Accurate measurement of the photonic crystal slab period using diffraction in the Littrow configuration

Photonic crystal slab devices with subwavelength periods can be tailored to provide remarkable functionality, such as ultrahigh reflectivity in a structure only 200 nm in thickness. Accurate measurement of the characteristics of these structures is essential to compare their performance to theoretical expectations and to better understand the origin of unexpected behavior. In this work, we present a simple non-invasive method employing diffraction of a visible wavelength reference in the Littrow configuration for measuring the period of a photonic crystal slab. We have measured periods of our devices with uncertainty below 0.5 nm and expect that the uncertainty could easily be improved by an order of magnitude. In addition to facilitating development, our approach can be used to explore possible variations in the period of the photonic crystal due to its operating environment and aging.