Domain Patterning Of Lithium Niobate Film And Its Application In Nonlinear Beam Shaping

Lithium niobate crystal has excellent nonlinear optical properties with large nonlinear coefficients.It becomes a typical crystal for studying nonlinear processes such as frequency conversion.In addition,lithium niobate crystal has ferroelectricity,that is,by applying reverse external electric field,its spontaneous polarization direction can be reversed,forming a reversed domain structure with opposite nonlinear optical coefficients.By modulating the second-order nonlinear coefficient of lithium niobate crystal and introducing the inverted lattice vector to compensate the phase mismatch caused by dispersion,the nonlinear effect could be greatly enhanced through quasi phase matching.However,aperiodically poled lithium niobate crystal has only one set of reciprocal vector,it is difficult to realize phase compensation and regulation of multiple parameters in complex nonlinear processes.Therefore,lithium niobate with specially designed domain pattern,i.e.,nonlinear optical superlattice has been proposed as an effective carrier for the general study of nonlinear optics,which has been widely used in beam shaping,electro-optical modulator,quantum communication and other fields.In this thesis,lithium niobate thin films with thickness of about 30-50?m is the main object of study.Preparation technology of domain structures are studied and applied to nonlinear beam shaping combined with nonlinear computer-generated hologram.They are included as follows:1.The development of nonlinear optics and optical superlattice,as well as the application and development of pattern poled lithium niobate crystals in beam shaping are reviewed.The three-wave coupling process,frequency doubling effect and quasi-phase matching technique in nonlinear optics are analyzed theoretically.Two types of nonlinear diffraction based on pattern poled lithium niobate crystals are introduced,namely Raman–Nath diffraction and cherenkov diffraction.The theory of nonlinear optical computer-generated holography is introduced and the distribution of nonlinear coefficient is derived,which lays a theoretical foundation for the follow-up work in chapter 3.2.The technological process and detailed domain patterning parameters of lithium niobate film in our group are mainly introduced.After many attempts,we have established a relatively mature method for preparing various domain structures into lithium niobate films.The whole process includes cleaning,lithography,coating and polarization etc.Compared with 0.5mm thick lithium niobate crystal,the domain structure prepared with 30-50?m thick lithium niobate film is more uniform,easier to penetrate,more stable,and can be prepared at normal pressure and temperature,which is safer.In addition,the film is also more conducive to the device integration.3.The generation process of second harmonic Ince-Gaussian beams is described in detail.The theory of nonlinear optical computer-generated holography and the quasi-phase-matching technique are introduced into beam shaping,and the hologram carrying the information of Ince-Gaussian beams can be simulated theoretically by selecting the appropriate modulation period.In the experiment,the domain structures consistent with the computed holograms are prepared via electric field poling.The domain structures can not only satisfy the transverse phase matching condition,but also record the amplitude and phase information of Ince-Gaussian beams.The 900 nm fundamental wave Gaussian beam is focused on the sample through a lens,and a short-pass filter is placed on the sample's exit surface to filter out the fundamental wave.The light spots received by CCD at the first order diffraction in the far field are the second harmonic Ince-Gaussian beams.In this process,the functions of frequency conversion and beam shaping are completed simultaneously.