Bulk photovoltaic effect in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>MoSe</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math> and Janus MoSSe sliding ferroelectrics

We present a first-principles study of the nonlinear optical properties of sliding ferroelectric bilayers based on <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msub> <a:mi>MoSe</a:mi> <a:mn>2</a:mn> </a:msub> </a:math> and Janus MoSSe. Two Janus configurations are considered: (i) one bilayer where the two intralayer polarizations caused by Janus chemical asymmetry cancel each other out, yielding photocurrent spectra comparable to pristine <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msub> <b:mi>MoSe</b:mi> <b:mn>2</b:mn> </b:msub> </b:math> bilayers; (ii) another bilayer where the intralayer polarizations add up, for which the photoresponses are strongly enhanced. Our results show that photocurrent generation in the polar Janus structures is predominantly governed by vertical chemical asymmetry, with limited dependence on the sliding direction. These findings highlight complementary design strategies: interlayer sliding enables sensitivity to external tuning, while the Janus intralayer polarization enhances photoresponses in the visible range. The interplay between composition and stacking therefore provides a versatile platform for tailoring light-matter interactions in two-dimensional ferroelectric materials.