Mid-wave Infrared Research Articles

Visible color camouflage and infrared, laser band stealth, compatible with dual-band radiative heat dissipation

Infrared stealth technology garners significant attention due to its value in military applications. With the rise of multi-band detection technologies, the infrared stealth target is easily detected. Therefore, it is urgent to study multi-band camouflage devices. Here, this paper proposes a multi-band camouflage device composed of ZnS, Ge, TiO2, crystalline Ge2Sb2Te5 (cGST) and Ag multilayer films, which can realize multi-band camouflage across visible, mid-wave infrared (MWIR, ε3∼5 μm = 0.22), long-wave infrared (LWIR, ε8∼14 μm = 0.16), and laser (R1.06 μm = 0.01) spectra. In addition, effective radiative heat dissipation was realized in two non-atmospheric transparent windows (ε2.5–3 μm = 0.51 and ε5∼8 μm = 0.65). Among them, visible camouflage in different background environments can be realized independently by adjusting the thickness of the ZnS film to produce different structural colors. Notably, this emissivity spectrum remains robust even as the incident angle increases to 60°. These results can guarantee the realization of multi-band infrared camouflage, laser camouflage and radiation heat dissipation. This study not only addresses high-efficiency infrared camouflage and thermal management but also achieves outstanding visible and laser camouflage performance, offering a comprehensive guidance for advancing multi-band camouflage technology.

Micropyramidal Si Photonics–A Versatile Platform for Detector and Emitter Applications

AbstractAnisotropic wet etching of silicon (Si) has been known for decades, but the extraordinary capabilities of this technology to provide highly ordered microphotonic arrays have received limited attention in previous studies. It is shown that these capabilities include: i) a unique self‐terminating etching nature − crucial for achieving uniform arrays over centimeter‐scale wafers, ii) an extraordinary smoothness of the slow‐etching (111) planes, and iii) a precise geometry control including the extent micropyramids or micropyramidal voids are truncated. The combination of these properties transforms this technology into a versatile platform for novel detector and emitter applications in Si photonics. The optical properties of such arrays are studied by finite‐difference time‐domain modeling in two realms represented by different boundary conditions (BCs). Periodic BCs result in Talbot self‐images experimentally observed in this work. Perfectly matched layer BCs describe mesoscale interference effects resulting in the subwavelength focusing properties of micropyramids. Enhancements in the performance of mid‐wave infrared (MWIR) focal plane arrays are demonstrated by monolithic integration of Si micropyramids with silicon‐platinum silicide Schottky barrier photodetectors. Finally, it is shown that Si micropyramidal arrays can be used to enhance light extraction and directionality of quantum sources and infrared scene projectors.

Research on Through-Flame Imaging Using Mid-Wave Infrared Camera Based on Flame Filter.

High-temperature furnaces and coal-fired boilers are widely employed in the petrochemical and metal-smelting sectors. Over time, the deterioration, corrosion, and wear of pipelines can lead to equipment malfunctions and safety incidents. Nevertheless, effective real-time monitoring of equipment conditions remains insufficient, primarily due to the interference caused by flames generated from fuel combustion. To address this issue, in this study, a through-flame infrared imager is developed based on the mid-wave infrared (MWIR) radiation characteristics of the flame. The imager incorporates a narrowband filter that operates within the wavelength range of 3.80 μm to 4.05 μm, which is integrated into conventional thermal imagers to perform flame filtering. This configuration enables the radiation from the background to pass through the flame and reach the detector, thereby allowing the infrared imager to visualize objects obscured by the flame and measure their temperatures directly. Our experimental findings indicate that the imager is capable of through-flame imaging; specifically, when the temperature of the target exceeds 50 °C, the imager can effectively penetrate the outer flame of an alcohol lamp and distinctly capture the target's outline. Importantly, as the temperature of the target increases, the clarity of the target's contour in the images improves. The MWIR through-flame imager presents considerable potential for the real-time monitoring and preventive maintenance of high-temperature furnaces and similar equipment, such as detecting the degradation of refractory materials and damage to pipelines.

