Second harmonic generation in microcrystallite films of ultrasmall Si nanoparticles

We dispersed crystalline Si into a colloid of ultrasmall nano particles (si milar to1 nm), and reconstituted it into microcrystallites films on device- quality Si. The film is excited by near-infrared femtosecond two-photon pro cess in the range 765-835 nm, with incident average power in the range 15-7 0 mW, focused to similar to1 mum. We have observed strong radiation at half the wavelength of the incident beam. The results are analyzed in terms of second-harmonic generation, a process that is not allowed in silicon due to the centrosymmetry. Ionic vibration of or/and excitonic self-trapping on n ovel radiative Si-Si dimer phase, found only in ultrasmall nanoparticles, a re suggested as a basic mechanism for inducing anharmonicity that breaks th e centrosymmetry. (C) 2000 American Institute of Physics. [S0003- 6951(00)0 5452-8].