Mid-infrared Broadband Research Articles

Disequilibrium Chemistry, Diabatic Thermal Structure, and Clouds in the Atmosphere of COCONUTS-2b

Located 10.888 pc from Earth, COCONUTS-2b is a planetary-mass companion to a young (150–800 Myr) M3 star, with a wide orbital separation (6471 au) and a low companion-to-host mass ratio (0.021 ± 0.005). We have studied the atmospheric properties of COCONUTS-2b using newly acquired 1.0–2.5 μm spectroscopy from Gemini/Flamingos-2. The spectral type of COCONUTS-2b is refined to T9.5 ± 0.5 based on comparisons with T/Y dwarf spectral templates. We have conducted an extensive forward-modeling analysis, comparing the near-infrared spectrum and mid-infrared broadband photometry of COCONUTS-2b with 16 state-of-the-art atmospheric model grids developed for brown dwarfs and self-luminous exoplanets near the T/Y transition. The PH 3 -free ATMO2020++, ATMO2020++, and Exo-REM models best match the specific observations of COCONUTS-2b, regardless of variations in the input spectrophotometry. This analysis suggests the presence of disequilibrium chemistry, along with a diabatic thermal structure and/or clouds, in the atmosphere of COCONUTS-2b. All models predict fainter Y-band fluxes than observed, highlighting uncertainties in the alkali chemistry models and opacities. We determine a bolometric luminosity of log(Lbol/L⊙)=−6.18 dex, with a 0.5 dex wide range of [−6.43, −5.93] dex that accounts for various assumptions of atmospheric models. Using several thermal evolution models, we derive an effective temperature of Teff=483−53+44 K, a surface gravity of log(g)=4.19−0.13+0.18 dex, a radius of R=1.11−0.04+0.03 R Jup, and a mass of M = 8 ± 2 M Jup. Various atmospheric model grids consistently indicate that COCONUTS-2b’s atmosphere likely has subsolar or near-solar metallicity and C/O. These findings provide valuable insights into COCONUTS-2b’s formation history and the potential outward migration to its current wide orbit.

Subwavelength broadband light-harvesting metacoating for infrared camouflage and anti-counterfeiting empowered by inverse design

Broadband mid-infrared (MIR) light harvesting is critical for a wide range of applications, including thermophotovoltaic conversion, thermal sensing and imaging, infrared camouflage and anti-counterfeiting technologies. In this study, we present the design and experimental validation of a deep-subwavelength broadband MIR light-harvesting metacoating (MMC), optimized through a genetic algorithm (GA)-based inverse design approach. The strength of this approach lies in its ability to automate and optimize the complex multilayer structure, encompassing both material selection and structural thickness, thereby achieving unparalleled performance in broadband MIR light absorption, with an average absorbance of approximately 0.85 across the 3–13 μm spectral range and nearly perfect absorption within the 4–12 μm range. This exceptional performance is attributed to strong electromagnetic localization within its multilayer configuration, facilitating efficient energy dissipation via high-loss materials such as bismuth and titanium. Notably, the MMC exhibits robust performance with respect to angle and polarization variations, maintaining high absorbance even at incident angles up to 70°. Its large-area fabrication capabilities and compatibility with various substrates further enhance its practical applicability. Two specific applications, long-wavelength infrared camouflage and anti-counterfeiting, highlight its potential for real-world deployment. In these applications, the MMC seamlessly integrates into high-emission environments and enables the modulation of patterned infrared emission, providing a lithography-free, cost-effective solution compared to conventional methods relying on artificially engineered structures. This work underscores the versatility of the developed MMC for a diverse array of MIR applications, ranging from camouflage technologies to advanced security measures.

