to enhance its tunability by manipulating the geometry parameter in the design process, and the related mechanism is explained using an equivalent circuit model. The simulation of the sensing model revealed that the slotted nanocircuit enhances the sensor's tunability and significantly improves its bandwidth and sensitivity, achieving peak enhancements at approximately 753 nm and 1311 nm/RIU, respectively. Due to the strong dual-band positive (and negative) responses of the CD values, flexible wavelength tunability, and nonlinear sensitivity enhancement, this design provides a new approach for the development and application of mid-infrared chiroptical devices. [ABSTRACT FROM AUTHOR]

The early detection of gynecological cancers, which is critical for improving patient survival rates, is challenging because of the vague early symptoms and the diagnostic limitations of current approaches. This comprehensive review delves into the game-changing potential of infrared (IR) spectroscopy, a noninvasive technology used to transform the landscape of cancer diagnosis in gynecology. By collecting the distinctive vibrational frequencies of chemical bonds inside tissue samples, Fourier-transform infrared (FTIR) spectroscopy provides a 'molecular fingerprint' that outperforms existing diagnostic approaches. We highlight significant advances in this field, particularly the identification of discrete biomarker bands in the mid- and near-IR spectra. Proteins, lipids, carbohydrates, and nucleic acids exhibited different absorption patterns. These spectral signatures not only serve to distinguish between malignant and benign diseases, but also provide additional information regarding the cellular changes associated with cancer. To underscore the practical consequences of these findings, we examined studies in which IR spectroscopy demonstrated exceptional diagnostic accuracy. This review supports the use of IR spectroscopy in normal clinical practice, emphasizing its capacity to detect and comprehend the intricate molecular underpinnings of gynecological cancers. [ABSTRACT FROM AUTHOR]

Guided-mode resonance (GMR) is a compelling scheme for optical sensing of various physical quantities, which holds great promise in non-invasive detection of chemical and biological species. Current GMR sensors are mostly based on detection of environmental refractive index change, which presents a challenge for obtaining sensing selectivity as required in practical applications. In this work, we report on a prototype demonstration of using GMR for detecting molecular fingerprint absorption. In a waveguide grating made of a ZnSe layer sandwiched between a Cu film and an Au grating, a mid-infrared GMR is excited and tuned over a wide frequency range of 617.4 cm−1. Coupling of the GMR with a polydimethylsiloxane (PDMS) analyte film reveals the signature absorptions of CH2 group, whose vibrational signal is enhanced by a factor of 2.37 at a frequency detuning of 30 cm−1 between the GMR and the molecular vibration. Our theoretical analysis reveals that both near-field enhancement of GMR and its coupling strength with molecules are responsible for the molecular detection performance. These results suggest that angle-tuned GMR presents an alternative design paradigm for surface-enhanced infrared absorption spectroscopy (SEIRAS). [ABSTRACT FROM AUTHOR]

This study proposes the design of a novel Metal-Insulator-Metal (MIM) nano-infrared emitter that uses a unique diamond-shaped grating to achieve selective infrared absorption. Diamond-shaped nano emitter (DNE) structure exhibits four narrow resonant peaks within key absorption windows such as short-wave infrared (SWIR) mid-wave infrared (MWIR), alongside with a wide absorption band in the Non-Transmissive Infrared Range (NTIR) for thermal camouflage applications compatible with radiative cooling. Moreover, the proposed DNE is polarization insensitive as it has an in-plane symmetric design. Using the 3D Finite-Difference Time-Domain (FDTD) simulations, we demonstrate the nanoantenna's superior performance characterized by its high absorption rates and tuned effective impedance matching. As of our knowledge, the findings suggest that this is the first time that a MIM structure achieved multiple narrow resonance peaks, located in SWIR and MWIR simultaneously, with a wide absorption range in NTIR. Represented DNE stands as a significant innovation in the field of stealth technology, providing a tunable, high-efficiency solution for managing and controlling thermal emissions across diverse applications. [ABSTRACT FROM AUTHOR]

