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Quasi-phase matched devices in Rb-doped KTiOPO4: counterpropagating nonlinear interactions, domain dynamics, and waveguides

Time: Fri 2022-12-09 09.15

Location: (Room 4205), Hannes Alfvéns väg 12, Alba Nova

Language: English

Subject area: Physics

Doctoral student: Patrick Mutter , Laserfysik

Opponent: Dr. Christopher Phillips,, Eidgenössische Technische Hochschule (ETH) Zürich, Switzerland

Supervisor: Professor Carlota Canalias, Biomedicinsk fysik och röntgenfysik

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Abstract Nonlinear interactions involving counterpropagating photons are gaining much attention in classical and quantum optics due to their unique properties that are very difficult, if not impossible, to realize in conventional co-propagating three-wave mixing. However, the large phase mismatch between the interacting waves demands an unnaturally large material birefringence or high-quality quasi-phasematched (QPM) devices with sub-µm periods. Fabrication of such devices is challenging and has been the main hindrance in demonstrating and further exploiting counter-propagating interactions. For that reason, most counter-propagating nonlinear interactions remain unexplored. Nevertheless, coercive field engineering in Rb-doped KTiOPO4 (RKTP) has proven to allow consistent fabrication of bulk sub-µm domain gratings. Despite the excellent results achieved by this poling technique, the physical mechanisms behind this method have not been thoroughly investigated. The goals of this thesis have been: First, to understand the ferroelectric domain dynamics when coercive field gratings are used; Second, using these insights, to fabricate QPM devices with shorter periodicities with the aim of enabling novel counterpropagating nonlinear interactions; and finally, to study waveguide implementation techniques that are compatible with periodic poling. On the basis of these studies, it was possible to show that the depth, shape, and critical ion concentration of the ion-exchanged volume govern the formation of sub-µm domain gratings independent of the poling period. These conclusions are reached by studying the domain morphology in coercive-field engineered sub-µm periodically poled crystals with periods ranging from 755 to 433 nm and correlating it to that of the ion-exchanged regions with nm resolution. The shortest bulk QPM periods ever reported were fabricated, namely 433 nm and 317 nm. The former was used to demonstrate the first phase-locked degenerate backward wave optical parametric oscillator (BWOPO). The phase-locked state was confirmed by interfering the frequency doubled backward wave with the pump wave. Moreover, when the BWOPO was operated at degeneracy, the sum frequency generation of the counterpropagating degenerate parametric waves occurred. The BWOPO exhibited a conversion efficiency of 40.7 %. The 317 nm crystal was employed to demonstrate first-order QPM backward second harmonic generation (BSHG) for the first time. The first-order QPM resulted in the highest conversion efficiency (18.7%) reported for BSHG. The high efficiency allowed scrutinizing the interaction, confirming the much narrower bandwidths compared to forward phase-matched second harmonic generation. Finally, a fabrication process for ion-exchanged ridge and channel waveguides in PPRKTP were developed. On the one hand, the ridge waveguides show a normalized conversion efficiency of 31 %/Wcm2 and high losses. On the other hand, the channel waveguides warrant high efficiency and low loss. 3.1 mW of continuous-wave blue second harmonic was generated from 74 mW of fundamental radiation at 940.2 nm, corresponding to a normalized conversion efficiency of 115 %/Wcm2 with a loss of 0.5 dB/cm at the fundamental wavelength. Additionally, the ferroelectric domain stability was investigated, and it was found that domain gratings with uncharged walls could withstand the ion exchange without deterioration.