Dynamic nonlocal metasurface for multifunctional integration via phase-change materials

Abstract Non-local metasurface supporting geometric phases at bound states in the continuum (BIC) simultaneously enables sharp spectral resonances and spatial wavefront shaping, thus providing a diversified optical platform for multifunctional devices. However, a static nonlocal metasurface cannot manipulate multiple degrees of freedom (DOFs), making it difficult to achieve multifunctional integration and be applied in different scenarios. Here, we presented and demonstrated phase-change non-local metasurfaces that can realize dynamic manipulation of multiple DOFs including resonant frequency, Q values, band, and spatial wavefront. Accordingly, a metasurface integrating multiple distinct functions is designed, as a proof-of-concept demonstration. Utilizing the geometry phase of quasi-BIC and the tunability of vanadium dioxide (VO2), a dynamic meta-lens is achieved by tailoring spatial light response at quasi-BIC in the temperature range from room temperature to 53 °C. Simultaneously, the sharp Fano resonance of quasi-BIC enables the metasurface to serve as an optical sensor in the mid-infrared band, yielding a sensitivity of 7.96 THz/RIU at room temperature. Furthermore, at the metallic state of VO2 (80 °C), the designed metasurface converts into a mid-infrared broadband absorber, achieving higher than 80 % absorptivity and an average absorption of 90 % from 28.62 THz to 37.56 THz. The proposed metasurface enabling multifunctional performances in different temperatures can effectively improve the availability of devices and find more new and complex scenarios in sensing, imaging, and communications.

Multilayer thin film mid-infrared broadband absorber with high visible light transmittance

Few reports on absorbers achieve both high transmittance of visible light and strong absorption of mid-infrared light. Here a multilayered absorber with high visible light transmittance and strong broadband absorption in the mid-infrared band is proposed. The absorber is four-layer films of ITO/SiO2/ITO/SiO2 successively deposited on single-sided conductive glass by electron beam evaporation technology. The experiment shows that the integral transmittance in the 400–800 nm range is 82.8 %, and the integral absorption in the 3–5 μm band is 88.2 %.

Mid-Infrared Supercontinuum Generation in Highly Nonlinear Chalcogenide Fibers

Interest in mid-infrared broadband laser light sources has surged due to applications in trace gas detection, free-space communications, and countermeasures. Progress in supercontinuum generation leverages fiber-based near-infrared and bulk-optic mid-infrared pump sources. In this paper, the Generalized Nonlinear Schrödinger Equation has been solved, using the Split Step Fourier Method, to simulate the pulse propagation and mid-infrared supercontinuum generation, inside a fiber composed of highly nonlinear As2Se3/As2S3 chalcogenide glass. The effect of various parameters, including fiber nonlinearity, Group Velocity Dispersion (GVD), input power and pulse-width, anomalous and normal dispersion pumping regime, etc. on the output supercontinuum bandwidth has been extensively studied. A tapered chalcogenide fiber is modeled to facilitate continuous simultaneous modification of the GVD and the Kerr nonlinearity parameter. Pumping the waveguides with 230-fs secant pulses at a peak power of 4.2-kW yields a mid-IR supercontinuum extending from [Formula: see text] to [Formula: see text] micrometers.

Near‐Field Nanospectroscopy and Mode Mapping of Lead Telluride Hoppercubes

AbstractLead 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.

Numerical study of mid-infrared broadband perfect absorber based on dielectric/aluminium doped zinc oxide multilayer films

We propose a lithography-free wide-angle polarization-insensitive ultra-broadband absorber by using multilayer films consist of five pairs of aluminium doped zinc oxide (AZO) and a dielectric film which is supposed to be fabricated on glass substrate. The absorption spectrum of the proposed model is calculated by using transfer matrix method. The analytic results show that the absorptivity is larger than 95% with normal incidence light in the wavelength range from 7.5μm to 20μm and having high transmittance at the visible wavelength range from 0.4μm to 0.8μm range. Further, the effect of incident angle and polarization on the absorption spectrum are also investigated. The absorption spectrum of the proposed design is highly tunable on changing the damping constant of the AZO film. The electric field and magnetic distributions at different wavelengths show that the absorption is mainly originated due to the constructive interference of electromagnetic waves in the multilayer films. Such physical mechanism of broadening bandwidth based on increasing the carrier concentration of the highly visible transparent conducting films have pave a new direction for the design of tunable broadband absorber in different frequency band. Meanwhile, this broadband absorber is a good candidate for the potential applications in daytime radiation cooling system in the mid-infrared frequency band and also used as transparent conducting electrode in solar cell applications.