Mid-infrared (mid-IR) photodetection is important for various applications, including biomedical diagnostics, security, chemical identification, and free-spacing optical communications. However, conventional "photon" mid-IR photodetectors require liquid nitrogen cooling (i.e., MCT). Furthermore, acquiring mid-IR spectra usually involves a complex and expensive Fourier Transform Infrared spectrometer, a tabletop instrument consisting of a meter-long interferometer and MCT detectors, which is not suitable for mobile and compact device applications. In this work, we present tunable photoresponsivity in the mid-IR wavelength in palladium diselenide (PdSe2) – molybdenum disulfide (MoS2) heterostructure field-effect transistors (FETs), operating at room temperature. Furthermore, we applied a tunable membrane cavity to modulate the Fabry–Pérot resonance to modulate the absorption spectrum of the device layer. We used a robust polyetherimide (PEI) membrane with CVD-grown graphene to electrically tune the membrane structure. For the next step, we will integrate the PdSe2-based photodetector and tunable membrane to increase detection sensitivity and spectrum tunability to realize the 'learning'-based spectroscopy. [ABSTRACT FROM AUTHOR]

Mid-infrared (mid-IR) photodetection is important for various applications, including biomedical diagnostics, security, chemical identification, and free-spacing optical communications. However, conventional "photon" mid-IR photodetectors require liquid nitrogen cooling (i.e., MCT). Furthermore, acquiring mid-IR spectra usually involves a complex and expensive Fourier Transform Infrared spectrometer, a tabletop instrument consisting of a meter-long interferometer and MCT detectors, which is not suitable for mobile and compact device applications. In this work, we present tunable photoresponsivity in the mid-IR wavelength in palladium diselenide (PdSe2) – molybdenum disulfide (MoS2) heterostructure field-effect transistors (FETs), operating at room temperature. Furthermore, we applied a tunable membrane cavity to modulate the Fabry–Pérot resonance to modulate the absorption spectrum of the device layer. We used a robust polyetherimide (PEI) membrane with CVD-grown graphene to electrically tune the membrane structure. For the next step, we will integrate the PdSe2-based photodetector and tunable membrane to increase detection sensitivity and spectrum tunability to realize the 'learning'-based spectroscopy. [ABSTRACT FROM AUTHOR]

Spectroscopy is a powerful tool to identify the specific fingerprints of analytes in a label-free way. However, conventional sensing methods face unavoidable barriers in analyzing trace-amount target molecules due to the difficulties of enhancing the broadband molecular absorption. Here, we propose a sensing scheme to achieve strong fingerprint absorption based on the angular-scanning strategy on an all-silicon metasurface. By integrating the mid-infrared and terahertz sensing units into a single metasurface, the sensor can efficiently identify 2,4-DNT with high sensitivity. The results reveal that the fingerprint peak in the enhanced fingerprint spectrum is formed by the linked envelope. It exhibits a significant enhancement factor exceeding 64-fold in the terahertz region and more than 55-fold in the mid-infrared region. Particularly, the corresponding identification limit of 2,4-DNT is 1.32 µg cm−2, respectively. Our study will provide a novel research idea in identifying trace-amount explosives and advance practical applications of absorption spectroscopy enhancement identification in civil and military security industries. [ABSTRACT FROM AUTHOR]

Perfluoride glass ceramics (FGCs) with SrF 2 /CaF 2 mixed crystals were prepared using a concurrent rapid-quenching-crystallization process. The FGCs contained dense, homogeneous, round crystals of sizes less than 1 μm with high transmittance (exceeding 80 % at 3–8 μm) and low phonon energy (508 cm−1). The fluorescence properties of the FGCs doped with Er and Dy were investigated. The energy transfer efficiency between Er and Dy reached 93.81 % and 98.13 %, corresponding to Er: 4I 11/2 to Dy: 6H 5/2 and Er: 4I 13/2 to Dy: 6H 11/2 , respectively. This enhancement is attributed to the enrichment of rare-earth ions in the crystals induced by phase-separation and low phonon energy of the host material, FGC. Moreover, the average lifetime of Er: 4I 11/2 in FGC reached 10.13 ms, which is the highest reported value for glass ceramics to our knowledge. This study enriches the research theory of FGC and provides guidance for expanding the properties of materials for mid-infrared photonics. [ABSTRACT FROM AUTHOR]

