Mid-infrared Atmospheric Window Research Articles

Thermochromic Nanocellulose Films for Temperature-Adaptive Passive Cooling.

Energy efficiency in habitation spaces is a pivotal topic for maintaining energy sufficiency, cutting climate impact, and facilitating economic savings; thus, there is a critical need for solutions aimed at tackling this problem. One viable approach involves complementing active cooling methods with powerless or passive cooling ones. Moreover, considerable scope remains for the development of passive radiative cooling solutions based on sustainable materials. Cellulose, characterized by its abundance, renewability, and biodegradability, emerges as a promising material for this purpose due to its notable radiative cooling potential exploiting the mid-infrared (MIR) atmospheric transmission window (8-13 μm). In this work, we propose the utilization of thermochromic (TC) materials in conjunction with cellulose nanofibrils (CNF) to confer temperature-dependent adaptivity to hybrid CNF films. We employ a concept where high reflection, coupled with MIR emission in the heated state, facilitates cooling, while high visible light absorption in the cold state allows heating, thus enabling adaptive thermal regulation. CNF films were doped with black-to-leuco TC particles, and a thin silver layer was optionally applied to the films. The films exhibited a rapid transition (within 1 s) in their optical properties at ∼22 °C, becoming transparent above the transition temperature. Visible range transmittance of all samples ranged from 60 to 90%, with pronounced absorption in the 8-13 μm range. The cooling potential of the films was measured at 1-4 °C without any Ag layer and ∼10 °C with a Ag layer. In outdoor field testing, a peak cooling value of 12 °C was achieved during bright sunshine, which is comparable to a commercial solar film. A simulation model was also built based on the experimental results. The concept presented in this study extends beyond applications as standalone films but has applicability also in glass coatings. Overall, this work opens the door for a novel application opportunity for green cellulose-based materials.

High-speed mid-infrared graphene electro-optical modulator based on suspended germanium slot waveguides.

The mid-infrared (MIR) region is attracting increasing interest for on-chip synchronous detection and free-space optical (FSO) communications. For such applications, a high-performance electro-optical modulator is a crucial component. In this regard, we propose and investigate a graphene-based electro-absorption modulator (EAM) and microring modulator (MRM) using the suspended germanium waveguide platform. The modulators are designed for the second atmospheric window (8 to 12 µm). The incorporation of double-layer graphene on the suspended slot waveguide structure allows for the significant enhancement of light-graphene interaction, theoretically achieving a 3-dB bandwidth as high as 78 GHz. The EAM shows a calculated modulation depth of 0.022-0.045 dB/µm for the whole operation wavelength range. The MRM exhibits a calculated extinction ratio as high as 68.9 dB and a modulation efficiency of 0.59 V·cm around 9 µm. These modulators hold promise for constructing high-speed FSO communication and on-chip spectroscopic detection systems in the MIR atmospheric window.

Broadband 1-GHz mid-infrared frequency comb

Mid-infrared (MIR) spectrometers are invaluable tools for molecular fingerprinting and hyper-spectral imaging. Among the available spectroscopic approaches, GHz MIR dual-comb absorption spectrometers have the potential to simultaneously combine the high-speed, high spectral resolution, and broad optical bandwidth needed to accurately study complex, transient events in chemistry, combustion, and microscopy. However, such a spectrometer has not yet been demonstrated due to the lack of GHz MIR frequency combs with broad and full spectral coverage. Here, we introduce the first broadband MIR frequency comb laser platform at 1 GHz repetition rate that achieves spectral coverage from 3 to 13 µm. This frequency comb is based on a commercially available 1.56 µm mode-locked laser, robust all-fiber Er amplifiers and intra-pulse difference frequency generation (IP-DFG) of few-cycle pulses in χ(2) nonlinear crystals. When used in a dual comb spectroscopy (DCS) configuration, this source will simultaneously enable measurements with μs time resolution, 1 GHz (0.03 cm−1) spectral point spacing and a full bandwidth of >5 THz (>166 cm−1) anywhere within the MIR atmospheric windows. This represents a unique spectroscopic resource for characterizing fast and non-repetitive events that are currently inaccessible with other sources.

Structurally Colored Radiative Cooling Cellulosic Films.

