In a recent paper [Physical Review B198, 085424 (2018)], A. Díaz-Fernández et al. have studied a topological boundary formed at the interface between a trivial and a topological insulator. The difference in the topological index across the junction leads to robust gapless surface states. Optical studies of these states are scarce in the literature, the reason being the difficulty in isolating their response from that of the bulk. In this work, theye propose to deposit a δ layer of donor impurities in close proximity to a topological boundary to help in detecting gapless surface states. As they show, gapless surface states are robust against this perturbation and they enhance intraband optical transitions as measured by the oscillator strength. These results help us to understand the interplay of surface and bulk states in topological insulators.