Strong optical nonlinearity of ultrathin graphitic films synthesized on dielectric substrates

We propose and demonstrate a scalable technique to grow a thin polycrystalline graphitic film directly onto a fused silica substrate. The technique is based on the pyrolysis of a photoresist in the presence of a sacrificial 10 nm thick nickel catalyst layer. The synthesized graphitic film with a thickness of about 50 nm possesses almost constant 40% absorptance over visual and near infrared spectral regions. By using Raman characterization, third harmonic generation spectroscopy, and the Z-scan technique we perform a comparative study of the films pyrolyzed with and without a Ni catalyst. We show that the amorphous carbon dominates the linear and nonlinear optical properties of the resist film pyrolyzed without the Ni catalyst. In contrast, in presence of a Ni catalyst layer, the pyrolysis leads to a graphitic film that demonstrates a strong saturable absorption behavior at 1550 nm wavelength and has a nonlinear refractive index comparable with that of graphene. Thus, the developed, transfer-free synthesis technique provides an alternative route towards the controllable growth of wafer scale graphitic films on the dielectric substrates for photonics applications.