Plasmonic spectral filters composed of periodic nanostructured metal films offer novel opportunities for the development of multispectral imaging technologies in the mid-infrared region. However, traditional plasmonic filters, which typically feature simplistic structures such as nanoholes or nanorings, are constrained by a narrow bandpass and significant crosstalk, leading to limited practical performance. Filters designed using inverse techniques allow a substantial degree of freedom in creating intricate structures that align with desired spectral characteristics, including a quasi-square spectral profile, high transmission, wide full width at half maximum, and reduced crosstalk. In this study, we have utilized an inverse design algorithm to engineer high-performance bandpass filters for the mid-infrared range, achieving an average transmittance exceeding 80% within the bandpass window and below 10% in the stop band, which is comparable to that of commercial multilayer Bragg filters. Nanofabrication processes were employed to transfer the designed pattern into the gold film on ZnS substrate that is transparent in the mid-infrared range. The resulting filters exhibit spectral performance analogous to that of the inversely designed models, making them suitable for direct integration with mid-infrared photodetector arrays in multispectral imaging systems. [ABSTRACT FROM AUTHOR]

It is extremely challenging to accomplish second‐harmonic generation (SHG) in the whole mid‐infrared (MIR) region for oxide crystals due to their short IR absorption cutoff wavelengths. Herein, a new MIR nonlinear optical (NLO) crystal of K4ZnV5O15Cl (KZVC) is designed and synthesized by the aliovalent substitution of [VIIIO5] with [ZnIIO4Cl] based on the K2(VO)(V2O7) matrix. KZVC features an ordered 2D [ZnV5O15Cl]∞ layered structure comprising [ZnO4Cl] / [VO5] square pyramids and [VO4] tetrahedron as functional units. Importantly, KZVC shows excellently balanced performances of strong SHG responses (12 × KDP @ 1064 nm, 1.1 × AgGaS2 @ 2.05 µm), broad IR transparency range (0.52‒10 µm), congruent‐melting behavior, and highest laser‐induced damage threshold (LIDT) (60 × AgGaS2 @ 1064 nm) among the reported vanadates. Most remarkably, KZVC represents the first example in the oxide‐based IR NLO crystals that can achieve phase matching in a wide wavelength range extending from 1.8 to 10 µm, fully covering the important MIR atmospheric windows (3‒5 µm). [ABSTRACT FROM AUTHOR]

The development of compact systems operating in the long‐wave infrared wavelength range is of high interest for spectroscopic and sensing applications. There is currently a wide interest toward the development of optical frequency‐combs to enhance the performances of these systems. Amongst the different techniques to obtain optical frequency‐combs, electro‐optic frequency‐comb generation presents major advantages thanks to the tunable repetition rate only limited by the bandwidth of the used electro‐optical modulator. However, the development of integrated and efficient electro‐optical modulators operating in a wide long‐wave infrared spectral band is still at its infancy, and electro‐optical frequency‐comb has not been demonstrated so far beyond 3 μm$\umu {\rm m}$ wavelength. In this work, a Schottky‐based modulator embedded in a Ge‐rich graded SiGe waveguide is used for electro‐optic frequency‐comb generation. Considering the limited efficiency of the modulator, harmonically‐rich RF signals are used to enhance the generation of comb lines around the optical carrier. Interestingly, this allows us to demonstrate the generation of electro‐optical combs spanning over 2.4 GHz around 8 μm$\umu {\rm m}$ wavelength. This paves the way toward fully integrated and tunable mid‐infrared electro‐optic frequency‐comb generation systems. [ABSTRACT FROM AUTHOR]

This study reports the experimental demonstration of the first waveguide-integrated SiGe modulator using a PIN diode operating in a wide spectral range of the mid-infrared region. At the wavelength of 10 µm, an extinction ratio up to 10 dB is obtained in injection regime and 3.2 dB in depletion regime. High speed operation is obtained, up to 1.5 GHz. Furthermore, the device can also operate as an integrated photodetector. Photodetection has thus been characterized from 5.2 µm to 10 µm wavelengths showing an internal responsivity around 1 mA/W, and a 3 dB electro-optical bandwidth of 32 MHz. These results show a significant advancement in integrated photodetectors and electro-optical modulators for mid-infrared spectroscopy. [ABSTRACT FROM AUTHOR]

