Non-linear optical phenomena in the large-sized nanocrystallites

Photoinduced second harmonic generation (PISHG) in SiC nanocrystallites (with sizes ranging from 10 to 30 nm) incorporated into an oligoetheracrylate photopolymer matrix was studied using an experimental photoinducing nonlinear optical technique and ab initio molecular dynamics simulations. PISHG measurements were made using a pulse YAG:Nd laser (gimel = 1.06 mum; duration time varying from 4 to 850 ps, laser power about 14 MW) as the probing beam and a nitrogen pulse laser (P = 25 MW; gimel = 0.377 mum) as the pumping beam synchronized with a probing beam. We found that with an increase in the nitrogen laser power, the PISHG output signal increased and achieved its maximum value at nitrogen laser photon flux of about 2.6 GW/cm(2). Maximum output PISHG signals were observed for parallel polarization of the photoinducing and probing light beams. The second order non-linear optical susceptibility observed at the maximum PISHG was about 1.2 pm/V. The PISHG signals increased as the sample temperature decreased. This increase became marked when the temperature fell below 30 K. Time-dependent probe-pump measurements showed the existence of a PISHG maximum at a time delay of about 20-25 ps. As a result of total energy structure optimization, a good correlation between the size dimensions and the degree of hexagonality within the nanocrystallites was observed. An increase in the degree of hexagonality was also found to cause an increase in the PISHG signal. At a temperature of about 4.2 K, and with a time delay of 25 ps, the maximum PISHG was observed for a degree of hexagonality of approximately 0.76. Our calculations suggest that hexagonal substructures play a key role in the photoinduced effects observed. Several discrepancies between theoretical and experimental data may be caused by specimen in-homogeneity which is usually present in such kinds of materials. ((C)) 2001 Elsevier Science B.V. All rights reserved.