Research On Nonlinear Optical Phenomena And On-chip Integration Of Ferroelectric Crystal

The traditional integrated circuit cannot satisfy the demand of information explo-sion,therefore people put forward to transmit and process signal by integrated optical chip.However,the current mature silicon chip cannot be used to achieve some key devices in communication system that based on second-order nonlinearities due to the central symmetry structure of silicon.Lithium niobate（Li Nb O₃,LN）crystal has been widely used due to its excellent nonlinear,electro-optic（EO）and acousto-optic prop-erties,low intrinsic absorption loss,and wide transparent windows.Recently,an ideal platform called Li Nb O₃thin film has attracted much interest for its good confinement and strong guiding of light due to the high-refractive-index contrast.Therefore,LN thin film can be a promising candidate for combining all the photonic components in a single chip.To explore new nonlinear effects and applications of Li Nb O₃,to study new waveguide fabrication techniques,to achieve the integration of current bulk devices on LN chip,are a very important research filed.In this dissertation,we demonstrated the research on nonlinear optical phenomena and its on-chip integration of Li Nb O₃.After a simple review of the development of nonlinear optics,the properties of ferroelectric crystals,the progress and applications based on Li Nb O₃,and the future development prospect of LN chip,we introduced the basis principles of second-order nonlinear effects in anisotropic crystal in detail.By solving the couple-wave equa-tions of second-harmonic generation（SHG）and sum-frequency generation（SFG）,we deduced that effective frequency conversion need to satisfy the phase matching condition,which can be matched by birefringence phase matching or by quasi-phase matching（QPM）methods.We analysed the effect of external electric filed on the in-dex ellipsoid of Li Nb O₃,as well as the light propagation properties of codirectional coupling and contradirectional coupling by couple-mode theory in periodic media.When an external electric field is applied along the y-axis of a periodically poled Li Nb O₃（PPLN）crystal,the energy coupling between the fundamental lights of differ-ent polarizations leads to a large nonlinear phase shift（NPS）due to the codirectional coupling.We proposed two applications of the cascaded linear EO effects.One is to enhance the Kerr EO nonlinearity,which is more than three orders of magnitude higher than the intrinsic value.Another one is to control the second-order nonlinear process.We designed a structure that can implement the cascaded linear EO effects and SHG simultaneously.The effective nonlinearity can be either positive or negative,causing the second-harmonic spectra to move toward the coupling center,which in turn,offered us a way to measure the effective electro-optic nonlinear refractive index.We reviewed the progress and applications of fast and slow light,then proposed two kind of electro-optic photonic crystals（EOPC）.In z-axis EOPC,the periodical inversion of refractive index leads to the generation of Bragg bandgap,around which the group velocity can be effectively controlled by the external electric filed.In y-axis EOPC,the NPS induced by the cascaded linear EO effects can also modulate the group velocity of light.The light experiences an anomalous dispersion near the transmission center and a normal dispersion away from the center,where it can be accelerated and slowed down,respectively.We also obtained the switch between fast and slow light by adjusting the external electric filed.The temporal gap can be modulated by phase mismatching and the external electric field.It puts forward a new solution for temporal cloak,controlled by external electric field instead of the light intensity.We demonstrated the applications of Li Nb O₃in on-chip integration.We intro-duced the method of direct wafer bonding to fabricate Li Nb O₃thin film.We proposed achieving SHG in Li Nb O₃thin film by mode-phase matching along with phase match-ing.We successfully observed type 0 mode-phase matching SHG between 0th funda-mental wave and 2nd second-harmonic wave in 900 nm thick Li Nb O₃thin film.We also studied the methods to fabricate strip waveguide based on LN thin film.We fabricated PPLN thin film by the direct wafer bonding method,which can solve the mode overlap issue of SHG in Li Nb O₃thin film by QPM technique.We presented the possibility of realizing broadband frequency conversion in PPLN thin film,by satisfying QPM and group-velocity matching（GVM）simultaneously,which exists in three type of SHG/SFG.We demonstrated the first type-0 broadband SHG/S-FG based on PPLN thin film.The broad bandwidth property was largely tuned from mid-infrared region to the telecommunications band by engineering the thickness of PPLN from bulk crystal to nanoscale.It provides SHG/SFG a solution with both broadband and high efficiency by using the highest nonlinear coefficient d₃₃instead of d₃₁in type-I broadband SHG/SFG.This could lead to the realization of more advanced and complicated photonic integration devices and circuits that based on the broadband properties,for example,chip-scale all-optical wavelength broadcast,broadband spec-troscopy,quantum information processing,LN-thin-film-based microresonator and optical nonreciprocity devices,etc.