Long-wave Infrared Research Articles

Spectrally Tunable Ultrafast Long Wave Infrared Detection at Room Temperature.

Room-temperature longwave infrared (LWIR) detectors are preferred over cryogenically cooled solutions due to the cost effectiveness and ease of operation. The performance of present uncooled LWIR detectors such as microbolometers, is limited by reduced sensitivity, slow response time, and the lack of dynamic spectral tunability. Here, we present a graphene-based efficient room-temperature LWIR detector with high detectivity and fast response time utilizing its tunable optical and electronic characteristics. The inherent weak light absorption is enhanced by Dirac plasmons on the patterned graphene coupled to an optical cavity. The absorbed energy is converted into photovoltage by the Seebeck effect with an asymmetric carrier generation environment. Further, dynamic spectral tunability in the 8-12 μm LWIR band is achieved by electrostatic gating. The proposed detection platform paves the path to a fresh generation of uncooled graphene-based LWIR photodetectors for wide ranging applications such as molecular sensing, medical diagnostics, military, security and space.

A gradient nanoporous radiative cooling ceramic with high spectral selectivity

Radiative cooling offers a sustainable solution for building space cooling. Cooling ceramics are receiving increasing attention due to their exceptional mechanical properties and environmental resistance. However, the limited spectral selectivity hinders their cooling temperatures, particularly in hot and humid climates. This study presents a gradient nanoporous magnesium oxide (MgO) ceramic with excellent spectral selectivity (1.69). It exhibits a high solar reflectance (0.96) and a selective long-wave infrared (LWIR) emittance (0.95), achieving a sub-ambient temperature reduction throughout the day in Changsha. Energy simulations demonstrate that the cooling ceramic can effectively alleviate cooling demands in hot and humid climates. The exceptional mechanical properties and environmental resistance ensure the long-term durability of the cooling ceramic. Moreover, the simple fabrication process and remarkable cleaning capability significantly reduce its cost, making it a highly promising candidate for large-scale radiative cooling applications in buildings.

Enhancing Passive Radiative Cooling Films with Hollow Yttrium-Oxide Spheres Insights from FDTD Simulation.

Passive daytime radiative cooling (PDRC) presents a promising avenue for efficient thermal management without relying on electrical power. In this study, the potential of integrating Hollow Yttrium-Oxide Spheres (HYSs) within a Polydimethylsiloxane (PDMS) matrix to enhance PDRC is investigated. Through a combination of experimental characterization and computational analysis, the optical properties and radiative cooling performance of PDMS films embedded with HYSs are evaluated. These results demonstrate that HYSs significantly improve both solar reflectivity and long-wave infrared (LWIR) emissivity of the PDMS matrix. Finite-Difference Time-Domain (FDTD) simulations confirm the scattering efficiency of HYSs across various wavelength ranges, highlighting their effectiveness as additives for enhancing the radiative properties of passive cooling materials. Experimental validation reveals enhanced reflectivity and emissivity of PDMS films with embedded HYSs, resulting in superior cooling performance compared to non-HYS counterparts. Overall, this study underscores the potential of HYS-infused PDMS films as a promising solution for passive radiative cooling, with broad applicability in diverse domains requiring efficient thermal management solutions. Additionally, these research insights pave the way for establishing an AI database for passive radiative cooling research, offering new avenues for further exploration and application in this field.

High-Performance Ultra-Broadband Photodetector Based on Fe3O4/CrSiTe3 Heterostructures.

Photodetectors based on advanced materials with a broad spectral photoresponse, high sensitivity, huge integration ability, room-temperature operation, and stable environmental stability are highly desired for diversified applications of imaging, sensing, and communication. Herein, a high-performance ultra-broadband photodetector based on an ultrathin two-dimensional (2D) Fe3O4 nanoflake heterostructure with high sensitivity was designed. The photodetector response light was from visible 405 nm to long-wave infrared (LWIR) 10.6 μm in ambient air. The competitive performances, including a high photoresponsivity (R) of 182.8 A W-1, fast speed with the rise time τr = 8.8 μs, and decay time τd = 4.1 μs, were demonstrated in the visible range. Notably, the device exhibits an excellent uncooled LWIR detection ability, with a high R of 1.4 A W-1 realized at a 1.5 V bias. In the full spectral range, the noise equivalent power is lower than 0.79 pW Hz-1/2, and specific detectivity (D*) is higher than 4.9 × 108 cm Hz1/2 W-1 in ambient air. This work provides alternative ultrathin 2D materials for future infrared optoelectronic devices.

