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We show that light emission from different systems of silicon nanocrystals does behave as expected for indirect-band-gap quantum dots. Photoluminescence excited on the low energy part of the distribution of Si nanocrystals exhibits a set of narrow peaks associated with Si TA and TO momentumconserving phonon-assisted optical transitions. These spectra allow us to determine the ratio of nophonon transitions to TA and TO phonon-assisted processes over a wide range of confinement energies. The ratio between these recombination channels changes by 2 orders of magnitude with increasing confinement energy. For confinement energies above 0.7 eV the radiative transitions are governed by no-phonon quasidirect processes. [S0031-9007 (98)07199-3] PACS numbers: 78.55.Ap, 78.66.Li

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We report on a strong intrinsic optical anisotropy of Si induced by its dielectric patterning. As a result, an in-plane birefringence for nanostructured (110) Si surfaces is found to be 104 times stronger than that observed in bulk silicon crystals. We found the value of birefringence to be strongly dependent on the dielectric surrounding of the silicon nanoparticles assembling these layers. Beyond numerous potential implications for realization of optical devices and sensors, this gives a favorable route for studying the physics of condensation phenomena in a mesoscopic geometrical scale.

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