Abstract [en]

This thesis explores the possibilities of controlling nonlinear optical interactions in ferroelectric materials for bandwidth tailoring of ultrashort pulses in the pico- and femto-second range. The control is achieved through quasi-phase matching, which is based on alternating the material’s spontaneous polarization into different domains. In the presented work, KTiOPO₄ (KTP) is the material of choice as it provides high optical nonlinearity, a wide transparency window in the near- and mid-infrared (IR), as well as a high damage threshold. Furthermore, KTP also enables fabrication of uniform high aspect ratio and fine-pitch domain structures of high quality. These qualities make KTP highly attractive for a vast range of applications and enabled much of the work presented in this thesis.The propagation of ultrashort pulses in domain-structured ferroelectrics was studied numerically with a model based on a single nonlinear envelope equation. This model accounts for the absorption and the dispersion of the material, as well as the second- and the third-order nonlinearities. Supercontinuum generation in the near- and mid-IR was studied in periodically structured KTP for femtosecond pulses at 1.5 μm. The numerical results showed the potential for pulse self-compression with octave-spanning spectral broadening. This process is enabled by cascaded second-order nonlinearities and can be tailored by the phase-matching parameters, which are set by the structure’s periodicity. The proposed design presented in this work resulted in a negative effective Kerr nonlinear coefficient with a magnitude of 1.65×10^-14 cm²/W in the positive dispersion regime, which is one order of magnitude higher than the natural Kerr coefficient in KTP. Experimental characterization of single pass propagation of 128 fs-long pulses at 1.52 μm through a periodically structured KTP sample, with a periodicity of 36 μm based on the proposed design, are also presented. The results show a spectral broadening from 1.1 μm to 2.7 μm and a simultaneous compression down to 18.6 fs, thus confirming the numerical findings. Bandwidth tailoring of ultrashort IR pulses in the picosecond range was also studied through devices known as backward-wave optical parametric oscillators (BWOPOs). These devices rely on sub-micrometer domain periods to generate counter-propagating signal and idler waves. In a BWOPO, the forward-generated wave inherits the phase modulation of the pump wave, while the backward-generated wave is inherently narrowband and basically insensitive to pump wavelength tuning. In this work, BWOPOs operated in a cascaded manner, with the forward-generated wave being employed as a pump in a single pass configuration, were studied. The tunability issue of the narrowband backward wave was solved by employing a broadband optical parametric amplifier seeded by the BWOPO forward wave. A tunability of 2.7 THz for a wave at 1.87 μm with a bandwidth of 28 GHz was demonstrated. The coherent phase transfer from the pump to the BWOPO forward wave was investigated in the context of pulse compression. In this experiment, a 220 GHz bandwidth was transferred from 800 nm to 150 ps-long pulses at 1.4 μm, which could be compressed down to 1.3 ps with μJ energy, in a single-grating compressor.