Domain Engineerng And Function-integrated Optical Devices Based On LiNbO₃ Superlatice

Lithium niobate has been a hot topic for its versatile properties in optics,acoustics and electrics.Electro-optical and acousto-optical modulators based on lithium niobate have already been used in lasers,optical communications,etc.Through ferroelectric domains modulation,optical and acoustical superlattices could be formed,which have interesting applications in different fields,such as quasi-phase matching(QPM)nonlinear frequency conversion and electro-optic(EO)polarization rotation.Lithium niobate's transparent window is quite large,and it is easy to realize waveguide optics.Recently,through the coupling of photon,phonon,and domain structures,new physical mechnisms and applications of lithium niobate superlattices have been revealed both in classical and quantum regimes.This traditional optoelectronic material is still playing an important role in up-to-date researches.In this thesis,in order to catch up with rapid development of photon integrations,we discussed the ferroelectric properties of lithium niobate thin film,proposed some functional integrated devices,revealed the transformation law of optical fields(photon state)in nonlinear processes,and prospected its application in terahertz band.1.Physical and chemical properties of lithium niobate are briefly introduced.The fabrication and inspection of ferroelectric domain are also discussed.Some commonly-used phase matching methods are reviewed,such as birefringence phase matching and QPM.If the electric fields are applied along x,y or z axes of lithium niobate,the electro-optical responses are different.Nonlinear frequency conversion based on lithium niobate superlattice employing QPM is also studied.Furthermore,some developing trends of lithium niobate are reviewed.2.A newly proposed lithium niobate material,called lithium niobate thin film on insulator(LNOI)is introduced,including fabrication,application,etc.The ferroelectric property of LNOI is discussed in detail.Two sets of specimens with different thicknesses,i.e.,micron(?28 ?m)and submicron(540 nm)thick LNOI.For micron thick samples,domain structures are achieved by pulsed electric field poling with electrodes patterned via photolithography.No domain structure deterioration has been observed for a month as inspected using polarizing optical microscopy and etching.As for submicron LNOI,large-area domain inversion is realized by scanning a biased conductive tip in a piezoelectric force microscope.Domain stability is investigated by measuring the pizeoresponses to stimulus applied on the tip.A domain life time of 25.0 h is obtained.Our study gives a direct reference for domain structure-related applications of LNOI,including guiding wave nonlinear frequency conversion,nonlinear wavefront tailoring,EO modulation,and piezoelectric devices.3.We propose three different functional integrated optical devices based on lithium niobate superlattice.Firstly,optical parametric amplification(OPA)of arbitrarily polarized light could be realized in a four-section periodically poled lithium niobate.The first and fourth sections are used for QPM OPA,while EO polarization rotation is realized in two central sections,respectively.In this case,not only pump,signal and idler waves coupled together through nonlinear interaction,ordinary and extraordinary lights also exchange their power for EO effect.Beams with complex wavefront could be generated and amplified using this structure.Secondly,tunable dual-wavelength filter is achieved using lithium niobate superlattice with a central defect.If electric fields are applied on the y surfaces of the sample,the transmission spectrum has two valleys,which are experimentally testified.Group delay dispersion is discussed.Furthermore,if the defect is not in the center,resulting in asymmetrical structure,flat-top filter could be obtained through structure optimization.The contrast ratio of the filter could also be tuned by external electric fields.4.Optical vortices carrying orbital angular momentum(OAM)are introduced,including generation,transmission,detection and applications.Then we propose and investigate the QPM nonlinear optical frequency conversion of optical vortices in periodically poled lithium niobate.Laguerre-Gaussian(LG)modes are used to represent the OAM states,characterized with the azimuthal and radial indices.Typical three-wave nonlinear interactions among the involved OAM modes are studied with the help of coupling wave equations.Being different from normal QPM process where the energy and quasi momentum conservations are satisfied,both of the azimuthal and radial indices of the OAM states stay constant in most of the cases.However,abnormal change of the radial index is observed when there is asynchronous nonlinear conversion in different parts of the beams.The QPM nonlinear evolution of fractional OAM states is also discussed showing some interesting properties.In comparison with the traditional birefringent phase matching,the QPM technique avoids the undesired walk-off effect to reserve high-quality LG modes.We believe the QPM is an effective way to convert,amplify,and switch OAM states in various optical vortex related applications.5.Some backgrounds of terahertz are briefly reviewed.Then,the refractive indices of ordinary and extraordinary waves(no and ne)of congruent LiNbO3(CLN)in the terahertz(THz)band are measured.Extended Cauchy equations are proposed to describe the temperature-and frequency-dependent refractive indices.With less than 0.5%deviation,both no and ne can be obtained accurately.The values of no and ne are?7 and?5.4,respectively,which are much larger than those in the visible band.The extremely high birefringence(?1.6)may lead to some interesting applications of linear,EO,and nonlinear optical effects of CLN.As an example,true zero-order THz waveplates could be obtained with considerable low loss based on thin CLN wafers.The working frequency can be tuned by changing the environmental temperature.Moreover,wide bandwidth achromatic THz WPs were also achievable by stacking four CLN wafers together with precise orientation control.