Tunable mid-infrared broadband absorption in the atmospheric window of wavelength in 3–5 μm using VO2 phase transition in a three-layer metamaterial

Tunable metasurfaces have garnered considerable attention for their remarkable ability to manipulate electromagnetic waves within carefully designed nanostructures. Achieving resonant absorption requires precise control over the geometry and size of micro-nano structures. This study introduces a highly versatile metasurface platform that leverages the unique phase transition properties of vanadium dioxide (VO2) for controlled absorption. Our design features a planar layered structure with a 0.1 μm thick VO2 top layer, enabling tunability in both thermal and intensity aspects. We demonstrate significant tuning of broadband light absorption across the mid-infrared spectrum, with absorption rates ranging from approximately 30 %–98 % in the critical atmospheric infrared window of 3–5 μm. This range is crucial for infrared sensing, imaging, and communication applications. Notably, the absorption rate increases as the temperature decreases, indicating a positive shift in dynamics, for which we provide a physical explanation. Numerical simulations highlight the ability to control absorption bandwidth by adjusting the thickness and temperature of VO2 layer. These adaptable broadband absorbers hold significant promise for a diverse range of applications, including absorption filters, thermal emitters, thermos photovoltaics, and thermal imaging. Furthermore, the introduced thermally switchable, spectrally selective metamaterials can find applications in dynamic infrared camouflage and active radiative thermal management, opening new horizons in the realm of metamaterial-based technologies.

Mid-infrared single-photon upconversion spectroscopy enabled by nonlocal wavelength-to-time mapping.

Ultrasensitive spectroscopy is an essential component in mid-infrared (MIR) technology. However, the drawbacks of MIR detectors pose challenges to robust MIR spectroscopy at the single-photon level. We propose an MIR single-photon frequency upconversion spectroscopy nonlocally mapping the MIR information to the time domain. Broadband MIR photons from spontaneous parametric downconversion are frequency-upconverted to the near-infrared band with quantum correlation preservation. Via the group delay of fiber, the MIR spectral information within a 1.18-micrometer bandwidth of 2.76 to 3.94 micrometers is then successfully projected to arrival times of correlated photon pairs. Under the conditions of 6.4 × 106 photons per second illumination, the transmission spectra of polymers with single-photon sensitivity are demonstrated using single-pixel detectors. The developed approach circumvents scanning and frequency selection instability, which stands out for its inherent compatibility for evolving environments and scalability for various wavelengths. Because of its high sensitivity and robustness, characterization of biochemical samples and weak measurement of quantum systems are possible to foresee.

Single‐Photon Time‐Stretch Infrared Spectroscopy

AbstractSensitive mid‐infrared (MIR) spectroscopy is highly demanded in various fields ranging from industrial inspection, biomedical diagnosis to astronomical observation. However, the detection sensitivity of conventional MIR spectrometers is severely limited by excessive noises for existing infrared sensors, which hinders widespread use in photon‐scarce scenarios. Here, a broadband MIR single‐photon time‐stretch spectrometer is devised and implemented based on high‐fidelity spectral upconversion and time‐correlated coincidence counting. Specifically, a nanophotonic supercontinuum illumination covering 2.4–4.2 µm is nonlinearly converted to the near‐infrared band, where low‐loss single‐mode fiber and high‐performance silicon detector can be leveraged to facilitate dispersive operation and sensitive detection, respectively. The arrival time for the dispersed upconversion photons is precisely registered with a low‐timing‐jitter photon counter, which enables us to obtain a high spectral resolution about 0.5 cm−1 under a low‐light‐level illumination down to 0.14 photons/nm/pulse. In comparison to previous MIR upconversion spectrometers, the presented time‐stretch architecture favors single‐pixel simplicity and high‐throughput acquisition for the single‐photon spectral measurement. The achieved MIR spectroscopic features of broadband spectral coverage, sub‐wavenumber resolution, single‐photon sensitivity, and room‐temperature operation would stimulate immediate applications in material and life sciences.

