Optical saturable absorption of conformal graphene directly synthesized on nonlinear device surfaces

• A transfer-free atomic carbon spraying provides large-area uniform surface coverage. • Strong nonlinearities owing to the high percentage of sp 2 bonding (73.94 %) • Rapid saturation with the energy-efficient response of the absorber (32.14 MW cm -2 ) • Conformal graphene greatly outperforms conventional transfer-free spraying process. • Efficient saturable absorption comparable to that of CVD-grown graphene. Optical saturable absorption (SA) in graphene has been intensively studied during the last decades to achieve graphene-based ultrafast pulse generation. We experimentally investigate the optical nonlinearities of transfer-free graphene synthesized by the atomic carbon spraying (ACS) method that is invented for conformal and uniform graphene coating on non-planar device surfaces to enhance the interaction of lasers with the nanostructures. Its successful implementation of nonlinear operations in practical photonic devices is confirmed. The SAs of two other different graphene structures including graphene prepared by chemical vapor deposition (CVD) and reduced graphene oxides (rGOs) by electro-spraying (ES) are quantitatively evaluated as references. All three nanocrystal structures are elucidated by transmission electron microscopy and Raman analysis comparatively and systematically. We confirm in X-ray photoelectron spectroscopy analysis that the contents of sp 2 bonds in ACS graphene as an indicator of the nonlinearity is 73.94 %. The measured value reaches the level of CVD graphene (76.70 %) and is much higher than 58.75 % of ES rGOs. Importantly, the ACS graphene displays a steep SA slope with saturation intensity of 32.14 MW cm -2 , an order of magnitude lower than conventional ES rGOs, verifying unimpaired nonlinear properties of graphene prepared by the suggested ACS method.