Inverse design of metamaterial for dual-band infrared camouflage along with dual-band thermal management

The optical metamaterial for infrared camouflage with thermal management is an artificial structure available to not only evade the detection of various thermal imagers operating in distinct infrared wavebands but also ensure the thermal stability of the target. However, designing such an optical metamaterial is challenging due to the necessity to optimize numerous structural parameters simultaneously, especially metamaterials that are compatible with multiple functions. Herein, this paper proposes a novel inverse design method, combining African vultures optimization algorithm (AVOA) with rigorous coupled-wave analysis (RCWA), to design an optical metamaterial capable of achieving infrared camouflage and thermal management compatibly. The proposed metamaterial enables dual-band infrared camouflage for mid-wave infrared (MWIR, 3–5 μm) and long-wave infrared (LWIR, 8–14 μm), along with thermal management through dual-band non-atmospheric windows of 5–8 μm and 14–17 μm. Comparing to other intelligent optimization algorithms, the AVOA is exceptionally efficient, which only takes less time than others to get better results. This work not only provides theoretical guidance to design an optical metamaterial for infrared camouflage with thermal management, but also has the potential to solve the multi-objective optimization problems of multi-band or wide-band radiative modulation.

Rectangular bismuth oxychloride with high mid-wave infrared reflectance to strengthen radiative cooling performance

Abstract Radiative cooling materials (RCMs) based on polymers, that reflect solar irradiance (0.24–2.5 µm) strongly and emit radiation within atmospheric window (8–13 µm) intensively, have yielded obviously cooling performance. However, RCMs reported failed to realize high mid-wave infrared (2.5–8 µm) reflectance (i.e., spectral selectivity), which play an importance role in reject the radiation from ambient especially in high relative humidity. Thus, enhancing the reflectance of RCMs in mid-wave infrared can further strengthen the cooling effect in the environment of high relative humidity. Additive of fillers with high reflectance in mid-wave infrared is effective, low cost and convenient to modified the cooling performance polymeric RCMs. In this paper, micron rectangular bismuth oxychloride (BiOCl) powders were proposed and synthesized by hydrothermal method. And, rectangular BiOCl boasts excellent mid-wave infrared reflective capacity (85%). Then, rectangular BiOCl was added into the top-layer of flexible hierarchically porous complex membrane, and improved the reflectance of porous membrane within 2.5–8 µm from 48.3% to 58.1%. While the emittance of the obtained bilayer membrane within atmospheric window was hardly affected. The membrane with BiOCl exhibited better cooling performance in humid ambient (70%), demonstrating the importance of spectral selectivity experimentally. This work proposed an effective strategy to modify spectral selectivity of RCMs and it could expand the application of radiative cooling materials in hot and humid areas.

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.

Van der Waals mid-wavelength infrared detector linear array for room temperature passive imaging.

Passive imaging for mid-wave infrared (MWIR) is resistant to atmospheric pollutants, guaranteeing image clarity and accuracy. Arrayed photodetectors can simultaneously perform radiation sensing to improve efficiency. Room temperature van der Waals (vdWs) photodetectors without lattice matching have evolved rapidly with optimized stacking methods, primarily for single-pixel devices. The urgent need to implement arrayed devices aligns with practical demands. Here, we present an 8 by 1 black phosphorus/molybdenum sulfide (BP/MoS2) vdWs photodetector linear array with a fill-factor of ~77%, fabricated using a temperature-assisted sloping transfer method. The flat interface and uniform thickness facilitate carrier transport and minimize pixel nonuniformities, showing an average peak detectivity (D*) of 2.34 × 109 cm·Hz1/2·W-1 in the mid-wave infrared region. Compared to a single pixel, push-broom scanning passive imaging is eight times more efficient and further enhanced through mean filtering and fast Fourier transform filtering for strip noise correction. Our study offers guidance on vdWs arrayed devices for engineering applications.

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.

