Studies On Nonlinear Optical Characteristics Of A New Organic Crystal: Urea L-Malic Acid

Nonlinear optical materials have been widely used in applications such as high-speed optical communication, optical signal processing, and high-density data storage. The research on organic nonlinear optical materials is attracting more and more attention because organic-based materials have larger nonlinear coefficients, faster response speed, higher damage thresholds, lower relative permittivity, and easier processability than those of inorganic materials. However, the larger nonlinear optical coefficients are always accompanied by longer maximum absorption wavelength. This correlation is called nonlinearity-transparency tradeoff. In this dissertation, the strategies of balancing the nonlinearity-transparency tradeoff are reviewed, including the utilization of special conjugated bridge, the optimal combination of donor-acceptor's strength, and the introduction of dual (mutiple) charge transfers in the design of chromophores and the construction of organic inclusion complex.The study on a new organic inclusion complex - urea L-malic acid (ULMA) - is described in this dissertation. ULMA crystal is constructed with L-malic acid held together by hydrogen bonds (O-H…O) with urea. It is a type of organic crystal able to optimize nonlinearity-transparency tradeoff.The Kurtz powder method was used to study the second order nonlinear optical properties of ULMA. Experiments showed that the nonlinear optical coefficient and laser damage threshold of ULMA were 1.57 times and 2 times, respectively, of those of KDP (@1064 nm). The broad band second harmonic generation (SHG) of ULMA was studied for the first time and the nonlinear coefficient dispersion relation was achieved in the range of 900~1300 nm. The first order hyperpolarizability of ULMA is within the same magnitude of that of urea measured by the Hyper-Rayleigh scattering method. Phase-matching of ULMA was studied. Both type I and type II phase-matching angles were numerically simulated. The results provided the foundation of phase-matching experimental study.The model of a single electron along a helical path for chiral molecular nonlinear polarization theory was chose to analyze the first order hyperpolarizabilities dispersion relation of ULMA based on its structure characteristics. Numerical simulation results were in agreement with the experiment data. The influences of microscopic parameter to the first order hyperpolarizability are discussed. Damping coefficient is a token of nonlinear conversion efficiency. The inherent frequencyω0, pitchξ, and radiusρare involved in the position, figure, and magnitude of the peak ofβ, respectively. The peak frequency ofβis proportional toω0 and less thanω0. The departure toω0 was determined byρ/ξ. Therefore, selecting different chiral molecular as host to adjustρ/ξcan achieve appropriate nonlinear optical response.ULMA crystalline film was prepared for the first time. Furthermore, the film keeps the nonlinear...