孤立電子對在氧化物、氮化物、鹵化物晶體之光學非線性所扮演的角色

Laser sources of different wavelengths have their own important use in industrial applications as well as fundamental researches, but since light is emitted as a result of transition between electronic states, the available wavelengths is limited in the entire EM wave spectrum. Frequency conversion techniques using Non-linear optical crystals allow the production of laser beam at different wavelengths, and are therefore very desirable. For example, harmonic generation can be used to produce laser beam at shorter wavelength which is essential for the next generation lithography, optical storage and fiber communication. Since each NLO crystal works at certain frequency range and has their own characteristics, it is also desirable to search for wider variety of them. A systematic study of the mechanism of NLO properties of these crystals will be beneficial. In UV/Visible range, oxide crystals are one of the most important types, which usually have high damage threshold, making them durable under high laser power operation. There has already some progress in the past on understanding the mechanism of NLO properties of oxides. Borate, for example, was found to have largely due to their anion group. In this work, we have proposed a new analysis scheme, which reveals the main factor of optical non-linearity from the picture of electron density. Using one of our analyzing tools band-resolved of second order susceptibility, one can identify which several orbitals or bands contribute most in a molecule. This work extends the idea further and makes it applied on solids even more easily. This is achieved by summing each orbital density based on the SHG weight. Through this SHG-density plots we can visualize the electron densities that have with significant contribution to the optical nonlinearity and reveal the SHG mechanism. Using this method that we found that lone-pair electrons play the major role in non-linear optics mechanism of oxide family NLO crystals. In order to identify lone-pair electrons, the technique of “Lone-pair identification scheme” is developed. After testing this scheme on a few simple molecules known to have lone-pair, the reliability of the scheme is confirmed. This method is then applied to the structures which we are interested in, and it dose prove our claim — lone-pair is one of the significant source leads to the non-linear optical properties of oxide. This leads to a further understanding on the mechanism of non-linear optical materials. Further more, non-linear optical Nitride and Halide crystals which are next to VIA group are also analyzed by this technique. Some preliminary results were obtained which might be useful toward a general understanding on the properties of various non-linear optical materials.