  Resonant periodic surfaces and films enable new functionalities with wide
applicability in practical optical systems. Their material sparsity, ease of
fabrication, and minimal interface count provide environmental and thermal
stability and robustness in applications. Here we report an experimental
bandpass filter fashioned in a single patterned layer on a substrate. Its
performance corresponds to bandpass filters requiring perhaps 30 traditional
thin-film layers as shown by an example. We demonstrate an ultra-narrow,
high-efficiency bandpass filter with extremely wide, flat, and low sidebands.
This class of devices is designed with rigorous solutions of the Maxwell
equations while engaging the physical principles of resonant waveguide
gratings. The proposed technology is integration-friendly and opens doors for
further development in various disciplines and spectral regions where thin-film
solutions are traditionally applied.