Daytime radiative cooling (DRC) materials offer a sustainable approach to thermal management by exploiting net positive heat transfer to deep space. While such materials typically have a white or mirror‐like appearance to maximize solar reflection, extending the palette of available colors is required to promote their real‐world utilization. However, the incorporation of conventional absorption‐based colorants inevitably leads to solar heating, which counteracts any radiative cooling effect. In this work, efficient sub‐ambient DRC (Day: −4 °C, Night: −11 °C) from a vibrant, structurally colored film prepared from naturally derived cellulose nanocrystals (CNCs), is instead demonstrated. Arising from the underlying photonic nanostructure, the film selectively reflects visible light resulting in intense, fade‐resistant coloration, while maintaining a low solar absorption (≈3%). Additionally, a high emission within the mid‐infrared atmospheric window (>90%) allows for significant radiative heat loss. By coating such CNC films onto a highly scattering, porous ethylcellulose (EC) base layer, any sunlight that penetrates the CNC layer is backscattered by the EC layer below, achieving broadband solar reflection and vibrant structural color simultaneously. Finally, scalable manufacturing using a commercially relevant roll‐to‐roll process validates the potential to produce such colored radiative cooling materials at a large scale from a low‐cost and sustainable feedstock.

VO2-based thermally tunable emitter and preliminary design of switching for mid-infrared atmospheric windows

An atmospheric window is greatly significant in the radiative cooling field. In this study, we preliminarily design a thermally tunable metamaterial emitter that can be used as an internal and external emission switch for a mid-infrared atmospheric window (3.5–5 and 8–14 μm). The proposed emitter has a multilayer metal–dielectric–metal micropyramid structure with thermochromic VO2 spacers. Its broadband emission and thermal tunability are due to the resonance of the hybridization of separate magnetic polaritons, surface plasmon polaritons, and the thermal phase-transition characteristics of VO2. The structure has a warming effect at the temperature below the phase-change temperature. On the contrary, it has a cooling effect at the temperature above the phase-change temperature. These phenomena show the emitter’s potential application in smart temperature control systems.

Large-area mid-infrared broadband absorbers based on spiral ITO resulting from the combination of two different broadening absorption methods

A large-area mid-infrared broadband absorber is proposed in this paper. The absorber is a spiral ITO structure grown on a hexagonal lattice arrangement of silicon nanopillars by using a glancing angle deposition method. The experimental results show that when the heights of the silicon nanopillars are 1.7 µm and the number of rotation depositions is n = 5, that is, the rotation angle is 150 degrees, the absorber absorbs more than 81% of electromagnetic waves in the 2.5-6 µm spectral range. In the atmospheric window of 3-5 µm, the integral absorption reaches 96%. The experimental results also show that the absorbing ability of the ITO structure in the mid-infrared atmospheric window is significantly stronger than that of the structure composed of silver under the same preparation conditions. The main reasons for the broadband absorption are that the spiral ITO structure has resonant absorption of electromagnetic waves with different wavelengths in the empty cavity regions with different sizes, and ITO has longer penetration depths than noble metals in the mid-infrared band, which brings about stronger broadband absorption. The combination of the two leads to a broadening of the total absorption spectrum. The higher heights of the silicon nanopillars enhance absorption further. Additionally, the loose spiral ITO distributions indicate lower mean plasma concentration and then increase penetration depths further, resulting in stronger light absorption. Such a large-area mid-infrared absorption structure with a simple preparation method has potential applications in mid-infrared cloaking and sensing.

Ultra-broadband and angle-insensitive perfect absorber in the mid-infrared atmospheric window

An ultra-broadband and angle-insensitive absorber is proposed and demonstrated in the mid-infrared (MIR) atmospheric window. The structure is coated with periodically arranged metal grating on top of a metal-insulator-metal (MIM) plate. The average absorption is up to 96.6% in the range of 8–14 µm. Due to the structural characteristics, the absorption properties are polarization-dependent, and the perfect absorption can be realized for a TM-polarized wave when the incident angle is smaller than 50°, bandwidth of 90% absorption reaches approximately 6 µm. The absorption characteristics can be feasibly manipulated via tuning the geometrical parameters. The electrical field intensity distributions are studied to disclose the broadband absorption mechanism. The proposed broadband absorber has many potential applications, including polarization detectors, heat radiators, infrared sensors and so on.

Octave mid-infrared optical frequency comb from Er:fiber-laser-pumped aperiodically poled Mg: LiNbO3.

In this Letter, we report an octave-spanning mid-infrared (MIR) comb generation with a difference frequency generation (DFG) approach optimized for aperiodically poled Mg:LiNbO3 and nonlinear spectral broadening. An Er:fiber comb is delivered to two branches and amplified in an Yb:fiber and an Er:fiber amplifier, respectively. We demonstrate that the two-branch DFG can yield the spectrum tuned over an octave in a fan-out periodically poled lithium niobate. Thus, we obtain an optimized poling period profile and design the aperiodically poled Mg:LiNbO3. The results demonstrate that broadband combs can be generated in the MIR atmospheric window.

