Crystalline Organic Films for Optical Applications

Marshall Space Flight Center, Alabama

Cells that contain thin, single-crystal films of photoresponsive organic materials [e.g., meta-nitroaniline (m-NA)] have been invented for use as nonlinear optics and especially as second-harmonic generators. In comparison with crystals of potassium dihydrogen phosphate (KDP) that have been used previously in such applications, the crystals of this invention are smaller and are capable of producing second harmonics of input light at lower input power levels.

A cell according to the invention (see figure) includes an upper and a lower plate made of fused quartz or other transparent material. A recess to hold the organic material is formed in the lower plate. The recess includes a groove around its periphery. The upper surface of the lower plate, the broad upward-facing surface of the recess, and the lower surface of the upper plate are optically polished to (1) eliminate defects that would otherwise act as seeds for undesired nucleation of multiple crystals in the final crystallization process described below and (2) enable the upper and lower plates to fit closely together. The dimensions of the final organic crystal are determined by the dimensions of the recess - typically a diameter of the order of 10 mm and a thickness between 0.5 and 500 μm.

The quantity of the organic material placed in the recess is chosen so that the material fills all of the recess except for the groove; this is because the groove serves to absorb any excess of the organic material during melting and/or thermal expansion, to prevent the material from flowing between (and thereby forcing apart) the faying surfaces of the upper and lower plates. Once the organic material is placed in the recess, the upper and lower plates are put together. The resulting cell containing the organic material is heated to melt the organic material, then cooled to freeze the organic material in a polycrystalline form with an even distribution of grains.

The cell is then placed on a controlled-temperature, heated stage under a polarizing microscope, so that the organic material can be heated and cooled for the final crystallization and the microscope can be used to observe the crystallization process. The temperature of the stage is first increased to the melting point, taking care that except for a single seed crystal of desired orientation, all the organic material is melted. The seed crystal can be singled out under the microscope, and while all other crystals melt, it can be kept solid by exposing it to a microjet of cool air. The temperature of the stage is then slowly decreased, causing the organic material to freeze as a single crystal that grows outward from the seed crystal.

This work was done by Alexander Leyderman of the University of Puerto Rico for Marshall Space Flight Center. For further information, please contact the innovator at This email address is being protected from spambots. You need JavaScript enabled to view it..

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