Design of ultra-broadband long-wave to ultra-long-wave infrared absorber based on machine learning

Abstract long-wave infrared (LWIR) and ultra-long-wave infrared (ULWIR) radiation find extensive utility in atmospheric monitoring, night reconnaissance, deep space exploration, and various other fields. However, achieving ultra-wideband absorption within these spectral ranges has persistently posed a significant challenge for researchers. Metasurfaces have attracted great interest due to their ability to manipulate electromagnetic (EM) waves with unprecedented precision and efficiency. In this paper, a metasurface absorber with a simple structure is proposed. Combined with machine learning (ML) of Random Forest algorithm for design optimization, the absorber achieves ultra-wideband perfect absorption of 42.54 μm within the LWIR-ULWIR band. Furthermore, the stability and accuracy of the Random Forest algorithm in absorber design are also evaluated and compared with other classical ML algorithms. The successful realization of this work can offer the advancement of thermal imaging, sensing, and communication systems operating within the LWIR and ULWIR spectra.

Germanium metalens for longwave infrared applications

Metalenses represent a paradigm shift in optics, offering unprecedented control over light manipulation. This study focuses on the design optimization of a polarization-insensitive germanium (Ge) metalens operating in the longwave infrared (LWIR) regime. Employing rigorous coupled-wave analysis (RCWA) and finite-difference time-domain (FDTD) simulations, a metalens with 1mm focal length was designed using nanopillars with 3.5µm height and radius ranging from 0.55µm to 1.2µm. Then, the impact of lattice size and numerical aperture (NA) on lens performance was investigated. The results indicate that smaller lattices allow finer phase control and enhanced transmittance stability across the phase profile if significant coupling effects are not verified. As the NA increases, the focal spot size decreases, albeit with diminishing returns towards the diffraction limit. To the best of our knowledge, it is the first work that shows high focal efficiency (~80%) across multiple NA’s for a LWIR metalens with a diameter under 1100µm. The proposed metalens is compatible with complementary metal-oxide-semiconductor (CMOS) technology and supports low-cost manufacturing.

Difference‐Frequency Generation of 0.2‐mJ 3‐Cycle 9‐µm Pulses from Two 1‐kHz Multicycle OPCPAs

AbstractIntense long‐wave infrared (LWIR) femtosecond pulses within the 8−14 µm atmospheric window present an array of applications, such as in strong‐field physics, ultrafast nonlinear spectroscopy, and self‐guided atmospheric propagation. However, the realization of an LWIR source capable of delivering millijoule‐class energy, few‐cycle duration, and kHz repetition rate concurrently remains challenging. Here, such an LWIR source via the combination of different nonlinear parametric processes is reported, driven by a 1 kHz Yb:YAG thin‐disk laser. The system comprises two parallel multi‐cycle optical parametric chirped‐pulse amplifiers (OPCPAs) operating at 2.3 and 3.1 µm, respectively, alongside a stage of ZnGeP2‐crystal‐based difference‐frequency generation (DFG). The resulting 9 µm DFG pulses have a record energy of 0.21 mJ, a 3‐cycle duration, a 1 kHz repetition rate, and long‐term energy stability. The simultaneous output of three synchronized intense lasers at short‐wave infrared (2.3 µm), mid‐wave infrared (3.1 µm), and LWIR (9 µm) renders the source particularly appealing for multicolor ultrafast applications.

Unipolar quantum optoelectronics for high speed direct modulation and transmission in 8–14 µm atmospheric window

The large mid-infrared (MIR) spectral region, ranging from 2.5 µm to 25 µm, has remained under-exploited in the electromagnetic spectrum, primarily due to the absence of viable transceiver technologies. Notably, the 8–14 µm long-wave infrared (LWIR) atmospheric transmission window is particularly suitable for free-space optical (FSO) communication, owing to its combination of low atmospheric propagation loss and relatively high resilience to turbulence and other atmospheric disturbances. Here, we demonstrate a direct modulation and direct detection LWIR FSO communication system at 9.1 µm wavelength based on unipolar quantum optoelectronic devices with a unprecedented net bitrate exceeding 55 Gbit s−1. A directly modulated distributed feedback quantum cascade laser (DFB-QCL) with high modulation efficiency and improved RF-design was used as a transmitter while two high speed detectors utilizing meta-materials to enhance their responsivity are employed as receivers; a quantum cascade detector (QCD) and a quantum-well infrared photodetector (QWIP). We investigate system tradeoffs and constraints, and indicate pathways forward for this technology beyond 100 Gbit s−1 communication.