Mid-Infrared Frequency Comb Generation and Spectroscopy with Few-Cycle Pulses and χ^{(2)} Nonlinear Optics.

The mid-infrared atmospheric window of 3-5.5 μm holds valuable information regarding molecular composition and function for fundamental and applied spectroscopy. Using a robust, mode-locked fiber-laser source of <11 fs pulses in the near infrared, we explore quadratic (χ^{(2)}) nonlinear optical processes leading to frequency comb generation across this entire mid-infrared atmospheric window. With experiments and modeling, we demonstrate intrapulse difference frequency generation that yields few-cycle mid-infrared pulses in a single pass through periodically poled lithium niobate. Harmonic and cascaded χ^{(2)} nonlinearities further provide direct access to the carrier-envelope offset frequency of the near infrared driving pulse train. The high frequency stability of the mid-infrared frequency comb is exploited for spectroscopy of acetone and carbonyl sulfide with simultaneous bandwidths exceeding 11THz and with spectral resolution as high as 0.003 cm^{-1}. The combination of low noise and broad spectral coverage enables detection of trace gases with concentrations in the part-per-billion range.

Absorption lines measurements of carbon disulfide at 4.6 μm with quantum cascade laser absorption spectroscopy

We report measured line intensities and air- and self-broadening coefficients for fifty-one carbon disulfide transitions in the ν1 + ν3 band near 4.6 μm. This spectral region was chosen due to the strong carbon disulfide absorption strength and in the range of the mid-infrared atmospheric window for laser-based sensing applications. Spectroscopic parameters were determined by spectra measuring with quantum cascade laser direct absorption spectroscopy and multi-line fitting with Voigt lineshape. These measured results would facilitate the development of calibrated-free mid-infrared carbon disulfide sensors.

Broadband switching of mid-infrared atmospheric windows by VO2-based thermal emitter.

Atmospheric windows play an important role in the field of infrared detection and radiative cooling. In this paper, the development of VO2-based metamaterial emitter brings broadband thermal-switching light to mid-infrared atmospheric windows. At room temperature, the emitter radiates light in both 3-5μm and 8-14μm atmospheric windows. At high temperature, the radiation peaks move out of the atmospheric windows and result a strong radiation at 5-8μm. The underlying mechanism relies on the relationship between VO2 metal-insulator transition (MIT) and resonant absorption modes coupling. Corresponding thermal imaging experiment exhibits two distinct phenomena. One is the observation of unchanged thermal radiation around MIT temperature. The other phenomenon regards the concealment of the emitter from Al background at specific temperatures. These two phenomena show potential application in infrared anti-detection.

Numerical investigation of an ultra-broadband coherent mid-infrared supercontinuum in a chalcogenide AsSe2-As2S5 multimaterial photonic crystal fiber

In this paper, we report on the simulation of an ultra-broadband coherent mid-infrared supercontinuum (SC) extending from 1.25 to 20 μm generated using a novel AsSe2-As2S5 multimaterial photonic crystal fiber (PCF). The proposed fiber is composed of a core made of AsSe2 glass and a surrounding cladding made of As2S5 glass. The hybrid PCF is designed to have a zero-dispersion wavelength (ZDW) of 3.3 μm with an overall highly engineered group velocity dispersion shifted to the mid-infrared wavelength region. The SC is generated by pumping 50 fs pulses at 4 μm emitted from an optical parameter amplifier with low energy of 0.625 nJ. The pumping wavelength is selected in the anomalous dispersion regime close to the ZDW. The widening of the SC is mainly based on the soliton effects in the anomalous dispersion region combined with self-phase modulation, cross-phase modulation, stimulated Raman scattering, four-wave mixing, and dispersive wave. The obtained SC shows a high degree of coherence and a 15 fs temporal compressed pulse is generated in only 5 mm long AsSe2-As2S5 hybrid PCF. The power proportion of the SC generated beyond 4 μm is 98% with its long wavelength edge up to 20 μm. To the best of our knowledge, the obtained SC is the first broadest spectrum reported in the mid-infrared region with very low energy. Our results highlight the potential of the novel chalcogenide AsSe2-As2S5 multimaterial PCF to emit across the ultra-broadband mid-infrared atmospheric windows and the molecular fingerprint region.

