Conventional materials of a wavelength converting element include inorganic materials, e.g., KH.sub.2 PO.sub.4. Recently, use of organic crystal materials having higher non-linear optical characteristics than inorganic materials, e.g., 2-methyl-4-nitroaniline (hereinafter abbreviated as MNA), 4,5-dimethyl-1,3-dithiol-2-ylidene cyanoacetate, and 1,3-dithiol-2-ylidene cyanoacetate, has also been extending.
Known techniques for producing an organic crystal include a method in which a fused liquid of the crystal is filled in a capillary tube by capillarity, once cooled to crystallize, fused again, and then cooled successively from one end of the capillary tube toward the other end to thereby allow a single crystal to grow in the capillary tube.
This method has been employed particularly for the production of a wavelength converting element which reduces by half the wavelength of a laser beam of, for example, a semi-conductor laser, by secondary harmonic generation (SHG) utilizing Cherenkov radiation.
However, some of the organic crystal materials, such as N-(4-nitrophenyl)-S-prolinol (hereinafter abbreviated as NPP), and N-(5-nitro-2-pyridyl)-S-penylalalinol (hereinafter abbreviated as NPPA), hardly crystallize on cooling of a fused liquid thereof only to be vitrified. Further, some of them, such as 9-methylcarbazole-3-carboxaldehyde, do not crystallize at all.
In addition, a single crystal obtained by crystal growth from a fused liquid of an organic crystal material having non-linear optical effects sometimes has a center of inversion symmetry and exhibits no secondary non-linear optical effects.
Hence, the above-described conventional method fails, in some cases, to produce an organic crystal having secondary non-linear optical effects. Further, since the method involves fusion of an organic crystal material, there is a fear that the organic crystal material is thermally decomposed. Furthermore, according to the method, since the organic crystal material is crystallized at the fusion point, the resulting single crystal is liable to undergo lattice defects or rearrangement, causing a great loss of a laser beam.