Of functional organic compounds as exemplified above, organic high polymers, such as polyacetylene and polydiacetylene, have recently attracting attention.  In particular, polydiacetylene series organic polymers (hereinafter referred to as "PDA organic polymers") comprising a repeating unit of formula (I) shown below turned out to have excellent electrical and optical characteristics, and intensive studies on their practical application have been made.  An optical element utilizing the cubic nonlinear properties is one of the important application field of the above organic compounds.  A thin film type waveguide element attracts attention as the most practical application of such an element. ## STR1##
In order to use organic compounds such as PDA as an optical element, it is important to produce an organic compound film having less light scattering and a size sufficient to form the optical element.  It is therefore necessary to manufacture an organic crystal film of a single crystal, which has less light scattering than polycrystals, on a smooth surface of a substrate having no unevenness which is a cause of light scattering. 
PDA organic polymers have great anisotropic characteristics due to their cubic nonlinear optical characteristics on account of the .pi.-electron conjugated system in the main chain direction and are therefore characterized by showing great functions in their main chain direction but no substantial function in the direction perpendicular to the main chain. 
In the production of a PDA organic polymer film, it is necessary to control the orientation of the PDA main chain so that the resulting film may perform a function as desired because of the above-mentioned great anisotropy.  However, PDA organic polymers have poor processability due to solvent insolubility and are therefore difficult to obtain in a thin film form while controlling the orientation as easy with general crystal materials. 
Some of diacetylene monomers represented by formula (II): EQU R.sup.1 --C.tbd. C--C.tbd. C--R.sup.2 (II)
which are starting compounds for PDA organic polymers exhibit solid phase polymerizability.  It is known that the crystals of such solid phase polymerizable diacetylene monomers are solid phase polymerized according to the following reaction formula by ultraviolet or .gamma.-rays irradiation or by heating to form a crystal of an organic high polymer without involving a great change in crystal structure. ## STR2##
Several approaches to production of an oriented PDA organic polymer film by utilizing the solid phase polymerizability have been proposed, in which a diacetylene monomer crystal is first made to grow under orientation control to obtain a film, which is then subjected to solid phase polymerization to obtain an oriented PDA organic polymer film. 
The above-mentioned production process of an oriented organic polymer film is also important as an application of other properties such as electrical conductivity. 
In forming a film of an organic compound crystal, not being limited to an organic polymer, on a substrate, with utility of the film as an element being taken into consideration, the organic compound crystal is required to be oriented as unidirectionally as possible over a sufficient area for application as an element on a substrate having no surface evenness.  For example, for an organic crystal film to be used as an element, the film is required to comprise a relatively large single crystal of the order of from several hundred microns to millimeters. 
Among conventionally known orientation techniques is included an epitaxial growth method utilizing a molecular level interaction between a crystal substrate and an organic compound.  In this method, since an organic compound shows crystal growth in the direction reflecting the symmetry of the substrate surface, it is necessary to use a substrate showing twofold symmetry for obtaining a unidirectionally oriented crystal film.  The problem here is that crystal substrates whose surface has twofold symmetry are limited in kind and the organic compounds capable of epitaxy on such a substrate surface are also limited in kind. 
J. Le Moigne, et al.  reported a method comprising depositing a diacetylene monomer of formula (II) wherein R.sup.1 and R.sup.2 both represent a 9-carbazolyl group on a cleavage plane of a KBr single crystal by vacuum evaporation and then subjecting the deposit to solid phase polymerization to obtain a crystal film of a PDA organic polymer with controlled orientation of its main chain (PDA-DCHD) (see J. Chem.  Phys.,  Vol.  88, p. 6647 (1988)).  However, this method cannot be a complete success since the PDA main chain is not oriented unidirectionally but bidirectionally (crossing directions) due to fourfold symmetry of the cleavage plane of a KBr single crystal.  Further, the resulting film has a polycrystalline structure, which is an aggregate of crystallites of several microns square, and cannot be applied to a practical element. 
T. Kanetake, et al.  reported preparation of a PDA crystal film having the PDA main chain oriented unidirectionally, which comprises depositing a diacetylene monomer on a glass substrate followed by solid phase polymerization, unidirectionally rubbing the surface of the resulting film with silicone cloth, etc.,  again depositing the same monomer thereon to obtain a unidirectionally oriented monomer crystal film, and solid phase polymerizing the deposited monomer (see Appl.  Phys.  Lett.,  Vol.  54, p. 2287 (1989)).  In this case, also, the resulting film has a polycrystalline structure composed of several microns square crystallites and is not applicable to a practical element. 
M. Thakur, et al.  tried making a diacetylene monomer crystal in a molten or dissolved state to grow between a pair of smooth glass substrates while sliding the substrates in contact with each other in their plane direction to give a shear force to the surface of the substrates (see Macromolecules, Vol.  18, p. 2341 (1985)).  According to the report, while a single crystal film reaching a millimeters square size can be obtained, the crystal growth direction cannot be controlled completely due to the use of an amorphous glass substrate. 
JP-A-3-59036 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method for producing an organic compound crystal film comprising immersing, at a slant, a plurality of smooth substrates laid on each other via spacers in a solution of an organic compound for single crystal growth, and slowly evaporating the solvent to make the organic compound crystal grow between the substrates. 
Y. Iwasa, et al.  proposed a method for producing a platinum compound film comprising setting a silicone flat substrate at an angle of about 10.degree.,  and casting a solution of a platinum compound over the substrate from the upper end thereof simultaneously with evaporation of the solvent to make the crystal to grow (see Appl.  Phys.  Lett.,  Vol.  59, p. 2219 (1991)). 
According to these methods using a tilted substrate, a single crystal film of millimeters square size can be produced efficiently.  However, these methods all involve the problem of incomplete control of the crystal growth direction due to the use of an amorphous substrate. 
An attempt to achieve orientation control by a geometrical macrostructure of the surface of a substrate has been made by T. Suhara, et al. ( see IEEE photo.  Tech.  Letter.,  Vol.  3, p. 241 (1991)).  The geometrical macrostructure used in this technique is composed of a pair of substrates having therebetween a pair or spacers parallel to the plane of the substrates, the pair of spacers forming a tapered recess therebetween, forming a groove area at one end which connects to a wide area at the other end.  A molten liquid of mNA is fed to the groove, and the mNA crystal is made to grow starting from the groove area toward the wide area.  A crystal film formed in the wide area shows crystal orientation controlled by the geometrically regulating force of the groove area. 
In order to control the crystal orientation in this method, the groove width is required to be relatively small with respect to the crystal nucleus generated in the initial stage of crystal development.  However, the groove in the above-mentioned technique is as wide as not less than 200 .mu.m in width, and the materials which may be controlled in crystal orientation on the substrate of that shape are very limited. 
JP-A-2-8823 discloses a method for crystal growth of a low molecular weight organic square nonlinear optical material by using a substrate having on the surface thereof grooves with a cyclic structure of a period of 3 .mu.m or less.  JP-A-4-12332 discloses a method for crystal growth of a low molecular weight compound by using a substrate having on the surface thereof irregular grooves formed by a diamond paste.  However, the resulting organic crystal films of these methods have a problem in applying to an optical element since they are formed on the grooves.