Continuously-tunable, compact, freespace notch-filter design using an all-dielectric metagrating capped with a low-loss phase change material.

Abstract Active metasurfaces utilizing phase change materials (PCMs) are currently
under investigation for applications in free-space optical communication, optical
signal processing, neuromorphic photonics, quantum photonics, and compact LiDAR.
Attention has now turned towards novel PCM like Sb2S3 which exhibit lower optical
absorption and reasonable values of refractive-index contrast in comparison to traditional
data-storage PCM. We propose and numerically study the class of all-dielectric
metagratings capped with low-loss PCM and predict the possibility of continuosly
tunable resonances whose quality factors degrade gracefully during the amorphous-
to-crystaline phase transition of the PCM. Specifically, we consider the CMOS-
compatible silicon-nitride on silica substrate material platform for simple and dimerized
metagratings (in particular, the symmetric-broken dimerization) and Sb2S3 capping.
Our numerical study predicts that notch-filters operating around the 1550 nm NIR
wavelength window can be achieved with tuning range of over 76 nm with Q-factors
ranging from 784 (amourphous-phase) to 510 (crystaline-phase) (a degration in Q
of about 35%) and insertion loss of about 0.9 dB. These performance figures are a
significant improvement over previously published designs utilizing data-storage PCMs
and other traditional notch-filter mechanisms. We examine the influence of grating
dimerization and geometrical parameters on performance metrics of the notch-filter and
predicts the possibility to trade-off rejection-band and in-band spectral transmission
properties. Lastly, we perform a study of all-optical phase change mechanism. Our
study is promising for the miniaturization of tunable notch-filter based optical systems.