Second-harmonic generation from thin films of three surface-anchored metal-organic frameworks stacked in an ABC manner

Second-harmonic generation (SHG) is a key nonlinear optical effect that only occurs in the absence of inversion symmetry. While centro-symmetric materials based on that definition would not sustain an SHG signal, the presence of an interface in a thin film can lead to a local deviation from the inversion symmetry and, hence, to a local second-order hyperpolarizability. To give rise to a notable SHG signal, the inversion symmetry needs to be broken at a global scale, suggesting that at least three materials must be stacked in an alternating manner when thin films are considered. Although such materials can be implemented with surface-anchored metal–organic frameworks (SURMOFs), their suitability for generating a strong second-order nonlinear response has remained unexplored so far. Here, we numerically investigate SHG from thin films of three different SURMOFs stacked above each other using a scale-bridging multi-scale framework combining precise quantum chemistry and fast Maxwell optical simulations. We determine the impact of the thickness of each SURMOF layer on the SHG signal. Our results indicate that interfaces between materials with similar SURMOF parameters contribute little to SHG, whereas interfaces to the surrounding dominate the response. Moreover, we demonstrate that the SHG signal can always be significantly boosted at specific total film thicknesses due to cavity resonances. At these resonances, reducing the individual layer thicknesses to the molecular scale, thereby allowing the film to behave as a bulk material composed almost entirely of interfaces, leads to a two-fold increase in the SHG signal compared to films with thicker layers. This introduces a trade-off between simpler fabrication and the enhanced SHG achievable with ultrathin layers. Our findings highlight the importance of interface effects in designing layered SURMOF structures with optimized nonlinear optical properties, offering guidance for future material explorations.