The melting process uses radio-frequency heating and an automatic diameter control system. Stoichiometric powder is supplied to the outer melt based on the weight increase of the crystal as determined by a load cell. Stoichiometric lithium niobate rests in the double-crucible melt cylinders the growth process. So far, LN and LT have been predominately used for frequency conversion into the infrared, but research is under way to extend their utility into the visible and ultraviolet. Nonlinear crystals require matching the phases of the interacting waves for efficient frequency conversion.
Conventional crystals utilize birefringence to overcome phase velocity dispersion.
In QPM, manufacturers take a raw crystal such as LN and switch the orientation of ferroelectric domains of the material 180 deg., forming sections with different periods. Although LN is easier to produce, its sister material LT has a shorter wavelength UV absorption edge, meaning that it can convert light to a shorter wavelength with less absorption of UV light, reducing the tendency toward crystal failure. Typical UV edges for LN are around 350 nm, while LT continues down to 280 nm. However, new manufacturing methods that produce stoichiometric LN and LT rather than congruent composition materials are reducing the UV wavelength edge while improving crystal quality. Nature's not perfect Myers, like others in the field of nonlinear optics, is closely watching developments in stoichiometric LN and LT manufacturing.