Dynamic selective emitter for low infrared signal over wide temperature range based on VO2

The infrared signal intensity of objects contains the thermal radiation from objects themselves and the reflected signal from their surface. Here, we theoretically propose the rule of regulating the infrared signal intensity in mid-wave infrared band, which is influenced by both the reflected solar irradiation and the self-emitted radiation. Then we propose a dynamic selective emitter (DSE) operating in the mid-infrared (MIR) band based on vanadium dioxide (VO2) for lowest infrared signal intensity within the low and high temperature ranges. In the dielectric state of VO2, the selective emitter exhibits the broadband selective absorption attributed to the excitation of magnetic polariton resonance, which can effectively reduce the solar irradiation reflection in 3–4.5 μm band (R‾3–4.5 μm = 0.18) and suppress the thermal radiation in 4.5–5 μm and 8–14 μm band (ε‾4.5–5 μm = 0.28, ε‾8–14 μm = 0.02). In the metallic state of VO2, the resonance absorption disappears and the structure exhibits broadband low emissivity in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) bands (ε‾3–5 μm = 0.28, ε‾8–14 μm = 0.21) for suppression of the thermal radiation. This work highlights the impact of solar irradiation on MWIR camouflage and offers new possibilities for solar and infrared spectral regulation.

High-Sensitive Uncooled Mid-Wave Infrared Detector Based on TiS3 Nanoribbon.

High-sensitive uncooled mid-wave infrared (MWIR) photodetection with fast speed is highly desired for biomedical imaging, optical communication, and night vision technology. Low-dimensional materials with low dark current and broadband photoresponse hold great promise for use in MWIR detection. Here, this study reports a high-performance MWIR photodetector based on a titanium trisulfide (TiS3) nanoribbon. This device demonstrates an ultra-broadband photoresponse ranging from the visible spectrum to the MWIR spectrum (405-4275nm). In the MWIR spectral range, the photodetector achieves competitive high photoresponsivity (R) of 21.1 AW-1, and an impressive specific detectivity (D*) of 5.9× 1010 cmHz1/2W-1 in ambient air. Remarkably, the photoresponse speed in the MWIR with τr = 1.3ms and τd = 1.5ms is realized which is much faster than the thermal time constant of 15ms. These findings pave the way for highly sensitive, room-temperature MWIR photodetectors with exceptionally fast response speed.

MWIRGas-YOLO: Gas Leakage Detection Based on Mid-Wave Infrared Imaging.

The integration of visual algorithms with infrared imaging technology has become an effective tool for industrial gas leak detection. However, existing research has mostly focused on simple scenarios where a gas plume is clearly visible, with limited studies on detecting gas in complex scenes where target contours are blurred and contrast is low. This paper uses a cooled mid-wave infrared (MWIR) system to provide high sensitivity and fast response imaging and proposes the MWIRGas-YOLO network for detecting gas leaks in mid-wave infrared imaging. This network effectively detects low-contrast gas leakage and segments the gas plume within the scene. In MWIRGas-YOLO, it utilizes the global attention mechanism (GAM) to fully focus on gas plume targets during feature fusion, adds a small target detection layer to enhance information on small-sized targets, and employs transfer learning of similar features from visible light smoke to provide the model with prior knowledge of infrared gas features. Using a cooled mid-wave infrared imager to collect gas leak images, the experimental results show that the proposed algorithm significantly improves the performance over the original model. The segment mean average precision reached 96.1% (mAP50) and 47.6% (mAP50:95), respectively, outperforming the other mainstream algorithms. This can provide an effective reference for research on infrared imaging for gas leak detection.

Image-driven evaluation metric for a space-based infrared diurnal detection analysis for flying aircrafts

The traditional evaluation calculation method fails to adequately consider the impact of image degradation and ignores the uncertainty caused by instrument noise on point target detectable status. This overestimates the detection capability of the system and cannot meet the needs of the point target detection evaluation under space-based observations. An image-driven evaluation metric (IDEM) is proposed in this paper, considering the effects of image degradation on the target, the background, and its clutter signals while providing coefficients of variation (CVs) for evaluation metrics. Image sequences of aircraft in different bands of the infrared imaging system were generated for cross validation. A comparative analysis against the traditional method shows that instrument noise significantly interferes with the point target signal in the image, and our method provides a more accurate and comprehensive evaluation of aircraft detectability under space-based infrared observations. Based on this, the IDEM maps under day and night in the mid- and long-infrared were computed and analyzed under space-based infrared observations of the flying aircraft. The results indicate that the mid-wave infrared (MWIR) is more sensitive to diurnal variation, whereas the long-wave infrared (LWIR) is more stable but has a lower daytime detectability compared to mid-infrared. Theoretically, both mid- and long-infrared enable night detection capabilities for point targets under the proper conditions, with mid-infrared offering higher detection potential. Our works provide new insight and approaches for the point target detection evaluation, system design, and on-orbit testing.

