Fano-resonant ultrathin film optical coatings.

Optical coatings are integral components of virtually every optical instrument. However, despite being a century-old technology, there are only a handful of optical coating types. Here, we introduce a type of optical coatings that exhibit photonic Fano resonance, or a Fano-resonant optical coating (FROC). We expand the coupled mechanical oscillator description of Fano resonance to thin-film nanocavities. Using FROCs with thicknesses in the order of 300 nm, we experimentally obtained narrowband reflection akin to low-index-contrast dielectric Bragg mirrors and achieved control over the reflection iridescence. We observed that semi-transparent FROCs can transmit and reflect the same colour as a beam splitter filter, a property that cannot be realized through conventional optical coatings. Finally, FROCs can spectrally and spatially separate the thermal and photovoltaic bands of the solar spectrum, presenting a possible solution to the dispatchability problem in photovoltaics, that is, the inability to dispatch solar energy on demand. Our solar thermal device exhibited power generation of up to 50% and low photovoltaic cell temperatures (~30 °C), which could lead to a six-fold increase in the photovoltaic cell lifetime.

Broadband infrared plasmonic metamaterial absorber with multipronged absorption mechanisms.

A broadband plasmonic metamaterial absorber with near perfect multiband absorption in the infrared region is designed using a metal-insulator-metal configuration and fabricated using photolithography. The metal-insulator-metal configuration consists of a Ti microdisk array, a SiO2 insulator spacer, and an Al bottom layer. The multiband absorption occurs with near perfect absorption at 4.8-7.5 µm and 9.7-10.5 µm. Ultra-broadband absorption in the mid-IR wavelength range between 3-14 µm is realized by adding a rough photoresist layer on top of the periodic microdisk structures. The multiband absorption is achieved through the combined mechanisms including plasmonic surface lattice resonance, gap plasmon resonance, Fabry-Perot cavity resonance, and the intrinsic phonon-polariton absorption of SiO2.