Ultra-broadband and wide-angle nonreciprocal thermal emitter based on Weyl semimetal metamaterials.

Nonreciprocal thermal radiation can violate Kirchhoff's law and exhibit different emissivity at symmetric polar angles relative to the normal direction. Realizing a mid-infrared broadband nonreciprocal thermal emitter with a wide emission angle range is a fundamental yet challenging task, particularly without the need for an external magnetic field. Here, we propose a nonreciprocal thermal emitter operating in the mid-infrared that achieves a significantly nonreciprocal thermal radiation in a wavelength range from 12 μm to 20 μm, spanning a wide angular range from 16° to 88°. This is achieved by utilizing a multilayered Weyl semimetal (WSM)/dielectric structure, which takes the advantage of the strong nonreciprocity of WSMs with different Fermi levels and epsilon-near-zero-induced Brewster modes. The results provide a wider angular range in the broad mid-infrared band compared to previous attempts. The robustness of the nonreciprocal radiation is confirmed through wavelength-averaged emissivity across the azimuth angle φ range from 0° to 360°. Some possible materials and nanostructures as dielectric layers are discussed, showcasing the flexibility and reliability of the design. This work holds promising potential applications such as enhanced radiative cooling, thermal emitters for medical sensing and infrared heating, energy conversion, etc.

Design and performance analysis of a mid-infrared broadband thermally tunable metamaterial absorption device based on the phase-change effect.

We propose a structurally simple, innovative, and multifunctional mid-infrared broadband thermally tunable metamaterial absorption device. The absorption device consists of a three-layer structure, from bottom to top: Ti substrate, SiO2 dielectric layer, and patterned VO2 layer. Through temperature control, the average absorption intensity of the absorption device can vary between 0.08 and 0.94. The absorption device's absorption mechanism is rooted in the thermal phase-change characteristics exhibited by the topologically patterned VO2. When the temperature is below 340 K, VO2 is in a dielectric state, resulting in near-total reflection performance in the mid-infrared range. When the temperature is above 340 K, VO2 undergoes a dielectric-to-metal transition, enabling the absorption device to achieve an average absorption rate of 94.12% in the ultra-wideband range of 6.26 μm-20.96 μm in the mid-infrared. This absorption range completely covers the atmospheric window wavelengths of 8 μm-14 μm, demonstrating high practical value. We explain the working mechanism of the absorption device from the perspective of the electromagnetic field. Subsequently, we study the variations in the absorption curve of the absorption device at different temperatures of VO2 and use the changes in the electric field at the same wavelength under different temperatures to explain the variations in absorption. Compared to previous work, our structure has only three layers in a single unit, making it easy to process and produce. Additionally, the absorption device's operating bandwidth and average absorption rate in the mid-infrared range have been significantly improved. Furthermore, the absorption device exhibits substantial incident angle tolerance and polarization insensitivity. We believe that this design has broad application potential in future optothermal conversion, infrared stealth, and thermal radiation.

Sensing Liquid- and Gas-Phase Hydrocarbons via Mid-Infrared Broadband Femtosecond Laser Source Spectroscopy.

In this study, we demonstrate the combination of a tunable broadband mid-infrared (MIR) femtosecond laser source separately coupled to a ZnSe crystal horizontal attenuated total reflection (ATR) sensor cell for liquid phase samples and to a substrate-integrated hollow waveguide (iHWG) for gas phase samples. Utilizing this emerging light source technology as an alternative MIR radiation source for Fourier transform infrared (FTIR) spectroscopy opens interesting opportunities for analytical applications. In a first approach, we demonstrate the quantitative analysis of three individual samples, ethanol (liquid), methane (gas), and 2-methyl-1-propene (gas), with limits of detection of 0.3% (ethanol) and 22 ppmv and 74 ppmv (methane and isobutylene), respectively, determined at selected emission wavelengths of the MIR laser source (i.e., 890 cm-1, 1046 and 1305 cm-1). Hence, the applicability of a broadband MIR femtosecond laser source as a bright alternative light source for quantitative analysis via FTIR spectroscopy in various sensing configurations has been demonstrated.