Precision measurements on molecules in the strong fundamental bands are of great importance. An ac curate mid-infrared light source is a key for these studies. By locking the signal and pump light to an optical frequency comb, a high-precision continu ous-wave optical parametric oscillator source is built, and the mid-infrared frequency drift is deter mined to be less than 1 kHz. As a demonstration, saturated absorption spectroscopy of the R(14) (00011)–(00001) line of 13CO2 is measured, and the transition frequency is determined to be 68786813496(29) kHz. The frequency-stabilized mid-infrared laser source provides an opportunity for precision measurements of molecules in fundamental bands. [ABSTRACT FROM AUTHOR]

Leveraging the low linear and nonlinear absorption loss of silicon at mid-infrared (mid-IR) wavelengths, silicon photonic integrated circuits (PICs) have attracted significant attention for mid-IR applications including optical sensing, spectroscopy, and nonlinear optics. However, mid-IR silicon PICs typically show moderate performance compared to state-of-the-art silicon photonic devices operating in the telecommunication band. Here, we proposed and demonstrated suspended nanomembrane silicon (SNS) PICs with light-guiding within deep-subwavelength waveguide thickness for operation in the short-wavelength mid-IR region. We demonstrated key building components, namely, grating couplers, waveguide arrays, micro-resonators, etc., which exhibit excellent performances in bandwidths, back reflections, quality factors, and fabrication tolerance. Moreover, the results show that the proposed SNS PICs have high compatibility with the multi-project wafer foundry services. Our study provides an unprecedented platform for mid-IR integrated photonics and applications.

Novel concepts for efficient compact spectroscopy are extensively researched due to their fundamental applications in prominent fields such as chemistry, biology, and physics. Here, with an unprecedented spectral-azimuthal resolution, such a concept is introduced and exemplified in the mid-infrared, in which its advantages are paramount and have yet to be established industrially. The concept is based on the design and instrumentation of optical absorption spectral tuning (or sensitivity) to the relative azimuthal component of light impinging on specifically designed metamaterials (MMs). The inversely-designed MMs offer perfect photo-absorption inside λ0/200 ultra-thin layer of lead telluride. Two small-footprint system designs are proposed to instrument the spectral-azimuth-angle tuning for spectrometry. The first is based on a single or few spinning MM layout elements, and the second, to avoid spinning, utilizes a fixed focal-plane-array approach. The latter exploits the inherent variations in the local azimuthal-incidence angle. While low absorption is the Achilles heel of conventional mid-infrared photodetector spectrometers, the optimized MMs, besides their unique spectral-azimuth-angle tuning functionality, provide giant absorption enhancement, facilitating higher resolution and even smaller in-plane form factor. The highlighted concept opens an additional dimension to encode-decode spectral information, yielding profound advantages over conventional designs, such as those based on diffraction gratings.

Soil’s pivotal role in environmental and agricultural processes underscores the importance of accurate soil property predictions for informed decisions and sustainable land management. Spectroscopic techniques, particularly mid-infrared (MIR) spectroscopy, have emerged as rapid and non-destructive tools for soil analysis. Despite advances in predicting soil properties using spectroscopy, quantifying prediction uncertainties has often been overlooked. Accurate uncertainty quantification helps risk assessment and decision-making processes. This study introduces an enhanced version of the variational inference technique to capture uncertainty when using Bayesian Convolutional Neural Networks (Bayesian CNNs). This Bayesian CNNs method was evaluated against two other methods — Bootstrapped Partial Least-Squares regression (Bootsrapped PLS) and Generalised Additive Models (GAM) for their ability to quantify uncertainty in six soil property predictions (clay, sand, silt, pH, phosphorus retention, and carbon) based on MIR spectroscopy. In terms of predictive performance and quality of prediction, our evaluation indicated that both GAMs and Bayesian CNNs outperformed PLS-BS for all six soil properties. The ability of GAMs and Bayesian CNNs to capture non-linear relationships in the data allowed for better fitting to the underlying patterns. Bayesian CNNs, in particular, demonstrated superior performance by combining accurate predictions with robust uncertainty quantification. Our results also showed that, on our dataset, bootstrapping failed to provide satisfactory prediction intervals. We suggest therefore that the evaluation of models should extend beyond standard validation metrics, which typically focuses on prediction accuracy, to include an assessment of the predicted uncertainty.

