Multispectral camouflage for infrared, visible, lasers and microwave with radiative cooling

The study addresses the challenge of achieving camouflage across multiple spectral bands—visible, mid-infrared (MWIR 3–5 µm and LWIR 8–14 µm), laser (1.55 and 10.6 µm), and microwave (8–12 GHz)—while maintaining effective thermal management. Traditional camouflage is generally single-band and cannot counter modern multispectral detection systems. For IR camouflage, low emittance is required within the atmospheric windows to suppress thermal signatures, yet high emittance in the 5–8 µm non-atmospheric window is desirable to radiatively dissipate heat and reduce surface temperature according to the Stefan–Boltzmann law. Conflicts arise because broadband low emittance blocks radiative heat dissipation, causing heat buildup, especially severe when absorbing energy from active detection (microwave and lasers). The research proposes a wavelength-selective emitter that simultaneously offers low emittance in IR atmospheric windows for camouflage, high emittance in the 5–8 µm band for radiative cooling, visible color tuning for background matching, and high absorbance at laser and microwave bands. The goal is to realize a single, compatible structure for comprehensive multispectral camouflage and efficient thermal management.

Conventional camouflage typically targets a specific spectral region: visible, MIR, or microwave. Recent multispectral detection platforms (e.g., combined visible–MWIR, SAR–IR fusion) necessitate multispectral camouflage. Prior work includes one-dimensional photonic crystals and metasurfaces for IR-radar compatibility and selective emitters for improved thermal management. Broadband low-emittance approaches reduce IR signatures but inhibit radiative cooling, leading to heat instability. Selective emitters with high emittance in the 5–8 µm non-atmospheric window have been proposed to reconcile thermal dissipation with IR camouflage, and some demonstrations combined IR compatibility with visible, microwave, or laser bands separately. However, no single material system previously met all requirements simultaneously across visible, MIR (both windows), lasers (1.55 and 10.6 µm), and microwave X-band while providing radiative cooling. This work fills that gap with a combined ZnS/Ge multilayer selective emitter and Cu-ITO-Cu microwave metasurface.

Design and structure:

• The combined device uses spectral separation and layer-wise functionality: a Ge/ZnS multilayer wavelength-selective emitter (SE) on silica for visible/MIR/laser camouflage and radiative cooling, and a Cu-ITO-Cu microwave absorptive metasurface for X-band camouflage.

Visible tailoring:

• The visible reflectance and perceived color are set by the thickness of the top ZnS layer (t_ZnS), since the underlying ~0.72 µm Ge film is opaque in the visible. Varying t_ZnS from 30–270 nm produces anti-reflection peaks at different wavelengths for background matching (e.g., ~30/140 nm for soil/desert; ~180 nm for clear water; ~220 nm for vegetation).

IR and laser design:

• The Ge/ZnS multilayers are optimized via a genetic algorithm to achieve: low emittance in MWIR/LWIR windows, high emittance in 5–8 µm for radiative cooling, and high absorbance at 1.55 µm and 10.6 µm (laser wavelengths). The SE forms a one-dimensional photonic crystal with bandgaps in MWIR and LWIR, suppressing emission. Outside these forbidden bands (notably 5–8 µm), fields penetrate and are absorbed in the lossy silica substrate, enabling radiative cooling.
• A narrowband dip in reflectance (high absorbance) at 10.6 µm is engineered by breaking the LWIR photonic bandgap locally using the 5th–7th Ge/ZnS layers, concentrating the electric field at 10.6 µm. At 1.55 µm, absorption is dominated by intrinsic loss in Ge (k≈0.0056 at 1.55 µm).
• Example multilayer thicknesses (top to bottom, excluding the very top color-tuning ZnS): 0.721/0.982/0.721/0.559/0.234/0.438/0.206/0.438/0.552/1.18/0.701 µm (Ge/ZnS sequence as designed).

Microwave metasurface (X-band 8–12 GHz):

• Frequency selective surface (FSS): periodic Cu square patches (thickness f_cu=0.18 µm, period P_cu=3.67 mm, side length a_cu=3.2 mm) on FR4 (t_FR4=1 mm) backed by a Cu ground plane.
• Lossy resonator: an ITO layer on 175 µm PET (t_ITO=175 nm, sheet resistance ~35 Ω/sq, square side a_ITO=8.1 mm). The magnetic field is enhanced between the FSS and ground, increasing ohmic loss in ITO.
• The combined structure maintains high X-band absorption when integrated with the SE stack.

Experimental setups:

• IR camouflage and radiative cooling test: Samples (SE vs. Cr film on identical silica substrate) are heated from below by a resistive heater placed on a copper spreader; silica aerogel underneath provides thermal insulation. Heater temperature (Th) and sample surface temperature (Ts) are measured while sweeping input power (1.25–25 W; corresponds to ~125–2500 W m−2 over 0.01 m² sample area). Radiative (apparent) temperatures are recorded using MWIR (3–5 µm) and LWIR (8–14 µm) thermal cameras under indoor conditions with ambient radiation.
• Convection–radiation coupling study: Identical heating experiments are performed under normal pressure (with natural convection, RH ~60%) and high vacuum (~1 Pa; convection suppressed). To equalize external radiative boundary conditions, a carbon surface is maintained near ambient temperature via forced air cooling outside the vacuum region. Power differences between SE and Cr at the same steady-state temperatures are compared across conditions.