Wideband Ge-Rich SiGe Polarization-Insensitive Waveguides for Mid-Infrared Free-Space Communications

The recent development of quantum cascade lasers, with room-temperature emission in the mid-infrared range, opened new opportunities for the implementation of ultra-wideband communication systems. Specifically, the mid-infrared atmospheric transparency windows, comprising wavelengths between 3–5 µm and 8–14 µm, have great potential for free-space communications, as they provide a wide unregulated spectrum with low Mie and Rayleigh scattering and reduced background noise. Despite the great efforts devoted to the development of mid-infrared sources and detectors, little attention is dedicated to the management of polarization for signal processing. In this work, we used Ge-rich SiGe alloys to build a wideband and polarization-insensitive mid-infrared photonic platform. We showed that the gradual index change in the SiGe alloys enabled the design of waveguides with remarkably low birefringence, below 2 × 10−4, over ultra-wide wavelength ranges within both atmospheric transparency windows, near wavelengths of 3.5 µm and 9 µm. We also report on the design of a polarization-independent multimode interference device achieving efficient power splitting in an unprecedented 4.5-µm bandwidth at around 10-µm wavelength. The ultra-wideband polarization-insensitive building blocks presented here pave the way for the development of high-performance on-chip photonic circuits for next-generation mid-infrared free-space communication systems.

Mid-infrared frequency comb via coherent dispersive wave generation in silicon nitride nanophotonic waveguides

Mid-infrared optical frequency combs are of significant interest for molecular spectroscopy due to the large absorption of molecular vibrational modes on one hand, and the ability to implement superior comb-based spectroscopic modalities with increased speed, sensitivity and precision on the other hand. Substantial advances in mid-infrared frequency comb generation have been made in recent years based on nonlinear frequency conversion, microresonator Kerr frequency combs, quantum cascade lasers and mode locking regimes. Here we demonstrate a simple, yet effective method for the direct generation of mid-infrared optical frequency combs in the region from ${2.5-4~\mu{\rm m}}$, i.e. ${2500-4000~{\rm cm}^{-1}}$ covering a large fraction of the functional group region, directly from a conventional and compact erbium-fiber-based femtosecond laser in the telecommunication band (i.e. ${1.55~\mu{\rm m}}$). The wavelength conversion is based on dispersive wave generation within the supercontinuum process in large-cross-section and dispersion-engineered silicon nitride (${\rm Si_3N_4}$) waveguides. The long-wavelength dispersive wave, with its position lithographically determined, performs as a mid-infrared frequency comb, whose coherence is demonstrated via optical heterodyne measurements. Such a simple and versatile approach to mid-infrared frequency comb generation is suitable for spectroscopic applications in the first mid-infrared atmospheric window. Moreover, the compactness and simplicity of the approach have the potential to realize compact dual-comb spectrometers. The generated combs have a fine teeth-spacing, making them also suitable for gas phase analysis.

Highly efficient and broadband mid-infrared metamaterial thermal emitter for optical gas sensing.

Development of a novel, cost-effective, and highly efficient mid-infrared light source has been identified as a major scientific and technological goal within the area of optical gas sensing. We have proposed and investigated a mid-infrared metamaterial thermal emitter based on micro-structured chromium thin film. The results demonstrate that the proposed thermal light source supports broadband and wide angular absorption of both TE- and TM-polarized light, giving rise to broadband thermal radiation with averaged emissivity of ∼0.94 in a mid-infrared atmospheric window of 8-14μm. The proposed microphotonic concept provides a promising alternative mid-infrared source and paves the way towards novel optical gas sensing platforms for many applications.

Design and analysis of infrared frequency selective surface with dual-stopband

A dual-band frequency selective surface (FSS) with double screen was designed to cover the mid-infrared atmospheric window (3 μm~5 μm) and the far-infrared atmospheric window (8 μm~14 μm). This structure is com-posed of hexagonal metallic mesh and hexagonal resonant ring array. Simulation results show that the average transmission of FSS in atmospheric window is lower than 5%, and this structure is insensitive to the incident angle. The effect of the parameters on transmission properties was analyzed. The results show that adjusting the size of hexagonal ring structure can effectively adjust the -10 dB stopband bandwidth in 3 μm~5 μm, and increasing the size of metallic mesh can move the stopband in 8 μm~14 μm to long wave direction.

