Second harmonic generation powder intensities

The large values reported for squaraines indicate that noncentrosymmetric crystals or films of these dyes should be capable of efficient second harmonic generation (SHG). A new class of chiral D -A-D squaraines 37-44 obtained by 1,2-substitution of squaric acid showed second harmonic generation intensities of up to 64 times that of urea (Table 3) in the powder form [124,125]. X-ray crystal analysis indicated that two hydrogen bonds between adjacent mol-... 

The Kurtz powder technique developed by S. W. Kurtz and T. T. Perry in 1968 offered the first possibility for the determination of nonlinear optical properties of crystalline organic componnds. In comparison with a standard such as quartz or urea, the capability of materials to generate the second harmonic is detected. The pulverized sample is irradiated by a laser and the intensity of frequency-doubled light is measured. As this method is influenced by many factors, such as particle size, it is only semi-quantitative. The results give no precise values for the hyperpolarizability p. A further disadvantage is that the method is limited to molecules that crystallize in noncentrosynunetric space groups. 

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