PhD Thesis Defenses

PhD Thesis Defense: Nonlinear optics in KTiOPO4 for spectral management of ultra-short pulses in the near and mid-IR

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.