The optical parametric amplification process in the mid-infrared (MIR) 3–5 μm region of nonlinear crystal LiInS2 (LIS) has been investigated. The LIS crystal was pumped by a 1064 nm laser with a pulse width of 30 picoseconds and seeded by a tunable laser generated from a KTP-OPG/OPA pumped by the second harmonic (SH) of the same 1064 nm laser. In the experiment, the 3.28–5.50 μm tunable idler was realized. When the fixed pump energy is 8.75 mJ, the idler energy obtained is 72.2 μJ at 3.28 μm and 63.3 μJ at 5.50 μm. The maximum idler energy of 179.4 μJ at 4.70 μm is reached at a pump energy of 12.5 mJ, and the optimal photon conversion efficiency is 6.36% at the pump energy of 11.5 mJ. Finally, the optical parametric frequency conversion parameters and performance in the 3–5 μm of several typical new crystals (BGSe, LISe, AGS, HGS, and LGS) developed in recent years are briefly compared and summarized.

Waveguide-integrated photodetectors (PDs) play a crucial role in mid-infrared (MIR) silicon photonics, serving vital functions in sensing and communication applications. III-V semiconductors are widely used in MIR PDs, and many state-of-the-art III-V PDs on Si still require complicated integration methods. Heteroepitaxial growth technology is a competitive approach for large-scale integration; however, buffers capable of simultaneously achieving heteroepitaxial growth and optical coupling are limited in the MIR region. In this paper, we report a waveguide-integrated InAs PD on Si, incorporating a GaAs/Ge buffer design based on interfacial misfit (IMF) technology. We optimize the geometric structure and calculate the optoelectronic properties at a wavelength of 3 μm. For our simulated parameters, the optimal PD achieves a responsivity of 2.77 A/W and a detectivity of 4.68×109 cm·Hz1/2·W-1 at -1V. This work suggests a promising avenue to further develop high-detectivity and high-speed PDs for MIR silicon photonics.

Fundamental limits of thermal radiation are imposed by Kirchhoff’s law, which assumes the electromagnetic reciprocity of a material or material system. Thus, breaking reciprocity can enable breaking barriers in thermal efficiency engineering. In this work, we present a subwavelength, 1D photonic crystal composed of Weyl semimetal and dielectric layers, whose structure was optimized to maximize the nonreciprocity of infrared radiation absorptance in a planar and compact design. To engineer an ultra-compact absorber structure that does not require gratings or prisms to couple light, we used a genetic algorithm (GA) to maximize nonreciprocity in the design globally, followed by the application of the numerical gradient ascent (GAGA) algorithm as a local optimization to further enhance the design. We chose Weyl semimetals as active layers in our design as they possess strong, intrinsic nonreciprocity, and do not require an external magnetic field. The resulting GAGA-generated 1D magnetophotonic crystal offers high nonreciprocity (quantified by absorptance contrast) while maintaining an ultra-compact design with much fewer layers than prior work. We account for both s- and p-polarized absorptance spectra to create a final, eight-layer design suitable for thermal applications, which simultaneously minimizes the parasitic, reciprocal absorptance of s-polarized light.

