Theoretical And Experimental Investigation Of Broadband Nonlinear Frequency Conversion

Materials with second-order optical nonlinearity are used in many important nonlinear optical processes.The critical route to achieve efficient nonlinear frequency conversion is to maintain phase matching of interacting lasers.The inherent dispersion in refractive index of nonlinear optical materials brings in serious phase mismatching problems.Birefringence phase matching?BPM?and quasi-phase matching?QPM?schemes are two popular routes to address this issue.engineering quadratic nonlinear photonic crystals?NPCs?are artificial materials with modulated distribution of second-order nonlinear coefficient and allow for natural while powerful means to implement QPM required for nonlinear frequency conversion.In recent years,broadband ultrashort pulse lasers have been extensively involved in nonlinear frequency conversion,however,previous theoretical approaches that are developed for monochromatic light nonlinear interaction are no longer accurate.For this cause,in this thesis we have focused on theoretical and experimental investigations of broadband nonlinear frequency conversion in NPCs,with the aim to gain a deeper insight on the underlying physics,mechanisms,and methodologies.This thesis is organized as follows.After a brief introduction to the mechanisms for phase-matching schemes and computing methods,the details of QPM sample preparation processes of NPCs are discussed.This is followed by an overview of the research progresses.In the next section,we have developed the effective nonlinear susceptibility model?ENSM?to analytically solve the general three-wave mixing nonlinear optical interaction problem.The model can be used to quantitatively evaluate the performance of various three-wave mixing interaction in general QPM structures.It is seen as a key enabling technology for controlling the phasing matching configurations of multiple nonlinear processes.Hence,we can freely design and optimize the QPM structure via engineering reciprocal lattice vectors?RLVs?bands for various specific functionalities of nonlinear frequency conversion.Owing to the high field confinement in photonic crystal fibers?PCF?and waveguides,the strong electromagnetic fields allow weak nonlinear processes to be significantly enhanced,which is benefit for the supercontinuum generation.Despite the unrivaled nonlinear response of PCF and waveguides,the pulse energy level of supercontinuum is inevitably restricted by the optical damage threshold of the material.We have reported a tunable up-conversion supercontinuum generation without the strong optical confinement in a single NPCs sample.Based on the proposed ENSM,the pattern of the chirped periodical poled lithium niobate?CPPLN?sample can be elaborately designed,which offers several continuous broadband RLVs bands.The phase mismatching of the multiple nonlinear processes can be simultaneously compensated.Driven by a near-IR femtosecond pump source,we have found the efficient up-conversion supercontinuum generation within a single CPPLN sample.Notably,the distribution of supercontinuum spectrum can be modulated by adjusting the central wavelength or the polarization of the pump source,which offers a promising way for the supercontinuum engineering and pulse shaping.Additionally,in order to illustrate the physical insight and quantitatively evaluate the nonlinear interactions of the ultrashort laser pulses,we develop a theoretical method called the broadband nonlinear coupled wave theory?BNCWT?,accompanied with the numerical algorithm.Particular emphasis is put on the theoretical derivation.Furthermore,the theory and the algorithm can be easily extended to handle the dispersion of the nonlinear medium,the spatial distribution of 2^nd nonlinear susceptibility and the non-collinear angle.The proposed theory has been successfully used to evaluate and optimize the second harmonic generation?SHG?and the difference frequency generation?DFG?of the ultrashort laser pulse.The consistency with the experimental results indicate the BNCWT is an instructive and efficient tool,which have great practical value for precise experimental prediction and optimization.