Mid-Infrared Bipolar and Unipolar Linear Polarization Detections in Nb2 GeTe4 /MoS2 Heterostructures.

Detecting and distinguishing light polarization states, one of the most basic elements of optical fields, have significant importance in both scientific studies and industry applications. Artificially fabricated structures, e.g., metasurfaces with anisotropic absorptions, have shown the capabilities of detecting polarization light and controlling. However, their operations mainly rely on resonant absorptions based on structural designs that are usually narrow bands. Here, a mid-infrared (MIR) broadband polarization photodetector with high PRs and wavelength-dependent polarities using a 2D anisotropic/isotropic Nb2 GeTe4 /MoS2 van der Waals (vdWs) heterostructure is demonstrated. It is shown that the photodetector exhibits high PRs of 48 and 34 at 4.6 and 11.0µm wavelengths, respectively, and even a negative PR of -3.38 for 3.7µm under the zero bias condition at room temperature. Such interesting results can be attributed to the superimposed effects of a photovoltaic (PV) mechanism in the Nb2 GeTe4 /MoS2 hetero-junction region and a bolometric mechanism in the MoS2 layer. Furthermore, the photodetector demonstrates its effectiveness in bipolar and unipolar polarization encoding communications and polarization imaging enabled by its unique and high PRs.

Detection of gaseous halocarbon refrigerants and extinguishing agent based on photoacoustic spectroscopy

A photoacoustic (PA) gaseous halocarbon refrigerants and extinguishing agent detector based on a mid-infrared broadband source was developed. H1301 (extinguishing agent), R134a and R410A (refrigerants) were the target gases. A mid-infrared thermal radiation source radiated along the longitudinal direction of the cylindrical PA cell and was reflected by a gold-plated mirror to excite PA signals. The concentrations of CO2 and H2O were measured to reduce their interference. The PA signals of H1301, R134a, R410A, CO2 and H2O were excited by using a thermal radiation source with five different optical filters, respectively. The fundamental-frequency intensity modulation (1f-IM) technique was used to measure the generated PA signals. The detection limits (1σ) of H1301, R134a and R410A were 49 ppb, 80 ppb and 211 ppb with an average time of 100 s, respectively. According to the absorption spectrum of the three target gases, there are strong cross interference around the wavelength of 8.5 µm. By using the proposed calculation method to obtain the concentrations of the target gases, the high cross interference among these gases is reduced to less than 2 %.

Routing to mid-infrared microcomb via near-infrared direct pump.

Mid-infrared (MIR) microcomb provides a new way into the "molecular fingerprint" region. However, it remains rather a challenge to realize the broadband mode-locked soliton microcomb, which is often limited by the performance of available MIR pump sources and coupling devices. Here, we propose an effective approach towards broadband MIR soliton microcombs generation via a direct pump in the near-infrared (NIR) region, through full utilization of the second- and third-order nonlinearities in a thin-film lithium niobate microresonator. The optical parametric oscillation process contributes to conversion from the pump at 1550 nm to the signal around 3100 nm, and the four-wave mixing effect promotes spectrum expansion and mode-locking process. While the second-harmonic and sum-frequency generation effects facilitate simultaneous emission of the NIR comb teeth. Both the continuous wave and pulse pump sources with relatively low power can support a MIR soliton with a bandwidth over 600 nm and a concomitant NIR microcomb with a bandwidth of 100 nm. This work can provide a promising solution for broadband MIR microcombs by breaking through the limitation of available MIR pump sources, and can deepen the understanding of the physical mechanism of the quadratic soliton assisted by the Kerr effect.