In this study, high-speed mid-infrared Mach–Zehnder electro-optical modulators in x-cut lithium niobate (LN) thin film on sapphire were designed, simulated, and analyzed. The main optical parameters of three types of Mach–Zehnder modulators (MZMs) (residual LN with thickness of 0, 0.5, and 1 μm) were simulated and calculated, namely, the single-mode conditions, bending loss, separation distance between electrode edge and lithium niobate waveguide edge, optical field distribution, and half-wave voltage–length product. The main radio frequency (RF) parameters of these three types of MZMs, such as characteristic impedance, attenuation constant, RF effective index, and the –3 dB modulation bandwidth were calculated depending on the dimensions of the coplanar waveguide traveling-wave electrodes. The modulations with residual LN thickness of 0, 0.5, and 1 μm were calculated with bandwidths exceeding 140, 150, and 240 GHz, respectively, and the half-wave voltage–length product achieved was 22.4, 21.6, and 15.1 V cm, respectively. By optimizing RF and optical parameters, guidelines for device design are presented, and the achievable modulation bandwidth is significantly increased.

Exoplanet science is one of the most thriving fields of modern astrophysics. A major goal is the atmospheric characterization of dozens of small, terrestrial exoplanets in order to search for signatures in their atmospheres that indicate biological activity, assess their ability to provide conditions for life as we know it, and investigate their expected atmospheric diversity. None of the currently adopted projects or missions, from ground or in space, can address these goals. In this White Paper, submitted to ESA in response to the Voyage 2050 Call, we argue that a large space-based mission designed to detect and investigate thermal emission spectra of terrestrial exoplanets in the mid-infrared wavelength range provides unique scientific potential to address these goals and surpasses the capabilities of other approaches. While NASA might be focusing on large missions that aim to detect terrestrial planets in reflected light, ESA has the opportunity to take leadership and spearhead the development of a large mid-infrared exoplanet mission within the scope of the "Voyage 2050" long-term plan establishing Europe at the forefront of exoplanet science for decades to come. Given the ambitious science goals of such a mission, additional international partners might be interested in participating and contributing to a roadmap that, in the long run, leads to a successful implementation. A new, dedicated development program funded by ESA to help reduce development and implementation cost and further push some of the required key technologies would be a first important step in this direction. Ultimately, a large mid-infrared exoplanet imaging mission will be needed to help answer one of humankind's most fundamental questions: "How unique is our Earth?"

Background: Accurately determining the age and survival probabilities of adult mosquitoes is crucial for understanding parasite transmission, evaluating the effectiveness of control interventions and assessing disease risk in communities. This study was aimed at demonstrating the rapid identification of epidemiologically relevant age categories of Anopheles funestus, a major Afro-tropical malaria vector, through the innovative combination of infrared spectroscopy and machine learning, instead of the cumbersome practice of dissecting mosquito ovaries to estimate age based on parity status. Methods: Anopheles funestus larvae were collected in rural south-eastern Tanzania and reared in an insectary. Emerging adult females were sorted by age (1-16 days old) and preserved using silica gel. Polymerase chain reaction (PCR) confirmation was conducted using DNA extracted from mosquito legs to verify the presence of An. funestus and to eliminate undesired mosquitoes. Mid-infrared spectra were obtained by scanning the heads and thoraces of the mosquitoes using an attenuated total reflection-Fourier transform infrared (ATR-FT-IR) spectrometer. The spectra (N = 2084) were divided into two epidemiologically relevant age groups: 1-9 days (young, non-infectious) and 10-16 days (old, potentially infectious). The dimensionality of the spectra was reduced using principal component analysis, and then a set of machine learning and multi-layer perceptron (MLP) models were trained using the spectra to predict the mosquito age categories. Results: The best-performing model, XGBoost, achieved overall accuracy of 87%, with classification accuracy of 89% for young and 84% for old An. funestus. When the most important spectral features influencing the model performance were selected to train a new model, the overall accuracy increased slightly to 89%. The MLP model, utilizing the significant spectral features, achieved higher classification accuracy of 95% and 94% for the young and old An. funestus, respectively. After dimensionality reduction, the MLP achieved 93% accuracy for both age categories. Conclusions: This study shows how machine learning can quickly classify epidemiologically relevant age groups of An. funestus based on their mid-infrared spectra. Having been previously applied to An. gambiae, An. arabiensis and An. coluzzii, this demonstration on An. funestus underscores the potential of this low-cost, reagent-free technique for widespread use on all the major Afro-tropical malaria vectors. Future research should demonstrate how such machine-derived age classifications in field-collected mosquitoes correlate with malaria in human populations. [ABSTRACT FROM AUTHOR]

