Second harmonic generation from the `centrosymmetric' crystals
What is it about?
Second harmonic generation (SHG) is a well known non-linear optical phenomena which can be observed only in non-centrosymmetric crystals due to non-zero hyperpolarizability. In the current work we observed SHG from a Zn(II) complex which was originally thought to have crystallized in the centrosymmetric space group C2/c. This has been attributed to the unequal antiparallel packing of the metal complexes in the non-symmetric space group Cc or residual non-centrosymmetry in C2/c giving rise to polarizability leading to strong SHG. The enhancement of SHG by UV light has been attributed to the increase in non-centrosymmetry and hence polarity of packing due to strain induced in the crystals. The SHG signals measured from these crystals were as large as potassium dihydrogen phosphate crystals, KH2PO4 (KDP), and showed temperature dependence. The highest SHG efficiency was observed at 50 K. The SHG phenomenon was observed at broad wavelengths ranging from visible to below-red in these crystals.
Why is it important?
In summary, this communication reports so-called `centrosymmetric' crystals of a Zn(II) complex, which is indicated surprisingly to have crystallized with a residual non-centrosymmetry in space group C2/c based on the observation of SHG. A small excess of polarity packing in the otherwise centrosymmetric single crystals appears to result in a large second-order nonlinearity which can be further enhanced by increasing the strain caused by the UV light exposure. These led to a strong dependence of χ(2) on the inhomogeneity created by the induced strain. The induced strain varies with temperature, as indicated from the temperature-dependent SHG measurements. By increasing the temperature from 10 to 50 K, a sharp increase in the SHG signal is followed by a slow decrease to its minimum at ∼ 175 K. A further increase in the temperature to 300 K is accompanied by a slow increase in the SHG signal. The underlying mechanism has been discussed.