Nonlinear Optical Crystals: A Complete Survey

The performance of urea as a new nonlinear optical crystal for phase-matched second-harmonic generation and sum-frequency mixing in the UV is reported. Tunable radiation down to 229 nm was obtained. In particular, efficient doubling at room temperature of CW argon laser lines from 488 to 528 nm was achieved. The nonlinear coefficient of urea was measured and found to be 2.5 times that of ADP. Angle tuning curves for type II SHG, several type II frequency mixing schemes, temperature tuning data, and comparison to other nonlinear materials are presented. More accurate Sellmeier constants were also obtained using the measured angle tuning curve for type I SHG.

Efficient phase-matched second-harmonic generation and sum-frequency mixing in urea

The validity of simpler additive models, accounting for nonlinear optical properties of van der Waals crystals in terms of independent molecular contributions is questionable in the case of more associated molecular structures such as examplified by the hydrogen‐bonded crystal lattice of urea. A number of one‐electron properties of single isolated urea molecules are first considered by means of a finite‐field perturbated LCAO Hartree–Fock model implemented at both semiempirical (INDO) and ab initio levels of approximation with different basis set extensions. Charge distribution, dipole (μ), multipoles, linear (α), quadratic (β), and cubic hyperpolarizabilities γ are computed and analyzed. The α tensor is shown to be quasi‐isotropic in the molecular plane while the β tensor exhibits a stronger anisotropy and a prevailing βzzz coefficient (z is the molecular symmetry axis). The negative value of the product βzzzμz is interpreted by a decrease of the dominant excited state dipole as compared to that of the g...

Nonlinear optical properties of organic crystals with hydrogen‐bonded molecular units: The case of urea

First-principles calculations of the linear and nonlinear optical properties of KH2PO4 (KDP) and CO(NH2)2 are presented. The calculations are an extension of methods we developed earlier and applied to borate crystals. Electronic band structure obtained from a pseudopotential method is input to the calculation. For two crystals considered, the resulting indices of refraction, birefringence, and nonlinear optical coefficients are in good agreement with experiments. The origin of nonlinear effects has been explained through real-space atom-cutting analysis. For KDP, the contributions of PO4 groups to second-harmonic generation effect are dominant, and the hydrogen bonds contribute much more to birefringence. For both KDP and urea, the contributions from the virtual electron process to nonlinear optical responses are dominant.

http://tkuir.lib.tku.edu.tw:8080/dspace/bitstream/987654321/27567/1/Mechanism+of+linear+and+non-linear+optical+effects+in+KDP+and+urea+crystals.pdf

Mechanism of linear and nonlinear optical effects of KDP and urea crystals

We present four molecular organic crystals which were recently proposed for applications in nonlinear optics (three-wave-mixing and electrooptic effects) : methyl - (2,4 - dinitrophenyl) - aminopropanoate (MAP), 2 - methyl - 4 - nitroaniline (MNA), 3 - methyl - 4 - nitropyridine - 1 - oxyde (POM) and urea. The nonlinear efficiency of molecular organic crystals is seen to depend on the molecular hyperpolarizability and on the crystalline geometry. Both features are discussed and shown to lead, for crystal point groups of lower symmetry, to new structural relations between crystalline non linear coefficients. Each of the four molecular crystals discussed here are shown to have specific properties connected to different types of applications.

D. Bäuerle

Papers

Phase-matched second harmonic generation in urea