Mid-infrared broadband metamaterial absorber based on van der Waals material

Van der Waals materials, such as α-phase molybdenum trioxide (α-MoO3), have promising prospects in modern optics technologies, such as nano-imaging, negative refraction, and infrared detection. Particularly, the natural hyperbolic properties of α-MoO3 make it an excellent candidate for perfect absorber. Here, we propose a design method for achieving broadband absorption based on van der Waals material (α-MoO3) in the mid-infrared band. The segmented cubic Hermite interpolation is used to generate various geometric structures. Numerical results show that the average spectral absorptance of the optimized structure is up to 0.993 in the wavelength range of 10.4–12.7 μm. The high absorption performance can be explained as the slow-light effect. The impact of incident angle on absorption performance is also investigated. Finally, we calculate the spectral absorptance of the proposed absorber when the crystal axes of α-MoO3 are rotated in the x-y plane. Our findings pave a novel path for designing broadband absorbers based on van der Waals materials, particularly in the mid-infrared band.

High‐Speed Mid‐Infrared Single‐Photon Upconversion Spectrometer

AbstractSensitive and fast mid‐infrared (MIR) spectroscopy is highly attractive in a variety of applications including astronomical observation, pharmaceutical synthesis, and environmental monitoring. However, the performance of conventional MIR spectrometers has long been hindered by the limited sensitivity of narrow‐bandgap detectors and/or the deficient brightness of broadband light sources. Here, an ultra‐sensitive and broadband MIR upconversion spectrometer, which integrates a supercontinuum source covering 1.5–4.2 m based on a silicon nitride nanophotonic waveguide, is devised and integrated. High‐efficiency and low‐noise nonlinear frequency upconversion is realized based on coincidence pulsed pumping with spectro‐temporal optimization, which enables leverage of silicon detectors for facilitating MIR single‐photon spectroscopy at 0.2 photons/nm/pulse. Furthermore, the upconversion‐based array spectrometer is manifested with high‐speed spectral acquisition rates beyond 200 kHz, which is about tenfold faster than the state‐of‐the‐art scan rates for FTIR‐based spectrometers at a comparable spectral resolution. The achieved features of broadband spectral coverage, single‐photon sensitivity, and sub‐MHz refreshing rate might open up new possibilities for infrared transient spectral measurements in combustion analysis, high‐throughput sorting, and reaction tracking, among others.

CH4, C2H6, and CO2 Multi-Gas Sensing Based on Portable Mid-Infrared Spectroscopy and PCA-BP Algorithm.

A multi-gas sensing system was developed based on the detection principle of the non-dispersive infrared (NDIR) method, which used a broad-spectra light source, a tunable Fabry-Pérot (FP) filter detector, and a flexible low-loss infrared waveguide as an absorption cell. CH4, C2H6, and CO2 gases were detected by the system. The concentration of CO2 could be detected directly, and the concentrations of CH4 and C2H6 were detected using a PCA-BP neural network algorithm because of the interference of CH4 and C2H6. The detection limits were achieved to be 2.59 ppm, 926 ppb, and 114 ppb for CH4, C2H6, and CO2 with an averaging time of 429 s, 462 s, and 297 s, respectively. The root mean square error of prediction (RMSEP) of CH4 and C2H6 were 10.97 ppm and 2.00 ppm, respectively. The proposed system and method take full advantage of the multi-component gas measurement capability of the mid-infrared broadband source and achieve a compromise between performance and system cost.

Low-noise synchronized mid-infrared upconversion spectrometer with a large spectral coverage

Mid-infrared (MIR) frequency upconversion presents a powerful tool for MIR photon detection at room temperature. However, the introduction of strong pump light will exacerbate the limitations of upconverted thermal radiation and upconverted parametric fluorescence on detectability in broadband MIR frequency upconversion. Here, a broadband synchronized MIR upconversion spectrometer system based on a step-chirped poled LiNbO3 crystal is demonstrated. This system is integrated with spontaneous parametric downconversion (SPDC) and sum-frequency generation (SFG) to form a broadband MIR light generation and synchronous pulsed upconversion, which are simultaneously driven by the same pulsed pump laser. The spectral coverage is close to 1.4 μm from 2450 to 3850 nm and the resolution of the system is about 4 cm−1 after deconvolution, as confirmed by a Fourier transform infrared spectrometer. In conjunction with synchronous pumping-based temporal gating, the system noise is suppressed to less than 10 counts per second per nm. This broadband synchronized SPDC–SFG configuration can be extended to different infrared wavelengths for low-noise broadband spectroscopic measurement.