Enhanced detection of circularly polarized photons with topological materials
Topological insulators (TI) are highly attractive platforms for next-generation optoelectronic and photonic devices. Spin-momentum locking of topological surface states enhances their nonlinear optical responses and sensitivities, especially to circularly polarized light. Until now, theoretical investigations of nonlinear responses in TIs have been limited to microscopic calculations on analytical continuum models, or leveraging density functional theory based Hamiltonians. In this work, we expand beyond these two approaches by employing a nonlinear Kubo formalism to calculate second-order nonlinear optical conductivity in a slab geometry using symmetry informed tight binding models that accurately reproduce the conduction, valence and topological surface bands in Bi$_2$Se$_3$. Our methodology enables us to study the layer resolved contribution to injection currents coupled to the incident electric field. We demonstrate that our technique can reveal how device engineering modifies elements of the nonlinear optical response such as the circular {and linear} photogalvanic effects by breaking inversion and time-reversal symmetry. {In particular, magnetization-induced symmetry breaking enables nonlinear conductivity tensor components (e.g., $σ_{xyz}$) that are normally forbidden by symmetry, thereby directly modifying the circular photogalvanic effect.} We find, in line with experiments, that the photogalvanic current is sensitive to field effects, Fermi level energy, gate voltage and the energy of incident light. Our computed midwave infrared (mid-IR) responsivity $R \approx 0.170~\mathrm{μA/W}$ is comparable to reported TI and intrinsic 2D-material photodetectors. We simulate experimentally unexplored methods to modify the circular photogalvanic effect such as proximitizing a magnetic field to one of the TI surface materials, suggesting a mechanism for optoelectronic tuning.