Mid‐infrared (mid‐IR) ultrafast lasers are widely employed in biomedicine, molecular spectroscopy, material processing, and nonlinear optics. With the improvement of fiber gain media and other fiber optical elements, low‐cost, compact, and high‐efficiency fiber lasers open up new opportunities for 2–4 µm pulse generations, which calls for a comprehensive review of their mode‐locking mechanisms, gain media, and fiber laser system performance. This paper, beginning with an overview of pulse‐generation technologies, reviews recent progress on 2–4 µm mid‐IR ultrafast fiber lasers, including Tm3+‐, Ho3+‐doped, Tm3+/Ho3+ codoped silicate fiber 2 µm lasers, and Er3+‐, Dy3+‐doped, and Ho3+/Pr3+ codoped ZBLAN fiber 2.5–4 µm lasers. Among them, the status of 2–4 µm ultrafast fiber lasers based on 2D material passive mode‐locking is emphatically discussed. Meanwhile, the novel advances on mode‐locking and gain fibers of mid‐IR ultrafast fiber lasers are explored. Furthermore, current and prospective applications of such laser systems are also introduced in detail. This review finally summarizes challenges associated with future development of mid‐IR ultrafast fiber lasers, which provides an outlook on how to achieve more desirable laser performance (e.g., higher average power, higher pulse energy, and longer emission wavelength) that can lead to more practical uses of such lasers. [ABSTRACT FROM AUTHOR]

A 2.03 μm gas-discharge fiber laser based on atomic xenon is investigated. Various gas mixtures, such as He–Xe, Ar–Xe, He–Ar–Xe, and He–Ne, are studied by optical emission spectroscopy. The possibility of extending laser generation further into the mid-infrared range is analyzed. [ABSTRACT FROM AUTHOR]

High-power quantum cascade lasers (QCLs) have a wide application prospect. In this paper, a high-power high-beam-quality device with a large ridge width is demonstrated. The effect of different ridge widths on mode loss was studied, and the results showed that the mode loss decreased as the ridge width increased. Furthermore, as the width of the ridge increased, the temperature of the active region rose. In the experiment, the wafers were grown by metal–organic chemical vapor deposition (MOCVD), and the ridge width of the device was controlled by wet etching. A laser with a ridge width of 15 µm and a length of 5 mm achieved an output of 2.2 W under 288 K continuous wave (CW) operation, with a maximum slow-axis divergence angle of 27.2° and a device wavelength of 5 μm. The research results of this article promote the industrial production of base transverse mode QCL. [ABSTRACT FROM AUTHOR]

A dry-etched pedestal chalcogenide waveguide platform, designed for use in long-wave IR spectrometer applications, is demonstrated, fabricated and optically characterized. The optical layers were deposited on pre-patterned dry-etched silicon pedestals. An exceptionally low waveguide propagation loss was measured, at around 0.1 dB/cm at λ = 10 μm. The modal thermo-optic coefficient of the waveguide was experimentally estimated to be approximately 1.1 × 10−4 C−1 at λ = 1.63 μm, which is comparable to that of Si and GaAs. Waveguide spiral interferometers were fabricated, proving the potential for realization of more complex, chalcogenide-based, integrated photonic circuits. The combination of low propagation losses and a strong thermo-optic coefficient makes this platform an ideal candidate for utilization in on-chip tunable spectrometers in the long-wave IR wavelength band. [ABSTRACT FROM AUTHOR]

Photonic sensors utilize light–matter interaction to detect physical parameters accurately and efficiently. They exploit the interaction between photons and matter, with light propagating through an optical waveguide, creating an evanescent field beyond its surface. This field interacts with the surrounding medium, enabling the sensitive detection of changes in the refractive index or nearby substances. By modulating light properties like intensity, wavelength, or phase, these sensors detect target substances or environmental changes. Advancements in this technology enhance sensitivity, selectivity, and miniaturization, making photonic sensors invaluable across industries. Their ability to facilitate sensitive, non-intrusive, and remote monitoring fosters the development of smart, connected systems. This overview delves into the material platforms and waveguide structures crucial for developing highly sensitive photonic devices tailored for gas and biosensing applications. It is emphasized that both the material platform and waveguide geometry significantly impact the sensitivity of these devices. For instance, utilizing a slot waveguide geometry on silicon-on-insulator substrates not only enhances sensitivity but also reduces the device's footprint. This configuration proves particularly promising for applications in biosensing and gas sensing due to its superior performance characteristics. [ABSTRACT FROM AUTHOR]

