Powerless tunable photonic crystal with bistable color and millisecond switching

This study demonstrated a tunable photonic crystal (PhC) with 70 nm-wide spectral tuning (535 nm to 605 nm) and 3 ms of response time. The tunable PhC is based on reciprocal capillary action of liquid in the nanoscale PhC voids. By wetting the porous silicon PhC with ethanol and water, the PhC can be bistably switched respectively between liquid-filled state (orange color) and vapor-filled state (yellow color). Owing to the energy barrier between the two wetting states, the tunable PhC can remain at either of the two states with no external power consumption.