| Since the discovery of laser,the nonlinear optical frenquency conversion technique has attracted considerable attentions.It utilizes the the nonlinear polarization of nonlinear optical crystals under strong light field to expand the available laser wave range,so that obtaining coherent laser sources with different frequencies.The second order nonlinear optical processes,which are always used for nonlinear optical frenquency conversion,include optical frequency doubling,sum frequency generation,difference frequency generation,optical parametric amplification and oscillation,etc.The efficiency of energy conversion in these processes depends on the phase relationship among the interacting light waves.High conversion efficiency requires the satisfication of momentum conservation,also named as phase matching.However,due to the dispersion property of nonlinear optical crystals,the phase mismatching always occurs,thus,phase matching condition can not be realized.There are two widely used methods to solve this problem,the first one is the birefringence phase matching which use the natural birefringence properties of nonlinear optical crystals,the second one is quasi phase matching(QPM)utilizing periodically modulated second order nonlinearity in nonlinear optical mediums.In 1962,the QPM theory was proposed by Bloembergen who won the Nobal prize later.It skillfully leads the reciprocal lattice vectors of periodic structure into frequency domain to realize phae matching.The QPM makes phase matching condition more neatly,and it is capable of reaching phase matching at multiwavelength.Now,the QPM technique has been widely researched and applied in the field of optical frequency conversion,modulation of pulse waves,THz wave generation and entangled photon generation,just name a few.The key factor of QPM is choosing appropriate period or quasi period ?(2)structures,always realized by ferroelectric domain engineering in ferroelectric crystals.On both sides of the antiparallel domains in ferroelectric crystals,the sign of second order nonlinear coefficient are contrary.Therefore,the period or quasi period arrangement of ferroelectric domains means the modulation of second order nonlinearity.Since the 1990s,with the development of electric field poling technique for domain engineering,the QPM technique has made great progresses.Most of all,the opcial superlattices consitituted by the one-dimensional period/quasi period structures fabricated by Nanjing University have been empolyed to realize various QPM processes.Based on this,the QPM theory is developed,many novel physics phenomenons were discovered,and the structures have been applied in the field of laser generation with three primay colours and optical chips.In the recent years,along with the progress of domain engineering,the two-dimensional QPM ferroelectric crystals and the related devices develop rapidly.The incremental modulation freedom of reciprocal lattice vectors have shown unique charm in multiwavelength and multidirection phase matching,and people's cognition on QPM and nonlinear optics were deepen.However,the three-dimensional(3D)QPM requires 3D domain engineer technique,and now the correlational research only consist in a few theoretical works.In this work,we intend to the development state of QPM,emphasise on the fabrication of 3D ferroelectric domain structure and experimental study about 3D QPM.Barium calcium titanate(Bai-xCaxTiO3,BCT)crystals which possess random 3D domain stacks were grown by Czochralski method,and the basic phsical properties were characterized.The natural 3D domain structure were observed,the 3D reciprocal lattice vectors and possible QPM wavelengths and conditions were calculated.The BCT crystal was employed as the nonlinear optical medium.Ultilizing its 3D reciprocal lattice vectores,broadband 3D QPM processes were realized.Contrable 3D ferroelectric domain structure were fabricated in BCT crystal by near infarad femtosecond laser writing,thus,the 3D nonlinear photonic crystals were realized.Then,we achieved 3D QPM in the written domain structures and analyzed the interaction between light waves and 3D reciprocal lattice vectors.1.Growth and characterization of BCT crystalsBCT crystal is a tetragonal perovskite ferroelectric at room temperature.Naturally grown BCT crystal possess antiparallel domains along three crystallographic axes,which can provide 3D reciprocal lattice vectors for 3D QPM.Thus,the BCT crystal has great research potential in the field of QPM.Compared with the classic perovskite crystal-BaTiO3,the phase transition property of BCT crsytals changes rapidly with the increase of Ca content.When it reaches 0.23,the orthorhombic-tetragonal transition(about 5? in BaTiO3)vanishes.At the same time,the Curie point(tetragonal-cubic transition tempearture)remains unchanged.In this way,the tetragonal phase of BCT maintains a stable state in a wide temperature range,which may avoid the influence of domain motion to the optical interactions.So compared with BaTiO3,the BCT crystal is more valuble for QPM.Normally,to obtain BCT crystals with different components,various polycrystalline materials should be synthetised.This may lead to a long crystal growth cycle and low economic benefits.In this part,BCT crystals were grown in N2 and air atmospheres.The contents of oxygen vacancis in crystals were controled by oxygen content in growth atmospheres,by ultilizing natural differences of the affinity of oxygen atoms on Ba and Ca atoms,BCT crystals with different components(Ca content of 0.189 and 0.225)were obtained from the same polycrystalline material.The valence state of Ti ions,crystal structure,thermal and dielectric properties of the samples grown from different atmospheres were investigated.At last,transmission properties of the crystals were studied and the results show that BCT crystal grown in air atmosphere exhibits a high transmittance at visable and near infrared spectrum,which laies a foundation for the works on 3D QPM experiment.2.3D QPM modulated by spontaneous domains in BCT crystalThe QPM properties of ferroelectric crystals depend on the domain structure.In this part,different crystallographic surfaces of BCT crystal were etched and the orthogonally distributed antiparall domains were observed under the optical microscope and SEM.Available 3D reciprocal lattice vectors were calculated based on the domain structure.Afterwards,the BCT crystal was illuminated by a focused infrared laser,and cross-like second harmonic patterns were recorded in a broadband wavelength range.By analyzing the QPM process in BCT considersing the domain structure,we can see that reciprocal lattice vectors along three crsytallographic axes participate in the nonlinear optical process simultaneously,which means the realization of 3D QPM.The properties of patterns dependents on the wavelength were studied experimentally and theoretically.The relationship between second-harmonic power and fundamental power was measured and found to be quadratic relation,which proves the frequency doubling process in BCT crystal is a second order nonlinear optical process.At last,we measured the polarization property of fundamental and frequency doubled laser.By the result a symmetry breaking is discovered which is leaded by the 3D domain stacks3.3D domain structure fabricated by femtosecond laser writing and the modulated 3D QPM processesThe femtosecond laser writing technique is a flexible,convenient,high-precision and simple method for domain engineering.In this part,we fabricated 3D domain structure in BCT crystal using the near infrared femtosecond laser writing.The domain structure constitutes 3D nonlinear photonic crystal which can provide abundant reciprocal lattice vectors and can compensate phase mismatch along any directions.The written domains were characterized under Cerenkov second harmonic microscopy,the 3D distribution is confirmed and the ? shaped domain walls were discovered.Combining with the all-optical poling theory,the poling mechanism in BCT crystal is predicted.Then 3D QPM processes were implemented employing the simple tetragonal domain structures with different periods,and the frequency doubled patterns constituted by many frequency doubled points were obtained.By analyzing QPM process theoretically combined with the 3D domain structure,we got that the nonlinear interaction in BCT contains both 3D QPM process and nonlinear Raman-Nath diffraction component.The patterns were reconstituted theoretically by Fourier analysis and the QPM process of each point was investigated in detail.In addition,the wavelength response of coll inear second harmonic power dependent on the fundamental power was anaylzed experimentally and theoretically. |
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