Due to the high cost and requirement for cryogenic cooling of Mercury Cadmium Telluride (MCT) detectors, there is a growing demand for cheap Long Wavelength Infrared (LWIR) detectors at room temperature. Graphene has shown great potential yet suffers from a low absorption (<3%) in the LWIR ranges. In this presentation, we demonstrate the asymmetric plasmon-assisted hot carrier generation and Seebeck photodetection in nano-patterned Graphene with an enhanced absorption (>60%). This novel Graphene photodetector exhibits a remarkable responsivity of 2900 VW-1, detectivity of 1.1 x 109 Jones with a fast response of 100ns in 8-12 μm band at room temperature.
Performance of adaptive infrared camouflage is usually parameterized in terms of cycle-ability, response time, actuation mechanism, stability etc., however, one of the key components that has not been addressed so far is the spatial density of infrared information that can be encoded and actively manipulated for camouflaging.
We report an adaptive infrared camouflage system that can be engineered to operate at any desired wavelength in the technologically relevant, infrared transparent 3 – 5 µm and 8 – 12 µm bands. We exploit the metal-insulator phase transition in VO2 to design an optical cavity coupled infrared absorber where the cavity length can be altered by controlling the VO2 phase. Cavity tuning is done by strategically placing the VO2 layer inside the optical cavity composed of a tri-layer architecture. In its insulating state VO2 is transparent to infrared such that incident light couples to the entire cavity length, however in the metallic state, VO2 behaves like a mirror and shortens the cavity length by reflecting ~80% of incident light. The Maxwell Garnett EMT describes the phase transition dependent optical response of the absorber better than the Bruggeman EMT when compared to the experimental results. We tailor the device parameters to demonstrate adaptive thermal camouflage of multispectral encoded infrared information on a pixelated designer surface with a pixel resolution (~20 µm) and density comparable to the industry standard for infrared sensors. We envision this work will pave the way for novel tunable optical devices for technological advancements in infrared tagging, camouflaging and anti-counterfeiting efforts.
The fast carrier relaxation time, high carrier mobility and electrostatic tunability make graphene a prospective ideal material for electronics and optoelectronics. However, its low optical absorption is a big obstacle. Moreover, for using graphene in the large area optoelectronic devices, any scheme for enhancing the light-matter interaction in graphene should be polarization and incident angle-independent.
Here, we demonstrate a novel design of an optical cavity-coupled hexagonal nanohole and nanodisk array to excite Dirac plasmon. We compare the Dirac plasmon lifetimes of the graphene nanohole and nanodisk arrays and their role in the enhanced light-matter interaction. Coupling the patterned graphene to an optical cavity creates a temporal and spatial overlap between the graphene plasmon and cavity modes. This complex geometry gives rise to an unprecedented polarization independent light absorption of 60% on nanohole and 90% on nanodisk arrays in low carrier mobility CVD-grown monolayer graphene in the 8-12 um atmospheric transparent infrared imaging band. Electrostatically doping of the patterned graphene tune the surface plasmon resonance wavelength up to 2.5 um by applying a small gate voltage (4V). We show theoretically, and also for the first time the experimental results of the enhanced light absorption for the non-normal incidence. While the light absorption up to 40° (incident angle) is almost constant, the trend of the angular optical response for s- and p-polarized light are different which is validated by our analytical coupled-dipole approximation modeling. This electronically tunable wide angle extraordinary light absorption paves the path towards new generation of graphene-based optoelectronics devices.
Conventional optical elements such as lenses, waveplates and polarizers function by adding phase delays to the propagating light. The thicknesses of these dielectric optical components are much larger than wavelength to accumulate 0-π phase shift. Moreover, spherical aberration and diffraction limit restrict their usage in integrated photonics circuits. Metasurface based lenses change the phase of transmitted and reflected electromagnetic waves significantly at resonance by exciting surface plasmons on the metallic arrays with thickness much lower than the wavelength of the incident light. However, previous demonstrations of plasmonic lens suffer from low transmission efficiency (< 20%) due to the high plasmonic losses. We overcame this shortcoming to some extend by engineering plasmonic coupling and demonstrated a relatively high 75% transmission in the mid infrared spectral domain. In this proposed work, coupled one dimensional array of gold disks with variable diameters have been employed to add varying phases to the transmitted light in order to create the phase front curvature in mid-IR wavelength range needed for the focusing of the incident radiation. The designed nanostructured surface achieves a resolution beyond the diffraction limit in thin-film planar geometry. The focal point, resolution and transmission efficiency can be tuned by various parameters such as period, diameters, and the size of the disks. The confocal measurement method has been performed to measure the far field focal volume of the fabricated lens, which is in good agreement with the theoretical results. Thin-film planar layout and subwavelength resolution mitigate the limitations of conventional optical elements.
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