3D MODELING OF GJ1214b's ATMOSPHERE: FORMATION OF INHOMOGENEOUS HIGH CLOUDS AND OBSERVATIONAL IMPLICATIONS

The warm sub-Neptune GJ1214b has a featureless transit spectrum which may be due to the presence of high and thick clouds or haze. Here, we simulate the atmosphere of GJ1214b with a 3D General Circulation Model for cloudy hydrogen-dominated atmospheres, including cloud radiative effects. We show that the atmospheric circulation is strong enough to transport micrometric cloud particles to the upper atmosphere and generally leads to a minimum of cloud at the equator. By scattering stellar light, clouds increase the planetary albedo to 0.4-0.6 and cool the atmosphere below 1 mbar. However, the heating by ZnS clouds leads to the formation of a stratospheric thermal inversion above 10 mbar, with temperatures potentially high enough on the dayside to evaporate KCl clouds. We show that flat transit spectra consistent with HST observations are possible if cloud particle radii are around 0.5 micron, and that such clouds should be optically thin at wavelengths > 3 microns. Using simulated cloudy atmospheres that fit the observed spectra we generate transit, emission and reflection spectra and phase curves for GJ1214b. We show that a stratospheric thermal inversion would be readily accessible in near and mid-infrared atmospheric spectral windows. We find that the amplitude of the thermal phase curves is strongly dependent on metallicity, but only slightly impacted by clouds. Our results suggest that primary and secondary eclipses and phase curves observed by the James Webb Space Telescope in the near to mid-infrared should provide strong constraints on the nature of GJ1214b's atmosphere and clouds.

Ground-based all-sky mid-infrared and visible imagery for purposes of characterizing cloud properties

Abstract. This paper describes the All Sky Infrared Visible Analyzer (ASIVA), a multi-purpose visible and infrared sky imaging and analysis instrument whose primary function is to provide radiometrically calibrated imagery in the mid-infrared (mid-IR) atmospheric window. This functionality enables the determination of diurnal fractional sky cover and estimates of sky/cloud temperature from which one can derive estimates of sky/cloud emissivity and cloud height. This paper describes the calibration methods and performance of the ASIVA instrument with particular emphasis on data products being developed for the meteorological community. Data presented here were collected during the Solmirus' ASIVA campaign conducted at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Climate Research Facility from 21 May to 27 July 2009. The purpose of this campaign was to determine the efficacy of IR technology in providing reliable nighttime sky cover data. Significant progress has been made in the analysis of the campaign data over the past several years and the ASIVA has proven to be an excellent instrument for determining sky cover as well as the potential for determining sky/cloud temperature, sky/cloud emissivity, precipitable water vapor (PWV), and ultimately cloud height.

InAs/GaSb type-II superlattices for high performance mid-infrared detectors

The superlattice (SL) design parameters of a 50 period InAs/GaSb SL structure with InSb-like interfaces (IFs) were systematically varied around the 26 Å InAs/ 27 Å GaSb design in order to explore the parameter space for maximum photoresponse in the 3–5 μ m mid-infrared atmospheric window. Using previously optimized growth conditions, the SL structures were grown on p-type GaSb substrates by molecular beam epitaxy with precisely calibrated growth rates. The electrical properties of the SLs were characterized by magnetic field-dependent Hall effect measurements below the carrier freeze-out temperature of the p-type substrate. Multi-carrier analysis at 4.2 K determined an electron sheet carrier concentration of 8.5 × 10 10 cm - 2 with a mobility of 8200 cm 2 /Vs. Two sets of SLs were used in the optimization process: the first set with a fixed InAs width of 26 Å, and the second with a fixed GaSb width of 27 Å . As the GaSb layer width varied from 15 to 27 Å , the photoresponse cut-off wavelength shifted from 6.47 to 5.24 μ m. Similarly, as the InAs width varied from 26 to 13 Å, the cut-off wavelength shifted from 5.08 to 3.05 μ m. The strongest photoresponse in the 3–5 μ m mid-IR window was achieved with the InAs (20 Å)/GaSb (27 Å) SL design.

A tandem periodically-poled lithium niobate (PPLN) optical parametric oscillator (OPO)

The operation of a synchronously pumped optical parametric oscillator (OPO) that employs two periodically poled lithium niobate crystals within a single cavity has been investigated. The tandem OPO device is pumped at 1.064 μm by an passively mode-locked Nd:YVO 4 laser that has been amplified at a pulse repetition rate of 100 Hz by a pair of diode pumped Nd:YAG rods. The second crystal in the OPO device converts the signal beam produced in the first crystal and consequently three tunable wavelengths are generated within the mid-infrared atmospheric transmission window (2–5 μm). The threshold for parametric oscillation is 30 mW and a conversion efficiency, from 1.064 μm to the 2–5 μm band, of 42% has been achieved at a pump power of 360 mW.