ABSTRACT
In this work, we investigate a pedestal tantalum oxide (Ta2O5) material platform for integrated nonlinear optics (NLO). In order to achieve low propagation losses with this material, pedestal waveguides with Ta2O5 cores were designed. The nonlinear refractive index n2 of this new platform was obtained by measuring the amount of spectral broadening due to self-phase modulation (SPM) of 23 fs optical pulses at 785 nm propagating through the waveguides. In this manner, a nonlinear index of (5.8 ± 2.0) × 10-19 m2W-1 was found for this material, which is in good agreement with values reported in related works where strip waveguides were used for a similar purpose. Furthermore, due to the pedestal configuration, propagation losses as low as 1.6 dB·cm-1 for narrow waveguides and 0.1 dB·cm-1 for large waveguides were obtained. Finite element method (FEM) mode analysis was performed to calculate the mode characteristics, as well as the effective areas of the waveguides. The high nonlinear and linear refractive indices, wide bandgap and low propagation losses make this platform ideal for applications extending from the visible into the mid-IR regions of the optical spectrum. Due the large gap, Ta2O5 should have low two photon absorption at the near-IR as well.
ABSTRACT
Rapid variations of the environmental energy caused by ultrashort laser pulses have induced phase transitions in carbon allotropes, therefore bringing the promise of revealing new carbon phases. Here, by exposing polycrystalline graphite to 25 fs laser pulses at 4 J/cm(2) fluence under standard air atmosphere, we demonstrated the synthesis of translucent micrometer-sized structures carrying diamond-like and onion-like carbon phases. Texturized domains of the diamond phase were also identified. Concerning different synthesized carbon forms, pulse superposition and singularities of the thermodynamical process, we pinpoint the synthesis mechanism by the laser-induced subsequent products energetically evolving to attain the diamond-like phase.