Square & H metasurfaces for SPR Increasing in long Wave-IR absorber

In the long-wave infrared (LWIR) spectrum, this paper suggests an electromagnetic (EM) waveband absorber design based on metamaterials. Germanium, gold, and magnesium oxide layers are arranged in a layered structure from top to bottom in the suggested model. Our proposed metamaterial structure’s upper surface is composed of metallic metasurfaces with H and square forms from various studies. The finite element method is used to evaluate the metamaterials’ electromagnetic properties in terms of absorbance and reflectance. It is observed that there is a particular size of the metamaterial at which extremely localized electromagnetic resonance occurs. Quantitative findings indicate that the suggested metamaterial design’s average absorption reaches 90 % in the 10 μm to 14 μm range across a wide variety of incidence angles (00 to 400 & 00 to 800) for both transverse electric (TE) and transverse magnetic (TM) polarization. It is evident from these data that the suggested model configuration has broad potential applications in manyoptoelectronic fields of study.

Enabling the Assimilation of CrIS Shortwave Infrared Observations in Global NWP at NOAA. Part II: OSEs and Results

Abstract The assimilation of data from hyperspectral infrared sounders in global data assimilation systems has historically been focused on observations in the longwave infrared (LWIR) region of the spectrum (650-1100 cm−1), despite the often concurrent availability of measurements from the shortwave infrared (SWIR) region of the spectrum (2150-2550 cm−1), because issues (like solar effects) have generally prevented the assimilation of SWIR observations. Recent advances in radiative transfer models have worked to address some of the previous challenges in simulating SWIR observations, and the assimilation of SWIR data (e.g. from potential future small-satellites) is now a feasible prospect. Still, a better understanding of how these observations perform in a data assimilation system and impact resulting analyses and NWP forecasts is necessary. In this study, the value of SWIR observations in global NWP is assessed by assimilating SWIR observations from the Cross-track Infrared Sounder (CrIS) in NOAA’s Global Data Assimilation System (GDAS). The methodologies used to enable the assimilation of these observations, including the implementation of a scene-dependent observation error and the enhancement of quality control procedures, are discussed, as are the results of Observing System Experiments (OSEs) conducted to evaluate the impact of assimilating SWIR observations on forecast skill. The overall results show that SWIR assimilation produces similar forecast impacts to LWIR assimilation. The ability to demonstrate that the assimilation or SWIR observations in NWP is a realistic prospect may help to shape future constellations of small-satellites to serve as a beneficial complement to the current constellation if hyperspectral IR sounders.

Optical Characterization of the Interband Cascade LWIR Detectors with Type-II InAs/InAsSb Superlattice Absorber.

The long-wave infrared (LWIR) interband cascade detector with type-II superlattices (T2SLs) and a gallium-free ("Ga-free") InAs/InAsSb (x = 0.39) absorber was characterized by photoluminescence (PL) and spectral response (SR) methods. Heterostructures were grown by molecular beam epitaxy (MBE) on a GaAs substrate (001) orientation. The crystallographic quality was confirmed by high-resolution X-Ray diffraction (HRXRD). Two independent methods, combined with theoretical calculations, were able to determine the transitions between the superlattice minibands. Moreover, transitions from the trap states were determined. Studies of the PL intensity as a function of the excitation laser power allowed the identification of optical transitions. The determined effective energy gap (Eg) of the tested absorber layer was 116 meV at 300 K. The transition from the first light hole miniband to the first electron miniband was red-shifted by 76 meV. The detected defects' energy states were constant versus temperature. Their values were 85 meV and 112 meV, respectively. Moreover, two additional transitions from acceptor levels in cryogenic temperature were determined by being shifted from blue to Eg by 6 meV and 16 meV, respectively.

