Strong near-infrared saturable absorption of nanocrystalline TiN for pulse generation

Abstract

The fast nonlinear optical (NLO) response generated by the transient thermalization/cooling of photon excited conduction electrons in plasmonic materials can be exploited as an efficient switching mechanism for all-optical devices, while most metallic nanostructures suffer from poor resistance to intensified laser irradiation. Here, we demonstrate that robust metallic nanocrystalline TiN (nc-TiN) thin films obtained by sputtering exhibit a strong nonlinear optical response in the near infrared region with a large negative NLO absorption coefficient. According to femtosecond transient absorption spectra, the strong NLO response is revealed to associate with a rapid carrier thermalization/cooling process in a sub-picosecond time scale. Leveraging the temporal absorption modulation, we demonstrate the generation of ultrashort pulses in the 1.5 μm region mode-locked using nc-TiN based switches operating in both transmission and reflection modes. Combined with a high laser damage threshold, our results suggest that the nc-TiN thin films obtained by a commercially viable sputtering process could find potential applications in the fabrication of NLO photonic devices.