Designer crystals double frequencies

17 Jun 2002

Second-harmonic generation of any frequency will be possible thanks to crystal research in today's Science.

Laser engineers could one day buy made-to-order polar crystals for second-harmonic generation (SHG) thanks to research at the University of Minnesota, US (Science 294 1907).

A group from the Chemical Engineering and Materials Science has designed organic crystals with predetermined polar symmetries by inserting guest molecules into a crystalline framework. It is the first time that structure (given by the host framework) and function (given by the guest) have been separated by crystal engineering.

In this way, the researchers, led by Michael Ward, have managed to make polar crystals from guest materials that do not normally crystallize or that normally exist as liquids at room temperature. This opens the door to new types of polar crystals which could be used for SHG.

Ward said: "We have been successful in the design of the most intricate details of a crystalline structure, namely the lattice parameter and space group symmetry. In particular we were able to design a series of compounds with prescribed polar crystallographic symmetries and SHG activity."

To date, the researchers have observed modest SHG from the structures by frequency doubling an Nd:YAG laser. Ward told Optics.Org: "There have been other organic materials with a greater SHG response, but the key difference is that our materials are designed so that the host frameworks must be polar."

"The SHG activities of these materials are still only modest, though there is some potential for future compounds," said Ward.