MEMS-based meta-emitter with actively tunable radiation power characteristic

We propose a meta-emitter based on micro-electro-mechanical system (MEMS) technology. The main structure of the meta-emitter unit cell is composed of four symmetrically split crosses of Au and SiO2 bilayer cantilevers. By changing the size of the cantilevers, this MEMS-based meta-emitter can realize the tunable perfect absorption, and the absorption spectrum is within the longwave infrared (LWIR) wavelength from 8.90 to 11.90 µm. When the surface temperature of the meta-emitter rises, the electrothermal actuation mechanism is performed through the different thermal expansion coefficient (TEC) of the bilayer cantilevers. Therefore, the cantilevers will be bent downward and the bending height of the cantilevers decreases linearly. In such case, the peak value of thermal radiation power can be tuned from the wavelength of 9.52 µm to 10.48 µm when the temperature of meta-emitter is increased from 293 to 1290 K. This proposed MEMS-based meta-emitter is an excellent LWIR light source and has potential application prospects in gas sensing, infrared spectroscopy analysis, medical care and so on.

Artificial Amacrine Retinal Circuits.

Event-based imaging represents a new paradigm in visual information processing that addresses the speed and energy efficiency shortcomings inherently present in the current complementary metal oxide semiconductor-based machine vision. Realizing such imaging systems has previously been sought using very large-scale integration technologies that have complex circuitries consisting of many photodiodes, differential amplifiers, capacitors, and resistors. Here, we demonstrate that event-driven sensing can be achieved using a simple one-resistor, one-capacitor (1R1C) circuit, where the capacitor is modified with colloidal quantum dots (CQDs) to have a photoresponse. This sensory circuit emulates the motion-tracking function of the biological retina, in which the amacrine cells in the bipolar-to-ganglion synaptic pathway produce a transient spiking signal only in response to changes in light intensity but remain inactive under constant illumination. When extended to a 2D imaging array, the individual sensors work independently and output signals only when a change in the light intensity is detected; hence, the concept of the frame in image processing is thereby removed. In this work, we present the fabrication and characterization of a CQD photocapacitor-based 1R1C circuit that has a spectral response at 1550 nm in the short-wave infrared (SWIR). We report on the key performance parameters including peak responsivity, noise, and optical noise equivalent power and discuss the operating mechanism that is responsible for spiking responses in these artificial retinal circuits. The present work sets the foundation for expanding the bioinspired vision sensor capability toward midwave infrared (MWIR) and long-wave infrared (LWIR) spectral regions that are invisible to human eyes and mainstream semiconductor technologies.

A Long Wave-Infrared Miniatured Quantum Dot Spectrometer.

In this work, a long-wave infrared (LWIR) quantum dot (QD) spectrometer was constructed for the first time by integrating a 255-element HgSe QD filter array with an LWIR array detector. The filter array was fabricated using a combination inkjet printing strategy with eight types of T-dodecyl mercaptan-terminated HgSe QD inks. The stability and morphology of the QDs were improved by optimizing the purification methodology and ligand modification. Combined with the compressive-sensing-based least-squares linear regression (CS-LS) algorithm, the LWIR QD spectrometer achieved a spectral resolution of 5.4 cm-1 over a wide spectral range of 8 to 14 μm, enabling the detection of the chemical warfare agent simulant dimethyl methylphosphonate. This technology is expected to facilitate the development of smaller volumes and more accurate identification of various targets in the future. This paper offers an approach to fabricating low-cost LWIR spectrometers and promoting the scale-up applications of QD devices.

Plasmonic Al, Ga and in-doped zinc oxide nanoparticles as key components for the design of electrophoretic inks with MWIR and LWIR absorbance properties

