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Due to the highly nonlinear optical properties and the
versatility in synthesis, organic materials are quite promising
in optics devices. Crystal properties, mainly determined by the
growth environment and impurities segregation, are very
important in application. The vertical dynamic gradient freeze
(VDGF) technique using a transparent furnance was applied to the
growth of benzil single crystal for nonlinear optics. The
effects of dopants on crystal growth, segregation and optical
properties were studied by growing observation and sequential
measurements of dopants concentration and second harmonic
generation (SHG). In the VDGF method, axial segregaton of
dopant is mainly controlled by diffusion and radial segregation
is mainly affected by the flow pattern near interface and
interface shape. Due to axial segregation, radial segregation
varies along the axial position. Doping benzoin to benzil can
reduce cracks. It is possible due to benzoin offering hydrogen
bonding and increasing mechanical strength of benzil. Because it
is not easy for $Nd^{3+}$ to be embeded in benzil crystal
lattice,doping high concentration of $Nd^{3+}$ will cause many
small shattered-like cracks. However the cracks caused by planes
glide were reduced at low $Nd^{3+}$ concentration, such as 1.24
ppm. $Nd^{3+}$-doped benzil has a higher SHG conversion
efficiency than the pure benzil. As the $Nd^{3+}$ concentration
increased, the conversion efficiency increased.The small cracks
tendency may also reduce radiation damage threshold.By releasing
the heat trapped in the crystal, the radiation damage threshold
may increase.
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