Design of mid-wave infrared achromatic double-layer metalens with enhanced efficiency

Achromatic metalenses in the mid-wave infrared (3–5 μm), known for their light weight, CMOS compatibility, and ultra-compactness, offer significant potential in astronomy, security inspections, and health security. However, previous endeavors have been hindered by underdeveloped material technology and relatively low efficiency. To address these challenges, this study introduces an enhanced-efficiency mid-wave infrared achromatic double-layer metalens, featuring a top-layer ZnS nanopillar array and a bottom-layer Si nanopillar array on a Si substrate. Utilizing this approach, we numerically demonstrate both polarization-insensitive and polarization-controlled varifocal broadband achromatic metalenses. For the polarization-insensitive metalens, the double-layer design provides achromatic focusing comparable to the all-Si counterpart, with a focal length of 133 µm, a focal length shift within ± 5.5%, and Strehl ratios above 0.8. However, the average focal efficiency improves from 40.8% (all-Si) to 50.2% (double-layer). Additionally, both all-Si and double-layer polarization-controlled varifocal achromatic metalenses show similar focusing abilities, with focal lengths of about 137 and 173 µm under X and Y linearly polarized light, respectively. Yet, the double-layer varifocal metalens achieves focal efficiencies of 46.4% and 52.7%, an improvement of 13.1% and 17.6% under X and Y linearly polarized light, respectively.

Multicolor Inks of Black Phosphorus for Midwave-Infrared Optoelectronics.

Black phosphorus (bP) based ink with a bulk bandgap of 0.33eV (λ = 3.7µm) has recently been shown to be promising for large-area, high performance mid-wave infrared (MWIR) optoelectronics. However, the development of multicolor bP inks expanding across the MWIR wavelength range has been challenging. Here a multicolor ink process based on bP with spectral emission tuned from 0.28eV (λ = 4.4µm) to 0.8eV (λ = 1.5µm) is demonstrated. Specifically, through the reduction of bP particle size distribution (i.e., lateral dimension and thickness), the optical bandgap systematically blueshifts, reaching up to 0.8eV. Conversely, alloying bP with arsenic (bP1- xAsx) induces a redshift in the bandgap to 0.28eV. The ink processed films are passivated with an infrared-transparent epoxy for stable infrared emission in ambient air. Utilizing these multicolor bP-based inks as an infrared light source, a gas sensing system is demonstrated that selectively detects gases, such as CO2 and CH4 whose absorption band varies around 4.3 and 3.3µm, respectively. The presented ink formulation sets the stage for the advancement of multiplex MWIR optoelectronics, including spectrometers and spectral imaging using a low-cost material processing platform.

Triple band diamond-shaped polarization insensitive plasmonic nano emitter for thermal camouflage and radiative cooling

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.

Exploring the impact of high-power infrared lasers on electro-optical systems performance: A field study with different wavelengths

The performance of different electro-optical (EO) and infrared (IR) imaging systems under the impact of high-power pulsed infrared lasers has been well evaluated and studied. The infrared lasers operating at 1.06 µm, 3.7–4.2 µm, and 9.6 μm wavelengths, including short-wave infrared (SWIR), mid-wave infrared (MWIR), and long-wave infrared (LWIR) sensors used in television, infrared imaging, and tracking systems. Our research is focused on the intended waveband of the infrared imagers at significant distances relevant to tactical security operations, with a range of 6 km–10 km. The study compares the effects of both dazzling and damaging laser radiation on EO systems using a straightforward theoretical framework supported by atmospheric modeling. It was observed that increased humidity and temperature diminish both dazzling and damage distances, with less impact at 1.06 µm and more significant at 9.6 µm, compared to 3.7–4.2 µm. The effect of atmospheric conditions on IR laser energy was studied using MODTRAN software to determine the effective laser parameters for EO imaging systems at different distances. The study also investigates the critical parameters of practical high-power pulsed infrared laser wavelengths to dazzle and damage imaging systems at different distances. Field investigations revealed that dazzling on TV seekers can extend beyond 10 km by 1.06 µm, with damages up to 8 km, depending on the sensor field of view, laser source pulse frequency, and environmental conditions. Lasers with wavelengths of 3.7–4.2 μm can produce high-dazzling spots at the center of images of MWIR seekers up to 6 km. Lasers with wavelengths of 9.6 μm can rapidly damage LWIR seekers’ pixels due to their varied energy outputs and pulse frequency-range responses up to 8 km. System functions were lost under the dazzling and damaging effects. The study revealed a strong correlation between experimental results and predictions, with a strong agreement between experimental and theoretical effective distances under similar environmental and atmospheric conditions.

