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 (similar to 30 degrees C), which could lead to a six-fold increase in the photovoltaic cell lifetime.

Automated summary

Optical coatings are crucial for optical instruments, with Fano-resonant optical coatings (FROCs) offering a new solution with photonic Fano resonance properties. FROCs allow control over reflection colors and can spectrally and spatially separate the thermal and photovoltaic bands of the solar spectrum, potentially addressing the dispatchability issue in photovoltaics.