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

Small\norganic molecules can possess extremely high hyperpolarizabilities.\nTheir potential use in nonlinear photonics has, however, been limited\nby the fact that their bulk second-order nonlinearities often vanish\nin thin film form due to the centrosymmetric arrangement that results\nfrom most fabrication processes. The typical approach to overcome\nthis problem has been to use electric field poling, which comes at\nthe cost of considerably increased complexity. In polar films, however,\nmolecules can spontaneously adopt an asymmetric out-of-plane orientation\ndistribution that breaks the centrosymmetry. This phenomenon is at\nthe origin of the spontaneous orientation polarization observed in\norganic thin films, a phenomenon that has recently attracted considerable\nattention from the organic optoelectronics community. In this work\nwe show that spontaneous orientation can be leveraged to obtain evaporated\nthin films with bulk second-order nonlinear coefficients of χ<sub>33</sub><sup>(2)</sup> ≃ 20\npm/V, on par with the inorganic nonlinear materials commonly used\nin integrated photonics. Additionally, we show that the evaporation\nrate and substrate treatments can be used to tune the nonlinear properties\nof these films. Finally, we show that the codeposition of a molecule\npossessing a large hyperpolarizability with a host molecule known\nfor its strong spontaneous orientation can favor the spontaneous orientation\nof the nonlinear molecule and lead to large nonlinearities. This technique\ncan lead to films with stronger nonlinearities than in neat films,\neven at low concentrations of nonlinear compounds (as low as 23%).\nThis work paves the way for the direct integration of evaporated organic\nsemiconductor thin films for second-order nonlinear optical processes\non optical chips and metasurfaces, without the need for electrical\npoling.