Spontaneously-Oriented Evaporated Organic Semiconductor Thin Films for Second-Order Nonlinear Photonics

Abstract

Small organic molecules can possess extremely high hyperpolarizabilities. Their potential use in nonlinear photonics has, however, been limited by the fact that their bulk second-order nonlinearities often vanish in thin film form due to the centrosymmetric arrangement that results from most fabrication processes. The typical approach to overcome this problem has been to use electric field poling, which comes at the cost of considerably increased complexity. In polar films, however, molecules can spontaneously adopt an asymmetric out-of-plane orientation distribution that breaks the centrosymmetry. This phenomenon is at the origin of the spontaneous orientation polarization observed in organic thin films, a phenomenon that has recently attracted considerable attention from the organic optoelectronics community. In this work we show that spontaneous orientation can be leveraged to obtain evaporated thin films with bulk second-order nonlinear coefficients of χ₃₃⁽²⁾ ≃ 20 pm/V, on par with the inorganic nonlinear materials commonly used in integrated photonics. Additionally, we show that the evaporation rate and substrate treatments can be used to tune the nonlinear properties of these films. Finally, we show that the codeposition of a molecule possessing a large hyperpolarizability with a host molecule known for its strong spontaneous orientation can favor the spontaneous orientation of the nonlinear molecule and lead to large nonlinearities. This technique can lead to films with stronger nonlinearities than in neat films, even at low concentrations of nonlinear compounds (as low as 23%). This work paves the way for the direct integration of evaporated organic semiconductor thin films for second-order nonlinear optical processes on optical chips and metasurfaces, without the need for electrical poling.