Doped zinc oxide nanocrystals (NCs) are halfway through semiconductors and metals. They exhibit unique optoelectronic properties from a high surface density of free-charge-carriers, which are responsible for localized surface plasmon resonance (LSPR). Here, a one-pot approach is presented to synthesize doped aluminum, gallium or indium zinc oxide NCs, making them all stable in non-polar media. The effect of doping on the growth mechanisms and the final crystalline structure were studied as a function of the aliovalent doping atom used. Doping atoms were integrated by substitution of Zn atom into the crystalline mesh of ZnO with a wurtzite phase identified as the primary crystalline phase for all samples by X-ray Diffraction (XRD). Typical aluminiun doped ZnO NCs (AZO) or Gallium doped NCs (GZO) nanoflowers were identified as a single crystalline structure by High-Resolution-Transmission Electron Microscopy (HR-TEM). Indium doped ZnO NCs (IZO) and pristine ZnO appeared significantly different with spherical and heart-like shapes, respectively. The doping level tendency was identified through Energy Dispersive X-ray (EDX) and increased form Al to Ga and In doping atoms. All plasmonic doped NCs have broadband infrared absorption in the Middle-wave (MWIR) and Long-wave infrared (LWIR) wavelengths, making them interesting for thermal regulation or thermal camouflage. As part of the latter application, dispersions of doped ZnO NCs were formulated as electrophoretic inks and their IR-absorbing performances determined by using a homemade setup including an infrared camera. AZO, GZO, and IZO NCs based inks achieved temperature contrasts of 15 °C and 6 °C in the MWIR and LWIR wavelengths.

Advancing LWIR FSO communication through high-speed multilevel signals and directly modulated quantum cascade lasers

This study investigates the potential of long-wave infrared (LWIR) free-space optical (FSO) transmission using multilevel signals to achieve high spectral efficiency. The FSO transmission system includes a directly modulated-quantum cascade laser (DM-QCL) operating at 9.1 µm and a mercury cadmium telluride (MCT) detector. The laser operated at the temperature settings of 15°C and 20°C. The experiment was conducted over a distance of 1 m and in a lab as a controlled environment. We conduct small-signal characterization of the system, including the DM-QCL chip and MCT detector, evaluating the end-to-end response of both components and all associated electrical elements. For large-signal characterization, we employ a range of modulation formats, including non-return-to-zero on-off keying (NRZ-OOK), 4-level pulse amplitude modulation (PAM4), and 6-level PAM (PAM6), with the objective of optimizing both the bit rate and spectral efficiency of the FSO transmission by applying pre- and post-processing equalization. At 15°C, the studied LWIR FSO system achieves net bitrates of 15 Gbps with an NRZ-OOK signal and 16.9 Gbps with PAM4, both below the 6.25% overhead hard decision-forward error correction (6.25%-OH HD-FEC) limit, and 10 Gbps NRZ-OOK below the 2.7% overhead Reed-Solomon RS(528,514) pre-FEC (KR-FEC limit). At 20°C, we obtained net bitrates of 14.1 Gbps with NRZ-OOK, 16.9 Gbps with PAM4, and 16.4 Gbps with PAM6. Furthermore, we evaluate the BER performance as a function of the decision feedback equalization (DFE) tap number to explore the role of equalization in enhancing signal fidelity and reducing errors in FSO transmission. Our findings accentuate the competitive potential of DM-QCL and MCT detector-based FSO transceivers with digital equalization for the next generation of FSO communication systems.

Photothermal synergistic modulation of patterned VO2-Based composite films for smart windows

Vanadium dioxide (VO2) emerges as promising material for smart windows due to its ability to dynamically modulate the characteristics of near-infrared light during phase transition, accompanied by a relatively high luminous transmittance (Tlum). However, traditional VO2-based smart windows have predominantly emphasized achieving high solar modulation ability (ΔTsol), often overlooking the modulation of emissivity (ε) of long-wave infrared (LWIR). This oversight makes it challenging to achieve optimal energy-saving effects. To address this, patterned smart window composite films are designed and optimized by using Fabry-Pérot cavity to achieve dual-band coordinated regulation of sunlight and thermal radiation, which can greatly improve the energy-saving efficiency. The obtained composite film has a great low-temperature luminous transmittance (Tlum-L=50.4 %) and a high solar modulation ability (ΔTsol = 13.9 %). Meanwhile, the composite film can adaptively adjust its emissivity, with emissivity values of 0.67 and 0.18 at high and low temperatures, respectively, endowing it with excellent thermal radiation regulation capabilities. The development of patterned composite films is of great significance in the field of thermal control smart windows, which has great potential in improving the energy efficiency of buildings and enhancing resident comfort.

