Second-order optical effects in organometallic nanocomposites induced by an acoustic field

Abstract

Acoustically stimulated second-order optical effects in Ru-derivative nanocomposites were discovered. The alkynyl ruthenium derivatives were embedded in a polymethyl methacrylate (PMMA) polymer matrix. As second-order optical effects we studied second-harmonic generation (SHG) and linear electro-optics (LEO) phenomena. The physical insight of the effect observed consists in a coexistence of nanocofined chromophore levels and localized $d$ states of ruthenium. A transverse acoustic field favors the occurrence of charge density noncentrosymmetry required for observation of the second-order optical effects, particularly SHG. We have found that acoustically induced SHG and LEO for fundamental YAB-${\mathrm{Gd}}^{3+}$ laser light $(\ensuremath{\lambda}=1.76\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ increases and achieves a maximum value at acoustic power density of about $1.45\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. The values of the SHG for several Ru chromophores were higher than those for well-known inorganic crystals. With decreasing temperature, the SHG signal strongly increases below 55 K and correlates well with occurrence of ``softlike'' low-frequency anharmonic quasiphonon modes responsible for the phase transitions. The SHG maxima were observed at acoustic frequencies of about 13 kHz. Increasing of acoustical frequencies up to the megahertz range suppresses the observed phenomena. Comparing the obtained results with the acoustically induced Raman spectra at different temperatures one can conclude that the observed effects are due to acoustically induced electron-vibration anharmonicity, and are observed at temperatures below 55 K. Varying the chromophore content within the embedded matrices we were able to use effective nanoparticle sizes within the range 5--60 nm. It is clearly shown that the enhancement of the effective nanosize effectively suppresses the observed second-order optical effects.