Light confined in a defect band in a 3D bandgap

It is well-known that fruitful analogies exist between the physics of photons at the nanoscale, and electrons, spins, and phonons in condensed matter. A famous example is the analogy between a 3D photonic band gap in a photonic crystal and the electronic band gap in a semiconductor like silicon. Here, we perform the first ever study of light hopping in a 3D superlattice of coupled cavities in a photonic band gap, whose defect bands are analogous to the impurity bands describing (Anderson) electronic transport in a semiconductor [1,2].
Our samples consist of 3D photonic band gap crystals with evenly spaced cavities, made from silicon by deep reactive ion-etching. We developed a setup to probe reflectivity and scattering, including position and polarization-resolved response [3]. Bright and broad reflectivity peaks appear at frequencies matching the 3D band gap, while sharp scattering peaks represent cavity-confined light leaking in the 3rd dimension. The sharp scattering peaks correlate positively with the focus being on or off the defects, while the maximum reflectivity and the gap width anti-correlate, both as expected. We discuss our observational evidence of impurity bands in a band gap, and interpret these with calculated band structures and extensive simulations [4].

[1] P.W. Anderson, Phys. Rev. 109 (1958) 1492
[2] S.A. Hack, et al., Phys. Rev. B 99 (2019) 115308
[3] M. Adhikary, et al., Opt. Express 28 (2020) 2683
[4] D. Devashish, et al., Phys. Rev. B 99 (2019) 075112