Thin film interference in ultra-thin layers: color coatings, tunable absorbers, and thermal emitters

Thin film interference is a ubiquitous and well-understood optical phenomenon responsible for the colorful, iridescent reflections from oil films on water, soap bubbles, and peacock feathers. This effect is typically observed in transparent thin films whose thickness is similar to the wavelength of light. I will show that nanometer-thick films made of optically absorbing materials can feature strong interference effects despite being much thinner than conventional optical thin films. Semiconductor films between 5 nm and 20 nm in thickness on metal substrates display an array of interference colors while absorbing a large fraction of incident light within a small volume of material [1]. This geometry is thus attractive for applications in visual design as well as for light harvesting and detection. I will also demonstrate a similar thin film structure comprising vanadium dioxide (VO2), a canonical phase change material, whose reflectivity and absorptivity in the infrared (IR) can be widely modulated as a result of enhanced interaction between IR light and the VO2 thin film [2]. This geometry also features a large temperature-tunable IR emissivity; as a result, a variety of unusual thermal phenomena can be observed such as large negative differential thermal emittance [3].

[1] M. A. Kats et al, Nature Materials 12, 20 (2013). [2] M. A. Kats et al, Applied Physics Letters 101, 221101 (2012). [3] M. A. Kats et al, Physical Review X 3, 41004 (2013)