Hybrid Signal Processing for Automatic Detection and Removal of Material Absorption and Noise From Microsphere-Lens-Enhanced Midwave Infrared Type II Strained Layer Superlattice Photodetectors

Hybrid Signal Processing for Automatic Detection and Removal of Material Absorption and Noise From Microsphere-Lens-Enhanced Midwave Infrared Type II Strained Layer Superlattice Photodetectors

All-dielectric grating-assisted absorption enhancement in a subwavelength mercury cadmium telluride layer for infrared photodetectors

Light absorption enhancement in a 1.5 μm thick mercury–cadmium–telluride (Hg0.762Cd0.238Te, MCT) layer at room temperature utilizing 1D dielectric grating at mid-wave infrared (MWIR) wavelengths (3–5 μm) has been theoretically investigated. The optimized dielectric grating facilitates light diffraction and scattering into the MCT-absorbing waveguiding layer resulting in an increased lateral optical path. The light absorption was improved from ∼37.5% to ∼71% (TE) and ∼70% (TM) at normal incidence. With enhanced absorption, the photocarrier generation rate in the thin layer would be comparable to a bulk 5 μm thick MCT layer. A ∼3× reduction in the MCT layer thickness without compromising absorption has the potential for realizing infrared photodetectors with improved sensitivity at conventional operating temperatures and/or elevated operating temperatures.

Optoelectronic Material Characterization Platform using RF Topological Edge States

AbstractEfficient characterization of optoelectronic material properties represents a crucial step in the development of next‐generation infrared (IR) materials and devices. For the first time, a sensitive, contact‐free optoelectronic material characterization platform is demonstrated that exploits the unique properties of radio frequency (RF) topological edge states implemented in a topological microstrip waveguide. The topological device is compatible with standard printed circuit board (PCB) processes with no additional materials, stubs, or resonators. The waveguide and topological properties computationally are designed and then experimentally the results are validated with S‐parameter and near‐field measurements. Finally, as a proof of principle, the room temperature spectral response of a micron‐scale thick mid‐wave infrared (MWIR) mercury cadmium telluride (Hg0.70Cd0.30Te) is measured using the topological microstrip and a five‐fold and 20‐fold increase is demonstrated in response when compared to a plain microstrip and trivial bandgap device, respectively. These results open the door to straightforward, low‐cost characterization of optoelectronic films.

Analysis of aero engine plume potential core infrared signature

Purpose Aero-engine exhaust plume length can be more than the aircraft length, making it easier to detect and track by infrared seeker. Aim of this study is to analyze the effect of free stream Mach number (M∞) on length of potential core of plume. Also, change in infrared (IR) signature of plume and aircraft surface with variation in elevation angle (θ) is examined. Design/methodology/approach Convergent divergent (CD) nozzle is located outside the rear fuselage of the aircraft. A two dimensional axisymmetric computational fluid dynamics (CFD) study was carried out to study effect of M∞ on potential core. The CFD data with aircraft and plume was then used for IR signature analysis. The sensor position is changed with respect to aircraft from directly bottom towards frontal section of aircraft. The IR signature is studied in mid wave IR (MWIR) and long wave IR (LWIR) band. Findings The potential plume core length and width increases as M∞ increases. At higher altitudes, the potential core length increases for a fixed M∞. The plume emits radiation in the MWIR band, whereas the aerodynamically heated aircraft surface emits IR in the LWIR band. The IR signature in the MWIR band continuously decreases as the sensor position changes from directly bottom towards frontal. In the LWIR band the IR signature initially decreases as the sensor moves from the directly bottom to the frontal, as the sensor begins to see the wing leading edges and nose cone, the IR signature in the LWIR band slightly increases. Originality/value The novelty of this study comes from the data reported on the effect of free stream Mach number on the potential plume core and variation of the overall IR signature of aircraft with change in elevation angle from directly below towards frontal section of aircraft.