Wide-angle camouflage detectors by manipulating emissivity using a non-reciprocal metasurface array.

Camouflage detectors that can detect incoming radiation from any angle without being detected are extremely important in stealth, guided missile, and heat-seeking missile industries. In order to accomplish this, the absorption and emission processes must be manipulated simultaneously. However, Kirchhoff's fundamental law suggests that absorption and emission are always in the same direction α(θ) = ε(θ), i.e., absorption and emission are reciprocal. This means that the emission from the detector always points back to the source, for example towards a laser source in a guided missile. Thus, detector emission serves as a complementary measure to hide an object. Here, we present a novel camouflage detector that uses a nonreciprocal metasurface array to independently detect the direction of the incoming radiation as well as manipulate its emissivity response. This is accomplished by using a magneto-optical material called indium arsenide (InAs), which breaks Lorentz reciprocity and Kirchhoff's fundamental law such that α(θ) ≠ ε(θ). This design results in the following absorption and emission: α(θ) = ε(-θ). Nine metasurfaces were designed, optimized, and operated at different incident angles from +50° to -50° at a wavelength of 13 µm. Furthermore, by keeping all metasurfaces in a pixilated array form, one could make a device that works over the full ±50° range. Potentially, this array of nonreciprocal metasurfaces can be used to fabricate thermal emitters or solar-harvesting systems.

Visibly transparent multifunctional camouflage coating with efficient thermal management.

Camouflage technology has attracted growing interest in many thermal applications. In particular, high-temperature infrared (IR) camouflage is crucial to the effective concealment of high-temperature objects but remains a challenging issue, as the thermal radiation of an object is proportional to the fourth power of temperature. Here, we proposed a coating to demonstrate high-temperature IR camouflage with efficient thermal management. This coating is a combination of hyperbolic metamaterial (HMM), gradient epsilon near zero (G-ENZ) material, and polymer. HMM makes the coating transparent in the visible range (300-700 nm) and highly reflective in the IR region, so it can serve as a thermal camouflage in the IR. G-ENZ and polymer support BE mode (at higher angles ∼50° to 90° in the 11-14 µm atmospheric window) and vibrational absorption band (in 5-8 µm non-atmospheric for all angles), respectively. So it is possible to achieve efficient thermal management through radiative cooling. We calculate the temperature of the object's surface, considering the emissivity characteristics of the coating for different heating temperatures. A combination of silica aerogel and coating can significantly reduce the surface temperature from 2000 K to 750 K. The proposed coating can also be used in the visible transparent radiative cooling due to high transmission in the visible, high reflection in the near-IR (NIR), and highly directional emissivity in the atmospheric window at higher angles, and can therefore potentially be used as a smart window in buildings and vehicles. Finally, we discuss one more potential future application of such a multifunctional coating in water condensation and purification.

Phase change material based hot electron photodetection.

We introduce a phase change material (PCM) based metal-dielectric-metal (MDM) cavity of gold (Au)-antimony trisulfide (Sb2S3)-Au as a hot electron photodetector (HEPD). Sb2S3 shows significant contrast in the bandgap (Eg) upon phase transition from the crystalline (Cry) (Eg = 2.01 eV) to the amorphous (Amp) (Eg = 1.72 eV) phase and forms the lowest Schottky barrier with Au in its Amp phase compared to conventional semiconductors such as Si, MoS2, and TiO2. The proposed HEPD is tunable for absorption and responsivity in the spectral range of 720 nm < λ < 1250 nm for the Cry phase and 604 nm < λ < 3542 nm for the Amp phase. The single resonance cavity and thus the sensitivity of the designed HEPD device can be changed to the double resonance cavity via the Cry to Amp phase transition. The maximum predicted responsivities for the single and double cavities are 20 and 24 mA W-1, respectively, at 950 nm and 1050 nm wavelengths which is the highest among all previously proposed planar HEPD devices. An anti-symmetric resonance mode at a higher wavelength is observed in the double cavity with 100% absorption. Owing to a high index of Sb2S3, an ultrathin ∼40 nm (∼λ/15) MDM cavity supports a critical light coupling to achieve high-efficiency HEPDs. Furthermore, a reversible and ultrafast (∼70 ns) Cry to Amp phase transition of Sb2S3 makes it suitable for many tunable photonics applications ranging from the visible to near-infrared region. Finally, we have introduced a novel scheme to switch between the single and double cavity by exploiting a semiconductor to metal phase transition in a PCM called VO2. The integration of VO2 as a coupling medium in the double cavity has increased the responsivity up to 50% upon phase transition to the metal phase. The proposed design can be used in optical filters, optical switches, ultrathin broad or narrow band solar absorbers, and other energy applications such as water splitting.

Phase change material-based nano-cavity as an efficient optical modulator.

Structural phase transition induced by temperature or voltage in phase change materials has been used for many tunable photonic applications. Exploiting reversible and sub-ns fast switching in antimony trisulfide (Sb2S3) from amorphous (Amp) to crystalline (Cry), we introduced a reflection modulator based on metal-dielectric-metal structure. The proposed design exhibits tunable, perfect, and multi-band absorption from visible to the near-infrared region. The reflection response of the system shows >99% absorption of light at normal incidence. The maximum achievable modulation efficiency with a narrow line width is ∼98%. Interestingly, the designed cavity supports critical resonance in an ultrathin (∼λ/15) Sb2S3 film with perfect, broadband, and tunable absorption. Finally, we proposed a novel hybrid cavity design formed of Cry and Amp Sb2S3 thin films side-by-side to realize an optical modulator via relative motion between the incident light beam and cavity. The proposed lithographic free structure can be also used for filtering, optical switching, ultrathin photo-detection, solar energy harvesting, and other energy applications.

Dynamic control of spontaneous emission rate using tunable hyperbolic metamaterials.

We numerically investigate the dynamic control over the spontaneous emission rate of quantum emitters using tunable hyperbolic metamaterials (HMMs). The dispersion of a metal-dielectric thin-film stack at a given frequency can undergo a topological transition from an elliptical to a hyperbolic dispersion by incorporating a tunable metal or dielectric film in the HMM. This transition modifies the local density of optical states of the emitter and, hence, its emission rate. In the visible range, we use an HMM consisting of TiN and ${{\rm Sb}_2}{{\rm S}_3}$Sb2S3 and show considerable tunability in the Purcell enhancement and quantum efficiency as ${{\rm Sb}_2}{{\rm S}_3}$Sb2S3 phase changes from amorphous to crystalline. Similarly, we show tunable Purcell enhancement in the telecommunication wavelength range using a ${\rm TiN}/{{\rm VO}_2}$TiN/VO2- HMM. Finally, tunable spontaneous emission rate in the mid-IR range is obtained using a ${\rm graphene}/{\rm MgF}_2$graphene/MgF2 HMM by modifying the graphene conductivity through changing its chemical potential. We show that using a metal nitride (for the visible and NIR HMMs) and a fluoride (for the mid-IR HMM) is important to get an appreciable change in the effective permittivity of the thin-film multilayer stack.