Fabrication and characterization:

• Selective emitter: ZnS/Ge multilayers deposited by e-beam evaporation on silica; deposition rates 1.5 nm s−1 (ZnS) and 0.5 nm s−1 (Ge).
• Spectral measurements: Visible/NIR reflectance via spectrophotometer (Agilent Cary7000); MIR reflectance via FTIR microscope/spectrometer (Bruker Hyperion 1000/Vertex 70, MCT detector); microwave reflectance via vector network analyzer (Keysight 8722C) on 200×200 mm samples.
• Temperature measurements: MWIR camera (Telops FAST M200, 3–5 µm), LWIR camera (Jenoptik Blackbird precision IR-OEM, 8–14 µm), thermocouples (Omega 5TC-TT-K-30-36) at ambient pressure, and Pt100 RTDs (TZ-CMEI) in vacuum (thermometer THTZ408R).

Simulations:

• Heat transfer and surface-to-surface radiation modeled in COMSOL Multiphysics; microwave metasurface and fields simulated with RF module (frequency-domain). Material permittivities provided in Supplementary Note 2.

• Spectral performance: Measured band emittance at 100 °C is 0.11 (MWIR) and 0.12 (LWIR), while the non-atmospheric 5–8 µm window emittance is 0.61, enabling radiative cooling without compromising MIR camouflage. Visible colors are tunable by the top ZnS thickness for background matching. The structure exhibits high absorbance at laser wavelengths 1.55 µm and 10.6 µm (narrowband at 10.6 µm to avoid degrading LWIR camouflage). X-band microwave absorption exceeds 90% (return loss < −10 dB) for the metasurface alone and remains >85% (return loss < −8.25 dB) for the combined stack.
• IR camouflage and thermal management: Under 25 W input (≈2500 W m−2), the selective emitter (SE) lowers the heater/surface temperatures by 8.4 °C / 5.9 °C versus Cr. At Ts ≈ 124.9 °C (SE) vs. 130.8 °C (Cr), radiative (apparent) temperatures are substantially lower for SE: MWIR Tr1≈69.1 °C (SE) vs. 98.1 °C (Cr); LWIR Tr2≈46.8 °C (SE) vs. 57.4 °C (Cr). Band-integrated radiation intensities show a 53.4% (3.32 dB) reduction in MWIR (22.6 vs. 48.6 W m−2) and a 13.0% (0.606 dB) reduction in LWIR (229 vs. 264 W m−2) compared to Cr. The SE tolerates at least 300 °C.
• Radiation-enhanced natural convection: Comparing normal pressure (with convection) and vacuum (≈1 Pa; no convection) at the same surface temperature demonstrates coupling between radiation (5–8 µm) and natural convection. At Ts=150 °C, the power difference between SE and Cr is 2.52 W under normal pressure vs. 1.36 W in vacuum, corresponding to an increase in cooling power density from 136 to 252 W m−2 over a 0.01 m² sample, consistent with simulations. This reveals that radiation in the non-atmospheric window can enhance natural convection, boosting total cooling power when forced convection and radiative emission in atmospheric windows are constrained.

The work demonstrates a single, layer-engineered structure that meets multispectral camouflage requirements while improving thermal management. By exploiting wavelength selectivity through a ZnS/Ge photonic crystal, the device maintains low emittance within MWIR/LWIR atmospheric windows (for IR concealment) and high emittance in the 5–8 µm non-atmospheric window (for radiative heat rejection), alongside visible color tuning and narrowband laser absorption at 1.55 µm and 10.6 µm. Integration with a Cu-ITO-Cu metasurface provides strong X-band microwave absorption without degrading optical/IR performance. Experiments validate significant reductions in surface and radiative temperatures relative to a broadband low-emittance Cr control, leading to marked decreases in detected IR signals. Importantly, radiative output in the 5–8 µm band couples to and enhances natural convection, increasing total cooling power beyond radiation alone—an advantage in scenarios where other heat transfer channels are limited. The fabrication relies on standard thin-film deposition and PCB-compatible processes, supporting scalability for large-area applications.

This study introduces a practical multispectral camouflage platform combining a ZnS/Ge wavelength-selective emitter with a Cu-ITO-Cu microwave metasurface to achieve: low MWIR/LWIR emittance for IR stealth, high 5–8 µm emittance for radiative cooling, visible color matching, high laser absorbance at 1.55 and 10.6 µm, and strong X-band microwave absorption. The device reduces surface and radiative temperatures and suppresses IR signatures compared to a conventional Cr surface. It further uncovers that radiation in the non-atmospheric window enhances natural convection, increasing overall cooling power. The approach is fabrication-friendly and suitable for large-area deployment, offering potential in military and civil stealth technologies, thermal management, and energy-efficient systems. Future work could explore durability at higher temperatures, environmental robustness, dynamic tunability, and extension to additional radar and optical bands.