Patterning of electrically tunable light-emitting photonic structures demonstrating bound states in the continuum

The authors report a scalable process to fabricate electrically tunable light-emitting photonic structures made of optically active and electrically conductive erbium-doped zinc oxide (Er:ZnO) deposited by magnetron sputtering. Such structures are expected to produce a dramatic amplification of the erbium fluorescence due to enhanced light-matter coupling at topologically protected states called bound states in the continuum (BIC). Our patterning approach circumvents roughening of the Er:ZnO during plasma etching by employing a metallic mask and a lift-off process. Etching with a polymer mask resulted in an unacceptable increase of the surface roughness, from a root mean square (RMS) roughness of 0.5 nm for the as-deposited sample to a RMS roughness of 25 nm after etching. Such surface roughness proves detrimental to the photonic crystal resonances and to the BIC mode in particular. Using a metallic mask instead allowed for the etching of Er:ZnO with a modest roughness increase (RMS value 4.1 nm). This patterning approach produced a photonic structure demonstrating a BIC mode close to the expected frequency of 1540 nm at normal incidence, in accordance with our simulations.