Inverse-design laser-infrared compatible stealth with thermal management enabled by wavelength-selective thermal emitter

With the fast development of multi-band detection technology, the demand for multi-band stealth technology with thermal management in military and civilian fields is increasing. Although the metasurfaces can achieve multi-band stealth and thermal management, their fabrication is complex and costly. In this paper, a simple multilayer structure is used to realize laser-infrared compatible stealth with thermal management. More importantly, the traditional method of optimizing metasurface structures by manually scanning parameters consumes a lot of computational time and resources. In order to speed up design of nanostructures, we propose a wavelength-selective thermal emitter (WSTE) composed of ZnS/Ge3Sb2Te6 (GST) multilayer films and a Ni reflector using inverse design method. The simulation results show that, in the crystalline phase, the WSTE has low spectral emissivity (ε3-5μm = 0.12, ε8-14μm = 0.22) in the mid-wave infrared (MWIR) and long-wave infrared (LWIR), high spectral emissivity (ε5-8μm = 0.72) in the non-atmospheric window and high absorptivity (A1.06μm = 0.90, A1.55μm = 0.97, A10.6μm = 0.91) in the laser wavelengths, which indicates that the WSTE can simultaneously achieve MWIR, LWIR and laser stealth with radiative heat dissipation. The method proposed in this paper overcomes the problems of single-band stealth and inefficient. Moreover, by varying the crystallization fraction of the GST, the WSTE can achieve dynamic control of thermal radiation in the wavelength range of 3–14 µm. The simulated infrared images verify that WSTE has good infrared stealth performance. Finally, the effects of incidence angle and polarization angle on the emission spectrum of WSTE are studied, which can demonstrate the infrared stealth is insensitive to both. In conclusion, the WSTE is attractive in advanced photonics applications, such as radiative cooling and infrared stealth.

Wafer Level Vacuum Packaging of MEMS-Based Uncooled Infrared Sensors.

This paper introduces a cost-effective, high-performance approach to achieving wafer level vacuum packaging (WLVP) for MEMS-based uncooled infrared sensors. Reliable and hermetic packages for MEMS devices are achieved using a cap wafer that is formed using two silicon wafers, where one wafer has precise grating/moth-eye structures on both sides of a double-sided polished wafer for improved transmission of over 80% in the long-wave infrared (LWIR) wavelength region without the need for an AR coating, while the other wafer is used to form a cavity. The two wafers are bonded using Au-In transient liquid phase (TLP) bonding at low temperature to form the cap wafer, which is then bondelectrical and Electronics d to the sensor wafer using glass frit bonding at high temperature to activate the getter inside the cavity region. The bond quality is assessed using three methods, including He-leak tests, cap deflection, and Pirani vacuum gauges. Hermeticity is confirmed through He-leak tests according to MIL-STD 883, yielding values as low as 0.1 × 10-9 atm·cc/s. The average shear strength is measured as 23.38 MPa. The package pressure varies from 133-533 Pa without the getter usage to as low as 0.13 Pa with the getter usage.

Photoelectric property degradation of graded barrier InAs/GaSb type II superlattice long-wave infrared detectors under 1 MeV electron irradiation.

Amid rapid advancements in aerospace science and technology, studying the effects of space radiation on an infrared detector is crucial for enhancing their reliability in radiation environments, particularly against electrons-one of the most damaging charged particles. Barrier structures significantly reduce dark current without any substantial degradation in the optical performance of the devices. Consequently, they are being investigated for use in extreme environments. This paper presents a study on the performance degradation of InAs/GaSb type II superlattice (T2SLs) long-wave infrared (LWIR) detectors with a graded barrier structure under 1 MeV electron irradiation and analyzes potential damage mechanisms. The findings indicate that 1 MeV electron irradiation causes both ionization and displacement damage to the graded barrier InAs/GaSb T2SL LWIR detectors. After irradiation with a fluence of 2 × 1015 e/cm2, the device's dark current density has increased by approximately two orders of magnitude, while the quantum efficiency has decreased by approximately one order of magnitude. As the device mesa shrinks, the sensitivity of dark current to radiation exposure increases. Electron irradiation notably exacerbates surface leakage and bulk dark current, with a pronounced increase in surface leakage current. The study also reveals that electron irradiation primarily enhances the dark current by introducing defect states, thereby leading to device performance degradation.