Understanding the role of excitons in the nonlinear Bulk Photovoltaic response of two-dimensional materials

It is well-known that exciton effects are determinant to understand the optical absorption spectrum of semiconductors and insulators, specially in low-dimensional (such as 2D) materials of recent interest. Absorptive properties can be understood using linear response theory, but on the other hand, the role of excitons in nonlinear optical responses has been much less investigated at both experimental and theoretical levels. One of these nonlinear effects is the so-called Bulk Photovoltaic Effect (BPVE), a second-order phenomenon in which incident light over a sample can be directly converted into a flow of current within the material. This effect only requires a non-centrosymmetric bulk sample and is a powerful platform to study a variety of fundamental phenomena, such as: in-gap (exciton-mediated) current transport, polarization and frequency-dependent responses, or the relation of nonlinear photocurrents to the quantum geometry and topology of electron wavefunctions, to name a few.

In this talk, I will go over the theoretical methods to understand excitonic effects in nonlinear optical responses from ab initio, including the BPVE or second-harmonic generation (SHG). I will present a methodology to calculate the excitonic second-order optical responses in 2D materials relying on: (i) ab initio tight-binding Hamiltonians obtained by Wannier interpolation and (ii) the Bethe Salpeter equation with effective electron-hole interactions. Focusing on MoS2 and GeS monolayer systems, our results help to understand the in-gap excitonic resonances in the shift current response, which is a part of the BPVE. Remarkably, we show that 2p-like excitons, which are dark in absorption experiments, yield a contribution to the nonlinear photocurrent comparable to that of 1s-like excitons, revealing different selection rules for nonlinear responses. Additionally, we show how excitonic effects can typically enhance the interband BPVE response by typically one order of magnitude in sufficiently clean samples.

[1] J.J. Esteve-Paredes, M.A. García-Blázquez, A. J. Uría-Álvarez, M. Camarasa-Gómez, and J.J. Palacios, “Excitons in nonlinear optical responses: shift current in MoS2 and GeS monolayers”, arXiv:2406.14215 (2024)

[2] A. Uría-Álvarez, J. J. Esteve-Paredes, M.A. García-Blázquez, and J.J. Palacios, “Efficient computation of optical excitations in two-dimensional materials with the Xatu code”, Comput. Phys. Commun. 295, 109001 (2024)