Currently, periodic structures on lithium niobate crystal surface exhibit extensive application prospects in related fields of on-chip photonic-integrated platforms and nanophotonics. We fabricated periodic ripple structures on the surface of lithium niobate crystal by femtosecond laser pulses irradiation and observed the evolution of these structures irradiated by 50,100,500,1000 laser pulses respectively. The morphology of surface structure became more uniform with the increasing number of laser pulses. Especially fabricated by 1000 pulses, regular ripples were found over the ablation area. The direction of ripples is perpendicular to the laser polarization and the period is around 190 nm, which was calculated by 2D-Fast Fourier Transform. By Finite-Difference Time-Domain method, we simulated the effect of the initial periodic structure on subsequent energy distribution. Numerical simulation results show that energy is deposited in the grooves between the ripples. Therefore, the ablation of grooves is more efficient, and the ripples morphology resulting from subsequent laser pulses irradiation becomes more uniform. The simulation results are consistent with the experimental results. This research is considerably valuable for controlling precisely periodic micro- and nanostructures formation and providing innovative laser manufacturing technology for wide bandgap material.
We present in this paper the fabrication and characterization of thermally stable double line waveguides in Z-cut periodically poled Lithium Niobate crystals. The waveguides were fabricated by using a femto-second laser, and utilized for second-harmonic generation. Our experiments have shown that a quasi-phase matching wavelength of 1548.2 nm, a tuning bandwidth of 2 nm, and a tuning temperature range of 150.4±1.6°C can be achieved.
Raman spectra of In0.65Al0.35As quantum dots (QDs) embedded between GaAlAs and GaP have been measured at room temperature. For the as-grown sample, in addition to the TO/LO modes from GaAs substrate, a weak broad peak appears from 165 cm-1 to 203 cm-1, corresponding to the interface mode of InAlAs QDs. The AlAs-like and GaP-like modes can be clearly seen at 382 cm-1. For the annealed samples, the AlAs and GaP-like modes disappeared, while the InAs-like modes become stronger, indicating strong intermixing between QDs and the matrix and the formation of uniform InGaAlAsP alloy.