Wide band gaps and large second harmonic generation (SHG) response are two crucial yet contradictory parameters for nonlinear optical (NLO) crystals. Herein, through exploring the influence of anion groups on structural crystallization dimension and space group, a series of new NLO Hg‐based chalcogenides AIIHgMIVS4 (AII = Sr, Ba, MIV = Si, Ge) with wide band gaps and large SHG responses have been successfully designed and synthesized. The structures of these compounds consist of 2D [HgMS4]2− (M = Si and Ge) layers composed of distorted (HgS4 and MS4) tetrahedra arranged alternately, and the layers are separated by eight‐coordinated A2+ (A = Sr and Ba) cations. Their band gaps and SHG responses meet the basic requirements (SHG > 1 × AgGaS2; Eg > 2.33 eV). Especially, SrHgSiS4 and SrHgGeS4 exhibit both large SHG effects (2 and 2.4 × AgGaS2) and wide band gaps (3.06 and 2.97 eV), respectively, indicating they are promising IR NLO crystals. This work opens a new avenue for new NLO material design to achieve the balance between strong SHG response and large band gaps. [ABSTRACT FROM AUTHOR]

Laser ablation of bio‐tissues is the key technology of future surgeries, owing to the merits of submicron accuracy, non‐contact operation, and precision control assisted by automation and computer intelligence. Excellent efficiency, minimal collateral damage, biological safety, and tissue selectivity are the ideal parameters for delicate surgical applications. Here, by exploring the mid‐infrared (MIR) resonant ablation and the femtosecond cold processing, tissue ablation with multi‐millimeter depth and cellular collateral damage, enabled by a tabletop femtosecond MIR optical parametric amplifier operating at the amide‐I resonant wavelength of 6.1 µm, is demonstrated. Remarkably, the collateral damage is observed to be 15, <4, and <1 µm in the porcine cornea, sclera, and articular cartilage, respectively, with multi‐millimeter incision depth. In addition, preliminary proof‐of‐concept experiments of tissue‐selective ablation, microchannels on cartilage for drug delivery, laser glaucoma, keratotomy, neurologic tissue incision, dentin deep crater formation, and hypertrophic scarring ablation are demonstrated, which pave the way for high‐precision surgical applications with a tabletop solution. [ABSTRACT FROM AUTHOR]

As the basic passive device in silicon photonic integrated chip, silicon-based optical waveguide is the channel for optical signal transmission. Because of its good performance and compatibility with CMOS(Complementary Metal Oxide Semiconductor) process, silicon-based optical waveguide has been widely used. Silicon photonic integrated circuits used in telecommunications and data centers have been gradually commercialized. The potential applications of mid-infrared band in free-space communication, sensing, environmental monitoring and other fields have attracted much attention from researchers in recent years. In this study, the research status of mid-infrared silicon-based optical waveguides in recent years is analyzed. The research results of waveguide material platform of SOI(Silicon on Insulator)、GOSI(Ge-on-SOI)、SOS(Si on Sapphire)、GOS(Ge-on-Si)、SGOS(SiGe-on-Si)、SON(Si-on Si3N4)、GON(Ge-on Si3N4) and manufacturing process platform of SOPS(Si on Porous Si), Pedestal, Freestanding, Suspended, LOCOS(Local Oxidation of Silicon), plasma structure are summarized. So far, the propagation loss of most monocrystalline silicon in MIR(Mid-Infrared) platform is about 0.7~3.0 dB·cm-1. The application prospects of different types of waveguides are discussed and compared, which provides a reference for the research and development, application and commercialization of mid-infrared silicon-based optical waveguides. [ABSTRACT FROM AUTHOR]